Day 058 - Choosing a mate

Submitted by Sam on 17 July, 2011 - 17:48

As far as the male's genes are concerned, the best strategy for propagation is to mate with as many females as possible, always abandoning one for another and forcing them to bring up his children themselves, affording him more time to mate by never having to expend resources in childcare. This strategy is only possible if the population of females permits it; if the majority of females choose instead to withhold mating until the male completes costly courtship tasks, he would never be rewarded for desertion as he would always face a repeated series of time and energy consuming trials before being able to mate again. Conversely, this strategy is unstable if there are many loose females in a population, as a male would always have someone to run to having deserted his mate.

Female elephant seals permit the male strategy 'mate with and abandon as many females as possible'. However, their social structure ensures that only one bull mates with the harem in any season, leaving the rest of the males with only opportunistic copulations when he is otherwise engaged. The other males will only stand a chance of spreading their genes if they can defeat this alpha bull in fighting, and become the leader of the harem themselves.

This subjugation by a single dominant male may have originally arisen through discrimination on the part of the females, who could have refused to copulate with all but the 'best' males, those who bore visual indications of carrying 'good' genes, that is, genes which confer survival advantages, like strong muscles or big, sharp teeth. By breeding only with these select males, females would increase the survival prospects of their children, and simultaneously benefit her own genes and creating a selection pressure for males with visual indications of health and strength, or promoting sexual attractiveness itself. As the females will be judging the males on the same criteria, only a few of them will be selected for breeding, and these will be those with characteristics that equip them well for fighting, or defending their mating rights from rival males: the harem structure is born.

Although not obviously the case in elephant seal populations, females selecting mates on the basis of physical indicators of survival prospects can create a selection pressure for males with these attractive qualities, whether they are actually useful for physical survival or not. The elaborate tails of the male birds of paradise, for instance, may have evolved this way: longer tails may initially have carried some indication of a male's health and ability to find food, and thus the suitability of his genes to create strong, healthy children. After generations of selection for bigger, more extravagant tails, males may actually have been left with tails so large that they became a physical disadvantage, possessing them merely because they were attractive to females, and no longer functioning as a truthful indication of physical prowess.

This is a self-reinforcing cycle, because if long tails were considered by the majority of the female population as desirable, any female mating with a male with a shorter tail would stand little chance of her own son being regarded as a potential mate, and therefore she would disadvantage the spread of her own genes. In this way, genes set a standard for sexual attractiveness which it pays to stick to.

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