From an evolutionary standpoint, menopause is a pretty puzzling thing. Why would you ever evolve to stop breeding? The traditional answer has been that older females may produce lower-fitness offspring themselves (e.g. due to more mutation-prone eggs), but still have the parenting skills to help other individuals’ offspring. If those others are her own sons and daughters, kin selection could then favour stopping breeding altogether and switching to a helping mode.
Still, menopause is extremely rare: only humans, short-finned pilot whales, orcas, and maybe chimps and gorillas go through it. And, oddly, there are also many species—not just mammals, but birds too—where younger females are the helpers and older ones are the breeders, often because the younger females are too small, too inexperienced, or not socially dominant enough to breed but can gain both experience and fitness by helping to raise their younger siblings.
So why menopause ever, and why in these few particular species?
A new study in Proceedings of the Royal Society B aims to answer that question using simulations of how relatedness within a group of animals changes with age. The researchers varied whether mating occurred within or between groups and whether males, females, both, or neither dispersed between groups. They then asked how closely related, on average, males and females in a group are to a given reproductive female, and how relatedness changed with that female’s age.
First, and this was surprising to me, across most parameter values, female relatedness did not change with age or changed only very little with age. However, the average relatedness of males within a group to a given female could vary quite a bit. If mating occurs within a group and females disperse but males don’t, a given female (1) is unrelated to group members when she joins a group and starts breeding, (2) stays (on average) unrelated to females in the group, because her daughters will disperse, and (3) becomes more related, on average, to males in the group as the group is “filled up” with her sons and their offspring. In this scenario, it becomes easier for kin selection to favour menopause because it could allow a female to help raise her sons’ offspring. This is a demographic situation that may have been common in early hominins.
In contrast, with within-group mating and high male but low female dispersal, a female is initially closely related to both males and females in her group. Relatedness to females stays the same, but as her sons disperse and new males arrive, a given female becomes less closely related to males in the group. Thus on average her relatedness to other group members declines with age, giving less of an evolutionary incentive to help other individuals raise offspring. This scenario, the authors point out, is common in mammal species.
If mating occurs between individuals in different groups, the situation changes. Low dispersal in both sexes, or (as before) high female and low male dispersal, allow stronger kin selection as a female ages. The former appears to be the situation in at least some orca populations and the short-finned pilot whale’s sister species: individuals find a mate from another group, but stay within the group to rear their offspring.
This paper is a useful contribution to an evolutionary question that’s proven difficult to solve. The next step, clearly, is better data. I’m astounded that we don’t know for sure whether chimpanzees experience menopause. Surely we’ve kept enough of them in captivity, not to mention tracked wild individuals, for their natural lifespans and would have some data on this? Additionally, the data on menopause in several other taxa are ambiguous: it has been reported in the likes of rhesus monkeys, elephants, mice, rats, and several other species, but these are observations of captive individuals that have lived much longer than they ever would have in the wild.
A further step after this is testing this paper’s predictions in a proper comparative framework. The authors note that the known cases in which menopause has evolved fit their model’s predictions, and that most/many other mammal species fit into the scenarios in which menopause is least likely to evolve, but this is a broad generalization. It would be more informative to see just how often the scenarios in which relatedness to the group increase with age occur versus whether menopause has evolved. If there are many cases in which it could evolve but hasn’t, that might be due, as the authors point out, to differences in the relative costs and benefits of helping/not reproducing.
Johnstone, R., & Cant, M. (2010). The evolution of menopause in cetaceans and humans: the role of demography Proceedings of the Royal Society B: Biological Sciences, 277 (1701), 3765-3771 DOI: 10.1098/rspb.2010.0988