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Vision

Most of the 5,000 or so living species of mammals have eyes and, in many, the keenness of their vision (visual acuity) is at least equivalent to that of humans. A few mammals have very limited vision, such as river dolphins (Platanistidae, Lipotidae, Pontoporiidae, and Iniidae) that live in extremely murky water or moles (Talpidae) that live in total darkness; indeed, in some moles, the optic nerve has actually degenerated. In mammals' eyes, a lens focuses light on the retina, a layer of light-sensitive cells in the back of the eye. Different chemicals (photopigments) in the cells of the retina convert optical information to electrical signals that are transmitted via the optic nerve to the brain. The retina has two main types of photosensitive cells: rods (that respond to black and white) and cones (that respond to color, which are different wavelengths of light). Color vision in mammals is uncommon, being present mainly in primates, some rodents, and some carnivores. In nocturnal mammals such as any microchiropteran bats, rodents (Muridae), and shrews (Soricidae), rods are often prevalent, while cones may be absent. To these mammals, the world is black, white, or shades of gray. The eyes of some diurnal mammals (for example, primates in the families Lorisidae and Leu-muridae, or rodents in the Sciuridae) have both rods and cones, and these mammals can see color. Other mammals such as some cats (Felidae) have color vision, but only perceive a few colors.

Mammals show a range of overlap between the field of view of left and right eyes—this is the degree of binocularity. The position of eyes in the face and the size and shape of the muzzle influence the degree of binocularity. Humans, with eyes side-by-side and no muzzle to speak of, have a high degree of binocular overlap, which means they have stereoscopic vision. Stereoscopic vision allows mammals (and other animals) to locate objects in space with accuracy. This is the ability to perceive depth, which plays an important role in hand-eye coordination. The distance between the eyes also affects binocularity. For example, African elephants (Loxodonta africana) or blue whales (Balaenoptera musculus), with eyes situated on the sides of huge faces, have almost no binocular overlap. In animals such as California leaf-nosed bats (Macro-tus californicus), the degree of binocularity depends upon the direction in which the bat is looking. There is minimal binocular overlap when the bat looks down its muzzle, and a high degree of overlap when it looks across the top of its muzzle.

Arboreal animals such as many species of primates (lemurs, galagos, and lorises) tend to have higher degrees of binocularity than more terrestrial species (horses, cows, and pigs, in the orders Perrisodactyla and Artiodactyla, respectively). Finally, in some cases, the significance of binocular-ity in the animal's life is not known (for example, in the case of the wrinkle-faced bat, Centurio senex, of South and Central America).

It is common for nocturnal mammals to have a tapetum lucidum behind the retina. The tapetum lucidum is a layer of cells on the back of the eye that reflects light back through the retina, amplifying the stimulation of retinal cells by ensuring one round of stimulation as the light goes through, and another as it is reflected back. Tapeta lucida account for the "eyeshine" when catching a house cat or raccoon (Procyon lo-tor) in a car's headlights or in the beam of a flashlight. Pinnipeds (Phocidae, Otariidae, and Odobenidae) and odontocetes (toothed whales and dolphins) also have tapeta lucida for

Structure Rods And Cones Aye
The mammalian eye. (Illustration by Michelle Meneghini)

helping gather available light at dark ocean depths, resulting in keen underwater vision. Visual displays from the tapetum lucidum are also common to the communication of diurnal mammals, but require that the individuals be close in proximity to each other.

Olfaction

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 scent-marking 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 mu-cosal surfaces of mesethmoid bones nose) in the nostrils con-

vert 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

Mammalian Taste receptors

Cetacean Taste Receptors
surface of the tongue. (Illustration by Michelle Meneghini)

Mammalian Sense of Smell

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Dendrite

Interna Structure Smell Receptor

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MLM ©2003

The sense of smell displayed by many mammals owes much to the internal structure of the nasal passages. These are most highly developed in predators such as dogs. (Illustration by Michelle Meneghini)

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 ex amination 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.

Acoustics

The importance of sounds (acoustics) to mammals should be obvious. As in vision, binaural cues are timing differences between the arrival of sounds at one ear before the other, and they assist in the localization of sources of sounds. Humans use acoustical information to recognize the voices of family and friends or to locate an accident from the wail of an emergency vehicle's siren. In odontocetes, the ability to use bin-aural hearing is improved by an evolutionary shifting of the bones of the skull so that the hearing anatomy of the skull is asymmetrical. This makes odontocetes particularly sensitive to the direction of an incoming sound.

Western Tarsier
The western tarsier (Tarsius bancanus) has a heightened sense of hearing to help it avoid danger and capture prey. (Photo by Fletcher & Baylis/Photo Researchers, Inc. Reproduced by permission.)
Western Tarsier
The American bison (Bison bison) uses its vomero-nasal gland located on the anterior palate in the roof of its mouth to sense females in es-trus. (Photo by © Layne Kennedy/Corbis. Reproduced by permission.)

The auditory system of most mammals consists of the following five main components:

• the pinnae, an external structure that acts as a sound collector

• the ear drum, or tympanum, that converts vibrations in air (sounds) to mechanical vibrations

• an amplifying system, the auditory ossicles (malleus, incus, and stapes) or bones of the middle ear

• a transducer (the oval window), where mechanical vibrations are converted to vibrations in fluid in the inner ear

• sites for converting vibrations in fluid to electrical stimuli (hair cells attached to the basilar membrane in the cochlea)

Through these components, electronic representations of the sounds are generated and transmitted to the brain via the auditory nerve.

Fossorial mammals, those that live most of their lives underground, may lack pinnae (which would only collect dirt). Many, but not all, aquatic mammals also lack pinnae (which would collect water). In fact, as a mammal progresses from amphibious (otters, seals, walrus, and sea lions) to totally aquatic (whales and dolphins), the pinnae go from small and valvular to absent. In fossorial mammals, considerable fusion of the auditory ossciles has reduced sensitivity to high-frequency sounds and emphasized the importance of low-frequency ones. In odontocetes, the lower jaw probably serves to conduct sounds to the middle ear and into the rest of the auditory system. Because water is a denser medium than air, it transmits sound more effectively (sound velocity in water is 4.5 times faster than in air), meaning that the auditory systems of odontocetes, even without pinnae, are no less sensitive than those of humans. In fact, the effective communication distance for all marine mammals is much greater than for any terrestrial animal because of the density of water. In contrast, the effective communication distance for fossorial mammals would be very small, being limited by the reflective tunnel/burrow environment.

Sounds used by mammals can be of very different pitch or frequency, depending upon the species and situation. African elephants are sensitive to sounds at frequencies below 40 Hz; blue whales produce sounds as low as 20 Hz. These are referred to as "infrasounds," because they are below the range of human hearing (arbitrarily, 40 Hz). Other mammals, notably many bats, most carnivores (Felidae, Canidae, Mustel-idae, Viverridae), and dolphins (Delphinidae), use sounds that are well above the range of human hearing (these are ultrasounds, theoretically >20,000 Hz). Humans hear best at frequencies from about 100-5,000 Hz, while some bats and dolphins hear very well at more than 200,000 Hz. In general, low-frequency sounds carry much farther (propogate) than high-frequency ones, and sounds greater than 20,000 Hz are rapidly eroded by the atmosphere (attenuated).

Touch

Mammals use their sense of touch in different ways. Tactile interactions are important for intraspecific communication, well known to a human who has benefited from the comfort of a hug. Often, touch plays an important role in female mammals recognizing their infants. Seal pups often reunite with their mothers by exchanges of vocalizations that

Human Dolphin Sound
The giraffes' 18-foot (5.5-meter) height and excellent vision make it easy for them to spot predators from a distance. (Photo by David M. Maylen, III. Reproduced by permission.)
Canis Lupus Eating Its Prey
The timber wolf (Canis lupus) uses its keen sense of smell to track its prey. (Photo by Wolfgang Baye. Bruce Coleman, Inc. Reproduced by permission.)

terminate in nuzzling. Grooming often involves touching, such as in two chimpanzees (Pan troglodytes) carefully stroking and picking at each other's fur. Primates have an especially well-developed sense of touch, having friction ridges (finger prints) on the tips of their digits used for careful investigation of objects. Although dolphins do not have limbs for grasping, their sleek, hairless skin is especially sensitive to touch at various locations on the body, specifically, the gape of the mouth, the gum, and tongue, and the insertion point of the flipper. Dolphins commonly swim close to each other, touching and rubbing their bodies together. The spectacular nasal appendages of the star-nosed mole (Talpidae) are extremely sensitive to touch and are used to locate and identify prey.

Vibration

Aye-ayes (Daubentonia madagascarensis) are among the mammals most obviously specialized to use vibrations. These Madagascar natives have long, slender third fingers. A foraging aye-aye taps branches with its elongated fingers and listens for reverberations that it uses to find hollows. The vibrations, combined with the noises made by insects moving through tunnels in wood or chewing to excavate tunnels, help aye-ayes find their prey.

Vibrations can also serve in communication. Vibrations tend to be low frequency, readily sensed by specialized hairs (whiskers) or other body parts. Nearly furless naked mole-rats (Heterocephalus glaber) live in a burrow system and announce their presence to nearby conspecifics in other burrows by tapping their heads against the roofs of tunnels. Elephants are thought to use vibrations to sense danger or intruders over long distances. Recent studies of captive elephants showed that male elephants in mating condition (musth) moved their foreheads in and out, movements coinciding with the production of low-frequency sounds. Although African elephants can detect acoustic signals of about 115 Hz at distances of 1.5 mi (2.5 km), they need to be closer (0.6-0.9 mi [1-1.5 km]) to extract individual-specific information about the sig-naler(s). Researchers also believe that elephants sense very-low-frequency vibrations with their large, flat feet, enabling them to detect the movements and signals of other elephants from great distances. Foot drumming is a common way to generate vibrations that are used in communication by mammals such as lagomorphs (rabbits and hares) as well as kangaroo rats and subterranean mammals.

Infrared

Around their nose leaves, vampire bats (Desmodus rotundus) have sensors sensitive to infrared energy. The bat's sensors

Vampyrum Spectrum Bat
Spectral bat (Vampyrum spectrum) locating prey. (Photo by Animals Animals ©Stephen Dalton. Reproduced by permission.)

lack a lens, so they provide poor spatial resolution of infrared sources. Vampire bats probably use infrared cues to locate places on a mammal or bird's body where blood flows close to the skin, ideal places to bite and obtain a blood meal. Among mammals, some felids have vision that extends into the infrared spectrum. Elsewhere among vertebrates, some pit vipers (rattlesnakes) use infrared sensors on the roofs of their mouths to locate and track warm-blooded prey in cool desert nights.

Chemoreception (taste)

Mammals detect a wide range of flavors as odors and tastes. Bottlenosed dolphins (Tursiops truncatus) readily detected different concentrations of bitter, sweet, and sour liquids presented to them. Unlike some terrestrial mammals, bottlenosed dolphins are not sensitive to subtle changes in salinity, suggesting that an animal living in salt water would not be averse to the taste of salt in its mouth.

Geomagnetic

The ability to orient to geomagnetic fields has been demonstrated in several species of migrating birds and in some rodents. In mammals, the geomagnetic sensing ability is correlated with the presence of magnetite in the brain. Some classic studies on trained rodents showed that the animals, when spun around 360°, could choose a particular orienta tion. Since the time of Aristotle, people have recognized that some odontocetes (toothed whales and dolphins) strand or beach themselves, often in large groups. Stranded animals may be completely out of the water and face certain death. Some locations where cetaceans often strand themselves are in areas with abnormal or unpredictable geomagnetic fields.

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