9.17–9.18: Communication and the design of signals evolve.

A mona monkey (Cercopithecus mona) in western Africa bares its teeth.

The challenge of communication is central to the evolution of animal behaviors, whether they relate to cooperation, conflict, or the many interactions inherent to selecting a mate. An animal must be able to convey information about its status and intentions to other individuals. And equally important, animals must be able to interpret and assess the information they get from other individuals so as to reduce the likelihood of being tricked or taken advantage of. In this section and the next, we explore the evolution of a variety of methods of animal communication and the ways in which communication can influence fitness and the evolution of other behaviors.

Not all animals need to open their mouths when they have something important to communicate. When the female silkworm moth is ready to mate, for example, she releases a potent chemical called bombykol into the air. As molecules of the chemical come in contact with the bushy antennae of a male silkworm moth—who might be as far as three miles away—he recognizes the mate attractant and responds by flying upwind, looking for the source of the chemical, until he zeroes in on the female and they mate.

This is just one example—a chemical one—of communication, an action or signal on the part of one individual that informs or alters the behavior of another individual (FIGURE 9-28). A huge range of animal behaviors requires communication, including defending territory, alerting individuals to threats, announcing one’s readiness for mating, establishing one’s position in a dominance hierarchy, and caring for offspring.

Figure 9.28: Chemical, auditory, and visual communication. Many animal behaviors require communication: the ability to convey and receive information from other animals.

Animals communicate with many different types of signals. Three types are most common.

1. Chemical. Molecules released by an individual into the environment that trigger behavioral responses in other individuals are called pheromones. They include the airborne mate attractant of the silkworm moth, territory markers such as those found in dog urine, and trail markers used by ants. They are even used by humans: some chemicals that influence the length of the menstrual cycle in women are responsible for the synchronization of menstrual cycles that occurs in women living in group housing situations, such as in dormitories and prisons.

2. Acoustical. Sounds that trigger behavioral responses are abundant in the natural world. These include the alarm calls of Belding’s ground squirrels described earlier, the complex songs of birds, whales, frogs, and crickets vying for mates, and the territorial howling of wolves.

3. Visual. Individuals often display visual signals of threat, dominance, or health and vigor. Examples include the male baboon’s baring of his teeth and the vivid tail feathers of the male peacock.

With increasingly complex forms of communication, increasingly complex information can be conveyed. One extreme case is the honeybee waggle dance. When a honeybee scout returns to the hive after successfully locating a source of food, she performs a set of maneuvers on a honeycomb that resemble a figure 8, while wiggling her abdomen. Based on the specific angle at which the scout dances (relative to the sun’s position in the sky) and the duration of her dance, the bees that observe the dance are able to leave the hive and quickly locate the food source.

The complexity and power of the waggle dance raises an important question: at what point does communication become language? Language is a very specific type of communication in which arbitrary symbols represent concepts, and a system of rules, called grammar, dictates the way the symbols can be manipulated to communicate and express ideas (FIGURE 9-29).

Figure 9.29: Dancing, signing, and speaking. A variety of ways have evolved in animals to convey complex information to others.

It’s not always easy to identify language. Consider one type of communication in vervet monkeys. Vervet monkeys live in groups, and when an individual sees a predator coming, it makes an alarm call to warn the others. Unlike in most other alarm-calling species, the vervet monkey’s alarm call differs depending on whether the predator is a hawk, a snake, or a leopard. And the response of other vervet monkeys to the call is appropriate to the type of predator approaching. The snake alarm causes individuals to stand on their toes and look down. The leopard alarm causes individuals to quickly climb the nearest tree. And the hawk alarm causes vervet monkeys to look to the sky.

Does alarm calling in vervet monkeys or the honeybee waggle dance constitute language? This is debatable. Is the form of American Sign Language taught to chimpanzees and gorillas a language (see Figure 9-29)? There are opinions on both sides. Interestingly, the great apes have shown a capacity for creating new signs, expressing abstract ideas, and referring to things that are distant in time or space—important features of human language. One thing that is certain, however, is that human language is near the most complex end of the communication continuum.

Language influences many of the other behaviors discussed throughout this chapter. The evolution of reciprocal altruism, for example, may be influenced by language, as this makes it easier for individuals to convey their needs and resources to other individuals. Similarly, in courtship and the maintenance of reproductive relationships, language gives individuals a tool for conveying complex information relating to the resources and value they may bring to the interaction.