Growing a human being inside you for nine months, and then expelling it with a force that can fracture your pelvis can take its toll on the body. (So can, you know, raising it.)
According to a branch of evolutionary biology called life-history theory, there should be a tradeoff for all this effort—namely, that having kids should speed up the aging process. Energy invested in reproduction, after all, is energy that isn’t being used toward keeping the body’s tissues all shiny and new.
Studies in other species—fish, birds, mice—have found this to be the case. These studies measured the length of the animals’ telomeres, the protective DNA caps at the ends of chromosomes. Telomeres are often compared to the plastic coverings at the end of shoelaces, in that they protect the ends of chromosomes from getting frayed or mucked up. And as organisms age, their telomeres shorten. If life-history theory holds, having more offspring should be associated with shorter telomeres and accelerated aging.
But a new longitudinal study published in PLOS One found just the opposite. It followed 75 women, all Kaqchikel Maya who lived in rural areas of southwest Guatemala, for 13 years. The Kaqchikel Maya tend to marry within the community, and all the women in the study had similar lifestyles, which helped limit possible confounding factors.
Researchers from Simon Fraser University and the University of British Columbia extracted DNA from the women’s saliva samples and cheek swabs and measured the length of their telomeres. After the 13 years had passed, once the researchers had controlled for telomere length at the beginning of the study, women who had more surviving offspring had longer telomeres than women who’d had fewer kids.
“These results suggest that, at least in our study population, having more surviving children acts as a protective factor, slowing the pace of telomere shortening,” the study reads. But why didn’t all those kids wear them out? One possibility the researchers suggest is that humans are “cooperative breeders,” and women who have children are likely to be supported by family members and friends in the community.
“More children may lead to greater support, which in turn may lead to an increase in the amount of metabolic energy that can be allocated to tissue maintenance, thereby slowing the process of cellular aging,” the researchers write.
A bird, on the other hand, may have trouble finding a babysitter.