(from Latin, ad: to, gradi: step) Aggression strictly refers to the act of making a first attack, or initiating a quarrel or fight, but it is used as an all-embracing term to cover a variety of states in animal behaviour relating to ATTACK and DEFENCE. Virtually all species engage in aggressive behaviour, the most common causes being to do with mate selection and the establishment of social status, recruitment and defence of territory (or other resources) and the capture of prey.
Aggressive behaviour can take a variety of forms. For example, much aggression involves threat behaviours in which various postures are taken or gestures made as forms of ritualized DISPLAY—and which may be followed by submissive behaviours. Such threatening and submissive actions serve to establish DOMINANCE between animals and often involve little or no actual FIGHTING. Threatening behaviours may also be followed by the production of species typical defensive behaviours, which might involve the production of ALARM CALLS to warn a CONSPECIFIC. Defence can take many forms, from straightforward ESCAPE by running away (see EXPLOSIVE MOTOR BEHAVIOUR) to the adoption of hostile defensive positions (hedgehogs for example curl up into prickly balls) or FREEZING, the adoption of an immobile posture held to avoid detection. The form of attack is also species typical, but two rather different classes can be differentiated: one is RAGE, involving the production of noise and threatening gestures, while the other form of attack is much quieter. Predation typically involves a stealthy attack (in rodents and other small animals it is often referred to as QUIET BITING ATTACK) in which the production of noise or gesture would alert the prey and initiate their escape or defence reactions. so CIAL BEHAVIOUR is also important in both defence and attack. Many animals live in groups—herds, flocks or shoals—which can be organized for effective defence. The behaviour of animals in herds, flocks or shoals can show an apprently remarkable degree of coordination, individuals seeming to act as one to avoid capture, and generating what is known as PREDATOR CONFUSION. (A good example of this is the movement of a herd of zebras—their movement as a group, combined with their striped pattern, is disorientating for predators.) Such group behaviour, while superficially complicated, is easily maintained by following simple rules: keep a more-or-less fixed distance from the animal adjacent and move in whatever direction it goes. These rules, and an additional one that prompts scattering when a predator strikes (followed by immediate group re-formation) allow large numbers of animals to avoid predation on the principle of ‘unity is strength’. On the other hand, by operating in a co-ordinated manner predators such as big cats or wolves, for example, can work to attack animals much larger than any of the individual predators. Some predators working socially have developed procedures for grouping their prey and attacking en masse: humpbacked whales for example engage in complex patterns of activity involving different functions for different members of the attacking group, that enable them to corral shoals of fish, driving them to the waters’ surface where they are devoured.
The neural and physiological mechanisms involved in aggression are not wholly clear. It is apparent that threats generate activity in the AUTONOMIC NERVOUS SYSTEM (see FIGHT-OR-FLIGHT) which serves to provide ENERGY for highly activated behaviour. HORMONES are also involved: in rodents, CASTRATION can reduce aggression, the administration of exogenous TESTOSTERONE serving to restore appropriate aggressive behaviour. Female sex hormones are also implicated, aggression by females being closely related to PREGNANCY. Female baboons for example have been shown to display higher levels of aggression during ovulation and when female mice are at their most receptive they appear to be least aggressive. Both male mice and male primates have been reported to show INFANTICIDE (of the offspring of others, not their own) which appears to be related to the hormonal state of females (usually females with whom they wish to mate but who have issue by other males). Explanations for infanticide typically engage arguments about the establishment of an individual’s reproductive success.
Neural mechanisms involved in aggression are widespread: there is no single site in the brain that controls aggression. Systems implicated in the control of FEEDING and FORAGING are involved, as are the neuronal systems involved in FEAR. It is also clear that BRAINSTEM mechanisms are involved in the production of appropriate species-typical defence reactions. The PERIAQUEDUCTAL GREY and nearby structures in the PONS and MEDULLA can, when stimulated, elicit defensive behaviours. It is useful when considering aggression to think in terms of rodents. Rats in the wild live in a hostile world: prey and predators need to be dealt with promptly by appropriate attack and defence behaviours. One likely mechanism involves the SUPERIOR COLLICULUS, a structure with direct input from the RETINA and direct output to brainstem motor systems. This structure is able to make a rapid analysis of visual input (the INFERIOR COLLICULUS can make a similar analysis of auditory input) and to initiate appropriate motor activities at very high speed. Stimulation of the superior colliculus, or the structures to which it is monosynaptically connected (such as the CUNEIFORM NUCLEUS) can elicit rapid defensive behaviours in rodents, including explosive motor behaviour (running away at speed). For rodents, overhead threat (the recognition of stimuli passing close above them, as birds of prey do) is a potent elicitor of escape. Of course, if time permits, more detailed analyses of incoming sensory data can be made, allowing the opportunity for FOREBRAIN systems involved in sensory analysis, LEARNING and MEMORY to be used.
Aggression is evidently a complex phenomenon. Virtually all species show aggression in some form, and it is beyond doubt that hormonal states, autonomic activation and complex neural systems throughout the central nervous system are involved in it. There is always a temptation to ascribe human aggression to simple biological causes, but it is a temptation that should be resisted. While it is likely that aggression and violence by humans has biological antecedents—the influence of hormones for example—it is also clear that humans have elaborate social systems to channel and control aggression and that as willful, self-aware, autonomous creatures, humans have to accept responsibility for their own actions. An example of this differentiation between humans (and indeed PRIMATES) and other animals comes from studies of overcrowding. Research indicated that overcrowding in rodent colonies could lead to heightened aggression, and it was assumed that this would be universally true. Recent research has shown that this is not the case (see de Waal et al., 2000). The appropriate suppression of antisocial biological urges is something that people must learn to do.
