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Hormones in Context:
Mice and rats


One of the cornerstones of the biological argument is that the high androgen level in a male human fetus, especially at about six weeks, directs the development of the brain towards a male, instead of female, path. The justification for this is largely derived from studies of laboratory rats and mice. Moreover, it is, at least partly, derived from measures of aggression, as if this was the only real measure of human masculinity.

If a young male rat is castrated within a few days after birth, it is much less aggressive, as it becomes older, than other male rats. Similarly, if a female newly-born rat is injected with male hormones, shortly after birth, it behaves more aggressively.

Aggressive behaviour, in this case, is often measured in terms of a behaviour called play fighting. One pounces on another, and a wrestling or boxing match ensues, which finishes with one animal on top of another. It is a behaviour common to both sexes; females perform the same behaviour, but initiate it less frequently

Rats are born virtually helpless and it is suggested that their brains, at this time, are at the same stage as a seven week old human embryo. There seems to be a critical period when the rat's brain is organised to develop as aggressive or otherwise.

In other words it suggests a developmental effect on the neurons of the brain, but it may be going too far to suggest that the behaviour is, as it were, 'wired in' by testosterone. Since females will fight if prompted by a playmate, without being injected with hormones, it may be that the behaviour is either learnt or a preset pattern. However, a normal male will initiate fighting as an adult and, given testosterone, is more likely to do so.

A castrated rat is unlikely to do so, when adult, even if given testosterone. Castrating a male rat after the critical first few days, does not reduce its propensity for fighting. In other words the organising effect would seem to involve initiation.

This is in contrast to sexual behaviour, which, having been organised, also needs testosterone in adulthood for the behaviour to occur. Sexual behaviour is measured by observing mounting in the male manner, or presenting in the female manner.

However the conclusion that aggressiveness in rats and mice is controlled by the presence or absence of testosterone is not a straightforward one. Both make a great deal of use of their sense of smell, in such situations, much more than their eyesight. Castrated mice produce urine which does not have the characteristic odour of male mice, so they are not attacked by other males. They also do not smell the odour of intact males, so do not themselves attack others. Prepubertal rats will fight each other, but won't fight adults, nor adults fight them.

A criticism that is often levelled is that such studies are performed on laboratory animals and are extremely artificial. The counter-argument is that they are comparative studies which provide fine detail for those carried on other species.

The situation is not as straightforward as this account may make it appear to be. Few biologists are willing to use the behaviour of one species to predict the behaviour of another, let alone humans. Moreover, when they write of aggression, they are referring to a precisely defined behaviour which may differ from one study to another. Thus one study may observe a particular type of fighting, another may observe territorial defence, while others measure sexual behaviors. In addition, tests which involve tinkering with levels of hormones, whether by castration or injection is complicated by the differences between species and the situation in which behaviour is assessed.

Another problem with injecting hormones is that they are given in one dose, whereas in the natural state, they tend to be released by the body in pulses in a graduated fashion. Homeostasis tends to oppose the administered dose, and different species vary in the hormones they produce in terms of quantities and timings. Where the study involves surgical removal of an organ, particularly part of the brain, that organ may be responsible for other effects, disruption of which may confound the results.

In some species, males fight for access to females, and only do so when ready to breed. Male hamsters, by contrast, fight to establish territories. As the breeding season approaches, their fighting decreases, just at the time when their testosterone is rising. Female hamsters are at least as aggressive as males. So much so, that hamster breeders have to introduce pairs for mating at exactly the right moment. And, while, in some species, castration reduces fighting, in others it does not necessarily do so. So testosterone appears to affect fighting but not in any predictable way between species.

In many of the 200-odd species of primate, either or both males or females defend territories or compete for resources, and in many the females are extremely aggressive.

Next Page: Primates.

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Bland, J.,(2002) About Gender: Mice and rats.
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Web page copyright Derby TV/TS Group. Text copyright Jed Bland.
06.05.98 Last amended 12.03.02