Definitions

Fitness – the ability of an organism to have reproductive success

Birth Interval – the time between the birth of a child and the following child

Reproductive Tenure – the total period between the first and last child

Altruism – the act of helping another individual without any benefit to oneself

The theory of evolution is the mainstay of human biology and is constantly shifting our perspectives on the development of all organisms. Evolution occurs when populations change over time and thus, the factors that contribute to evolution are heavily analyzed. Most organisms are capable of reproducing and passing on their genetic information. However, unlike some other animals, human females can live long past their reproductive years, which raises the question ‒ what is the evolutionary benefit of females living past reproductive age? Post-reproductive age refers to the lifespan of a female after they have stopped reproducing. Previous studies have found that evolution will favour longer post-reproductive life spans in females if there are conferred benefits to their offspring and their reproductive abilities. [1,2,3] The presence of a “helper” can aid in another individual’s ability to reproduce and their frequency of breeding. Additionally, the helper can provide care during the offspring’s upbringing, thereby increasing its chances of survival. This theory is called the Grandmother Hypothesis as it aims to explain the benefits of grandmothers ‒ post-reproductive females ‒ in their children and grandchildrens’ lives. Existing literature has explored the ability of post-reproductive females in aiding their offspring’s reproductive efforts; however, information is lacking on whether post-reproductive life spans result in greater numbers of grandchildren and the specific  advantages that are seen since data does not typically cover generations. The following study aims to bridge this gap in knowledge by exploring the specific benefits that post-reproductive females confer to their own offspring.

Methods

Mirkka Lahdenperä in the Department of Biology at University of Turku, Finland and colleagues from the University of Cambridge, University of Quebec, University of Sheffield, as well as fellow department colleagues conducted a study to assess the fitness benefits of post-reproductive females. [5] Data was collected from Finnish and Canadian farming communities in the 18th and 19th centuries as they typically feature greater grandparent involvement and close familial proximity. In the Finnish population, the Lutheran Church collected data on births, marriages, and deaths from the 17th century onwards, which was used for this study. In the Canadian population, the BALSAC population registers in Quebec collected information from baptisms, marriages, and deaths. However, the analysis of a female’s offspring’s reproductive success was unavailable using the Canadian data, so information regarding reproductive histories was conducted on the Finnish population only. The study measured the following parameters:

1. Does the length of a female’s post-reproductive stage impact the number of grandchildren she has?
2. Does the presence of a post-reproductive female impact her offspring’s ability to reproduce, number of offspring produced, and timing of reproduction? (Finnish data only).
3. Whether the presence of a post-reproductive female affects their offspring’s age at reproduction and subsequent birthing intervals.

Results

It was found that the length of post-reproductive years was related to the number of grandchildren, with a positive correlation between increased post-reproductive lifespan and number of grandchildren, as seen in Figure 2.

The study also found that the presence of a grandmother impacted her offspring having an increased fecundity ‒ number of their own children (grandchildren) (Figure 3a). Additionally, the grandmothers’ presence also resulted in greater lifetime reproductive success (LRS) in her offspring, which measures survival of grandchildren to age 15 (Figure 3b). Similarly,  the longer the grandmother lived, the more grandchildren survived to 15 years. Figure 3f shows the impact of alive and dead grandmothers on their grandchildrens’ survival, in which the presence of grandmothers, specifically younger ones, positively impacted survival.

Figure 3c measured the total number of grandchildren in offspring who lived close to or away from the grandmothers’ house and shows the increase in grandchildren of locally-situated grandmothers. It was also found that offspring reproduced earlier if the grandmothers were alive (Figure 3d).

Furthermore, if a grandmother was present, her offspring reproduced earlier, had shorter birth intervals (for the first three births) (Figure 3e), and longer spans of reproductive tenure.

Discussion

Evidence supports the benefits of the presence of a grandmother in her offsprings’ lives. The grandmother hypothesis may explain the increased post-reproductive life spans seen in females over time. Grandmothers do not affect their grandchildrens’ survival between birth and age 2, but their presence resulted in an increased survival probability between the ages of 2 and 5 years. Since newborns spend the first few years of their lives dependent primarily on their mother, the increase in survival after this period of time suggests that the presence of a grandmother does influence the grandchild’s life during its subsequent upbringing.

The results presented in Figure 3 include both sons and daughters, and each sex shared similar benefits from the grandmothers’ presence. Figure 3c further demonstrates that more grandchildren were produced when the offspring lived closer to the grandmother, thereby suggesting the effectiveness of a helper on frequency and success of breeding.

As shown in Figure 2, a grandmother roughly gains two grandchildren for every ten years she survives past her reproductive life; this statistic is shared amongst the Canadian and Finnish populations despite geographical and cultural differences. However, the mean lifespan of the grandmothers coincides with the age at which their offspring ceased reproduction, which suggests that grandmothers are beneficial until their own children become the following generation’s grandmothers (Figure 4).

The grandmother hypothesis has thus far appeared to account for the increasing post-reproductive lifespan of females, and, while there is still not enough evidence to make concrete conclusions, it may also provide insight regarding the evolution of menopause.

However, there are objections to the grandmother hypothesis, including how the father’s role should negate the altruistic aid provided by grandmothers. Hunter-gatherer males have historically provided food resources to their own offspring, thereby improving survival. However, common objections to this theory cite male promiscuity and hunting as a mating effort rather than survival necessity as explanations on why fathers may not be as important as caregivers. [6] Overall, the role of males and their own parents brings more nuance into the grandmother hypothesis theory.

A study done in 2011 by Strassman et al., found that only maternal grandparents, including grandfathers, were associated with lower grandchild mortality rates; paternal grandmothers did not have any significant effects on their grandchild’s survival rates, which may suggest a more narrow version of the grandmother hypothesis that focuses on maternal family influence. [7]

Future studies should re-examine the grandmother hypothesis using current global data and ascertain whether it could definitively explain the presence of menopause. Additionally, studies should also examine the mental and emotional impact of having a grandmother present to help with the responsibilities of child-rearing as most studies focus solely on the physical benefits. Despite the uncertainties that may still surround this field, the data suggests that the presence of post-reproductive females generally provides substantial benefits to their offspring and further bolsters the strength of familial bonds.

References 

1. Blell M. Grandmother hypothesis, grandmother effect, and residence patterns. The International Encyclopedia of Anthropology. 2017 Sept 29;1–5. doi:10.1002/9781118924396.wbiea2162

2. Gibson MA, Mace R. Helpful grandmothers in rural Ethiopia: A study of the effect of kin on child survival and growth. Evolution and Human Behavior. 2005 Jul 25;26(6):469–82. doi:10.1016/j.evolhumbehav.2005.03.004

3. Sear R, Mace R. Who keeps children alive? A review of the effects of kin on child survival. Evolution and Human Behavior. 2007 Dec 10;29(1):1–18. doi:10.1016/j.evolhumbehav.2007.10.001

4. Laisk T, Tšuiko O, Jatsenko T, Hõrak P, Otala M, Lahdenperä M, et al. Demographic and evolutionary trends in ovarian function and aging. Human Reproduction Update. 2018 Oct 20; doi:10.1093/humupd/dmy031
5. Lahdenperä M, Lummaa V, Helle S, Tremblay M, Russell AF. Fitness benefits of prolonged post-reproductive lifespan in women. Nature. 2004 Mar;428(6979):178–81. doi:10.1038/nature02367

6. Watkins A. Reevaluating the grandmother hypothesis. History and Philosophy of the Life Sciences. 2021 Aug 24;43(3). doi:10.1007/s40656-021-00455-x

7. Strassmann BI, Garrard WM. Alternatives to the grandmother hypothesis. Human Nature. 2011 Jul;22(1–2):201–22. doi:10.1007/s12110-011-9114-8