The activity of the brain stem circuits appears to be controlled by the hypothalamus and the amygdala, which also influence many other species-typical behaviours. The activity of the limbic system is controlled by perceptual systems that detect the status of the environment, including the presence of other animals. In short, subjective judgements about a person’s social rank drive a frontal lobe-amygdala-hypothalamus-pituitary-gonad axis, modulating testosterone production and thus regulating the expression of certain genes. Direct connections link part of the brain’s frontal lobes. Nerve bundles link the amygdala to the hypothalamus, generating hormones appropriate to motivated behaviours (e.g. aggression).
The electrical activity of the amygdala ‘increases during social aggression in monkeys.’ Damage to the amygdala reduces aggressive behaviour in animals and makes monkeys lose social rank. Similarly, damage to the hypothalamus reduces both aggressive and sexual behaviours in male rats, whereas implanting testosterone there restores these behaviours in castrated males. Thus, sex hormones play an important role in translating social contexts and events – via the frontal lobes, amygdala, hypothalamus, and gonads – into social behaviours such as inter-male aggression.
Aggression seems to be more predominant in males than in females due to simple genetics. The fact that social aggression in many species occurs more commonly among males than among females is usually explained with reference to the organisational and activational effects of testosterone. Men’s short-term testosterone levels respond to competitive situations, such as a tennis or football match, a chess game, or a competitive task. Levels rise in preparation for the competition, and then go up afterwards in the winners and down in the losers. This applies to any changes in an individual’s perceived status in a social hierarchy. The brief period of testosterone release that occurs around birth in genetic males is thought to organise their nervous systems along masculine lines and hence to create the potential for male patterns of social aggression (after puberty).
Does the testosterone response to competition occur in women as well as in men? Evidence in scant, but suggests it does not. Testosterone levels rose before a male-male competition in a video game, but not before a female-female competition. Outcomes of aggressive interactions affect testosterone levels among animals. When male rodents fight over status and territory, the winner of the fight produces more testosterone and the loser produces less.
Since winning social conflicts increases testosterone levels, winners are presumably more sexually motivated than losers. In some species, high-status males who win conflicts (and, sometimes, control territory) do most of the breeding. This may be the original evolutionary reason for testosterone to rise in winners. The lingering effects on our physiology could help explain Henry Kissinger’s claim that ‘power is the great aphrodisiac.’
Testosterone is, however, only a minor influence on changes in status hierarchy, as compared with ‘social context.’ This context includes the formation and shifting membership of coalitions (especially important near the top of a social hierarchy where an ‘alpha male’ often needs allies to stay in power), individual deaths and coming of age, individual intelligence and learning, scarcity or abundance of resources, and other complex elements that affect hierarchical social relationships.
In the animal world, human no less than nonhuman, competition is often intense. Males typically threaten, bluff, and if necessary fight one another in their efforts to obtain access to females. Among vertebrates in particular, males tend to be relatively large, conspicuous in colour and behaviour, and endowed with intimidating weapons (tusks, fangs, claws, antlers etc.).
For animals aggression is displayed through fear and anger. Fear and anger responses at the right level and at the right time are of great importance in enabling animals to survive through numerous potentially dangerous situations. The anger responses may take different forms that may be classed together under the term aggression. It is however important that we do not confuse predation – the hunting and killing of animals for food – with aggression.
Animals may become aggressive when defending territory. A group of animals usually occupy a particular area, or territory to live, gather resources and breed. Aggressive actions may be used to repel intruders. Males tend to become more aggressive in the presence of another male. This can be called dominance aggression. If an animal is dominant, it will reproduce more often and more effectively. An animal may show aggression when it is afraid. If an animal is ‘cornered’ it will attempt to escape. Male animals display a lot of sexual aggression. In some species, a male will threaten and attack females to ensure mating (e.g. the mal hamadryas baboon). In human beings, many aggressive attacks have been due to a sexual nature. Males are in constant competition with each other to find a suitable female. Arguments and disputes can occur which in turn can lead to an attack of some sort.
Aggression can also be instrumental. When aggression was used successfully in a given situation in the past, the animal will use this behaviour again (in the same or similar situation – therefore the behaviour has been reinforced via learning). Aggression can also be maternal. When there is a potential threat to an animal’s young, it may react with aggression in order to protect its young from harm (also displayed by females). Aggression also frequently stems from the need to reciprocate after being provoked by aggressive behaviour from another animal/person. Sometimes it is not so easy to ‘turn the other cheek.’
Pain and discomfort may well be social factors that increase aggression. If an animal experiences pain and cannot flee the scene, it will almost invariably attack; this is true of rats, mice, hamsters, foxes, monkeys, crayfish, snakes, racoons, alligators and a host of other creatures (Arzin, 1967; Hutchinson, 1983). In those circumstances animals will even attack members of their own species. Could this be true for human beings as well? Most of us have experienced becoming irritable when subjected to sharp, unexpected pain and we may lash out at the nearest available target (e.g. when you stub your toe).
Therefore the environment, the specific aggression, the inducing stimuli, the species and which neural (brain) circuits are involved, all must be taken into account when attempting to define aggressive behaviour. Aggression does seem to be more widespread in most mammalian species, due to the factors that have just been covered. However, the important point is that as humans (unlike animals), we can always say no to our instinctual behaviours, just like we can say no to our learned ones!
Bibliography
Carlson, N. (2001) Physiology of Behaviour, USA: Allyn &Bacon, 7th Edition
Pinel, J. (2003) Biopsychology, USA: Allyn & Bacon, 5th Edition
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