There are 3 species that experience menopause: humans, short-finned pilot whales and killer whales. Scientists now have a theory why killer whales do.
LA Times wrote:As it turned out, older mothers who had calves around the same time as their daughters had a far greater risk of losing their offspring before age 15 (which is around the time when males and females reach sexual maturity). The older mothers’ offspring were 1.67 times as likely to die as their daughters’ offspring were.
The scientists think that this dynamic may arise because of the structure of orca pods. Killer whales, male and female, tend to stay with their maternal pod; males actually go visit other pods to mate and then return to their own. So as a female orca ages, she ends up being more related to members of the group than her daughters are. The study authors explain:
“At the start of her reproductive life, a female’s relatedness to males in her local group is relatively low, because her father is from a different social group,” they wrote. “As a female reproduces, her sons will remain in her group, increasing her overall age-specific local relatedness.”
So an older female orca might be motivated to ensure the survival of many members of her family, not just her own. Her daughter, however, is not as related to the group overall – and so competing with them to feed her own offspring makes more evolutionary sense.
In short, the benefits of grandmothering may have played some role in the success of older female killer whales – but the costs of being outcompeted by their daughters may have played a key role in the emergence of menopause. Scientists say they’ll have to take both aspects of that complex relationship into account to understand the behavior and life cycle of these remarkable animals.