Smell (comparative physiology)
Smell, or olfaction, is a critical sensory modality that varies significantly across different biological organisms, playing essential roles in survival, communication, and behavior. In animals, olfactory receptors are highly specialized for detecting chemical signals from their environment, either through air or water. For insects, these receptors are often located on antennae, allowing them to sense pheromones that facilitate mating, foraging, and social interactions. Fish also rely heavily on smell, utilizing olfactory sacs to navigate, find food, and recognize other species through waterborne chemicals. In terrestrial vertebrates, including humans, olfactory receptors are located in the nose, where they respond to airborne odorants and send signals directly to the brain's limbic system, linking smells to memory and emotional responses. The interplay between smell and taste is crucial for flavor perception, as both senses contribute to the overall experience of food. Understanding these mechanisms highlights the diverse adaptations of olfaction across species and its importance in ecological interactions.
Smell (comparative physiology)
Responses to chemicals are fundamental at all stages of biological organization. Chemotaxis, an oriented response toward or away from chemicals, has been observed in species ranging from single-cell animals, such as bacteria and protozoa, to very complex multicellular animals, including humans. An attraction to a chemical is called positive chemotaxis, whereas a rejection or repulsion is called negative chemotaxis. The development of sensitivity (both positive and negative) to particular chemicals is the dominant sense in most animals. In general, receptor organs that have very high sensitivity and specificity, and which are distance chemical receptors, are called olfactory; the receptors of moderate sensitivity, usually found in the mouth, which are associated with feeding and are stimulated by dilute solutions, are called taste receptors. Smells can be delivered to the olfactory receptors through air, as is the case with terrestrial animals ranging from insects to humans. On the other hand, smells can be delivered to the olfactory receptors through water, as is the case with aquatic animals such as insects and fish.
Smell in Insects
In insects, the olfactory (smell) receptors are located on the antennae. Because of the superficial location of their receptors and, more especially, because of their suitability for electrophysiological studies, insects have contributed much basic information about the mechanisms of olfaction. The olfactory receptors of most insects are highly specialized and can detect trace amounts of compounds that are biologically important to the animal.
Olfaction is an important sensory modality for insects, particularly in mating, egg-laying, and food selection. Numerous male insects, such as moths and cockroaches, are attracted by species-specific compounds called pheromones that act as chemical stimuli at a distance. Pheromones can be thought of as a language based on the sense of smell. Pheromones are often divided into two categories. Releaser pheromones initiate specific patterns of behavior. For example, they serve as powerful sex attractants, identify territories or trails, signal danger, and bring about swarming or similar types of grouping behavior. Primer pheromones trigger physiological changes in metabolism related to sexual development, growth, or metamorphosis. These changes are usually mediated through the endocrine system.
Male silk moths and gypsy moths may be attracted from a distance of a mile or two by a releaser pheromone from the scent glands of the females. Males will attempt to mate with any object that has touched the female scent gland; however, males deprived of their antennae do not even orient toward the female. Synthetic releaser pheromones are now being used in traps to attract pest insects such as the gypsy moth and the Japanese beetle.
Chemical communication in social insects is used for alarm, attraction, recruitment, and recognition of nestmates and castes. Ants give off alarm releaser pheromones from mandibular glands and so can warn other ants of impending danger. Army ants deposit releaser pheromones on trails to food sources or to nest sites. Primer pheromones secreted by the queen bee cause the worker bees to cluster and swarm, and they suppress the rearing of other queens in the hive.
Mosquitoes are attracted chemically to warm-blooded animals and are sensitive to several chemicals. Carbon dioxide (the metabolic waste product excreted through the lung) attracts them, and they can orient themselves and fly to the source of this compound. They also react positively to other mammalian body products. Most common insect repellants work by interfering with the olfactory ability of the mosquito so that the insect can no longer follow an odor toward its source.
Smell in Fish
The olfactory receptors in most fish are located in olfactory sacs in a pit on the head. Chemicals are brought to the receptors while swimming or during respiratory movements. Odors and the olfactory sense play a major role in the life of many species of fish. For example, homing in salmon is controlled mostly by the “smell” of the water where the fish was born. By following the smell trail composed of the minerals found in the water, salmon can return to breed in the same stream in which they were born. Fish can also become rapidly conditioned to odors. For example, once a pike has attacked a school of minnows, the odor of other pike in the water becomes associated with an alarm response in the minnows. They can use this same ability to identify other friendly fish or members of their species. Fish also use their sense of smell to find their food, and in migratory species, they use their sense of smell to navigate.
Olfactory receptors in most fish are located near the eye in pits on their snout. Their olfactory rosette consists of many sensory epithelial folds called lamellae. The openings in the olfactory pits allow water into the fish's nose to pass over the olfactory rosette, then a chemical in the water contacts the rosette, which sends signals to the brain's olfactory lobe, and the water is passed back out the nostril opening.
The strength and accuracy of a fish's sense of smell varies significantly between species. Catfish have an excellent sense of smell because they have more than 140 folds in their rosette. They can sense food up to 100 yards (91 meters) away. Conversely, largemouth bass have sensory organs to facilitate the sense of smell, but they only have eight to thirteen folds, so their abilities are much less useful.
Smell in Terrestrial Vertebrates
In vertebrates, the olfactory receptor cells are located in the nose along the respiratory airflow path. As a result, when air is brought into the nose either during breathing or sniffing, odorant molecules are delivered to the headspace above the mucus-coated olfactory receptors. The odor molecules then bind to hairlike cilia on the olfactory receptors, producing a signal that is transmitted to the central nervous system. Because they stimulate different receptors, different smells produce different patterns of electrical activity. These odorant-specific patterns are used by the brain in smell identification.
Olfactory receptor cells are primary receptors, with axons running directly to the brain. This makes olfactory receptor cells unique since most other sensory cells send their signals through processing centers (called synapses) before the message is carried to the brain. In the case of the olfactory receptor cells, all the information recorded by the cell is transmitted to the central nervous system. Once in the brain, the output of the olfactory receptors is sent to the limbic system (a portion of the brain involved with memory), the endocrine system, and the rest of the central nervous system. The connections to the limbic system result in the very strong association that odors have in memory recognition. In humans, smells can often trigger very vivid memories. The rest of the brain also sends messages back to the bulbs, amending the pleasure of a food aroma when the stomach is full. Unlike other neurons, olfactory receptor cells constantly replicate. As a result, after a life span of about thirty days, olfactory receptor cells are replaced.
Odors help bond mothers to their newborn babies. A mother cuddling her infant will invariably brush her nose in the baby’s hair to inhale its sweet aroma. She can identify her baby by its smell as much as by its cry. Additionally, one-day-old infants of many species are able to recognize the smell of their mothers. A mother rat licks her nipples so that her blind pups can follow the scent of her saliva to the milk. Likewise, a mother kangaroo produces a saliva trail so the newborn and blind babies can follow the trail from the uterus to the mother’s pouch. Wash the nipples and eliminate the saliva trail, and the pups are lost.
Female rodents who periodically smell male urine will progress more quickly into puberty than females who do not. If a pregnant female mouse smells the urine of a male in another colony, she will immediately terminate her pregnancy. Also, if the olfactory nerves of a newborn rat pup are cut, the rats will never develop sexually.
A diminished sense of smell is termed hyposmia. Hyposmia can occur following a cold or after head trauma, and humans experience some reduction in the sense of smell with age. Also, most conditions that reduce the flow of air through the nose will reduce olfactory acuity. For example, a stuffy nose as a result of an allergy, a cold, or a nasal polyp often creates hyposmia. Anosmia is the complete loss of the ability to detect airborne odorants. Head trauma and severe nasal obstructions can produce anosmia. If the cause of hyposmia or anosmia is related to a blockage in the nasal airflow passageways, treatment with steroids or surgery can often restore the olfactory loss.
Human experience seems to draw a sharp contrast between taste and smell. Taste is the chemical sense related to sampling compounds that come in direct contact with the inside of the mouth, whereas smell is the ability of the nose to monitor airborne chemicals, often from distant sources. However, the sensations of taste and smell are not completely independent since smell can influence taste and vice versa. For example, a lemon smell in the nose can make distilled water appear to “taste” bitter, and a sugar solution in the mouth can affect the perception of a fruit smell such as cherry. Much of what is usually perceived as being a taste is really a smell. For example, with the nose blocked, it is difficult to tell coffee from bitter water or an onion from a potato. As humans chew, volatile compounds in the food are released into the air in the back of the throat. These compounds then make their way up the back of the nasal cavity, stimulating the olfactory receptors, producing a smell sensation that dramatically enriches the perception of the taste. This combination of smell and taste is referred to as flavor. What is often thought of as “taste” is actually a combination of smells and tastes, with additional contributions to the flavor coming from temperature and pain receptors in the nose and mouth.
Principal Terms
Anosmia: the clinical term for the inability to detect odors
Chemotaxis: an oriented response toward or away from chemicals
Olfaction: the sense of smell
Olfactory Receptors: receptor organs that have very high sensitivity and specificity and which are “distance” chemical receptors
Pheromones: species-specific compounds (odors) which, acting as chemical stimuli at a distance, have a profound effect on an animal’s behavior
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