Day 057 - Differences between the sexes

Submitted by Sam on 17 July, 2011 - 01:11

Relative to each other, males tend to produce small sex cells and females tend to produce large sex cells; sperm are small and many, eggs are large and few. Apart from being one of the most convenient ways of distinguishing males from females in both animals and plants, this difference in gamete size is symptomatic of a fundamental asymmetry between the sexes. Each sex has to establish a compromise between time and energy spent raising children to ensure they are healthy and that their genes will continue to propagate, and time and energy spent fighting with rivals for mates and other resources, to maximize the number of children containing these genes. If one sex is to settle on a different balance between these two opposing needs, perhaps initiated by a difference in resource allocation so slight that it arose at random, it is likely to be selected for so that parents who are successful in fighting breed fighters who benefit their genes more from fighting more than parenting, and vice versa. This selection will naturally lead to an escalation in differences until one sex achieves reproductive success primarily through investing in parental behaviour whilst the other ensures their reproductive success primarily through fighting rivals to mate with the most.

This difference is apparent right from conception, where the unequal size of the male and female gametes informs an uneven distribution of resource investment in the child in the earliest possible stages – the male has invested less than his fair share of half of the necessary energy to produce the fertilized egg, whilst the female has had to invest comparatively heavily in her large, energy-rich egg. Whilst both parents have contributed an even number of genes to the child, their sex cells contribute an uneven share of resources.

Unconsciously impelled by their genes, parents try to exploit each other to invest more than their fair share in their child. It is advantageous, from their gene's point of view, to let the other half do all the work and take the burden of child rearing, allowing the shirking parent more time and energy to pursue other sexual partners, and thereby spread more copies of their genes. The basic characteristics of the male sex cells reflect this disparity, as many small, fast sperm allow him to potentially sire many more children than a single female could raise. As eggs are larger than sperm, the female is evolutionarily exploited at the earliest stage, and she is placed in the position where she must invest more in the child's rearing than the male. In mammals this is especially pronounced, as the mother must incubate the foetus herself, feeding it from her own body, investing more than the father and therefore becoming more 'committed' (resource-wise) to the child. She stands to lose much more from the loss of the child than the father does, and is therefore much less likely to abandon the child when it is born. The father, who has much less to lose if he abandons mother and child, will have more time and energy to fight for another mate and beget more children if he does so. There is a battle between the parents as to who deserts first, and who can get away with the least amount of parental investment.

Perhaps the only option the female has to redress this unfavourably weighted balance is to refuse to mate with the male until he pays a high price for her prized commodity, her large, nutritious egg for his child, making him invest time and energy in her and the unborn child before conception. Such pre-copulation investment take the form of courtship rituals, and include nest building and offerings of substantial amounts of food. By extracting a price for mating, the female is in a position to tie the male in through his investment, perhaps decreasing the chances that he will desert after mating, and instead continue to devote himself to the well-being of his child.

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