LONGEVITY, WELL… BUT OH WHAT A LIFE!

Across the animal world (humans are animals, among other things) one fact is stark and consistent. Females outlive their male counterparts. A recent study, described in the Economist magazine, and to be presented to the Proceedings of the Royal Society, demonstrates that the more time males spend hitting on females, the less time they spend on earth.

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.

POLYGAMISTS AND POLYGYNOSTS OF THE WORLD, ARE YOU LISTENING? OR LIKE ROGER DALTREY, DO YOU HOPE TO DIE BEFORE YOU GET OLD?

Evolution is the ultimate tuner (or selector) for efficiency. If it doesn’t help you mate or outcompete the others, it likely is no longer there. It long ago decayed and was taken up by other organisms. So species often trade longevity for sexual proclivity.

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.

There is no reason why that logic should not work between the sexes as well as between species. And this is what Dr Clutton-Brock and Dr Isvaran seem to have found. The test is to identify a species that has made its environment so safe that most of its members die of old age, and see if the difference continues to exist. Fortunately, there is such a species: man.

YES GUYS, WE ARE STILL CARRYING AROUND OUR LIBIDINOUS PASTS.

Dr Clutton-Brock reckons that the sex difference in both human rates of ageing and in the usual age of death is an indicator that polygyny was the rule in humanity's evolutionary past—as it still is, in some places. That may not please some feminists, but it could be the price women have paid for outliving their menfolk.

YOU CAN RUN, BUT YOU CANNOT OUTRUN, YOUR EVOLUTIONARY DISPOSITIONS IN THE DESERT OF THE REAL!