"Individuals that invest more into reproduction must pay some cost in terms of their longevity. And that matters because that determines how many kids you can have." says Penn.
When it comes to humans, women are naturally thought to suffer fitness costs from reproduction, given the stresses of pregnancy, childbirth and breast-feeding, but evidence for whether men do has been mixed and controversial. Prior studies on European aristocrats suggested they did not experience costs from raising a family. Other investigations, however, hinted that most men in preindustrial societies were not so sheltered, perhaps becoming more prone to infections the more children they had in their household, for instance.
Penn and Smith found that having more children was linked with decreased survival for both parents, although this effect was more pronounced in women. After age 50 the number of offspring had no effect on the likelihood that fathers would die, but continued to have an impact on the mortality of mothers. This difference in cost between men and women fits with evolutionary explanations for the observation that women tend to be choosier than men when selecting mates.
In addition, the more offspring parents had, the less likely each child was to survive to reproductive age. Smith speculates that children in larger families might not have been breast-fed for as long as in other families or at all, and therefore may not have received immunities passed on through breast milk. "Or it could simply be that in larger families, all things being equal, not as many resources are available to all those children," he added.
These findings may help shed light on "the question of the evolution of menopause," says Smith, which humans experience but chimps and many other species do not. Menopause could enable women to stop reproducing around the age when giving birth becomes harmful for their health or for any offspring. In contrast, men do not experience menopause "because as we've demonstrated, the fitness costs of reproduction are significantly less for men than women," says Smith.
Intriguingly, an environment in which resources were scarce, such as that faced by preindustrial Mormons, may have placed selective pressures on the innate psychology of women that at least partly explain why birth rates are so low in modern industrialized societies.
"One of the other problems we've been trying to explain in human reproduction is why women all over the world have been having fewer children whenever they get access to contraception or more education," says Dr. Penn. "If women bear a higher cost for reproduction than men, then this might help explain why, when they get control over their reproduction, they would have fewer children."
"The authors have conducted an imaginative and very useful study," says human geneticist Alan Bittles of Edith Cowan University in Australia. "What is interesting here is a confirmation of some ways in which humans are unusual," says behavioral ecologist Bobbi Low of the University of Michigan at Ann Arbor. "In most species--and especially most mammals--it is the males who die off more, and earlier, than females."
Penn and Smith reported their findings online this week via the Proceedings of the National Academy of Sciences USA.