I have a hypothesis that explains why in many (most?) species, males have a shorter life expectancy than females. My apologies if this has been thought of before, or if it’s already well-known. It’s quite likely that I’m re-inventing the wheel here, that I’ve come across the current explanation before somewhere, and have simply forgotten. I have a keen interest in evolutionary theory, but I’m not a biologist.
The hypothesis is this: males are subject to more exploitation by parasites than females, because in general parasites “want” their host species to thrive. Over the course of a lifetime, this greater exploitation takes its toll.
In non-monogamous species, males are useful for fertilizing the eggs of the females, but not much else. In effect, after donating sperm most of them are redundant. They use up the food supply that could otherwise swell numbers of individual members of the species, and hence safeguard the species itself. In non-monogamous species, too many males are “bad for the species”. Drone bees consume as much nectar as honey-producing females. Male elephant seals consume far more fish than their smaller female counterparts, and few of them even get to donate sperm.
Farmers — in effect, human parasites of animals used as food — know all this, and so they usually kill males apart from the few needed to fertilize females. In doing so, they strengthen the species they parasitize, in the sense of increasing their numbers and assuring their future. Through domestication, the humble jungle fowl of Asian forests has become the mighty chicken, found in huge numbers all over the world. Much the same applies to cattle and sheep, which now occupy much of the earth’s surface.
Most parasites (such as microbes) are brainless, but through the process of natural selection they adopt “strategies” which can promote their numbers. In most cases, these strategies ensure that their host species do well enough to function reliably as hosts. The parasites aren’t actually thinking as human farmers think, of course, but over many generations they stumble upon similar strategies, which become established as the parasites that benefit from them proliferate.
With sex ratios, the “interests” of species and genes conflict. What’s “good for the species” is a much larger proportion of females than males, at least in non-monogamous species. But what’s “good for the genes” is a roughly equal number of males and females (as explained by Fisher’s Principle). The fact that in most species the ratio of males to females is indeed 1:1 makes a compelling case for a gene-centered understanding of evolution (a la Richard Dawkins’ Selfish Gene), and against group selectionism.
This hypothesis (I hesitate to call it “my” hypothesis) should be easy enough to test, as it entails that there should be a greater difference in male–female life expectancy in non-monogamous species than in monogamous species. It also entails that many of the diseases we associate with early male mortality (such as coronary heart disease, possibly suicide) may in fact be partially caused by infection by microbes.