Scientific data on taste and smell in animals. Chemical communication

The cause of ocean water poisoning.

American scientists from the state of Michigan believe that bacteria are the main cause of mercury poisoning of the world's oceans.

The secret of frog survival.

American scientists managed to figure out how frogs manage to continue living even after deep freezing.

The secret of the night bat's longevity.

Biologists have long believed that the lifespan of an animal is determined very simply: the larger it is, the longer it lives.

Biological characteristics of the class

Page 5

The role of smell in the behavior of amphibians. In various behavioral actions of animals, the processes of communication, searching for mating partners, marking boundaries, etc. are associated with smell. There are many ways to transmit information, and especially in the living world, the “language” of smells is widespread. Amphibians use special chemical marks for this - pheromones. These biologically active substances are automatically released by the animal’s body at the right time. And the olfactory system, for example, of a female or fellow tribesman, with the help of its receptors, perceives information about the traces left. Then the obtained data is compared with the odor standards stored in memory. And only then does the animal receive a command for certain purposeful actions - say, the female approaching a place prepared by the male for laying eggs, etc. Many amphibians mark and protect their territory. And some of them, like, for example, the American lungless amphibian - the ashy ground salamander, not only perfectly recognize and distinguish their own marks from others, but also the scent traces of salamanders of their species. The red-backed salamander always sniffs attentively near its home site. And if he inadvertently crosses the property of his neighbors, he tries to return to his site as quickly as possible. But she simply ignores the boundaries of the territories of salamanders of other species. And salamanders protect their possessions only from uninvited guests of their own species. When they invade an area, the amphibian immediately releases a special chemical that signals that the territory is occupied. The sense of smell is especially important for amphibians with poor vision or blindness. For example, tailed amphibians - European proteas, living in cave rivers and streams, when traveling through dark underground reservoirs, necessarily leave their pheromone marks on the substrates. And then they are guided by these odors or similar chemical traces of other proteas, which persist for at least five days. The female follows the trail left by the male and searches for him. By smell, the protea recognizes all its closest neighbors and is careful not to enter the territory of an aggressive male.

The sense of smell can play an important role in the orientation of amphibians in the area when they search for their permanent spawning reservoir in the spring. After all, each pond or swamp has its own smell due to a different combination of surrounding vegetation, the amount and type of algae, etc. Studies have shown that, for example, a leopard frog in a T-shaped maze (with two diverging corridors with different compositions of water at their end) accurately determines at a fork which side the water from its pond is on. Feeling a pleasant aroma for it, the frog turns towards the pond water.

Response to natural phenomena.

Amphibians, like many living beings, are characterized by an as yet inexplicable sensitivity to various natural phenomena. Frogs, for example, thanks to their analyzers, clearly respond to any weather changes. Even with the impending weather situation, the color of the frog's skin changes: before the rain it acquires a grayish tint, and in clear weather it turns a little yellow. And thus, frogs prepare in advance for the future light spectrum, and the necessary pigment grains appear in their skin cells. But it remains a mystery how amphibians learn about weather changes several hours in advance. Scientists suggest that their bodies have electrosensitive analyzers that are capable of detecting even small changes in the charges of atmospheric electricity. The search continues to confirm that frogs can perceive information about upcoming weather changes through the interaction of natural fields with the body's own electric field.

Orientation and navigation ability

Thanks to orientation, animals are able to determine their location in space and carry out purposeful movements. The most complex form of spatial orientation is navigation. This is the ability of animals to choose the right direction of movement during long-distance migrations. When navigating, three methods of orientation are used: laying a path along familiar landmarks; compass orientation - moving along a certain azimuth, etc., without using landmarks; true navigation is the ability to get to a goal (place of breeding, food source, etc.) without using a compass and familiar landmarks. Amphibians can use all three orientation methods. Their orientation and navigation are almost always the result of analyzing and comparing information they receive from the outside world.

Taste analyzer

The organ of taste is one of the chemical sense organs and contributes to a preliminary analysis of the quality of various substances entering the oral cavity. Nutrients have an irritating effect on hair follicles only in a dissolved state. The solvent for them in the oral cavity is saliva. On the surface of the tongue, areas of specific sensitivity can be identified, with taste buds containing receptor cells that respond to chemical compounds that have a specific taste.

The sensation of taste arises as a result of the action of chemical solutions on the chemoreceptors of the taste formations of the tongue and oral mucosa; in this case, sensations of bitter, sour, sweet, salty or mixed taste arise. The sense of taste in newborn babies awakens before all other sensations.

The main part of the taste organ is the so-called taste buds (taste buds), which are in taste buds tongue, as well as in the soft palate and pharynx. Taste buds consist of special cells, around which taste (sensitive) nerve fibers end. Special glands located between the taste buds secrete a liquid that washes the taste buds.

Entering the oral cavity, food chemicals dissolved by saliva enter the recesses of the taste buds, where they come into contact with microvilli formed by the membranes of sensitive cells. They contribute to the formation of receptor potential in them, which passes along the nerve fibers of the taste nerve first to the medulla oblongata, and from it to the cerebral cortex. This is where the sensation of taste is created. Brain department taste analyzer located in the temporal lobe. The sense of taste plays a very important role in the digestion process. It stimulates the food center, which is perceived as a feeling of appetite. Stimulation of the food center has a stimulating effect on the digestive tract.

In the sense of taste of food, its smell is of great importance. When food enters the animal's mouth, it determines whether it is edible. The nature of the saliva secreted depends on this. When edible substances come in, thick mucous saliva is released, and when inedible or irritating substances come in, liquid saliva is released (some substances can cause vomiting). Edible food has a stimulating effect on the digestive system.

Taste perception is directly related to the sense of smell.

Olfactory communication, sense of smell

Smell - this is the perception by animals through the corresponding organs of a certain property (smell) of chemical compounds in the environment. The sense of smell differs from taste perception in that the odorous substances perceived with its help are usually present in lower concentrations. They serve only as signals indicating certain objects or events in the external environment. Land animals perceive odorous substances in the form of vapors delivered to the olfactory organ with air flow or by diffusion, and aquatic animals - in the form of solutions. For many animals: insects, fish, predators, rodents, smell is more important than sight and hearing, since it gives them more information about the environment. Sensitivity to odors is sometimes simply fantastic: for example, the males of some butterflies react to several molecules of the female sex pheromone in a cubic meter of air.

The degree of development of the sense of smell can vary quite greatly even within the same taxonomic group of animals. Thus, mammals are divided into macrosmatics, whose sense of smell is well developed (most species include them), microsmatics with a relatively weak development of the sense of smell (seals, baleen whales, primates) and anosmatics, who lack typical olfactory organs (toothed whales).

The sense of smell serves animals to search and select food, track prey, escape from an enemy, for bioorientation and biocommunication (marking territory, finding and recognizing a sexual partner, etc.). Fish, amphibians, and mammals are good at distinguishing the odors of individuals of their own and other species, and common group odors allow animals to distinguish “friends” from “strangers” (Fig. 4.4).

The number of odorous substances is huge, and the smell of each of them is unique: no two different chemical compounds have exactly the same smell. Based on the effect of odors, they can be divided into attractive and exciting, repulsive and indifferent. Attractive and stimulating odors have a positive physiological significance for the animal’s body. Such odors include the smell of food, the smell of female secretions during the breeding season, the smell of the owner for the dog, etc.

Rice. 4.4.

Repulsive odors do not have a positive physiological meaning and cause reactions in the body aimed at freeing themselves from their effects. An example of such odors can be the strong odors of perfumes, tobacco, and paint. For some animals, this smell will be the smell of a predator.

Olfactory analyzer vertebrates consists of a perceptive apparatus, pathways and a cortical center.

Olfactory organ vertebrates is a peripheral apparatus of the olfactory analyzer. It is located in the nasal cavity and occupies a relatively small area in the area of the upper nasal passage and the posterior part of the nasal septum. The mucous membrane of the olfactory region is covered with olfactory epithelium, which is the direct receptor apparatus of the olfactory analyzer. The olfactory epithelium consists of olfactory cells that have a spindle-shaped shape due to the presence of one dendrite and one axon. The dendrite ends on the surface of the mucous membrane with olfactory vesicles equipped with cilia. The cilia are immersed in a layer of mucus covering the surface of the olfactory epithelium. Molecules brought by the air flow come into contact with the membranes of the cilia and stimulate them to generate an impulse.

Olfactory bulbs - These are protrusions of the medulla of the brain, which are a collection of nerve cells. In the olfactory bulbs, the fibers of the olfactory nerve end and the processing of sensory information coming from the olfactory receptor cells occurs. Direct contact of odorant particles with olfactory cells occurs in the olfactory region of the nasal cavity.

The perception of smell is possible only when air, including molecules of odorous substances, moves through the nasal cavity. Still air, even if it contains them, does not cause any olfactory sensations. The appearance of sensations depends not only on the concentration of the odor and the time of its exposure, but also on the speed with which the odorous mixture passes through the nasal cavity. The speed at which odor passes through the nose can vary widely depending on the animal's breathing rate. That is why the animal, trying to obtain maximum odor information, sniffs intensely, often drawing in air, and thereby accelerates the flow of air containing odor particles.

In many macromatic mammals, the olfactory area of the nose is enlarged due to additional shells of the bony wall of the nasal cavity. In reptiles and some mammals, in the nasal septum, in addition to the main organs of smell, there is vomeronasal, or Yakobson organ. Amphibians, most reptiles, and many mammals have it. In the latter, it consists of two thin tubes at the base of the nasal septum, opening into the nasal cavity. The inside of these tubes is lined with sensitive epithelium, from the receptors of which a special branch of the olfactory nerve departs. The olfactory receptors of the vomeronasal organ are selectively tuned to the most important odors for the animal, associated with danger, the search for food and a sexual partner, and have incredible sensitivity. The modern concept of dual olfaction provides for the existence of primary and additional olfactory systems in vertebrates. The first plays an important role in nature in the perception of odors associated with feeding, behavior in the “predator-prey” system, as well as in recognizing individual odors of individuals, odors of a “group”, etc. The additional olfactory system is responsible for the perception of pheromones and plays a key role in regulation of sexual and maternal behavior.

The receptor role in it is played by the vomeronasal organ already mentioned above.

In fish, the olfactory organs are presented in pairs nasal pits, or bags located on the head in the vicinity of the mouth opening and including numerous connective tissue plates covered with olfactory epithelium. In insects, the olfactory organs are sensitive formations - olfactory sensilla, located mainly on antennas. A number of mollusks have special organs osphradia.

Olfactory acuity (absolute threshold) is measured by the minimum concentration of odorants that causes an olfactory response. The sensitivity of the sense of smell to the same odor in an animal can vary depending on its physiological state. It decreases with general fatigue, runny nose, as well as with fatigue of the olfactory analyzer itself, which occurs when a sufficiently strong odor is exposed to the animal’s olfactory cells for too long.

To determine the direction of the source of the smell, the moisture in the animal's nose is important. It is necessary to determine the direction of the wind, and therefore the direction from which the smell comes. Without wind, animals detect odors only at very close distances. The side cutouts on the nose of mammals are designed to perceive odors brought by side and rear winds.

Pheromones

A special group of odorous substances consists of pheromones, which are secreted by animals into the environment, usually with the help of special glands, and regulate the behavior of representatives of the same species. Pheromones– biological markers of their own species, volatile chemosignals that control neuroendocrine behavioral reactions, developmental processes, as well as many processes associated with social behavior and reproduction. If in vertebrates olfactory signals act, as a rule, in combination with others - visual, auditory, tactile signals - then in insects the pheromone can play the role of the only key stimulus that completely determines their behavior.

Communication with the help of pheromones is usually considered as a complex system, including mechanisms of pheromone biosynthesis, its release into the environment, its distribution in it, its perception by other individuals and the analysis of received signals.

Interesting ways to ensure species specificity of pheromones. A pheromone always contains several chemicals. Typically these are organic compounds with low molecular weight - from 100 to 300 amu. Species differences between their mixtures are achieved in one of three ways:

1) the same set of substances with different ratios for each species;
2) one or more common substances, but different additional substances for each species;
3) completely different substances for each species.

The most famous pheromones are:

epagons – “love pheromones” or sexual attractants;
odmihnions - “guiding threads” indicating the way to the house or to the prey found, they are also markers on the boundaries of the individual territory;
toribons – pheromones of fear and anxiety;
gonophions - pheromones that change sexual properties;
gamophions – pheromones of puberty;
etophions – behavioral pheromones;
lychneumones are taste pheromones.

Individual smell

The smell is a kind of “calling card” of the animal; it is purely individual. But at the same time, the smell is species-specific; by it, animals clearly distinguish representatives of their species from any other. Members of the same group or flock, in the presence of individual differences, also have a common specific group smell.

The individual smell of an animal is formed from a number of components: its gender, age, functional state, stage of the sexual cycle, etc. This information can be encoded by a number of odorous substances that make up urine, their ratio and concentration. Individual odor can change under the influence of various reasons throughout the life of the animal. The microbial landscape plays a huge role in creating an individual scent. Microorganisms living in the cavities of the skin glands take an active part in the synthesis of pheromones. The sources of odor are the products of incomplete anaerobic oxidation of secretions secreted by animals in various body cavities and glands. The transfer of bacteria from individual to individual can occur during the interaction of group members: mating, feeding of the young, childbirth, etc. Thus, within each population a certain group-wide microflora is maintained, providing a similar smell.

The role of smell in some forms of behavior

The sense of smell is extremely important in the lives of animals of many taxonomic groups. With the help of smell, animals can orient themselves relative to certain physiological states that are currently inherent in other members of the group. For example, fear, excitement, degree of saturation, illness, are accompanied in animals and humans by a change in the usual body odor.

Olfactory communication is especially important for processes associated with reproduction. Specific sex pheromones have been found in many vertebrate and invertebrate animals. Thus, some insects, fish, and tailed amphibians have pheromones that stimulate the development of female gonads and secondary sexual characteristics in females. Pheromones from males of some fish accelerate the maturation of females, synchronizing population reproduction.

Termites and closely related ants are endowed with a functional system for inhibiting the development of females and males. While the worker ants lick the required doses of gonophions from the abdomen of the oviparous female, there will be no new females in the nest. Its gonophions suppress the development of ovaries in worker ants. But as soon as the oviparous female dies, some worker ants immediately begin to bear fruit. In 1954, R. Butler discovered that the jaw glands of the queen bee secrete a special uterine substance, which she spreads over the body, then allowing the workers to lick it off. The biological activity of this pheromone is so high that a worker bee only needs to touch the body of a living or dead queen with its proboscis, and the development of the ovaries is inhibited. Its main role is to suppress the development of ovaries in worker bees. But as soon as the uterus disappears, and with it this pheromone, many ordinary family members immediately begin to develop ovaries. These bees then lay eggs, even though they are not fertilized. The same thing happens when there is not enough queen pheromone for all members of the bee family.

Pheromones secreted by females to attract males are of great importance for sexual behavior. During the period of estrus in female mammals, the secretion of many skin glands increases, especially those surrounding the anogenital zone, in the secretion of which sex hormones and pheromones appear at this time. During estrus, these substances are found in even greater quantities in the urine of females. They help create odors that attract the attention of males.

A number of pheromones - gonophions, described in invertebrates, contribute to the change in the sex of an animal during its life. The marine polychaete worm Ophriotroch is always male at the beginning of its life, and when it grows up, it turns into a female. Adult females of these worms secrete gonophion into the water, causing the females to turn into males. Something similar happens in some gastropods. They are also males when young and then become females.

The males of many insects (flies, crickets, grasshoppers, cockroaches, beetles, etc.) carry glands on different parts of their bodies, the secretion of which gives the females an incentive to reproduce. Adult male desert locusts, releasing special pheromones, accelerate the maturation of young locusts.

In mammals, gamophions are described, perceived mainly by smell. They play a significant role in reproduction. Mice have been the best studied in this regard. The urine of aggressive males contains an aggression pheromone, which contains metabolites of male sex hormones. This pheromone can promote aggression in dominant males and a submissive reaction in low-ranking males. In addition to aggression, the smell of urine from male house mice causes many other behavioral and physiological reactions in individuals of the same species. For example, the smell of an unfamiliar male suppresses the exploration of a new territory by other males, attracts females, blocks pregnancy, causes synchronization and acceleration of estrus cycles, accelerates puberty in young females and suppresses the normal development of spermatogenesis in young males.

Since the sex hormones and pheromones of all mammals are basically the same, similar phenomena can be observed in animals of other species.

Smell is one of the earliest senses that “turns on” in ontogenesis. Cubs already in the first days after birth remember the smell of their mother. By this time, the nervous structures that provide the perception of smell have already fully developed. The smell of the cubs plays an important role in the development of normal maternal behavior in the bitch. During lactation, females produce a special maternal pheromone, which gives a specific smell to the cubs and ensures normal relationships between them and the mother.

A specific smell also appears when the animal experiences fear. With emotional excitement, the secretion of sweat glands sharply increases. Sometimes animals experience an involuntary release of secretion from the odorous glands, urination, and even feces. The scent marks that animals use to mark their possessions are of great informational value.

Marking territory

The sense of smell plays a huge role in the territorial behavior of animals. Almost all animals mark their areas with a specific smell. The source of odor can be almost all animal secretions: urine, feces, secretions of special glands. Marking is an extremely important form of behavior for many species of terrestrial animals: by leaving odorous substances at different points in their habitat, they signal about themselves to other individuals. Thanks to odorous marks, a more uniform and, most importantly, structured distribution of individuals in the population occurs; opponents, avoiding direct contacts that could lead to injury, receive sufficient complete information about the “master,” and sexual partners find each other more easily.

Marking behavior. This phenomenon is widespread among mammals and is carried out by leaving their traces in visible places: marks in the form of secretions of odorous glands, excrement, scratches or scratches on the bark of trees, stones or dry soil, preserving the smell of secretions of the plantar glands (Fig. 4.5).

Rice. 4.5.

Deer and some antelopes mark the territory they occupy with the abundantly secreted odorous secretion of the preorbital glands, for which they rub their muzzles against branches and tree trunks. During the rutting period, roe deer, chamois, and snow goats butt the bushes, leaving odorous secretions of the subcorneal gland on them. The musk peccary lays out an odorous trail, wiping off the secretion of the dorsal musk gland on the hanging branches along the way. The bear also sometimes leaves an odorous trail, rising on its hind legs near tree trunks and rubbing its muzzle and back against them, but more often it tears off the bark with its claws, applying the secretion of the plantar glands to the scratches. Animals living in burrows constantly leave odorous traces on the walls of the burrow. In rural areas and cities it is easy to observe markings in domestic cats. Passing by the marked object, the cat stops, turns its back to it and splashes out a little urine with a particularly pungent odor, while making characteristic movements of its tail. All “outstanding” objects are subject to marking: roof ridge, corners of buildings, pillars, hummocks, tree trunks, car wheels, etc. Subsequently, all other cats in the area leave their marks in these places. Marking urination is fundamentally different from “hygienic” urination, when the cat first digs a hole in the substrate and then carefully buries its secretions to mask the smell.

All members of the canine family also mark territory using urine. Males raise their legs and mark all possible prominent objects: trees, pillars, stones, etc. Each subsequent male always tries to leave his mark higher than the previous one. Bitches also mark territory. Marking behavior is especially intensified before and during estrus. In places of mass walks of domestic dogs, specific urinary points. By sniffing marks left by other dogs while walking, dogs receive a lot of valuable and interesting information. Cal also has informational value. When defecating, many animals try to leave it on the highest possible places, sometimes even tree trunks or stones.

The boundaries of the habitat of a pack of dogs or wolves are intensively marked with urine. This is usually done by the dominant male. F. Mowat, who studied the behavior of polar wolves in Alaska, points out that a pack of wolves walks around the “family lands” approximately once a week and refreshes boundary signs. One day, when the wolves went hunting at night, the scientist decided to “stake out” “his” territory, an area of about three hundred square meters, in the same way. Returned!) from the hunt, the male wolf immediately noticed F. Mowat’s marks and began to study them: “Rising to his feet, he sniffed my sign again and, obviously, made a decision. Quickly, with a confident look, he began a systematic walk around the area , which I had staked out for myself. Approaching the next “border” sign, he sniffed it once or twice, then carefully made my a mark on the same bunch of grass or on a stone, but from the outside. After some fifteen minutes the operation was smoke-filled. Then, the wolf came out onto the path where my domain ended and trotted towards the house, providing me with food for the most serious thoughts."

This example shows that the marks of an individual of one species can be understandable and informative for individuals of another species.

A.e.m. – atomic mass unit.
Mowat F. Don't cry wolf! M., 2002. P. 75.

Invertebrates. Chemocommunication plays a very important role for most invertebrates. The most primitive of them do not have special olfactory organs, but most of the surface of the body is sensitive to the presence of chemicals in the water. Among aquatic invertebrates, chemical signals are used by the ciliated ciliates suvoika and barnacles - sea acorns; they secrete chemicals that attract individuals of their species. Some aquatic invertebrates, such as lobsters and crabs, have specific chemocommunication organs - taste buds at the base of their legs. Widespread land mollusks, grape snails shoot thin, dart-shaped “love arrows” into each other during mating. These miniature structures contain a substance that prepares the recipient for sperm transfer.

Insects. Chemical communication is the most widespread in insects. This applies primarily to bees and ants, which is due to the complex structure of communities and the division of functions between their members.

In ants, different forms of chemical sensitivity play a greater role than other types of reception. Communication between individuals is carried out with the help of long antennae, or antennae, which perform a dual function, being simultaneously organs of touch and smell.

Smells play an extremely important role in triggering behavior in ants. Well-trodden paths stretch from an anthill or nest to food gathering places. Ants mark their paths by releasing specific pheromones. An ant, coming out of an anthill, probably feels like a person standing at a fork in a brightly lit street or highway, equipped with signs leading into the distance. But a person lives mainly in the world of vision, and an ant in the world of smells, touches and tastes, and therefore for them pheromone marks are the same as well-lit store signs for us. Scent marks also indicate which path to take to find more food. Well-trodden paths have a stronger smell because more ants have passed along them, touching the soil with their abdomens, leaving an odorous trail.

In addition, ants also have a topochemical sense, when they can determine the shape of a mark or a smelling object by smell. Pheromones control the entire life of ants. For example, the alarm pheromone released by a disturbed ant instantly excites others. This, like a chain reaction, spreads further, and now hundreds of ants are ready to rush at the enemy.

Not only do the females secrete special pheromones that dictate certain behavior to the worker ants, but the larvae also secrete specific substances that encourage the adults to feed them. The inhabitants of an anthill recognize “friends” and “strangers” by smell. Even its relatives know whether an ant is alive or dead by specific secretions. When two ants come face to face, one of the insects often “licks” the head and abdomen of the other. It is assumed that this facilitates the transfer of secretions that have their own specific odor within each colony. Apparently, it is thanks to this smell that ants are able to easily distinguish members of their anthill from “strangers”. In many species of ants, an alien who accidentally ends up in the territory of another anthill is simply killed by the hosts.

Not only for ants, but also for many insects, scent indicators are the most important and necessary landmarks that they always use. When bumblebees fly out of the nest in the morning, they circle around it for some time, busily humming with their wings and leaving odorous droplet marks on neighboring plants and soil. Each type of bumblebee places the sign differently, so as not to get confused when returning home: on different plants, at different heights from the ground and with different smells.

Each beehive, like each anthill, has its own smell. When young bees learn to fly near their native hive, adults often help them by sitting on the entrance, turning out the scent gland and dispersing the smell around them with the movements of their wings. The native smell prevents young bees from getting lost.

Bees, having found abundant food, mark this place with the help of an odorous gland. For every bee returning with a good bribe, there is an invisible trail of scent in the air. Other bees follow this thread. If you look closely at the bees collecting honey in a flowering meadow, you will notice a striking fact: one bee hurries from clover to clover and does not pay attention to the other flowers, another flies to the thyme, and the third is only interested in the forget-me-not. Biologists call this behavior “floral persistence.” This applies to individuals, not the entire family.

Floral consistency is beneficial for both bees and plants. For bees - because, while remaining faithful to certain flowers, they everywhere encounter the same working conditions to which they are already accustomed. But this behavior of bees is even more important for flowers, since their rapid and successful pollination depends on it; it is clear that clover pollen, for example, would be completely unsuitable for thyme.

The relative role of scent and color in attracting bees depends in each individual case on the intensity of the flower's scent and its color. But in general, we can say that from afar the color of the flower serves as a guide for bees, and they are guided by it during the flight, but in the immediate vicinity of the flower they recognize by the smell whether it is the plant they are looking for.

Fish. Fish have two pairs of nostrils, the location of which on the head can be completely different, depending on the type of fish. The flow of water passes through all the holes, flowing into the front nostrils and flowing out through the rear nostrils, irritating the sensitive cells that tell the fish about the smell. Fish have a well-known reaction to the so-called “scare substance” released into the water. The “fright reaction” (dispersal of the school, sudden, rushing movements) is easily reproduced in an experiment if, for example, an extract of the skin of fish of the same or systematically similar species is added to the aquarium. The biological certainty of the reaction to the “frightening substance” is confirmed by the fact that, for example, among the Verkhovkas the reaction to the real appearance of a predator (pike), its visual image (pike in a glass vessel) and the scaring substance were exactly the same. The reaction to substances released by fish wounded (or killed) by a predator is an undoubted adaptation at the population level, when the effect of avoiding a predator that is beneficial for the population is achieved at the cost of the death of one or more individuals.

As for the nature of the “fright substance,” it could be blood or tissue fluid (the fear reaction to fish with damaged skin is known), but some authors believe that this substance may be the secretion of special skin glands. It was shown, in particular, that a substance secreted by special flask-shaped cells of the epidermis caused a fear reaction in representatives of Ostariophysi, Kneriidae and Phractolaemidae, and the action of this substance was equally effective regardless of the systematic affiliation of the “donor” and “recipient”. In this case, it is possible that secretion of the “fright substance” is also possible in intact fish under the influence of nervous stimuli associated with the reaction to the appearance of a predator; No objective confirmation of this possibility has yet been made. The biological adequacy of the production of the “fright substance” is clearly demonstrated by observations of some fish. Thus, in Piniephales promelas, males during the spawning period lose epidermal cells containing this substance, which is most likely due to behavioral characteristics: they actively clean the substrate for laying eggs and could scare away females as a result of accidental scratches. The regulation of the production of the “fright substance” is hormonal in nature: males artificially exposed to testosterone stopped producing this substance.

Experiments conducted with tadpoles of 8 species of tailless amphibians revealed a fear reaction to a specific skin secretion in toad tadpoles. The secretion of this substance (“bufotoxin”) is produced by special skin cells.

It is characteristic that the “smell of fear” was also found in house rodent mice, which, as shown above, lead a group lifestyle. If usually in a group the smell of individual individuals has attractive properties, then the smell of mice that were previously frightened (by blowing or shaking) caused a clear defensive reaction.

The sense of smell is of great importance for the so-called migratory fish, which make long migrations from the upper reaches of rivers, in which they hatched, to the sea. In the sea they grow and live until sexual maturity, and then return to their place of birth to lay eggs. Thus, every spring, millions of Pacific salmon return to their waters to spawn. They have to make a difficult journey up the rivers, several hundred kilometers long. For example, the Pacific Chinook salmon, which the Japanese call the “prince of salmon,” travels 4,000 kilometers along the largest river in Alaska, the Yukon. An experimental study of the physiology of fish during migration has shown that the main guideline in the search for a home for fish is the smell of the water of their native river. An unusually sensitive sense of smell helps salmon escape from numerous predators, which gather in large numbers on the banks of the river during their passage. Thus, it has been shown that if you put the hand, paw of a dog or a bear into the water, the salmon located downstream immediately freeze, move back and only resume movement after 15-20 minutes. Scientists call the substances they capture “animal skin factor.”

Many other fish species also have an unusually sensitive sense of smell. For example, an eel in such a mass of water that fills Lake Ladoga would recognize a spoonful of phenylethyl alcohol. Even humans do not have such ultra-sensitive devices. And sharks are able to smell blood dissolved in water at a concentration of 1: 10,000,000.

Fish, like insects and some other animals, use pheromones - chemical signaling substances. Catfish recognize individuals of their species by tasting the substances they secrete, probably produced by the gonads or contained in the urine or mucous cells of the skin. After the first meeting of catfish, they remember the taste of each other's pheromones. The next meeting of these fish may end in war or peace, depending on the previously established relationship. Quite a few fish have spines equipped with poisonous glands that protect them from attacks by predators.

Amphibians. Many amphibians have special glands that secrete a caustic and sometimes poisonous secretion. Some toads, in defense, emit a highly acidic fluid produced by the parotid glands (one behind each eye). The Colorado toad can spray this poisonous liquid up to 3.6 m away. At least one species of salamander uses a special “love drink” produced during the mating season by special glands located near the head.

Reptiles. The sense of smell and taste is well developed in snakes and lizards; in crocodiles and turtles it is relatively weak. Rhythmically sticking out its tongue, the snake enhances its sense of smell, transferring odorous particles to a special sensory structure - the so-called so-called sensory structure located in the mouth. Jacobson's organ. Some snakes, turtles and alligators secrete musky fluid as warning signals; others use scent as a sexual attractant.

Like migrating fish, sea turtles also navigate by the smell of water and also make long migrations to their breeding grounds. The shortest route for turtles from Brazil to Ascension Island goes exactly towards the Equatorial Countercurrent, the moving mass of water of which creates a “zone” or “wedge of smell”. It is realistic to assume that their sense of smell allows them, once caught in a “wedge of smell,” not to lose it and move against the flow.

Birds. The majority of birds are almost completely devoid of smell. However, among the entire mass of birds, there are some species that are exceptions to the rule. Such exceptions include the famous New Zealand kiwi, which has a good sense of smell. Obviously, this feature of the kiwi is associated with its terrestrial lifestyle in the thickets of the tropical forest. This bird has a very special beak structure that distinguishes it from other taxonomic groups of birds. So, the nostrils of a kiwi are located not at the base of the beak, but at its end. While searching for prey, the bird uses its beak to sniff out worms and insects in the ground.

American turkey vultures, which are common in the forests of North America from the Canadian border to Patagonia and fly low above the ground, have a well-developed sense of smell. The dense crowns of trees do not allow them to look out for carrion, like Asian vultures living in open spaces. What allows a large feathered predator-scavenger to survive in such conditions is that it sniffs out “smelling” prey in the thickets. The presence of a sense of smell in some species of ducks, as well as some species of tits, has been experimentally proven.

Mammals. In the animal world, olfactory abilities reach their greatest development in mammals with their highly developed brain. Scientists study the sense of smell and its role in the entire complex of behavior in great detail. One of the challenges facing researchers is to model this ability in order to create olfactory sensors that can detect a variety of odors. However, until now this problem has not actually found its solution and the dog’s nose remains the most accurate olfactory device. The mucous membrane of the olfactory organs in dogs contains thousands of times more sensitive cells than the human nose; their olfactory lobes of the brain are also better developed. With the help of its nose, a dog can recognize a variety of both natural and synthetic odorants. Based on the slightest nuances, it is able to distinguish the individual odors of people and animals, so a dog can be trained to recognize, for example, a specific person by smell. It is this feature that people use when training dogs for detection service. An individual smell is, of course, not unique to humans, and a dog can identify individual individuals by their scent, be it a tiger, a bear or a mouse. Thanks to clear individual identification, specific individuals, such as man-eating tigers, can be removed from the population. Dogs' keen senses are also widely used for other purposes, such as detecting explosives or drugs. Currently, not a single customs service can do without them. Various rescue services actively use dogs to help find people in rubble after earthquakes, under avalanches, or tourists lost in the mountains. Since 1966, our country began to use dogs to search for minerals. Employees of the Karelian branch of the USSR Academy of Sciences, with the help of dogs, found tungsten deposits on the Kola Peninsula, nickel deposits in the Ladoga region, and others. In some countries, dogs are quite successfully trained to search for gas leaks from city gas mains.

In human life the chemical senses play a very insignificant role and are therefore difficult to study; Perhaps this is precisely why scientists did not pay attention to them for a long time. Until now, we don’t even know exactly why “smells” smell; and yet, as has now been established, many animals live in a world dominated by odors.

The sense of smell is the perception of airborne chemicals that we take in when we breathe; therefore, it is a distant feeling. Taste is very closely related to the sense of smell; on the contrary, it is a contact sense: with the help of taste we determine the chemical nature of substances that are in contact with receptors. However, the capabilities of a taste analyzer are very limited, and what we usually perceive as the taste of food is in fact mainly its smell. When you have a cold and your nose is stuffy, food often seems tasteless. If you hold your nose and stop chewing, it is very difficult to distinguish turnips from onions. We are given only four types of pure taste sensations; The human tongue distinguishes only sweet, sour, salty and bitter tastes. Perhaps the word “bouquet” is more suitable for the combination of smell and taste; however, we must not forget that all three of these terms are subjective; they describe only our own sensations, and it would be wrong to apply them to animals.

Insects have chemical sense organs in their mouths, on their antennae, and even on their legs, so it is difficult to determine whether they smell or taste food. This question does not arise in the scientific literature. The chemical feeling is called chemoreception, and the corresponding sensory organs - chemoreceptors. This is not a very good term; after all, no one would think of saying that a blowfly “perceives chemical signals” when it crawls over a piece of meat. If we remember that, strictly speaking, there are only four types of taste sensations, then it is more appropriate to consider that blowflies and other insects perceive the smell of food.

The sense of smell probably arose in animals before all other senses. The first living organisms that swam in the world ocean (the primordial broth) that covered the Earth must have had the ability to somehow react to various chemicals dissolved in water: to swim away from harmful compounds and look for those that served them as food. As has now been established, even bacteria react to chemicals: they look for an environment with suitable concentrations of oxygen and sugar for them; We also know that in the lives of many animals - from insects to mammals - the sense of smell plays a huge role. Animals use their sense of smell when obtaining food, detecting enemies, to recognize individuals of the opposite sex and their own offspring, and in various rituals preceding mating. There have even been suggestions that humans are not as “short-nosed” as previously thought, and that smells influence our emotional behavior.

Studying the sense of smell is fraught with many difficulties. Since humans have little use of our sense of smell, we do not have precise terms to describe smells. Terms like “floral”, “musky”, “musty” are quite vague and different people interpret them differently. Additionally, different people may describe the same smell differently. There is no absolute basis for the classification of odors, analogous to the wavelength spectrum for colors or the frequency spectrum for sounds. Finding such an objective basis for classification is the goal of all research into the mechanism of smell, since any theory can be considered proven only if it allows predictions to be made. A theory of olfaction should be able to predict the odor of a chemical based on other properties of that substance.

Over the past two decades, several theories of smell have been proposed. Each of these theories corresponds to one or another data from the physiology of smell, but they all have shortcomings; Further research is needed before it can be decided whether any of these theories, albeit in modified form, are suitable to explain the mechanism of smell, or whether an entirely new theory will have to be created. The solution to this problem is of great importance not only for physiologists who study the functioning of the sense organs. Knowledge of the physiological mechanisms of smell, which plays a very important role in the lives of many animals, is also necessary in order to study the behavioral characteristics of these animals.

The development of a system for classifying odors and studying the physiological mechanisms of smell are hampered by the fact that we do not have any equipment for recording and measuring odors that could be compared with a camera or tape recorder used to record images or sounds. The currently available device for recording odors is very bulky and insensitive even in comparison with the human nose: after all, our nose, although it does not distinguish odors very well, is capable of detecting surprisingly small concentrations of odorous substances - on the order of millionths of a gram per cubic meter of air. None of the instruments for chemical analysis has such sensitivity; however, some odorants can be analyzed if large enough samples are collected. Human body odors can be analyzed by placing a person in a hermetically sealed cylinder through which purified air is passed. Then the “polluted” air is passed through some solvent that captures these odorous substances, and the resulting solution is analyzed. In this way, you can determine the differences between the body odors of men and women.

Fig. 25. Sensitive cilia of chemoreceptors are washed with mucus. Odor molecules from the air penetrate the nasal mucosa, where they stimulate chemoreceptors

Structure nose unlike the structure eyes and the ear does not have any features that would help us understand the mechanism of its functioning. There are no supporting structures in the nose, and the olfactory receptors are so small and the nerve fibers extending from them are so thin that they are very difficult to study using electrophysiological methods. Chemoreceptors in humans and other mammals lie in special groove-shaped pits located in the uppermost part of both nasal cavities. During quiet breathing, the main air flow bypasses these pits and only small portions of air enter there - turbulences of the main flow, but when we sniff, the air is drawn into this part of the nasal cavity and passes over the yellowish tissue, the area of which is about 3 cm 2. This tissue contains several million chemoreceptors, which represent are long thin cells covered with hair-like cilia; these cilia form a dense plexus on the surface of the olfactory epithelium, washed by mucus (Fig. 25). Chemoreceptors are associated with an area of the brain called olfactory bulb, the size of which indicates how great the role of smell is in the life of a given animal. A dog, for example, has much larger olfactory bulbs than a human.

As we can see, one of the main problems in studying the physiological mechanisms of vision and hearing is to find out how the huge amount of information that enters the brain from receptors in the form of nerve impulses is analyzed. However, when studying the mechanism of smell, the main task is to understand how the molecules of the odorant substance stimulate the receptors. Of course, we do not know in detail how exactly the other receptors are stimulated, but we do know for sure that light entering the eye destroys visual pigments, and in the cochlea, sound waves deform hair cells. We do not have similar data on how exactly chemoreceptor stimulation occurs, although there are many hypotheses on this matter. One of the difficulties, as already noted, is that we do not know what smell is; Therefore, studying the physiological mechanisms of smell can be compared to trying to figure out how this or that part of a car engine works, without knowing what it serves or where its place is.

While going about our daily activities, we do not pay attention to various kinds of smells; but as soon as we think about them, we will immediately feel that there are many smells surrounding us. You can count hundreds, if not thousands of different odors that we distinguish: the smell of soup, coffee, gasoline, fish, tobacco smoke, various flowers, etc. Any theory of the physiological mechanism of smell should be able to explain what different odors have in common with each other . Many scientists have tried to classify odors based on the assumption that there are certain “primary odors,” just as the variety of colors we see can be reduced to combinations of several “primary colors.” They proposed that each primary odor stimulates a specific receptor mechanism in the same way that three primary colors are perceived by three different pigments, and that mixtures of primary odors are perceived as novel odors. It was further assumed that the molecules of each odorous substance have certain specific characteristics, due to which each substance stimulates only its own specific receptor mechanism. The interaction between the molecules of the substance and the chemoreceptors seemed to occur like a “key and lock” (Fig. 26).

Fig. 26. Schematic representation of the principles of the olfactory theory of “lock and key”

On the left are three “keys”, which are molecules of three odorous substances that fit “lock A” and do not fit “lock B”. Despite the fact that these substances consist of molecules of different shapes, they have the same odor.

This assumption led to the study of the molecular structure of many odorous substances in order to find out whether there was any common feature in the formula of the molecules of all substances that have, for example, the smell of musk, but are absent in the molecules of all substances with the aroma of mint. If it were possible to identify such features, then we could assume that the general geometric shape of the molecules is precisely the “key” to the “lock” of the receptor mechanism. According to one of the latest theories proposed by J. Eymour, there are seven main olfactory receptors that are sensitive to camphoric, ethereal, floral, musky, minty, pungent and putrid odors. There is fairly strong evidence that all substances that have one of these odors have molecules of similar shape, and Eimour suggests that they fit into one of the seven "locks" in the chemoreceptors, causing the receptors to somehow produce electric charge. It is assumed that the “locks” have a very simple shape, due to which a number of similar, but by no means identical in shape molecules, “keys,” fit into them. Having studied the structure of the molecules of various odorous substances, Eimur suggested that ethereal “keys” have the shape of a stick, musk - the shape of a disk, camphor - a spherical shape, etc.

According to another theory, developed by R. Wright, a characteristic feature of the odorant molecule is its vibrations, resulting from the vibrational motion of all its constituent atoms. Thus, each substance is characterized by a specific type of vibration, and therefore chemicals with similar vibrations should have very similar odors.

To give preference to one theory or another, it is necessary to collect information about the configurations and vibrational characteristics of the molecules of many different odorants and see whether substances with similar odors actually have the same properties.

In parallel with the development of these theories, research was carried out on chemoreceptors to determine how they are stimulated by odors. The mechanism of this stimulation was found to be very complex. Each chemoreceptor responds to several odors, and it responds differently to different odors; in all likelihood, the chemoreceptor functions in much the same way as the ommatidium in the insect eye. A single ommatidium contains all the elements of a compound eye and transmits to the brain fairly detailed information about some part of the visual field directly in front of it. Then the information from all ommatidia is combined, resulting in a holistic picture of the external world. In a similar way, perhaps individual chemoreceptors perceive individual components of the odor acting on them, and all chemoreceptors collectively detect the odor, so to speak, “as a whole,” after which this odor is analyzed by the brain.

Insect chemoreceptors, especially contact ones, have been studied much better than vertebrate chemoreceptors, mainly due to the relative simplicity of their structure. The blue fly's chemoreceptors are located in hollow hairs located on its legs and proboscis (the mouth parts elongated into a tube through which the fly sucks in food). If the fly is hungry or thirsty, then in response to irritation of such a hair with the appropriate chemical substance, the proboscis is straightened and brought into a position in which food can be absorbed. Therefore, by applying droplets of various substances to the tip of the hair, you can quite easily determine which substances stimulate the chemoreceptors. Each hair has from two to five receptors; experiments in which the straightening of the proboscis was regarded as an indicator of chemical sensitivity, as well as experiments studying the electrical activity of individual nerve fibers using an oscilloscope, showed that there are four types of receptors. Some respond to hair bending, others to stimulation with clean water, others to certain sugars, and others to certain salts. Thus, if a hungry or thirsty blue fly crawls on food or any wet surface, its proboscis will automatically extend and the fly will begin to drink or eat.

With the help of its strong antennae, this larva searches for a suitable place to settle. Having found such a place, it “stands on its head”, attaches its antennae to the stones and turns into an adult sea acorn. She pushes food into her mouth with the help of her “legs”.

At the beginning of this chapter we already noted how difficult it is to distinguish contact and distant chemoreceptors from each other; If we look at the behavior of a sea acorn, it is difficult to even figure out what sense it uses: smell or touch. Sea acorns are barnacles - crustaceans related to shrimp and crabs; like these animals, in their development they pass through the stage of a free-swimming larva (Fig. 27), which eventually turns into a sexually mature form. Anyone who has looked at stones on the seashore is well aware of dense clusters of sea acorns, and therefore the message that the larvae of these crustaceans prefer to settle near adults of their species is unlikely to cause surprise. The antennae of the larva have peculiar discs surrounded by hairs, which are used when examining the surface of stones in search of a suitable place for attachment. A “good” place is considered to be one where another sea acorn once sat: after all, if it managed to survive and leave its mark, then this place will probably be suitable for other acorns. The trace left by the previous occupant is a protein similar to that which is part of the hard integument of all crustaceans and related animals: wood lice, insects and spiders, but sea acorns are able to recognize that special protein that only their species has. The peculiarity of this protein is that it is completely insoluble in water; therefore, sea acorns have to deal not with individual protein particles in the water, but with a continuous protein mass. Perhaps the sea acorn larva somehow “determines by touch” the configuration of protein molecules. If it turns out that the receptors located on the antennae of the barnacle respond only to those molecules that are structurally similar to the molecules of the proteins mentioned above, this will serve to confirm the suitability of the theory based on the “lock and key” principle to explain the mechanism of smell.

It is important for us not only to understand what determines the smell of chemicals and why they have different odors, but also to establish at what concentrations of these substances in the air their smell becomes noticeable; this is necessary in order to calculate the acuity of smell and determine what role it plays in the lives of various animals. Olfactory acuity is measured by the minimum concentration of a substance at which its odor can be detected, and is usually expressed as the number of molecules per 1 cm3. This value is not so easy to measure: even if it is possible to obtain a certain concentration of a substance in the air, it is difficult to introduce it into the nose in exactly this concentration, without allowing it to decrease due to the air in the nasal cavity. The sensitivity of the olfactory organs to different substances varies. Hydrogen sulfide (a gas that smells like “rotten eggs”) is every bit as toxic as hydrogen cyanide, but it is much less dangerous because we can smell the gas at extremely low concentrations.

The human olfactory organs have a surprisingly high sensitivity to certain odors. An experiment was conducted in which one person walked barefoot on sheets of clean paper spread on the floor, and after half a minute another was able to determine by smell which sheets he was stepping on. If even a person can find such a trail by smell, even if it is completely fresh, then it is not surprising that dogs cope with this very well.

Many experiments were carried out in which a dog's ability to find someone's scent by smell was studied. The first experiments were carried out back in 1885 by D. Romanes. He led a chain of twelve people who walked in single file, each stepping exactly in the footsteps of the one in front. After walking some distance, these people split into two groups, and each group went its own way to the place of shelter. Then Romanes' dog was released, and she managed to find her owner almost without stopping along the way. In other experiments, she followed the tracks of a person wearing her owner's shoes, but lost the trail when the shoes were wrapped in paper.

The acuity of dogs' sense of smell was even more clearly demonstrated in experiments with identical twins: a group of people, including two twins, walked across a field and then split in half, so that each new group contained one twin; the dog followed the scent of the twin whose scent it had been introduced to before the experiment. If one of the twins, whose scent the dog was given to smell, did not participate in the experiment, it followed the trail left by the other twin. This apparently means that the smells of identical twins are very similar and a dog can only tell them apart if it encounters them at the same time.

We are so accustomed to dogs being used as trackers that evidence of their keen sense of smell comes as no surprise to us. It may seem that the experiments of Romanes and other researchers prove only obvious things, but in scientific research this is completely inevitable. We do not have the right to take anything on faith, without careful experimental verification, since any phenomenon we observe may have some not very obvious reasons that could go unnoticed. Without being convinced of the reliability of any fact or theory, you cannot use them as the basis for further work, no matter how obvious they may seem. Otherwise, all the work may go down the drain. Nevertheless, unproven statements have been made and continue to be made quite often.

For half a century it has been said that kiwis find food by smell. This assumption seems reasonable because kiwis feed on earthworms, which they search for in damp soil with their long beaks and because they are the only birds with nostrils located at the tip of their beaks. However, birds' sense of smell is very poorly developed, and if kiwis find their food by smell, they are an exception.

It was not until 1968 that data was published (Nature, December 1968) demonstrating the kiwi's ability to detect food by smell. At a bird sanctuary in New Zealand, kiwis were trained to find food sealed in aluminum tubes and buried in the ground. Kiwis learned this quickly. Then some of the tubes were filled with earthworms or some other food, and the rest with earth. Aluminum tubes were tightly tied on top with pieces of nylon fabric and covered with a large amount of earth. In the morning it was discovered that during the night the kiwis had perforated only those tubes that contained food, while the tubes filled with soil were left untouched.

Compared to the kiwi's ability to forage for food in the ground, the ability of migratory fish such as salmon to find their way upriver to their home waters to spawn seems almost uncanny. In fact, magic has nothing to do with it; in all likelihood, fish find their way to their homeland, guided first by the sun and then by smell. However, no one has yet shown exactly how they do this. Every year, salmon make the journey from the ocean, where they fed, to the mouths of their native rivers, and then with incredible determination they swim up the river, repeatedly overcoming rapids and striving at all costs to reach their spawning grounds.

Salmon migration consists of two stages. They first swim from the sea "pastures" to the river mouth, and then travel up the river to their spawning grounds. The path to the mouth of a river can be many hundreds of kilometers, such as for Scottish salmon, which feeds off the coast of Greenland, almost 4000 km from Scotland. It is believed that at this stage of migration, salmon orient themselves by the sun, much like birds. Caught salmon lose their ability to navigate when the sky is covered with clouds, and in laboratory conditions they are guided by an artificial “sun”.

Navigation by the sun is probably accurate enough for salmon to be within about 100 km of their home river. Starting from this place, the salmon finds its way up the river, guided by some other landmarks. Scientists have long been interested in this ability of salmon to find their home spawning grounds, choosing the right direction at each fork in the river. Salmon that were tagged before they left the pond where they hatched returned to the same location to spawn several years later. Sometimes up to 10,000 or more tagged salmon returned to the same place and not a single one went astray. It has now been proven quite accurately that salmon somehow captures the special smell of their native reservoir. Chemoreceptors in salmonids lie in shallow U-shaped tubes located just in front of the eyes (Plate XIV). Water enters one end of the tube, passes over the chemoreceptors and exits the other end, driven by the pulsating movements of microscopic cilia or the flow of water across the surface of the skin caused by the movements of the fish. Experiments by American researchers who plugged the “nostrils” of salmon with cotton swabs showed that these fish are guided by olfactory signals on their way to the spawning ground. Salmon with tampons completely lost the ability to find the right path (sometimes they succeeded purely by chance); at the same time, salmon, whose “nostrils” were not closed, found their way to the “home” even if they were released into the river above their native tributary. They swam downstream towards the masses of other salmon that were rising upstream until they found the right path.

These data were supported by the results of electrophysiological studies. Several salmon, caught at their spawning grounds, had water taken from different parts of the river passed through their nostrils. When a salmon's chemoreceptors were washed with water from its native tributary, the olfactory bulb at the base of the brain showed strong electrical activity, while water from foreign spawning grounds produced no response; at the same time, under the influence of water taken from the river below the spawning grounds of the experimental salmon, a weak reaction could be observed in its olfactory bulb.

It follows that water from salmon’s native spawning grounds has a certain odor that differs from the odors of foreign spawning grounds. It was calculated that even from a very small tributary of the river, a sufficient amount of various substances enters its mouth so that fish can catch the specific smell of their native places. However, identifying this odor will require lengthy and expensive research. The concentrations of the corresponding substances are so low that this makes their analysis very difficult; In addition, there are too many different substances in the water that can be involved in creating the specific smell of the reservoir. It may be caused by algae or substances washed out of river sediment. Chemical analysis did not reveal any significant difference between water samples taken from various tributaries of the river, but when studying the reaction of fish to variously treated water, it was possible to establish that the substance dissolved in the water that attracts fish is an organic compound, i.e. plant or animal origin.

Recently, a number of insect-attracting chemicals have been identified; however, this took a lot of time and effort. Finding such substances was very important because they can be used as “bait” to catch pests. The sense of smell plays a very important role in the life of insects: they use it not only when obtaining food, but also when looking for partners for mating, and recognize members of their community or family by smell; In addition, the sense of smell is of great importance for organizing community activities. In all these cases, odors serve as a means of communication between individual insects; the corresponding substances are called pheromones. Just as hormones serve as chemical messengers that transmit commands from one part of the body to another, pheromones transmit information outside the body - from one individual to another. The queen bee, for example, attracts drones by the smell of secretions from special glands located on her mouth parts. This specific smell, which attracts only males, is so strong that it can attract them at a distance of several hundred meters. Moreover, this smell not only attracts drones, but also encourages them to mate with the queen: for example, if you moisten a piece of blotting paper with the secretion of the queen bee glands and hang it at a height of about 5 m from the ground, i.e. at the level of the queen’s flight, then drones will try to mate with the paper. During the mating flight, the queen is surrounded by many drones; During the mating process, the smell of the queen is transmitted to them, and other drones mistakenly begin to pursue them.

One of the pheromones that needs to be identified is secreted by female silkworms. It's called bombicol(from the Latin name for the silkworm - Bombyx mori). To isolate bombycol, it was necessary to remove the scent glands from more than three hundred thousand female silkworms. This was a very difficult task, followed by even more painstaking work: analyzing the liquid extract of the glands and determining exactly what substance attracts male silkworms. To do this, the extracted liquid was divided into two parts so that each part contained different chemicals. Each part was then tested on male silkworms to determine which one caused a reaction and therefore contained the pheromone. Repeating this procedure many times, the liquid was gradually cleared, and in the end a drop of oily liquid remained - a total of 4 mg. It was pure bombicol; one millionth of a picogram (one picogram is equal to a millionth of a gram) of this substance was enough to cause a male silkworm to become excited.

It is now well known that in many insects pheromones serve to attract a male to a female and vice versa; sometimes insects can attract individuals of the opposite sex at a distance of several kilometers (Fig. 28). At this distance they cannot determine which direction the smell is coming from, but they fly against the wind, and as soon as they lose the smell, they begin to move in a circle and eventually get close enough to the source of the smell to detect a slight increase in its concentration; this allows the insects to find the right direction that leads them to the goal. To attract insects, pheromones are now used, found through trial and error. This is much simpler than isolating them by systematic analysis of raw material extracted from insect bodies; The chemical analysis of this material is so laborious that testing the insects' response to a wide variety of chemicals usually takes much less time. Usually, first, several compounds are found that cause at least a weak reaction in insects, and after that it is relatively easy to narrow down their circle and find one among them that has a very strong effect.

Fig. 28. The feathery antennae of the moth are lined with chemoreceptors, with the help of which the male can detect the smell of a female located 2...3 km away from him.

Mediterranean Piedwings (Ceratitis capitata) are pests of orange and lemon plantations; studies have shown that they are attracted to angelica oil. Unfortunately, this substance is quite rare and also expensive; however, further testing resulted in a cheap synthetic compound that is surprisingly good at attracting male moth flies. One of the advantages of using chemical attractants to reduce pests is that they are very powerful: for example, one female sawfly (a wood pest) can trap up to 11,000 males. Another very important feature of these substances is that each of them attracts individuals of only one species, which means that such a trap will not kill beneficial or harmless insects, which during normal spraying of crops often suffer more than pests. If pests fall into a trap and die instantly, this eliminates the possibility of pesticides passing from their bodies to predatory animals, which often die by eating poisoned insects. Another way of controlling pests with the help of pheromones is to “envelop” the area with a corresponding smell, as a result of which the chemoreceptors become overexcited, the insects become confused and lose the ability to find mating partners.

Pheromones help social insects unite into communities and organize their activities within the community. The same smell that attracts drones to the queen during her mating flight regulates the behavior of bees in the hive, such as preventing the appearance of new queens. The pheromone geraniol, which worker bees secrete during the waggle dance, complements the information conveyed by the dance. If a worker bee stings an “intruder”, it leaves a mark at the site of the bite in the form of a tiny droplet of pheromone, after which other bees begin to sting this target and the concentration of the poison increases.

Ants and termites leave chemical trails to help their fellows locate their food source. Thief Ant (Solenopsis), which has a bad reputation in America for causing severe pain to unwary people, leaves marks by periodically sticking out its sting and touching the ground with it. Any worker ant that stumbles upon these marks immediately follows the trail and thus comes to the food source. This trail is not just a series of “landmarks”: ants leave such trails on their way home only if they managed to find food. Thus, the scent of a trail increases if a rich food source is found: the stronger the scent of a trail, the more ants follow it. Then, as food supplies become depleted, some ants return home hungry and do not leave their scent mark, so the trail gradually weakens. Finally, when the food source dries up, the trail disappears. This is a very precise way of regulating the labor required to carry food home, depending on the quantity of food; Thanks to the pheromone, the entire operation is carried out with maximum savings.

Some ants also have an "alarm odor" that they emit when disturbed; this smell, like the smell of ant tracks, is associated with a certain type of behavior that is very important for the ant family. When the smell of alarm appears, it spreads in all directions within a matter of seconds and puts all ants within a radius of 10...15 cm into a state of extreme excitement. The reaction of worker ants is that they go to the place where the disturbance arose. If the danger has passed, the pheromones gradually disappear and the ants calm down; otherwise, more and more pheromones are released and more and more ants become alarmed. Thus, minor violations of order in the anthill are quickly eliminated, and serious attacks lead to general mobilization.

The behavior of social insects often makes people wonder and wonder whether these insects are intelligent. A complex social organization, caring for offspring, the ability to obtain and store food, and protect their home create the impression that animals that behave in this way are very “intelligent.” However, insects have a very small “brain”, which is just a small thickening of the central nervous system. It is quite obvious that this excludes the possibility of any complex nervous activity and, as we have already seen, the behavior of insects is characterized by extreme simplicity. For example, in order to induce feeding behavior in a blowfly, it is sufficient to stimulate one chemoreceptor. In fact, all the examples of behavior of social insects described in this chapter show that their apparent intelligence is simply the result of a “blind” reaction to external stimuli; however, both these stimuli themselves and the reactions to them perfectly correspond to the biological needs of animals.

Compared to what we know today about the behavior of social insects and the role the sense of smell plays in it, our understanding of the social behavior of mammals is only just beginning to take shape. Everyone knows that dogs use scent marks all the time; Since a dog never passes a scent mark without examining it (and often makes its own contribution to it), we can conclude that these marks must be very important to dogs. Based on careful observations, it has been established that most mammals live in a world of smells and many of them use smells to exchange information with each other.

First of all, the smell serves to announce its right to territory. Day after day, the animal's shelter is saturated with its smell, but this natural smell is often enhanced by odorous marks created during urination, defecation or the secretion of special glands. Many deer and antelope have preorbital glands, which look like small dimples located in front of the eyes. In October, male deer mark their territory: they loosen the ground with their antlers and strip bark from trees. The deer rubs its muzzle against the tree, leaving an odorous substance on it from its preorbital glands. The same thing happens when a deer hits the foliage of trees or tall grass with its head. Many other animals have scent glands at the base of their tail. Badgers, for example, mark their territory by pressing the back of their body against rocks or tree trunks.

It would be tedious to enumerate the different types of scent glands that mammals have, and the methods used by animals to disseminate their odors; in all cases, the function of the smell is the same: to “build a fence” around the individual territory that would prevent the invasion of strangers (unless, of course, they are individuals of the opposite sex, since in this case the odorous marks have the opposite effect). In addition, the smell creates a feeling of confidence in the animal; Once in a new cage, the animal begins by marking the entire cage with its scent. A male hamster, entering the territory of a female, disperses its scent mark as it moves and even marks the female’s nest; other mammals also mark mating partners with their odorous substance.

Some mammals consider their domain not a specific territory, but a whole network of paths; in this case, paths belonging to different individuals often intersect and overlap. At these intersections, animals leave their scent marks, thus informing other animals that use the same paths not only that they have been here, but also what sex they are and the state of their reproductive system; the freshness of the mark allows us to judge the animal’s movements. Dogs are the animals best known to us for marking their properties and examining the “autographs” of all those who have passed through them; cats also leave scent signals, although we don't see them when they do it. The hippopotamus ensures the spread of its smell in a very original way: during defecation, it waves its tail from side to side, spraying droppings over a fairly large area, so that it falls on the surrounding vegetation at the height of the hippopotamus's nostrils.

Mammalian pheromones are as specific as those of insects. Animals usually do not pay any attention to signals from animals of other species. Deer mice, for example, use scent to prevent interbreeding. The American deer mouse is similar in appearance and lifestyle to the common wood mouse. In vast areas of the country - from swampy places to semi-deserts - 55 species of mouse-like rodents live, and in some places several species live together, but even in such places interspecific crossing does not occur. How animals use smell to maintain species identity was shown in experiments with special cells divided into three compartments. For example, a deer mouse from the Rocky Mountain region was placed in one of the compartments, and a deer mouse from Florida was placed in the other. When both compartments acquired odors characteristic of mice, the mice were removed. Then a new mouse, belonging to one of these two species, was placed in the third compartment, allowing it to move through all the compartments. By recording the time the mouse spent in each compartment, it was possible to show that it preferred that part of the cage in which the smell of a mouse of its own species was preserved. It is almost certainly the smell that attracts mice of the same species to each other, even if they occupy the same habitat with mice of other species. In the same experiments, it was found that male mice were especially attracted to that part of the cage in which there was the smell of a female of their species, who was in a state of readiness to mate.

Smell is also very important for social animals. Animals belonging to the same group are constantly in close contact and rub against each other, as a result of which they acquire a common smell. Sometimes this smell is spread deliberately: for example, male rabbits mark the young rabbits of their group by rubbing them with their chin, on which there are glands that produce an odorous secretion. Group scent reduces aggression between group members and makes it possible to instantly identify a stranger. If a new rat is placed in a room where a well-established group of rats lives, the owners will immediately attack it. They will accept her (if, of course, she survives) only after she acquires the smell of this group. Bees also use group scent to recognize their hive mates. Any foreign insect attempting to enter the hive will be killed by the guard bees.

It was recently discovered that group odor has another, perhaps even more important, function. If you keep a colony of rats in a spacious room, then their numbers do not increase indefinitely, but stabilize at a certain level. This is not due to an increase in mortality or simply increased mortality of young animals. The population size stabilizes when the number of individuals per square meter becomes slightly higher than in natural habitats, and it is the population density that is the factor inhibiting further increase in the number of rats. When the numbers of rats or other animals increase excessively, fights between them become common and group members show signs of physical and mental stress. The functions of the endocrine glands are disrupted, and the behavior of animals becomes abnormal. This has a particularly noticeable effect on marital behavior. The courtship procedure is disrupted, females lose the ability to give birth, and those who succeed often do not take proper care of their offspring. In short, the life of the community is disrupted, the birth rate falls, and the mortality rate of young animals increases.

There are fairly good reasons to believe that pheromones play an important role in these changes. If, four days after mating, a foreign male is placed with a pregnant female, her pregnancy is terminated. In this case, it is not at all necessary that the new male come into contact with her. To terminate pregnancy, it is enough that its smell penetrates the female’s cell. It appears that the male's scent somehow prevents the secretion of a hormone that regulates the female's sexual cycle.

Notes:

More precisely, the sensitivity of the olfactory organs. - Note translation