While philosophers and poets muse on the meaning of life, natural selection casts a dispassionate eye on the whole affair. From the viewpoint of evolution, there is only one thing that matters – that we survive long enough to pass our genes on to the next generation, as many times as possible. And from the viewpoint of evolution, we are not doing a very good job.

Birth rates in several countries around the world – the UK, Japan, China – are falling dramatically. Women are having fewer children and they are having them later, close to the end of their fertile period. But the fact that women undergo menopause at all seems strange, and the reasons for this reproductive expiry date has long puzzled biologists. There doesn’t seem to be any obvious benefit to ending a woman’s child-bearing potential with many years or decades to spare. Nor is menopause a symptom of our healthy modern lives – even in traditional societies, women often survived long past this point.

The favoured idea is that women retire early from child-bearing for the same reasons that athletes retire from their sports at a young age – their bodies cannot handle the strain. Childbirth is a taxing process for a woman and at some point, it becomes too risky for mother and child. Scientists have suggested that menopause is an evolutionary respite from the burdens of having children. Now, Dustin Penn at the Austrian Academy of Science and Ken Smith from the University of Utah have found compelling evidence to support this idea.

They looked at comprehensive records on over 21,000 couples living in Utah in the 19th century. This time and place in history was not an easy one. Families migrated across the Western frontier, facing the hardships of uncertain food supplies, poor medical care and unknown dangers. In spite of this, families were very large, with many couples having 10 children or more.

By studying these families, Penn and Smith found that both parents paid a physical price for having children, but mothers particularly so. The more children a woman had, the lower her chances of survival became, and this extra risk lasted well into later, post-menopausal life.The children’s health suffered too as their mothers went through more pregnancies. Those with more siblings were less likely to make it to their eighteenth birthdays, and the youngest proved to be the most vulnerable.

The duo reasoned that having more children gave mothers less time to recover from the many physical difficulties of pregnancy, such as nutritional deficiency and weakened immune systems, and the extra burdens of birth and lactation. These burdens are even higher if women have to raise chronically unhealthy children, as the youngest ones in large families often were. Even at the cellular level, they tend to show the signs of stress and damage. And the actual effect may even be greater than Penn and Smith found since many of these Utah settlers were Mormons and would have received considerable community help in raising their children.

A halt in the ability to have more children would allow women to focus on their existing families. Penn and Smith found that children were 78% more likely to die before the age of 18 if they lost their mothers first. Even across just two generations, this small survival difference meant that women who died early left behind about three fewer grandchildren than those who survived to care for their kids.

Together, Penn and Smith’s results provide strong evidence that menopause evolved to allow older women to avoid the high cost of giving birth to more children, and concentrate on their existing ones. But it also explains why women tend to be choosier about their partners than men are. Childbirth carries greater costs for them than for men, and they have more to lose by making a bad choice.

This could explain why women are drawn to indicators of resource and future investment, while men are more likely to look for youth and waist-to-hip ratio – signs of reproductive potential and the ability to tolerate the stresses of childbirth.

The high cost of childbirth may also go some way to explain the modern decline in fertility. Low birth rates are more commonly found in countries where the sexes are relatively equal, and where women enjoy independence and greater opportunities for education. Given these opportunities, it may be that women prefer to have smaller families, perhaps instinctively to reduce the costs of reproduction.

[Since this research was published, another theory has been put forward to explain the origins of menopause. It doesn't necessarily contradict Penn and Smith's ideas, and it can complement them. I've written about it here]

Reference: D. J. Penn, K. R. Smith (2007). Differential fitness costs of reproduction between the sexes Proceedings of the National Academy of Sciences, 104 (2), 553-558 DOI: 10.1073/pnas.0609301103

October 11th, 2008 by Ed Yong in Evolution, Human evolution, life history | 6 comments | RSS feed | Trackback >