Animal Chemical Sensitivity

What is the Chemical World of Animals?

Jakob von Uexküll, in the early 1900s, emphasized that the “umwelt,” or sensory world, of each animal species is unique to it, being shaped through evolution to include those stimuli important to that species. Animals live everywhere, from the bottom of the deepest oceans, to the frozen polar caps, to the bowels of other animals, and each is adapted to detect stimuli that are present in that habitat and are of ecological significance to that species. Chemical information dominates the umwelt of most animals. Chemicals called pheromones reveal the nature of members of their own species, including sex, age, social status, health, and individual identity. Chemicals also inform the identity and location of food, shelter, home, predators, competitors, and more. The molecules that convey chemical information are diverse—they can be protons, such as sour-tasting acids in unripe fruit, or they can be macromolecules, such as sweet-tasting proteins in some ripened fruit.

Many animals are much more sensitive to odorants than are humans. Dogs and cats can detect some odorants at thousands of times lower concentrations than humans can. Although the olfactory systems of dogs, cats, and humans have a similar basic organization, the greater sensitivity of dogs and cats comes from their having many more olfactory receptor neurons in their noses. Moths have different “noses” than humans or dogs do, but their olfactory sensitivity to some odorants such as pheromones is also extremely high compared with that of humans.

However, some animals lack sensitivity to specific chemicals that are a central part of the sensory world of humans. Cats lack a sweet taste, a result of a loss of functional receptors in their mouths that detect sugars. Given cats' carnivorous diet, a lack of sweet taste is not a significant deficit.

High sensitivity of animals to particular chemicals is a proven or promised method of controlling populations of species that are pests to humans. Use of specific blends of pheromones in traps is a standard method of controlling some insect species. A similar application holds promise for an aquatic pest, the lamprey.

How Are Chemicals Detected?

Animal chemosensors are diverse in their structure and organization. This reflects differences in their phylogeny and life history. Sensors include nose and tongue of mammals, antenna of insects and crustaceans, pectin of scorpions, tentacles of sea anemones, osphradium of snails, and olfactory pits of squid. Terrestrial animals have olfaction and taste as their two major chemical senses, and these are distinguishable by the chemicals' carrier medium: olfaction uses volatile molecules delivered in air, and taste uses water-soluble molecules delivered in liquid. Aquatic animals mostly use water-soluble molecules, thus raising the question, can olfaction exist underwater? The answer is yes, as exemplified by frogs, which use their “nose” both underwater and in air. Chemical sensors, with their vast diversity, go beyond the dichotomy of olfaction and taste. Fish such as catfish have an olfactory organ; gustatory organs in the form of taste buds inside their mouth, on their barbels, and on their entire body surface; trigeminal chemosensors; and solitary chemoreceptor cells. Crustaceans have many types of chemosensors based on structural organization and innervation, even on a single antenna, mouthpart, or leg. Even the nose of a rodent has multiple chemosensory pathways that differ in structure and function, including the main olfactory epithelium, vomeronasal organ, trigeminal system, septal organ, and Grueneberg ganglion.

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