Aggressive behaviour

Aggressive behaviour, animal behaviour that involves actual or potential harm to another animal. Biologists commonly distinguish between two types of aggressive behaviour: predatory or antipredatory aggression, in which animals prey upon or defend themselves from other animals of different species, and intraspecific aggression, in which animals attack members of their own species. Intraspecific aggression is widespread across the animal kingdom, being seen in creatures as diverse as sea anemones, rag worms, wolf spiders, field crickets, lobsters, salmon, tree frogs, lizards, songbirds, rats, and chimpanzees. Given that so many different kinds of animals fight, aggression takes a variety of forms. Sea anemones lash at one another with tentacles armed with stinging cells, rag worms batter each other with the proboscises that they use for digging burrows, lobsters use their large claws for hitting and grasping, tree frogs wrestle, robins peck, red deer use their antlers to push and batter one another, and one chimpanzee, made famous through the work of British behavioral scientist Jane Goodall, intimidated rivals by banging two oilcans together. Usually just two or a few individuals participate in fights, but there are cases in which larger groups of animals are involved. Spectacular examples are the lethal fights that take place between neighbouring ant colonies in the spring and the killing raids by coalitions of male chimpanzees against smaller neighbouring groups.

This article follows the common practice of biologists by considering only intraspecific attacks under the title aggressive behaviour. The emphasis here is on biological context—that is, the roots of aggression in competition for food and mates; the influences of the nervous system, hormones, genetics, and environment; and scientific models for analyzing the likely outcome of aggressive interactions.

The nature of animal aggression

Aggression sometimes occurs when parents defend their young from attack by members of their own species. Female mice, for example, defend their pups against hostile neighbours, while male stickleback fish defend eggs and fry against cannibalistic attack. More frequently, however, animals fight over resources such as food and shelter—e.g., vultures fight over access to carcasses, and hermit crabs fight over empty shells. Another important resource over which fighting commonly occurs is potential mates. In this case the biology of gamete production has an influence on aggressive behaviour: because a female’s eggs are larger, are fewer in number, and require more energy to produce than a male’s sperm, competition among males over females is usually more frequent and intense than competition among females over males. As a result, the most spectacular fights among animals, whether they are crickets, salmon, tree frogs, chaffinches, or stags, occur between males over fertile females.

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Aggression may be focused on a specific area, such as a defended territory from which rivals are vigorously excluded. A notable example is shown by mudskippers, intertidal fish that defend small territories where they browse on microscopic plants. The fish build mud walls around the borders of their territories, and at low tide water is retained within the walls (incidentally permitting the human observer to visualize the mosaic of territories in a colony of these fish). Territorial behaviour is also shown by rag worms and fiddler crabs when they defend their burrows, by male dragonflies and sticklebacks defending breeding grounds, by male tree frogs, sage grouse, and Uganda kob defending high-quality sites for courting and mating, and by spiders, reef fish, and hyenas when they defend feeding areas.

A common feature of aggression in most species is that fights tend to start with relatively harmless displays or postures. For example, aggressive interactions between two red deer stags begin with an exchange of deep roars followed by a display of “parallel walking,” in which the stags strut side by side assessing their relative size. The aggression may then escalate to direct attacks during which the stags charge at each other, stabbing and wrestling with their antlers. Most confrontations are resolved early while displaying, but many others continue to the point of intense and dangerous fighting.

Contrary to previous assumptions, injury and death during animal fights are not uncommon. In species where animals live in established groups, however, overt fighting is often replaced by a set of relationships in which a subordinate individual consistently defers to a dominant one. Wolf packs, for example, are known for their clear hierarchical relationships. When two group members meet, the dominant animal adopts an upright stance, with raised ears and tail, while the subordinate flattens its body to the ground with the ears against the head and the tail lowered, a submissive posture that serves to protect it from attack. In a number of bird species, variations in plumage act as “badges of status,” especially in large winter flocks. The black throat patch or bib of the house sparrow and the dark chest stripe of the great tit are signals of status; dominant individuals have more-conspicuous bibs or stripes than do subordinates and thus have preferential access to food.