The role of aggression

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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.

Bibliography

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.

Citation

Bland, J.,(2002) About Gender: The role of aggression.

http://www.gender.org.uk/about/06encrn/63aaggrs.htm

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Hormones in Context: Testosterone and Aggression - Mothers

About Gender. Notice: You can find the original article here

Clearly the motivation to fight is influenced by the situation. A male may fight another male that threatens his mate, or approaches one that he courting. A female might fight to protect her young when, at other times she might flee. Rats that were deprived of nourishment when very young were more likely to fight when they were older. In experiments where they were trained to press a bar to obtain food, they also continued much longer. However, since they were undernourished when suckling, by depriving the mother, she was affected, which meant that she was less attentive, and they interacted less with their peers.

In earlier chapters, we described how biologists are concentrating their attention on the extent to which learning influences behaviour. Even the behaviour of rats and mice is influenced by the sort of maternal care they receive. It has been suggested that, when newborn rats have their hormones tinkered with, the difference in their smell may well affect maternal care. In addition, a study has demonstrated that the general behavioural characteristics of the mother can have an effect.

Laboratory rats and mice can be selected for specific physiological or behavioural characteristics. In one study,(1) there were two strains. One exhibited a generally low level of aggression, measured on the frequency of chasing, attacking and fighting. The other strain exhibited high levels under the same conditions.

When a low aggression female was mated to a high aggression male, the male offspring consistently had a low aggression score. When a low aggression male was mated to a high aggression female the opposite result was obtained. In other words, the mother's 'personality' had a significant effect on the offspring.

This could, of course, have been a genetic effect of the mother's X chromosome, or prenatal hormonal effects. However, the experiment was repeated by cross-fostering the young. Thus the male offspring of parents that were both from the high aggression strain, were fostered by a low aggression mother. The offspring displayed a significantly lower level of aggression than those that stayed with their original mother, and vice versa. It would seem, then, that a significant effect arose from the way the mother treated the young after they were born.

Similar studies were carried out with other species, including rhesus monkeys. Mitchell and Brandt concluded that mothers encourage greater independence and activity in males. Mothers of males act as "punishers", of females as "protectors". Young males are "doers"; young females are "watchers."(2)

There is no ethical way that one can carry out experiments on human beings. However, the side effects of various drugs used to help women in pregnancy is said to support the idea that prenatal hormones affect the personality of the child. A problem for women with diabetes is that they often suffer spontaneous abortions. For a while this effect was countered by treating the mothers with a hormone, diethylstilbestrol. Later, it was discovered that little boys, born to these mothers, were much less assertive as they grew up. For a while, too, male hormone had been administered to mothers with toxemia and, again, if the child was a girl, she was said to show measurable behavioural differences.

However, in such studies, no account was taken of the effect on the mother after the birth, or her emotional state. In some cases the mother's condition was severe enough to require daily attention. Were mothers' behaviours different towards their offspring? In other cases, the baby had genital abnormalities. What effect did these have on the children's view of themselves? Once again, there were several studies and the results were, by no means, as straightforward as is sometimes suggested.

Fausto-Sterling(3) is clear that "The claim that clear-cut evidence exists to show that fetal hormones make boys more active, aggressive or athletic than girls is little more than fancy, although harmless it is not."

The effects of socialisation on animals, even rats and mice, is considerable. Some time ago, experiments were carried out on young Rhesus monkeys that might now be considered unethical. One group was brought up from birth with an artificial 'mother' made out of cloth, without any living companions.

When introduced into the community, they exhibited behavioural problems, ranging from hyperaggression to withdrawal, with sexual and parental incompetence, along with other social disorders.

A second group had real mothers but no age peers. They also exhibited hyperaggression. They were extremely mother-dependent, although they were more willing to explore.

In other studies, Rhesus monkeys developed within a normal social structure, but lived in what is known as a restricted environment. This was bare and monotonous with little to stimulate the senses, rather like an inner city housing estate. The monkeys were more gregarious but, at the same time, more aggressive, and there was less social grooming.

Bibliography and good reading.

1. Hall, H., Halliday, T., eds, (1992, 1995) Biology, Brain and Behaviour: Behaviour and Evolution, (p112) Milton Keynes: The Open University.
2. Mitchell, G., Brandt, E.M., (1970) Behavioural Differences Relate to Experience of Mother and Sex of Infant in the Rhesus Monkey, Developmental Psychology, 3(1970):149 in Fausto Sterling, A., (1992) Myths of Gender, Biological Theories about Women and Men,(p143) New York: Basic Books
3. Fausto Sterling, A., (1992) Myths of Gender, Biological Theories about Women and Men, (p141) New York: Basic Books

Citation

Bland, J.,(2002) About Gender: Testosterone and Aggression - Mothers.

http://www.gender.org.uk/about/06encrn/63daggrs.htm

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