The part of womanhood that only happens to humans.

By MARK ELGAR

Research claiming that men are to blame for menopause has gone viral in the popular media in the past week. But does the theoretical model’s fundamental assumption – that men prefer young women – stack up?

It may surprise some to discover that, in many respects, humans are remarkably unremarkable. Our physiology and morphology is similar to other higher primates. In fact, it follows a broadly similar blueprint for primates and mammals more generally.

Nevertheless, we clearly lack the impressive size dimorphism of gorillas, the extraordinarily large testes of chimpanzees, and only a tiny fraction of our species share the fabulous hair colour of orangutans.

Humans also align unremarkably with conventional life-history theory, which attempts to understand how natural selection has shaped the principal events in an organism’s lifetime. Recent comparisons of mortality schedules in natural populations of primates, including humans, for instance, reveal similar patterns.

But we are unusual in at least one respect. In the vast majority of species, males and females die shortly after they cease producing and caring for dependent offspring. Several species of small marsupials of the genus Antechinus spectacularly illustrate this effect. Male and female Antechinus typically survive only one reproductive cycle.

In contrast, human females typically remain alive for many years after they cease to reproduce. This change in reproductive activity, menopause, is unusual and seems to be confined to only a handful of species of mammals, such as killer whales and humans.

Menopause represents something of an evolutionary puzzle. Natural selection will favour reproductive or life-history strategies that maximise the number of surviving offspring. As our bodies deteriorate with age, we expect the end of female reproductive activity to coincide with the time when our bodies can no longer function.

More bluntly, males and females are expected to be able to produce offspring until they die.

Explanations for menopause treat it as either an adaptive reproductive strategy, or a non-adaptive consequence of extended lifespans. Chief among the former is the “grandmother” effect. Here, females can improve their fitness through the survival of grandchildren, without bearing the risks of giving birth at an older age.

Empirical tests of this idea have had mixed success. But importantly, these studies recognise the challenge of controlling for cultural, socioeconomic effects when examining human life-history features.

The recent, novel explanation of menopause takes a more cavalier approach. Using a computer simulation model, Rama Singh and his team claim that menopause is the consequence of male mating preferences for young females, which leads to the accumulation of mutations that are deleterious to female fertility.

The novelty of this model is that it doesn’t assume that diminished fertility in older females predates menopause, or that offspring survival is enhanced by the presence of grandmothers.

Ultimately, the success of theoretical models that make novel claims must stand on their underlying assumptions. The crucial assumption of this model is that males prefer to mate with young females. Apparently, Singh and colleagues believe this is self-evident, because they detail no supporting evidence.

Perhaps this isn’t surprising, since the required evidence may not be available. Popular wisdom comes from David Buss’ classic 1986 survey of human mate preferences across 33 countries.

This study suggested that females prefer relatively older and males prefer relatively younger partners. This is not quite the same kind of mating preference that appears to characterise the Singh model.

In any case, it’s impossible to discern, from these kinds of data, whether male mate preferences have a genetic basis or are influenced by the local culture. Indeed, we don’t really know whether these preferences reflect real life.

This is a crucial issue. If the Singh model has any evolutionary significance, mate preferences must have a genetic basis.

A recent study of human mate preferences suggests not. The degree of preference for many mate preference attributes, including age, vary predictably between countries according to an index of gender parity (the Global Gender Gap Index).

This means that either mate preferences are strongly influenced by society, or the preference can experience truly impressive rates of evolution.

More generally, explanations that rely on male mate choice in mammals are skating on thin ice. Sexual selection theory is unlikely to predict male mate choice in mammals, because the typically lower reproductive rate of females predisposes them to be the more choosy sex.

And the relative rarity of menopause among mammals begs the question of why a male preference for young female mates is confined to humans and a handful of other species. In this context, it is telling that male chimpanzees in their natural environment prefer to mate with older females.

The point is that humans are remarkable in one crucial aspect. In all other animals, behaviour is shaped by a combination of genes and environment, but in humans the sociocultural influence is particularly potent.

Distinguishing between these effects is crucial for unravelling the evolutionary significance of those aspects of human biology that involve behaviour.

Studies of human life-history strategies that carefully negotiate through this challenge reveal intriguing patterns that offer broad insights. Those that don’t, can’t.

Mark Elgar  is a Professor of Evoluttionary Biology at the University of Melbourne. He receives funding from the Australian Research Council. 

Note: This article was originally published at The Conversation and has been republished with full permission. You can read the original article here.