As far as we know, only four species experience menopause: humans, killer whales, short-fin pilot whales, and false killer whales. So how did this rare biological process, which leaves females unable to bear children, evolve? A new study of one species without menopause—the bottlenose dolphin—may hold some of the answers.
Bottlenose dolphins care for their young longer than most mammals—sometimes more than 8 years for the last calves they give birth to. And because they are closely related to those cetaceans (the group that includes whales and dolphins) that do go through menopause, they’re a good species for exploring its origins, says Caitlin Karniski, a Ph.D. student at Georgetown University in Washington, D.C.
So she and other scientists turned to a unique data set: more than 34 years of observations of 229 female dolphins and their 562 calves living off the coast of Monkey Mia in Western Australia. Females generally had their first calf when they were 11 years old, and then gave birth at increasing intervals until their last recorded birth, usually in their early 40s. (These dolphins typically live to their mid- to late-40s.)
Calves born to older mothers were more likely to die than those born to younger mothers; late-in-life calves were more likely to die by age 3. The interval between births also increased as females aged—a change that has also been documented in chimpanzees, Barbary macaques, and Hamadryas baboons.
To help ensure the survival of their last-born offspring, older mothers nurse longer and wean later—and that was the case with the dolphins Karniski studied. Dolphin mothers on average weaned their offspring at 4 years old. But mothers with late-born calves nursed their offspring longer than those born earlier, on average for almost 5 years; some nursed for more than 8 years—perhaps as a way of compensating for the likelihood that they would not have another calf, Karniski says.
That extended maternal care by older mothers—together with the reproductive decline brought on by aging—might over time cause menopause to evolve, Karniski and her colleagues report today in the Proceedings of the Royal Society B. Because later-born calves are more likely to die, eventually it makes more sense for a mother to invest her energy into her already existing offspring, rather than continue reproducing.
“Mothers teach their calves where to forage, how to hunt, and [they] protect them from predators,” Karniski says. Older mothers who aren’t as good at these tasks might nurse their calves longer in order to “hedge their bets” and make sure that their last calf survives, Karniski adds.
Andrew Foote, an evolutionary ecologist at Bangor University in the United Kingdom, agrees. “[This study] adds to our understanding of why in some species, such as humans and killer whales, females stop reproducing and instead invest in their existing offspring,” he says. The new work is also the first to show that some marine mammals have lower infant survival rates as mothers age—common among land-based mammals.
It’s a challenge to try to distinguish reproductive aging from age-related changes in maternal care, adds Lauren Brent, a behaviorist at the University of Exeter in the United Kingdom, because “the same hormonal and neurochemical pathways underlie both. But by asking these questions, the scientists have pushed an overdue debate forward.”