Becoming a parent isn’t easy. Okay, conceiving can be far too easy. But I mean all that stuff about nappies and midnight feeds, and the germs they bring back from daycare, and trying to understand school newsletters. That’s the difficult bit. And for mums, there’s the metabolically taxing business of carrying a baby to term and breastfeeding it. Not to mention the perilous nature of child birth.
In biological terms, reproduction constitutes the costliest thing most animals do. The more eggs an animal lays, chicks it hatches or babies it gives birth to, the less time and energy it has to devote to other things. Like looking after its body.
In the strange corner of evolutionary biology where I spend much of my research time, we talk of trade-offs. Investments in reproduction today exact their toll on the body, reducing both future reproduction and how long an animal can expect to live into the future.
But in humans, these trade-offs tend to be difficult to detect. The evidence rather tantalisingly suggests they exist, but it takes decades to see the effects of reproduction on a trait like survival. On top of that, variation in socioeconomic status, healthcare and diet can obscure trade-offs. Parents who are well-off or who eat well can, potentially, have more kids and live longer than less well-off or well-fed parents, even if having kids imposes a big cost.
Which is why I’m excited about a study published late last year of 40,454 mothers who gave birth to a total of 125,842 children in contemporary Jerusalem. Uri P. Dior and a team of Israeli collaborators followed mothers for up to 37 years after the birth of their first child. And the results provide evidence that having lots of children can hasten a mother’s mortality.
Mums who bore between two and four children were at lowest risk of mortality from all causes. Mothers who had five or more children lived shorter lives, on average. Analysis of the three types of disease responsible for most deaths showed that risks of cancer, circulatory disease and heart disease all rose dramatically in mums who had five or more children.
In fact, mothers with between five and nine kids had about two and a half times the risk of dying of heart disease or circulatory disease as mothers with fewer than five children.
This study adds to historic analyses showing similar costs but far less directly. For example, British aristocrats living between the 8th and 19th Centuries were far more likely to make it to the ripe old age of 80 if they were childless. While the Israeli study doesn’t compare mortality risks with childless women, it does provide a serious estimate of the mortality costs of reproduction in contemporary women.
I would love to see a similar study following fathers. I wrote last year about the effects of castration on male lifespan, and how they support the existence of a link between reproduction and longevity. But how does the effort men make in fathering their children alter their risks of dying?
Such a study is probably some way off, though, because fathers vary far more in the size of their contribution to child-rearing and the family. From the most devoted dads to men whose only contribution to the family was a few ounces of semen. That is not to say mothers’ contributions don’t vary. They certainly do, but gestation, child birth and to a lesser extent breast-feeding at least are currently inescapable aspects of reproduction that exact physiological costs on the body.