Women live longer than men. Why has it been so is a question of debate. Whether it is because the nature makes men more aggressive and so more prone to disease and destruction may be one cause. Or is it because of the attention males pay to the females and fight to stay in the “limelight” or the competition for their attention? Well, here are two researchers who tried to test this idea. The result: Chasing Females can take years off your life!
To test that idea, Tim Clutton-Brock of Cambridge University and Kavita Isvaran of the Indian Institute of Science in Bengalooru decided to compare monogamous and polygynous species (in the latter, a male monopolises a number of females). They wanted to find out whether polygynous males had lower survival rates and aged faster than those of monogamous species. To do so, they collected the relevant data for 35 species of long-lived birds and mammals.
As they report this week in the Proceedings of the Royal Society, the pattern was much as they expected. In 16 of the 19 polygynous species in their sample, males of all ages were much more likely to die during any given period than were females. Furthermore, the older they got, the bigger the mortality gap became. In other words, they aged faster. Males from monogamous species did not show these patterns.
The point about polygyny, according to Dr Clutton-Brock, is that if one male has exclusive access to, say, ten females, another nine males will be waiting to topple the harem master as soon as he shows the first sign of weakness. The intense competitive pressure means that individuals who succeed put all their efforts into one or two breeding seasons.
That obviously takes its toll directly. But a more subtle effect may also be at work. Most students of ageing agree that an animal’s maximum lifespan is set by how long it can reasonably expect to escape predation, disease, accident and damaging aggression by others of its kind. If it will be killed quickly anyway, there is not much reason for evolution to divert scarce resources into keeping the machine in tip-top condition. Those resources should, instead, be devoted to reproduction. And the more threatening the outside world is, the shorter the maximum lifespan should be.