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Hormones in Context:
The role of aggression

Systems and Controls.
Estrogens and Androgens.
Testosterone and Aggression.
The role of aggression.
Mice and rats.
Flooded with testosterone?
Crimes of violence.

It may seem a superfluous question, but what is the evolutionary reason for aggressive behaviour? Fitness in evolutionary terms is the ability to produce as many offspring as possible. This depends not only on virility, but on staying healthy long enough. In order to do this, an organism requires resources. If these are in short supply, the more likely they are to be the subject of competition. In this context, even sea anemones have been observed to fight. If one anemone encroaches on another's piece of rock, the latter can be seen to manoeuvre so that it can use its stinging cells.

In most species, aggressive behaviour follows distinctly stereotyped behaviour patterns, involving threat, which may be followed by fighting, leading to submission.

A given individual is likely to lose some disputes and win others. The less damage incurred in losing, the better chance of winning the next time. For the winner, just winning is enough. Prolonging the encounter, uses time and energy that could be used in obtaining food. Besides the loser is still potentially dangerous, and could rally. For this reason, threat displays allow each opponent to assess each other, and submission is damage limitation for both. Throughout the engagement, both protagonists assess risks and potential benefits and losses, while having stereotyped behaviours ensures that the signals are mutually understood.

If it actually comes to a fight, there may be costs to the victor as well as the vanquished. Even if it doesn't lead to death, wounds are susceptible to infection. They may make the animal more vulnerable to predators.

Among group living species, there is a turnover of top males, so that any one may be dominant for a few years during its life. Yet, as in the case of humans, there are males who may never mate. Although such encounters are often symbolic, it is not true, as was once thought, that they rarely lead to injury. In a study of deer on the Isle of Rhum, Clutton-Brock(1) reported that all the stags sustain some injury at some time in their life.

Whether the stags were reaching their population limit is unclear. In studies of rats crowded together it is noted that fighting invariably ensues. Whether it escalates because individuals can't get away from each other, or whether continual close proximity produces greater arousal is impossible to say. Fausto-Sterling suggests that rats and mice rarely fight in the wild. In other words, the situation does not arise where fighting would be necessary. There is an often recounted story of Chimpanzees in Arnhem zoo, where two males formed a coalition to oust the leader. Normally such a dispute would be settled by considerable violence, but this went to the extreme. The leader was savagely mutilated, and died a few days later from his wounds. The park at Arnhem is large and spacious, but not as much as the jungle. In a possibly similar situation, in Gombe National Park in Africa, the defeated male was able to retreat many miles away and stayed away from the tribe for several years.

An event on the Isle of Rhum gives an interesting insight into the costs of fighting. The stag, Congal, received a wounded leg during a fight. Whether he won or not is immaterial. Although he was mobile and could feed, he was, for two years, unable to keep a group of hinds gathered. Thus in evolutionary terms, the fight had a disastrous effect on his reproductive fitness for during that period.

In social and group living species, the ability to recognise and assess one another, allows the formation of dominance hierarchies with minimum fighting. While high-ranking males may achieve a majority of copulations, there is a turnover as males get older and younger ones take over. It pays low ranking youngsters to defer to the high ranking males, since they avoid injuries which might prevent them from later taking over.

Next Page: Mice and Rats


  1. Clutton-Brock, T.H., Albon, S., Gibson, R.M., Guinness, F.E., (1979) The Logical Stag: Adaptive Aspects of Fighting in Red Deer, Cervus elephus L.), Animal Behaviour, 27,(211-225) in Toates, F., (1992,1995) Biology, Brain and Behaviour: Control of Behavior, (p118) Milton Keynes: Open University.
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Bland, J.,(2002) About Gender: The role of aggression.
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Web page copyright Derby TV/TS Group. Text copyright Jed Bland.
06.05.98 Last amended 12.03.02