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.
(From Science / By Virginia Morell)– “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.
Read the full article here: http://www.sciencemag.org/news/2017/01/study-suggests-surprising-reason-killer-whales-go-through-menopause