Sensory systems

Introduction

Mammals, like other animals, can be expected to use whatever information is available to them when making decisions about activities such as foraging, mating, navigating, selecting shelter, or locating habitats. The range of information actually used by any one species can be predicted from its sensory apparatus—the stimuli they can perceive. Lifestyle plays an important role here, so that moles and other fossorial (also known as subterranean) mammals, including golden moles, some rodents, and at least one species of marsupial, can be expected to use vision less than species that are active aboveground, including the lion (Panthera leo), vervet monkey (Cercopithecus aethiops), or moose (Alces alces). Everyone who has walked a dog (Canis familiaris) or experienced the spraying of a male housecat (Felis cattus) knows the importance of odor in the lives of these mammals. The important role that sound plays in the lives of mammals becomes obvious when listening to the echolocation calls of a bat attacking an insect or to the bugling of a male elk (Cervus elaphus) during the rut.

The media

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 Leumuridae, 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 (Macrotus 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 binocularity 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 lotor) 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

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 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.

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 binaural 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.

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, Mustelidae, Viverridae), and dolphins (Delphinidae), use sounds that are well above the range of human hearing (these are ultra-sounds, 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

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 signaler(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

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 orientation. 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.

Role of sensory data

Mammals sense or gather information about their environment and use it to make decisions that affect their survival and reproduction. Sometimes, species initially respond to one type of cue. For example, female hammer-headed bats (Hypsignathus monstrosus) in Africa locate groups of males by listening to their distinctive calls. Picking a male to mate with, however, is a decision females appear to make only after visiting several in the line of displaying suitors. A female's actual choice may involve more than just her response to the males' calls.

Mammals typically use clues collected from several modalities. For example, vervet monkeys must cope with different predators. Social animals, vervets have keen vision and extensive vocal repertoires that include several types of warning calls, which indicate the presence of a predator. Using different warning calls, vervets can alert group members to specific threats. One alarm call is given in response to snakes, another to mammals such as leopards (Panthera pardus), and yet another to raptors such as eagles. These predators pose different kinds of threats. Vervets typically see the predators, but use sound to alert their group mates to the danger. Because each type of predator requires different defensive behavior, the vervets have specific acoustic signals to increase the precision of their communication.

The ability of recognizing other individuals in mammals begins at birth. Female mammals are expert at recognizing

their own young. This is a valuable behavior because milk is expensive to produce and vital to the survival of young. The level of challenge to the mother varies with different mammals. Ewes recognize their lambs by smell, and she usually finds her own lamb quite easily. The lamb imprints on its mother within in a few days of birth. A female Brazilian free-tailed bat (Tadarida brasiliensis) faces a more difficult challenge. She typically leaves her single young in a creche with hundreds or thousands of others. When she returns from foraging and looks for her young, she initially relies on spatial memory to locate the general area where her young might be, then she uses the calls of her young to pinpoint their location, and finally ensures that she is feeding the right young by smelling its scent. When females depend upon odor to recognize their offspring, the distinctive smell could be something produced by the young, something in the milk she has fed it, or her own distinctive aroma.

In mammals, distinctive odors do more than mediate inter-actions between mothers and young. Young piglets (Sus scrofa) can recognize other piglets by the odor of their urine, which allows them to distinguish between familiar and strange individuals. In summer, Bechstein's bats (Myotis bechsteinii) live together in small groups (colonies). Individuals have specific odor signatures produced by a gland located between their ears. In other bats, such as big browns (Eptesicus fuscus), lesser-crested mastiff bats (Chaerephon pumila), or pipistrelles (Pipistrellus pipistrellus), odors allow bats to identify their home groups or roosts. It is typical for individual aromas to reflect a combination of odor sources, not just the products of a single gland.

Groups of mammals can also have distinctive vocalizations. Greater spear-nosed bats (Phyllostomus hastatus) use group-specific screech calls to locate other group members when they are feeding. In big brown bats and little brown bats (Myotis lucifugus), echolocation calls provide cues to group membership. Humpback whales (Megaptera novaeangliae) have distinctive songs that consist of phrases and arrangement of phrases in a music-like organization. Songs within a pod slowly change, so that last year's song is slightly different than the current year's song. Furthermore, phrases come and go in the overall repertoire. Songs also play a role in maintaining group cohesion. Male humpbacks sing from an inverted position and project their sounds over large ocean expanses. In the Antarctic, Weddell seals (Leptonychotes weddellii) have a large vocal repertoire of 34 underwater vocalizations, 10 of which are used only by males.

Echolocation

Animals, including some mammals and birds, use echoes of sounds they produce to locate objects in their surroundings. This is echolocation behavior. Microchiropteran bats produce echolocation signals by vibrating their vocal cords— exactly the same operation humans use in speaking. Echolocating mammals hear echoes through their auditory systems, just as humans hear sounds. While echolocating bats collect echoes of their own sounds by their pinnae (external ears), odontocetes appear to collect sounds via their lower jaws.

An echolocating animal uses the differences between the sound it produces and the reflected echoes to collect information about its surroundings. Echoes arrive sometime after the production of the outgoing signal, providing time cues to the echolocator. Echoes can differ in frequency composition from the outgoing signal, encoding information about the target's surface. Echolocating mammals, including some toothed whales and many bats, use echolocation to detect targets (food items) in their path, whether fish or insects, as well as objects such as trees or seamounts. They use echolocation to determine three factors about detected targets: identity; distance; and movement, whether toward or away from the echolocator. Some of this information is obtained by comparing information from

sequences of calls and their echoes. Some microchiropteran bats and toothed whales also use echolocation to obtain fine details about objects. Echolocating dolphins can distinguish between echoes from same-sized spheres of aluminum and glass (down to accuracy of 0.001 in [0.0025 cm]), while some echolocating bats detect insects smaller than midges and can distinguish flying moths from flying beetles.

It is not obvious that other echolocating mammals (some species of shrews and tenrecs) collect and use such detailed information. Shrews and tenrecs appear to use echolocation while exploring, providing another medium for collecting general information about their surroundings rather than about specific targets. Most species of pteropodids, plant-visiting flying foxes of the Old World, do not echolocate. Furthermore, not all bats echolocate. Or, while Egyptian fruit bats (Rousettus aegyptiacus) and perhaps some other species in this genus (Rousettus) echolocate, they produce echolocation sounds by clicking their tongues. A further complication is that the role echolocation plays in the lives of some other bats is not known. Then, some phyllostomid, nectar-feeding bats, visit flowers that are specialized to deliver strong echoes of ultrasonic (echolocation) calls and thus guide the bats to the nectar they seek.

While toothed whales living in turbid waters may use echolocation to find prey, it is not clear how often these animals use echolocation to find food in clear waters. It is not known if any of the mysticete (baleen) whales use echolocation because none has been held in captivity to conduct the necessary perceptual studies.

The echolocation signals of toothed whales, shrews, and tenrecs are short, click-like sounds composed of a range of frequencies (broadband). The echolocation signals of microchiropteran bats are tonal, because they show structured changes in frequency over time. The echolocation signals of toothed whales and some microchiropteran bats are very intense, measured at over 110 decibels; in the toothed whales, it is measured at over 200 decibels. Other microchiropteran bats and shrews and tenrecs produce signals of low-intensity (<60 decibels). Many species of echolocating bats that hunt flying insects and other species that take prey from the surface of water change the details of their calls according to the situation. Longer calls, often consisting of a narrow range of frequencies (narrowband) and dominated by lower frequency sounds, are produced when the bats are searching for prey. Once prey has been detected, the bats often produce shorter, broadband signals. Over an attack sequence, the calls get progressively shorter as does the time between them. The end sequence (or terminal feeding buzz) has closely spaced signals for precisely timed capture of the prey. Odontocetes can adjust the frequency and amplitude of their echolocation signals, depending on the amount of environmental noise. In areas of noise from snapping shrimp, dolphins produce louder and higher frequency signals to avoid the masking sounds of the shrimps' snaps.

To ensure that they can hear faint returning echoes, echolocating mammals typically separate pulse and echo in time. In other words, most echolocating mammals cannot transmit signals and receive echoes at the same time because strong outgoing pulses deafen the echolocator to faint returning echoes, which means that collecting information about close objects requires signals that are short enough to be over before the echoes return. Differences in the density of water and air indicate that sound travels much faster in water, and cetacean echolocation signals are much shorter than those commonly used by bats.

Some microchiropteran bats, including species of horseshoe bats (Rhinolophidae), Old World leaf-nosed bats (Hipposideridae), and Parnell's moustached bat (Pteronotus parnellii; Mormoopidae), take a different approach to echolocation. They separate pulse and echo in frequency, meaning that they can transmit signals and receive echoes at the same time. These bats depend upon Doppler shifts in the frequencies of their echolocation sounds to collect information about their surroundings and targets.

Microchiropteran bats that separate pulses and echo in time produce short echolocation calls separated by long periods of silence. Separating pulse and echo in frequency produces much longer signals that are separated by shorter periods of silence. Some bats produce short signals that are called low-duty cycle, while others produce longer signals that are called high-duty cycle, reflecting the percentage of time that the signal is on (10% versus >50%, respectively). Anatomical evidence from Eocene fossil bats indicates that both high-duty cycle and low-duty cycle approaches to echolocation had evolved over 50 million years ago.

The distance over which an animal can use echolocation to collect information will depend upon the strength of the original signal and the sensitivity of the echolocator's auditory system. As a signal moves away from the source (an echolocator's mouth), it loses energy through spreading loss and, for higher frequency signals in air, by attenuation. The same rules apply to the echo returning from the target. For a big brown bat, this means that the effective range for detecting a spherical target 0.7 in (19 mm) in diameter is 16.4 ft (5 m). This assumes that the initial signal was 110 decibels and that the bat's hearing threshold is 0 decibels. For 0.7-in (19-mm) diameter spheres located 32.8 ft (10 m) in front of the bat, the original signal would reach the target, but the echo would not have sufficient energy to return to the bat.

The same general situation applies to echolocating porpoises and dolphins, although the distances are greater because of a combination of original signal strength and the sound-conducting properties of water. A false killer whale (Pseudorca crassidens), for example, can detect a 3-in (76-mm) diameter water-filled, aluminum sphere at 377 ft (115 m), a range that is far beyond visual detection. Similarly, an echolocating big brown bat would detect a 0.7-in (19-mm) long insect at 16.4 ft (5 m), but see it only at 3.2 ft (1 m). However, because of spreading loss and atmospheric attenuation over longer distances, the same bat would detect a tree-sized object at 49.2 ft (15 m), but would have been able to see it at over 328 ft (100 m).

Echolocation is an orientation system that allows animals to detect objects in front of them, and some species also use it to detect, track, and assess prey. The short operational range of echolocation in terrestrial mammals means that it is of much less value in navigation. But short operational range used in combination with local knowledge can be effective in a longer-range orientation. Greater spear-nosed bats deprived of vision and taken 31 mi (50 km) from their home caves could find their way back. Odontocetes use echolocation for navigation in murky, deep, dark waters and a varied underwater topography. Beluga whales (Delphinapterus leucas) use echolocation to survey the irregular undersurfaces of their ice-covered habitats.

Information leakage is an important disadvantage to echolocation. The signals one animal uses in echolocation are available to any other animals capable of hearing them. Many species of insects (e.g., some moths, beetles, mantids, crickets, and lacewings) have ears that allow them to detect the echolocation calls of bats. Moths with bat-detecting ears evade capture in 60% of attacks by echolocating bats, while deaf moths are most often caught. Some herring-like fish change their behavior when they hear the echolocation clicks of dolphins. Weddell seals dramatically reduce their underwater vocalization rate from 75 calls per minute to no calls when they hear sounds from their predators, killer whales (Orcinus orca) and leopard seals (Hydrurga leptonyx).

The most obvious eavesdroppers on echolocation calls are members of the same species. Bats often use the echolocation calls of other bats to detect patches of prey or vulnerable prey. Echolocation calls also can serve as communication signals promoting cohesion in groups of flying bats.

Echolocating animals also have repertoires of social calls. While echolocation signals can serve a communication function,

many social calls are too long to be useful in echolocation. A long signal masks echoes that return as the call is being produced.

Navigation

Some species of mammals make long-distance migrations, typically repeated annual movements between summer and winter ranges. Gray whales (Eschrichtiidae), bowhead whales (Balaena mysticetus), right whales (Eubalaena species), and humpback whales make predictable, long-range migrations between summer feeding areas and winter breeding areas. Manatees (Trichechus manatus) make shorter seasonal migrations up and down the east coast of Florida. Other migrating mammals include some species of bats, caribou (Rangifer tarandus), and antelopes that regularly move between summer and winter ranges.

The navigational cues used by migrating mammals appear to vary. Over shorter distances, perhaps first traveled with their mothers or group members, young may learn the landmarks that guide them from one place to another. This appears to be true of manatees off the east coast of Florida. Over longer distances, geomagnetic cues may be more important. However, compared to the situation in migrating birds, relatively little is known about the mechanisms of navigation in mammals.

Resources

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Periodicals

Deutsch, C. J., J. P. Reid, R. K. Bonde, D. E. Easton, H. I. Kochman, and T. J. O'Shea. "Seasonal Movements, Migratory Behavior, and Site Fidelity of West Indian Manatees along the Atlantic Coast of the United States." Wildlife Monographs, 151 (2003): 1–77.

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Scully, W. M. R., M. B. Fenton, and A. S. M. Saleuddin. "A Histological Examination of Holding Sacs and Scent Glandular Organs of Some Bats (Emballonuridae, Hipposideridae, Phyllostomidae, Vespertilionidae and Molossidae)." Canadian Journal of Zoology, 78 (2000): 613–623.

Voight, C. C. "Individual Variation in Perfume Blending in Male Greater Sac-winged Bats." Animal Behaviour, 63 (2002): 907–913.

Jeanette Thomas, PhD

M. B. Fenton, PhD