A ladybird larva is on the prowl on a witch hazel plant. The youngster is a voracious predator and it’s hunting for aphids. It seems to have found a bountiful feast – a swollen structure called a gall that houses an entire aphid colony. With so many meals in one place, the colony seems easy prey, but it has staunch defenders.
As the ladybird approaches, aphids pour out of the gall and grab the predator by their jaws and legs. It’s a suicide defence. The aphids secrete massive amounts of waxy liquid from their bodies, which quickly solidifies and glues the ladybird to the plant. Unable to walk or bite, the ladybird dies and the aphids go with it. In the video below, you can see what happens when one of these aphids is prodded with a needle.
There is more to these suicidal protectors that meets the eye. Keigo Uematsu and University of Tokyo found that all of them are ‘menopausal’. They are the parents of the other aphids in the gall but their reproductive days are long behind them. With no further opportunities to raise the next generation, their final role is to defend their offspring, with their lives if necessary.
You could argue that life is all about cheating death and having enough sex to pass on your genes to the next generation, as many times as possible. From this dispassionate viewpoint, human reproduction is very perplexing for our reproductive potential has an early expiry date. At an average age of 38, women start becoming rapidly less fertile only to permanently lose the ability to have children some 10 years later during menopause.
From an evolutionary point of view, this decline is bizarre. Other long-lived animals stay fertile until close to the end of their lives, with elephants breeding until their 60s and the great whales doing so in their 90s. In comparison, a human woman is exceptional in losing her child-bearing potential years or decades before losing her life. Even in hunter-gatherer societies that lack our access to modern medicine and technology, women who pass through menopause can expect to live well into their sixties.
Now, a pair of scientists have proposed a new model to explain the origins of menopause. Michael Cant from the University of Exeter and Rufus Johnstone from the University of Cambridge suggest that the loss of fertility helps to lessen reproductive conflicts between successive generations of women.
A few theories have already been put forward to resolve this conundrum. I’ve previously blogged about one of these, which suggests that the menopause reduces the health risks that repeated childbirth brings to both mother and child. This idea complements the most popular theory, known as the “grandmother hypothesis“, which suggests that older, infertile women can still boost their reproductive legacy by feeding, teaching and caring for their existing children and grandchildren.
The basic idea makes sense and while some studies have backed it up, it’s clearly not the whole story. Some analyses of hunter-gatherer populations have found that the indirect advantages of helping your family don’t outweigh the potential benefits of having more children yourself. Alone, the grandmother hypothesis can explain why women continue to live past the menopause, but not why they go through it in the first place.