A female killer whale can breed at 15 years of age. She will live on for decades and may even celebrate her 90th birthday, but she stops having babies at 40. Only two other species are known to go through menopause — short-finned pilot-whales and humans.
Young female mammals tend to settle close to where they were born, while their brothers seek their fortunes elsewhere. Killer-whale youths, however, do things differently; they remain tied to their mothers’ apron strings for life. A killer-whale pod will have a matriarch, her daughters, grand-daughters, sons and grandsons. None of the males present will be members’ daddies because males don’t mate with the females of their own pod, but leave for brief encounters elsewhere. Meanwhile, their sisters entertain visiting Casanovas.
A young female is only loosely related to the other members of her pod, because they all have different fathers. She has little incentive, therefore, to share resources with them. By favouring her own calf, quartering food for it and seeing off rivals, she gives her genes a survival advantage.
Male killer-whales have much shorter lifespans than females. As time goes by and her older half-brothers die off, a female’s sons gradually replace them, increasing her relatedness to the group as a whole. Soon many of the local males will be close relatives, so she has more incentive to work for the benefit of everybody.
When her daughters begin breeding, however, her relatedness to the pod starts to decline; offspring and grand-offspring born from then on will be less closely related to her. She used be a mother or grandmother to new arrivals. Now she is only a great-grandmother to many of them. To maintain her genetic dominance she might continue having babies, but there’s a problem; a mother-daughter conflict has begun to emerge.
If she were to produce a youngster, it would have to compete with those of her daughters and grand-daughters. As a senior female, who is trying to look after the pod as a whole, she can’t devote herself exclusively to motherhood and so her calf would lose out in competition with those of younger mothers. The prospects of transmitting more of her genes to future generations will be better if she stops breeding and devotes herself exclusively to looking after the group. This is what theory suggests she should do, but is it borne out by observations in the field?
Darren Croft and colleagues at Exeter University have studied the lives of 200 killer whales in the Pacific. Analysing over 40 years of data on killer-whale relationships, they found a matriarch’s relatedness to the other members of her pod does indeed increase with age. It is also true that the calves of older females tend to lose out when their daughters start breeding. They are about 1.7 times more likely to die without reaching maturity than those born to young mothers. ‘Reproductive conflict’ between matriarch and daughters is real.
The ‘grandmother hypothesis’ suggests that a similar conflict may be responsible for the human menopause. By ceasing to have more babies of her own and helping those of her daughters instead, a granny’s own genetic dividend is increased.
Older people, with a lifetime of experience behind them, are like filing cabinets full of useful information, especially valuable at times of crisis. The benefits an offspring gains from the attentions of a granny outweigh the resources the matriarch consumes. Natural selection has kept her alive for these services.
Not everyone agrees with the theory. It doesn’t explain, critics argue, why old males continue to be fertile. Also, rising numbers of elderly people result in stagnant populations, less well equipped to adapt and change. The jury is still out as to why killer whales, and ourselves, evolved menopause.
- Darren Croft et al. Reproductive conflict and the evolution of menopause in killer whales. Current Biology. January 2017.