Signals or displays carry two basic kinds of information: nonbehavioral (who
and where) and behavioral (what and how). Nonbehavioral messages identify the animal by species, gender, rank, and sometimes even individual identity.
Examples of nonbehavioral signals are distinctive striping patterns, hormone-laden odors, or territorial calls. Knowing the gender, status, and identity of the sender gives the receiving animal a context for future behavior. Since males behave differently than females, and dominants differently than subordinates, knowing gender and status will help the receiver know what to expect. A nonbehavioral message also tells the receiver where the sender is—in the next canyon, on top of the hill, or right over its shoulder. Behavioral messages, on the other hand, deal with action. They tell what an animal is likely to do next and how it is going to do it (weakly or vigorously, for example, and in what direction).
To put the nonbehavioral and behavioral messages in human terms, think of referees on a football field. Their uniform (nonbehavioral) identifies who they are, and blowing a whistle and throwing a flag (behavioral) signify that they have spotted an infraction of the rules and are about to call a penalty.
THE MEDIUM AND THE MESSAGE It’s easy for us to grasp that the referee is giving a signal because we naturally communicate in sounds and gestures.
Animals have other ways to get their messages across, however, ways that may be too subtle for our senses to detect.
Communication signals have evolved to fit each animal’s lifestyle and to carry effectively in its habitat. Animals with territories that take days or weeks to patrol, for instance, need to advertise with something long-lasting, something that will keep on broadcasting even when they are gone. Since odors have the ability to linger, animals often anoint their territories with scent marks that other animals can read as they pass by. Animals that operate in the dark depend on sounds as well as odors, whereas day-active animals are more likely to use visual means to communicate. You’ll see the most dramatic visual signals exchanged between animals of the open plains, since, in their native habitat, they have the luxury of unobstructed views. Birds in dense forests usually can’t see their receivers, however, so they have perfected a rich repertoire of songs instead.
Low-frequency songs are preferred, since higher frequencies tend to hit and bounce back from objects such as tree trunks. Songsters usually perch in the tree
canopy, where an “acoustic window” carries their messages most effectively. A similar sound channel is found in shallow seas, as well as in the surface layers of warm oceans. When whales broadcast on this channel, they can send their songs to other whales up to 2,000 miles away.
For the sender, broadcasting a clear signal to the right audience is as critical to survival as webbed feet or powerful lungs. To complete the circuit, it is equally important for the receiver to be able to hear, see, smell, or feel the message coming through. However a species communicates, it is endowed with complementary adaptations that allow it to pick up the messages of its species.
You can often see how animals at your zoo are tuned in to their particular communication channels by noticing their sensory organs. Visual animals have large, light-gathering eyes; tactile animals have sensitive whiskers; and vocal animals have large, twitching ears.
SCENT MARKING. With surgical precision, a black buck inserts a twig into his facial gland, wetting it with a strong-smelling secretion that will mark his territory.
PRINCIPLE OF OPPOSITES. The dominant wolf’s inflated posture contrasts sharply with the subordinate’s quivering crouch. This clearcut difference helps wolves avoid misinterpretation.
THE PRINCIPLE OF OPPOSITES Here’s a helpful concept to remember when you are trying to interpret signals: signals that carry opposite messages typically have opposite characteristics. Harsh, deep calls, for instance, indicate threat, whereas soft, high calls are used for appeasement or begging. In the same way, if a wolf raises its body, bristles its fur, thrusts its ears forward, and stares intently to show aggression, it does exactly the opposite to show that it is submissive. It lowers its body, droops its tail between its legs, averts its eyes, and flattens its ears back against its head. This principle of opposites is a universal rule that holds true for most species groups. Its main benefit is clarity; it dramatically differentiates the two behaviors so there is no mistaking them.
FACIAL CLUES Although many social encounters occur over long distances (howl to howl or scent mark to scent mark), those that occur face-to-face are far more intense. For animals in the bird, reptile, or amphibian clans, a lack of facial mobility limits how expressive they can be. A gape may be all that these animals can muster. Since the gape exposes a mouthful of sharp teeth, however, it is
often enough said.
Mammals, having the most facile faces in the animal world, are often able to move their lips, ears, eyes, and even noses with meaning. Many of these movements are characteristic not just of one species, but of an entire group of species. Primates, for instance, be they monkeys, orangutans, or humans, share a common repertoire of facial expressions. The human smile is a good example;
both human and nonhuman primates bare their teeth in a grin when they are delighted or feeling frisky. The similarities in mammal expression don’t end here, though. Carnivores, such as wolves and cats, and hoofed animals, such as antelopes and horses, use their eyes and lips and ears in many of the same ways that we primates do. In his excellent book Animalwatching, zoologist Desmond Morris explores the similarities in the facial expressions of primates, carnivores, and hoofed animals. Keep these principles of animal gesture in mind when you’re watching not only the species in this book but other mammals as well.