Why is aggression more widespread in the males of most mammalian species?

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Why is aggression more widespread in the males of most mammalian species?

Psychologists have found that attempts to define aggression are fraught with difficulties.  Animal studies can shed a great deal of light regarding aggressive behaviour in humans.  Almost all species of animals engage in aggressive behaviours, which involve threatening gestures or actual attack directed toward another animal.  Human aggression takes a tremendous range of forms – from ignoring another person or spreading false rumours about him/her to destroying the target’s property or attacking this person directly, either verbally or physically.  Aggressive behaviours are species-specific and patterns of movement are organised by neural circuits whose development is largely programmed by an animal’s genes.  There are therefore many kinds of aggressive behaviour.  Freud defined aggression as ‘the instinctual behaviour exhibited when a drive constantly seeks a release.’  The social situation does play a crucial part in determining our aggressive actions towards other human beings as it does for animals too.  Social environments differ considerably throughout each day, sometimes causing spontaneous and harmful actions.

A team of scientists from the University of Akron found that male rat aggression results from reduced levels of serotonin in the brain.  These researchers therefore suggested that aggressiveness in human males is also the result of reduced levels of serotonin in the brain and is not a learned behaviour.  Research was conducted on a colony of rats. By introducing new male and female rats to an established colony, the resulting behaviour could be observed.  As expected, the males in the group reacted strongly to the introduced rats whereas the females did not.  After the experiment, norepinephrine, dopamine and serotonin were measured in various regions of the brains by high performance liquid chromatography.  Serotonin is one of several substances known to affect mood and aggression.  The decreased serotonin in the male brain was associated with increases in aggressive behaviour.  The Y chromosome governs serotonin levels.  It can be concluded that when the Y chromosome is stimulated, serotonin levels decrease and testosterone increases.  The result is aggressive behaviour!

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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).

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