Only three known species go through menopause: killer whales, short-finned pilot whales, and humans. Two years ago, scientists suggested whales do this to focus their attention on the survival of their families rather than on birthing more offspring. But now this same team reports there’s another—and darker—reason: Older females enter menopause because their eldest daughters begin having calves, leading to fights over resources. The findings might also apply to humans, the scientists say.
“What an interesting paper,” says Phyllis Lee, a behavioral ecologist at the University of Stirling in the United Kingdom, who was not involved in the study. “It brings two perspectives on menopause neatly together, and provides an elegant model for its rarity.”
The new work came about when Darren Croft, a behavioral ecologist at the University of Exeter in the United Kingdom, and his colleagues looked back on their 2015 killer whale menopause study. “That showed how they helped and why they lived so long after menopause, but it didn’t explain why they stop reproducing,” he says, noting that in other species, such as elephants, older females also share wisdom and knowledge with their daughters, but continue to have calves.
To find out, the scientists analyzed 43 years of data on two populations of orcas in the Pacific Northwest. In killer whale families, sons and daughters stay with their mothers while mating with whales in other families. Males generally live about 30 years, whereas female killer whales stop reproducing in their 30s and 40s and live for many more decades. (“Granny,” the oldest known orca in these populations, died recently at the grand old age of 105; she hadn’t reproduced in some 40 years, but was still the leader of her pod.)
That family structure means that at some point in the senior mother’s life, she’s likely to birth calves at the same time as one or more of her daughters in the same pod. During the 43 years of data collection, 525 calves were born. Of these 161 were co-generation births and about 31% of the babies died. In these cases, the older mother was most likely to lose her calf before it reached 15 years of age. The mortality of the older mothers’ offspring was 1.7 times that of the younger mothers’, the scientists report in Current Biology.
“That’s a high cost,” says Croft, “and it’s led to the evolution of menopause.” There’s no point in the older mother putting time and energy into a new calf that will most likely die; she’ll do better—in terms of her genetic legacy—by not having calves of her own, but helping her older offspring and their calves survive by sharing food and knowledge about where to forage, and by babysitting.
The main competition between mothers is over food, the scientists say. A mother whale needs some 42% additional sustenance to nurse her calf, and because orcas share the food they catch, she can get those calories by demanding a bigger share. The knowledgeable senior mother probably finds and catches most of the salmon, Croft and his colleagues think, but her daughters and grand-calves likely end up with most of it on their plates, possibly through fighting and hoarding, something the researchers plan to investigate. Senior mothers are also under pressure to share food with their sons, so that they’re able to reproduce as well. It also makes sense for these mothers to assist their sons because as the mothers grow older, they are increasingly related to whales in other, neighboring pods via their sons’ calves. But it’s just the opposite for the young daughters: Their closest kin are in the immediate pod. All of these pressures combined mean calves born late in a mother’s life may be neglected and likely die from that and starvation, the scientists say.
“It’s a cool study because we usually think about these kinship issues in terms of helping behavior, not competition,” says Richard Connor, a behavioral ecologist at the University of Massachusetts in Dartmouth who was not involved with the work.
Menopause in these killer whales, thus, results from their unusual family structure, which leads to both cooperation and reproductive competition between generations, the scientists say. And because this kind of family is rare in the animal kingdom, menopause is also rare.
Although human families are not structured exactly like those of the killer whales, ours (like theirs) are built around the provisioning and sharing of food, Croft notes. The same dynamics of cooperation and intergenerational reproductive conflict might well underlie human menopause, too, the scientists say.