To this day, menopause remains a puzzling concept but researchers may be getting closer to the truth by studying orcas – killer whales.

Killer whales also reach menopause later in their life. Image credits: Robert Pittman / NOAA.

Menopause has only been identified in a handful of species, including humans, killer whales, and chimps. Killer whales start having calves when they’re around 15 and stop when they’re 30 or 40 even though they can live up to 100 years old. Now, a team led by Darren Croft of the University of Exeter published data from a 40-year-old study which offers an intriguing clue as to why the whales stop reproducing later in life.

What they found is that there seems to be some sort of biological contrast between orca generations. When older females reproduce at the same time as their daughters, the calves of the older generation have are twice as likely to not reach the age of 15. But when older whales had calves in the absence of a reproducing daughter, their offspring did just fine. Researchers believe this happens because of competition.

“It’s not that older mothers are bad mothers, that they’re not able to raise their calves as younger mothers,” says Croft. “It’s that when they enter into this competition with their daughters, they lose out and their calves are more likely to die.”

The competition, as it so often happens in nature, is centered around food. Older females are more likely to share their food with others because they feel a stronger sense of kinship to the group, while younger females feel more detached (they haven’t yet formed such a strong social bond). In fact, older females are key to the survival of others, while young mothers focus only on their calves.

“It’s not that older mothers are bad mothers, that they’re not able to raise their calves as younger mothers,” says Croft. “It’s that when they enter into this competition with their daughters, they lose out and their calves are more likely to die.”

The idea that menopause emerges as a genetic safeguard to the legacy of the family is not new and has been proposed in the case of humans as well – but in a different form. In many human cultures (both ancient and modern), grandmothers take extra care of their grandchildren. If they would have children of their own, then they wouldn’t place such an emphasis on their grandchildren and in time, the family is better off without the grandmother’s kids – this is called the Grandmother Theory. However, these new findings somewhat contradict the Grandmother Theory and come up with a different reason for menopause.

Anthropologist Kristen Hawkes, at the University of Utah doubts that this study tells the full story. She believes killer whales are very difficult to study and they’re doing all sorts of stuff we don’t yet understand so any parallel between them and humans is forced.

“They’re doing all kinds of stuff where you can’t see it, and even to get demographic data is just so tricky, because they’re all underwater and they’re long-lived,” she says.

Whatever the truth may be, menopause remains an intriguing mechanism — one for which biologists and anthropologists will doubtlessly argue for years to come.

Journal Reference: Darren P. Croft et al – Reproductive Conflict and the Evolution of Menopause in Killer Whales. DOI: http://dx.doi.org/10.1016/j.cub.2016.12.015