Mammals communicate with one another by various means, including those that depend on sight, smell, tactile, and auditory effects. An act of communication is an action on the part of one organism (the sender) that alters the probability of occurrence of behavior patterns in another organism (the receiver) in a fashion adaptive to one or both of the participants. There usually is a genetic component. The sender usually is considered as at least potentially receiving a benefit; the receiver may either benefit, have no change, or be harmed. Frequently, a signal is ritualized so the receiver easily recognizes the message without the sender having to spell it out.

One classification of animal signals involves discrete versus graded types. Discrete is an either/or type signal. For example, in a zebra, ears back when encountering another zebra is a threat signal; ears forward indicates a friendly greeting—thus discrete signals. On the other hand, the intensity of either of these signals is communicated by how wide the mouth is opened; a partially opened mouth conveys a moderate threat (or friendliness) and a widely open mouth a strong threat (or a strong friendliness of the greeting).

Generally, mammalian species have between about 20 and 40 different displays, but variations in a display may give additional information. Some signals may act over long distances (e.g., low-frequency sounds of whales or elephants). Visual displays usually are over shorter distances. The length of a display may vary greatly, also. The alarm call of ground squirrels, etc., may be short and its effects quickly terminated. On the other hand, such displays as antlers and primate color displays may last over a breeding season. Signals may have various meanings depending on the context: a lion's roar may be a signal that a territory is held; other lions keep out; or it may be an aggressive display when two males are engaged in fighting; or it may be a way to maintain contact among members of the pride.

Odor is probably the oldest method of communication evolutionarily. Pheromones and other chemical elicit responses from other individuals, usually of the same species. Effects may be involved in mate identification and attraction, identification of relatives, spacing of individuals, indication of territory, or alarm.

Some examples of the effects of pheromones (in mice in the example) are priming pheromones (pheromones that produce a generalized effect): estrus inducer, estrus inhibitor, adrenocortical activator; and signaling pheromones (pheromones that produce an immediate motor response): fear inducer, male sex attractant, female sex attractant, aggression induces, and aggression inhibitor.

Many animals produce odors from various glands on and in the body. Among deer, for example, tarsal and metatarsal glands, tail glands, and urine are involved in secreting and depositing signaling odors. Likewise, we're all familiar with scent marking by canines and felines.

Sounds used as communication vary with the animal kind and with the environment. Howler monkeys, with relatively large home ranges, use low frequency sound that will travel for kilometers to communicate; mammals with smaller home ranges tend to use higher frequency sounds that quickly dissipate. Ultrasonic sounds (above human hearing range; that is, above about 20 kHz) are widely used among rodents, shrews, and canines, among others. Whales produce complex songs; judging from where and when (males on the breeding grounds), presumably either to entice females or to challenge other males. Elephants produce very low frequency sounds (14 to 35 Hz) that can be detected over several kilometers.

Social mammals and diurnal mammals tend to use sight a great deal. Many cursorial mammals display flagging behavior: erecting white rump-patch or tail hairs and making a prominent display in the presence of predators. Functions have been argued. Some suggested functions include distracting predators from others in the group, warning other members of the herd, confusing the predators when a number of the prospective prey items do this, signaling the predator that it's been noted, and eliciting premature pursuit. Another argument is that it's primarily an intraspecific social communication.

Touch may be important. Social grooming is widespread in primates,for example. In many of these, at least, those of high social status get groomed more often and those of lower status may compete to be allowed to groom the high status individuals. Kin are groomed more often than non-kin, though non-relatives may form long-term grooming partnerships. In many mammals, tactile stimulation may be important in copulation, and cats and rabbits require vaginal stimulation to induce ovulation.

Functions of communication include group spacing and coordination of movement, species recognition, and kin recognition. The latter occurs in some mammals even if not raised together. Clues to relationships may come from the major histocompatibility complex (MHC) that tells the immune system what is self and what is not. At least in rodents, urine odor is affected by the MHC and allows recognition of genetic similarity. Humans and some other mammals are known to prefer mates with unlike MHC genetics, possibly as a way to avoid inbreeding. Stages of sexual readiness often are communicated by any of the methods noted. Aggression often is indicated by signals that usually act to avoid fighting. A threat display often is sufficient, particularly when a social hierarchy has been established. Such rituals serve to decrease actual combat where even the victor is apt to be injured.

Alarms are common communications. They usually are simple and may converge in type among different species in an area, allowing the alarum signal of one to be understood by all. In some cases, the alarum calls may be quite specific. Among vervet monkeys, there are different calls for leopard, eagle, and snake. Some rodents when stressed produce a substance in their urine that cause other rodents to avoid the area.

Various other social activities are communicated. Social hunters may engage in "fellowship" interactions before a hunt, and one observer strongly suggests that hyaenas are able to convey the type of prey they're after. Care-soliciting signals are widespread among mammals, usually between offspring and parents, but sometimes otherwise. Playing often includes signals to insure that everyone knows it's not the real thing.

Whereas a home range is the area where an animal carries out most of its everyday activities, a territory is actively defended by an individual or a group. Defense usually is by signalling rather than actively fighting. Thus visible presence during patrols, scent posts, and vocalizations are common. Usually once a territory is established (often aggressively, but by posturing rather than actual fighting), the signals are enough to keep intruders out. Active fighting usually occurs only when two individuals of approximately equal strength are involved (or the territory holder is the weaker).

Many social mammals, show dominance rather than territory to maintain order. There are several patterns. One is where a single individual dominates all, with the subordinates being more or less equal. A second pattern is where we have a linear hierarchy, with A dominating B, B dominating C, C dominating D, etc. Rarely among primates, a circular pattern may appear, where A dominates B who dominates C who dominates A. Some primates also form coalitions, where A may dominate B and C separately, but C and B may gang up to dominate A. In some primates, other individuals may intervene either to support the dominant individual or the subordinate individual. When two individuals repeatedly form coalitions, the term alliance is used; alliances are known from primates and bottlenose dolphins. Dominance has value. In studied groups, the dominant individuals tend to be well fed and healthy, whereas the subordinate groups more often are malnourished and diseased.


Last Update: 4 Feb 2008

Centennial Museum and Department of Biological Sciences, The University of Texas at El Paso