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The sense of smell in mammals

About mammals > Sensory systems mammal
Mammalian Sense of Smell

Terrestrial mammals often have distinctive scents. In some societies, humans go to great lengths (and expense) to mask or alter olfactory information, as is reflected in the sales of deodorants and perfumes, respectively. Zookeepers recognize the importance of smell in mammals because, immediately after they have cleaned a cage, the animal often defecates, urinates, or otherwise marks its area again. Individual olfactory signatures may be less likely in mammals that spend most of their lives in water, which would at the least dilute, if not wash away body odors. Water does not allow permanent scentmarking locations, whereas land provides many places to position a long-term scent mark. In fact, whales and dolphins have completely lost the olfactory-sensing portions of their brains.

Mammals use their noses to collect information about odors. Specifically, olfactory epithelium (sensors on the mucosal surfaces of mesethmoid bones nose) in the nostrils convert chemical signals to electrical ones that are conveyed to the brain via the olfactory nerves. Many species of mammals also use Jacobson’s organs (structures in the roof of the mouth) to obtain additional olfactory data through the “Flehman” response (the curling of its upper lip as a male horse [Equus caballus] or an impala [Aepyceros melampus] smells the urine of a female). One advantage of olfaction is that some odors are persistent and may continue to produce signals for long periods of time, unlike visual displays, which are immediate.

Distinctive aromas signal the locations of the permanent dens of river otters (Lontra canadensis or Lutra lutra) or the burrows of shrews (Soricidae). Other olfactory materials such as mating pheromones in rodents are volatile and persist for only a short period of time. Pheromones can be quite potent, causing the “strange male (or “Bruce”) effect” in some rodents (e.g., house mice, Mus musculus). With this effect, the mere presence of another male’s urine can cause a female to miscarry a litter.

An individual’s olfactory signature is often the product of the interaction of odors from different sources. Familiar examples include the aromas of sweat and breath and, in some situations, body products such as urine, feces, or oil from glands. An animal’s scent can reveal a great deal about its condition and status, while yet more detailed information can be obtained from the aromas of its urine and/or feces. Bull elk during the rut rub urine on their chests, providing a conspicuous signal to females and other males of their condition. Male white-tailed deer (Odocoileus virginianus) leave urine and feces in specific locations in the woods to announce their presence to other deer. Male pronghorn antelopes (Antilocapra americana) mark the boundaries of territories with piles of feces, as do male white rhinos (Ceratotherium simum), which both spray urine and kick feces at specific locations (middens) in their territories.

Many species of mammals also have glandular organs that contribute to their olfactory signatures. These organs typically include sebaceous and/or sudoriferous glands that synthesize odoriferous molecules. Behavior that transfers the glandular product(s) to other surfaces, sometimes to other animals, is called “scent marking.” An example is the chinning behavior of male rabbits, which serves to place the products of exocrine glands located on the chin on the surfaces being marked. Scent glandular organs often are visually conspicuous, enhancing their role in advertisement. The behavior of mammals rubbing scent glands on surfaces makes the glands even more conspicuous, as in male white-tailed deer marking twigs with scent from their tear ducts during rut. In some mammals, scent glandular organs are associated with specialized hairs called osmotrechia, which are typically quite different from body hairs, being larger in diameter, sometimes longer, and often with a different scale structure; osmetrichia hold and transfer odoriferous molecules.

Although the wing sacs of some sheath-tailed bats (family Emballonuridae) have been referred to as glands, closer examination reveals that they lack glandular tissue. Rather, the wing sacs are fermentation chambers to which the bats (adult males) add various ingredients to enhance their personal scent. Greater sac-winged bats (Saccopteryx bilineata) put saliva, urine, and products of glands located near the anus into the mix in the sac, where fermentation produces the distinctive odors. Using their wing sacs, males can mark objects ranging from females in their group to their roosting sites.

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