Saturday science subjects: Polygyny and male lifespans

It's common knowledge that women live longer than men, but why? Some say testosterone makes men age quicker, while others blame evolutionary forces for weeding out males to conserve resources. Others yet think man's polygynous past has something to do with it. As Nature reports, Tim Clutton-Brock and Kavita Isvaran of Cambridge University in the United Kingdom have looked at data for a number of species to test the third hypothesis, and they found a surprising correlation:

In monogamous species, they found no consistent sex differences in breeding lifespans, annual rates of mortality or rates of ageing. But the more polygynous a species was, the more short-lived the male was likely to be, and the shorter their duration of effective breeding, the team reports in Proceedings of the Royal Society of London B 1.

In polygynous red deer, for example, males live 75% as long as females, and have an effective breeding period of less than half that of the females. In swans, which almost always pair for life (with only a 6% divorce rate), lifespans and effective breeding periods are nearly equal. (See Table 1)

According to Nature, the short lifespans of males in polygynous species could be a result of evolution favoring genetic traits that help males win fights, so they can breed with more females, over ones that help longevity. Stephen Stearns of Yale University explains, "The results can be simplified as fight hard, and not only die young but evolve to die young."

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