Displays (animal behavior)

Why do birds sing? Not because they are happy. They sing to communicate. By singing, a bird communicates its location, its species, its sex, its approximate age and size, and, perhaps, its current reproductive status, territory ownership, health, dominance status, and motivational state. Birdsong and other nonlinguistic forms of communication are called displays.

Types of Displays

Some displays involve, literally and simply, the visual display of a physical feature. Among insects, for example, green or brown coloring is often used for camouflage. Insects that are poisonous do not need camouflage and often advertise themselves with warning colors, such as black and red, or black and orange. This is referred to as aposematic coloration or an aposematic display.

Physical features can also indicate an individual’s sex, age, and reproductive status—as do peacock tails, turkey wattles, deer antlers, the canine teeth of male baboons, and the swollen genitals of estrous female chimpanzees. Many such features vary in size, shape, or color in relation to an animal’s health, hormones, or social status and are, therefore, referred to as status badges or signs.

Often, meaningful physical features are further highlighted by behavioral displays. A courting peacock or turkey will fan open his tail and shake it back and forth for emphasis. Similarly, a challenged buck will load plant material onto his antlers to exaggerate their size. An angry baboon will curl back his lip to further expose his canine teeth, and an estrous chimpanzee will approach a friendly male and assume a posture displaying her fertile state. One species of pufferfish creates art on the sea floor to attract a mate rather than using its body for a physical display. The male draws a large circle, adds a smaller circle in the middle, and then uses his fins and tail to place rocks and ridges in the sand to decorate his work. If the female is interested, she will lay her eggs in the middle circle.

A particularly energetic or dramatic behavioral display not only calls attention to a physical feature but indicates the health and vitality of the performer. The principle of honest signaling refers to the fact that large and healthy individuals tend to have brighter, more contrasting colors, make deeper-pitched and louder sounds, and produce longer, more intense performances than small or weak ones. Such differences in display quality are readily noticed by predators, potential competitors, and potential mates.

Most displays are performed by individuals and are one way—sender to receiver. Threat displays, however, may involve reciprocal signaling between two challengers or between two groups of challengers. Courtship displays also may occur in groups. In some species, males gather to perform in what is called a lek or a lekking display. Courtship of monogamous species may include long sequences of frequently repeated, ritualized interactions in which both partners participate. Such pair-bonding displays may continue well into the breeding season and the mateship, initially serving to familiarize the pair with one another and to synchronize their hormones and breeding behavior, but they may later serve as greeting displays after separation.

Interpreting Displays

Darwin noted that displays having opposite characteristics often signal opposite meaning. In humans, for example, a face with upturned corners of the mouth (a smile) signals friendliness, whereas a face with downturned corners of the mouth (a frown) signals displeasure. In most animals, loud, deep-pitched sounds (for example, roars and growls) indicate aggression, whereas quiet, high-pitched sounds (for example, mews and peeps) indicate anxiety or fear. Similarly, body postures exaggerating size tend to signal dominance, whereas postures minimizing size tend to signal submission. Darwin called his observation the Principle of Antithesis.

Although some rules of display can be applied across species, most displays are specialized for intraspecific (within-species) communication— male to female, parent to offspring, or dominant to subordinate—and are, therefore, species-specific. That is, the ability to perform and interpret a particular display (such as a particular birdsong) is generally characteristic only of individuals of a particular species and is either innate (inborn) or learned from conspecifics (individuals of the same species) during an early critical period of development.

To ensure meaning is easily and quickly conveyed, most displays tend to be highly ritualized—they are performed only in certain contexts and always in the same way. This consistency in communication prevents errors of interpretation that could be disastrous. It would be a grave mistake, for example, to interpret an aggressive signal as a sexual overture, or an alarm call (predator alert signal) as an offspring’s begging call. Mistakes of interpretation are also minimized by signal redundancy, that is, messages are often conveyed simultaneously in more than one sensory modality.

Display Modality

Displays utilize every sensory modality. Visual displays involve the use of bright, contrasting, and sometimes changing colors; changes in body size, shape, and posture; and what ethologists call “intention movements”—brief, suggestive movements which reveal motivational state and likely future actions. Auditory displays include vocal songs and calls, as well as a variety of sounds produced by tapping, rubbing, scraping, or inflating and deflating various parts of the body. Tactile displays include aspects of social grooming, comfort contacts (such as between littermates or parents and offspring), and the seismic signaling of water-striders, elephants, frogs, and spiders which, respectively, vibrate the water, ground, plants, or web beneath them. Olfactory displays include signals from chemicals wafted into the air or water, rubbed onto objects, or deposited in saliva, urine, or feces.

Olfaction (sense of smell) is the most primitive, and therefore the most common and most important, sense in the animal kingdom. Species of almost every taxonomic group use smell to signal their whereabouts and, generally, their sex and reproductive state. (Most birds seem to be an exception.) Animals may also use smell to identify particular individuals, to recognize who is related to them and who is not, and to determine the relative dominance status of a conspecific.

Chemicals used in displays are called pheromones. They may be derived from waste products or hormones, acquired by ingesting certain food items or obtained directly from plants or other animals. Releaser pheromones communicate information and have immediate physical effects on their receivers, including attracting a mate or sending a warning signal. For example, moths can sense another moth's pheromones from several miles away. Primer pheromones impact animals' physiology and produce short and long-term responses. These pheromones help build structure in social insects and encourage bonding.

Principal Terms

Aposematic Display: Use of bright, non-camouflaged colors to indicate toxicity or dangerousness

Pair-bonding: Prolonged and repeated mutual courtship display by a monogamous pair, serving to cement the pair bond and to synchronize reproductive hormones

Pheromone: A modified hormone that, through sense of smell, communicates information to, and has effects on, individuals other than the individual producing it

Principle of Antithesis: The observation that signals communicating opposite meaning tend to be expressed using displays having opposite characteristics

Ritualization: An evolutionary process that formalizes the context and performance of a display so that its meaning is clear and straightforward

Species-specific: A behavior or trait that characterizes members of a species, is innate, and is exclusive to that species

Status Badge: A visual feature that, based on its size or color or some other variation, indicates the social status of the bearer

Bibliography

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