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.