As single income families become rarer and aging baby boomers begin to play a greater role in caring for their grandchildren, people have increasingly come to appreciate how much help a doting grandmother can provide. In fact, interest in the helpful role played by the elderly has given rise to the so-called grandmother hypothesis, which posits that women have evolved to live well past their reproductive years because, free from the costs of childbearing, they are able to invest more time into benefiting their grandchildren and other younger family members, raising the odds that their genes will be carried on to future generations.1 While the strength of the evidence for the grandmother hypothesis is still being debated2, it’s certainly got some intuitive appeal (especially, perhaps, to harried young parents).
What’s also quite fascinating is that the long post-reproductive life of human females – up to a third of a woman’s lifespan or more – is extremely rare: menopause appears to be unique to humans and (somewhat controversially) certain other great apes, as well as to certain toothed whales, including short-finned pilot whales and killer whales. (It’s possible that other species of cetacean may undergo menopause, but this hasn’t been established yet; also, more to come about elderly elephant matriarchs in a later post…)
So, why is post-reproductive life is so rare? If the grandmother hypothesis applies to great apes and toothed whales, why isn’t it at work with other long-lived animals who live in socially-cooperative societies? Also, if evolution favors post-reproductive life because it provides distinct social advantages, why did menopause evolve in humans and toothed whales, given the very different social structures of humans and whales?
A fascinating study published last year in Proceedings of the Royal Society B3 by Rufus Johnstone of the University of Cambridge and Michael Cant of the University of Exeter may offer plausible answers to these questions.
In a nutshell, they found that, although humans, pilot whales and killer whales have quite different social systems, in each case older females become, on average, more genetically related to those with whom they associate. By contrast, in most other long-lived complex mammal societies, older females become increasingly less related to those in their local groups as they age.
The researchers began by developing a mathematical model that would allow them to draw general conclusions about age-related changes in the genetic relatedness of long-lived social animals as individual group members disperse, die and are replaced over time. (For those interested in such things, they based their approach on the “infinite island” model that is commonly used in considering the process of gene flow among a set of subpopulations.)
With their model in hand, the researchers analyzed three relevant social scenarios:
- Males Move On. In the large majority of social animal societies, males tend to move on as they mature, ultimately mating with unrelated females they find within new social groups. In this type of society, the researchers’ model determined that, over time, an older female will become less related to her group mates as she ages. She starts out in a highly related group that includes her father, but over time her older male relatives die, and her sons, and the sons of her relatives, leave the group and are replaced by unrelated males from other groups. Her average genetic relationship to the females in the group doesn’t change much, but since her relatedness to local males declines, overall her genetic connection to the group lessens as she gets older.
- Females Move On. Conversely, evidence suggests that during the course of human evolution, women were the ones that were more likely to move on to start families in new environments. (In support of this proposition, Johnstone and Cant cite the behavior of other great apes, human DNA variation patterns, and social patterns among human forager societies, evidence they concede is “far from conclusive.”) In this type of society, where males stay at home and females disperse, an older female tends to become more related to her fellow group members over time. She begins her reproductive life in new surroundings where she has few genetic ties to those around her, but as she produces sons who are likely to remain in the group, her relatedness to local males builds up over time. Again, because the degree of her relatedness to other females stays fairly constant – she starts out with little relation to the females in her new group and this doesn’t change much as her daughters leave and are replaced by new unrelated females – her overall genetic connection to the group increases as she ages.
- Males and Females Stay Put, But Mating Occurs Between Different Groups. In the resident killer whale and pilot whale societies studied, males and females stay with their natal groups for life, but mating occurs non-locally, that is, between females and males from other groups. In this final scenario, even though the social structure is quite different from “female moves on” societies, the results are the same: an older female tends to become more related to her fellow group members over time. A female begins her reproductive life separate from her father and her paternal relatives (who belong to a different group), but as she has male offspring her relatedness to males within her group grows over time. Once again, her relatedness to other females stays more or less constant, meaning that her overall genetic affinity with her group increases as she grows old.
Thus for human and certain whale societies, in contrast to most other social animal groupings, a female’s relatedness to her group increases as she becomes older.
Johnstone and Natal next considered the fitness costs of reproduction. They noted that having children imposes costs on other breeders within one’s group due to increased competition for food, resources and mating opportunities, whereas cessation of reproduction confers a benefit, due to a corresponding reduction in competition. Then, using a using a statistical model involving an “inclusive fitness” approach to generate quantitative results for the three scenarios described above, they reached a not-surprising conclusion: in scenario 1 (males move on), it is less advantageous for older females to “help” younger generations by stopping their own breeding, whereas in scenarios 2 and 3 (the human and toothed whale scenarios), non-breeding “help” is favored by evolution, as it confers advantages on a younger generation that is progressively more related to the older helper.
So there you have it. Does Johnstone and Natal’s analysis sound plausible? It certainly offers a neat way of finding an underlying similarity in great ape and whale societies that may explain menopause and support the grandmother hypothesis in these very distinct groups.
No wonder cetaceans often look like they’re grinning – they’ve been spoiled by their grandmothers!
1See, e.g., Lahdenperä, M., Lummaa, V., Helle, S., Tremblay, M., & Russell, A. (2004). Fitness benefits of prolonged post-reproductive lifespan in women Nature, 428 (6979), 178-181 DOI: 10.1038/nature02367; Shanley, D., Sear, R., Mace, R., & Kirkwood, T. (2007). Testing evolutionary theories of menopause Proceedings of the Royal Society B: Biological Sciences, 274 (1628), 2943-2949 DOI: 10.1098/rspb.2007.1028.
2See, e.g., Kachel, A., Premo, L., & Hublin, J. (2010). Grandmothering and natural selection Proceedings of the Royal Society B: Biological Sciences, 278 (1704), 384-391 DOI: 10.1098/rspb.2010.1247.
3Johnstone, 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.