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Reevaluating the grandmother hypothesis

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Abstract

Menopause is an evolutionary mystery: how could living longer with no capacity to reproduce possibly be advantageous? Several explanations have been offered for why female humans, unlike our closest primate relatives, have such an extensive post-reproductive lifespan. Proponents of the so-called “grandmother hypothesis” suggest that older women are able to increase their fitness by helping to care for their grandchildren as allomothers. This paper first distinguishes the grandmother hypothesis from several other hypotheses that attempt to explain menopause, and then develops a formal model by which these hypotheses can be compared and tested by empirical researchers. The model is then modified and used to respond to a common objection to the grandmother hypothesis: that human fathers, rather than grandmothers, are better suited to be allomothers due to their physical strength and a high incentive to invest in their own children. However, fathers—unlike maternal grandmothers—can never be sure that the children they are caring for are their own. Incorporating paternity uncertainty into the model demonstrates the conditions under which the grandmother hypothesis is more plausible than a hypothesis that focuses on the contributions of men.

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Notes

  1. Technically, fathers aren’t alloparents; they are allomothers.

  2. For a model that combines post-menopausal mothering and grandmothering, see Peccei (2005, p. 53).

  3. Of course, nothing I have said so far requires that the grandmother hypothesis is about maternal grandmothers, but early literature on the grandmother hypothesis does tend to focus on the role of maternal grandmothers in particular. Why this might be is the subject of Sect. 4.

  4. This explains the matrilocality of toothed whales, species which also exhibit post-reproductive grandmothering behavior (e.g., Johnstone and Cant, 2010; Ward et al., 2009).

  5. Hrdy (1989) also says, “what if patrilocal residence patterns did not reflect million-year-old lifestyles so much as demographic and historical trends of the last tens of thousands of years?” (p. xxv).

  6. See Alvarez (2000) for a comprehensive discussion of life history theory pertaining to the grandmother hypothesis.

  7. This terminology is inspired by West et al. (2007), although their definitions are slightly different. For example, they define indirect fitness as “the component of fitness gained from aiding related individuals,” which would apply to helping grandmothers. Rather, my use of direct and indirect is meant to distinguish between direct descendants (children, grandchildren) and other relatives.

  8. The model could be easily modified to represent the fitness of a “first generation” grandmother whose mother did not herself help out as a grandmother. In this case only the term pertaining to the grandchildren would differ from the baseline scenario. One way to accomplish this modification would be to examine four generations, rather than three: the first generation (and any earlier generations) would not have the grandmothering trait, but the second and subsequent generations would. Women in the second generation thus would not be helped by their own mothers, but women starting in the third generation would be. This model would be better able to address questions of the initial appearance of the grandmothering trait.

  9. Note that one other way to enable comparison with the baseline would be to allow \(x,y<0\) and \(s,q<p\). These would involve grandmaternal “help” actually having a detrimental effect on their fitness, which I don’t take to be particularly realistic. I therefore think that adding in parameter t is a better way to accomplish comparison with the baseline.

  10. One potential problem for this objection, which is not my focus in this paper, is the extent to which other primates actually do hunt and share meat (Hawkes et al., 2000, pp. 246–7).

  11. For a lively debate of this issue, see Gurven and Hill (2009), Hawkes et al. (2010), and Gurven and Hill (2010).

  12. See Hrdy (2009, pp. 153–156) for a discussion of various cultural practices that involve mothers using paternity uncertainty to elicit the support of multiple adult males. Hrdy (1989) reviews the theoretical fitness benefits of polyandry. Blurton Jones et al. (2000, p. 82) also mention the extent to which stepfathers are willing to provide for children, which must be for reasons other than their relatedness.

  13. A move which is not unprecedented; indeed, as Hrdy (1989) says, “Whole chapters of human history could be read as an effort to contain the promiscuity of women and thus to establish, from circumstantial evidence, the paternity that could never be proved directly (before the advent of sophisticated laboratories)” (p. 179).

  14. Later in the same article, Hrdy does say, “Because it is often difficult to know for sure which male is actually the genetic father...” but this is in the context of defending her use of “allomother” instead of “alloparent” (p. 299).

  15. For easy comparison with the other models, I am sticking with measuring fitness in number of grandchildren. This particular comparison does not require that we do so; comparisons between fathers can be made by only looking at number of children. However, the subsequent comparison between grandmothers and grandfathers will require that we use grandchildren for both fitness measures.

  16. We can always concoct situations in which \(k<1\) for a mother. I will ignore these improbable situations.

  17. In this case, k is actually also restricted: there are no solutions at all unless \(k>\frac{1}{2}\sqrt{p^2s^2-2p^2s+p^2+4s^2}+ps-p\). In ordinary language, paternal credence k has to be greater than a certain number—not just greater than 0—for it to be at all possible that \(g_{3b'}<f_{1b}\).

  18. Note that we must restrict n to be \(>2-y\) in order for the number of additional children a father has to be greater than the number of additional children a grandmother’s daughters each have. This will be true for all \(n>2\) because \(y>0\) by stipulation.

  19. Notice that I’m returning to only using y and s, instead of z and v, although the latter could be easily used instead.

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Acknowledgements

This paper greatly benefited from feedback from Rory Smead, Victor Kumar, and Pierrick Bourrat, as well as a number of anonymous reviewers.

Funding

This material is based upon work supported by the National Science Foundation Graduate Research Fellowship Program under Grant No. DGE- 1840990. Any opinions, findings, and conclusions or recommendations ex- pressed in this material are those of the author and do not necessarily reflect the views of the National Science Foundation.

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    Aja Watkins

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Watkins, A. Reevaluating the grandmother hypothesis. HPLS 43, 103 (2021). https://doi.org/10.1007/s40656-021-00455-x

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