The selfish grandma gene: the roles of the x-chromosome and paternity uncertainty in the evolution of grandmothering behavior and longevity

How prenatal cortisol levels relate to grandmother-mother relationships among a cohort of Latina women

INTRODUCTION

As part of the human reproductive strategy, mothers receive childcare assistance from others. For kin, allomothers are adaptively incentivized to provide assistance due to inclusive fitness benefits. Previous studies across a broad range of populations identify grandmothers as particularly consistent allomothers. Minimal attention has been paid to the possibility that allomothers may begin investing in offspring quality during the prenatal stage of life. Here, we innovate within the area of grandmother allocare research by examining the prenatal stage of life and biopsychosocial mechanisms by which prenatal grandmother effects may be enacted.

METHODS

Data derive from the Mothers' Cultural Experiences study, a cohort of 107 pregnant Latina women in Southern California. At <16 weeks' gestation, we administered questionnaires, collected morning urine samples, and measured cortisol by enzyme-linked immunosorbent assay, correcting for specific gravity. We measured the soon-to-be maternal and paternal grandmothers' relationship quality, social support, frequency of seeing each other, communicating, and geographic proximity to pregnant mothers, that is, their daughters and daughters-in-law. These measures were self-reported by the pregnant mothers. We assessed how grandmother constructs related to the pregnant women's depression, stress, anxiety, and cortisol levels.

RESULTS

We observed benefits conferred by maternal grandmothers for mothers' prenatal mental health and lower cortisol levels. Paternal grandmothers also conferred mental health benefits to pregnant daughters-in-law, but higher cortisol levels.

CONCLUSION

Our results suggest that grandmothers, especially maternal grandmothers, are able to improve their inclusive fitness by caring for pregnant daughters, and allomother support may positively impact prenatal health. This work extends the traditional cooperative breeding model by identifying a prenatal grandmother effect, and, by examining a maternal biomarker.

Do Step- and Biological Grandparents Show Differences in Investment and Emotional Closeness With Their Grandchildren?

Human children are raised by a variety of caregivers including grandparents. A few studies have assessed potential differences in direct caregiving, financial expenditures, and emotional closeness between biological and step-grandparents. Drawing upon kin selection theory, we hypothesized that step-grandparents would provide less care and be less emotionally close to grandchildren than would biological grandparents. A sample of 341 heterosexual U.S. adults 25-35 years of age in a long-term partnership and with a biological child 5 years of age or younger were recruited via Amazon Mechanical Turk. Subjects provided sociodemographic information and answered questions about the dynamics between their own parent/stepparent and their own youngest biological child (hence, biological/step-grandparenting dynamics). Main analyses were restricted to within-subject comparisons. Results showed that biological grandmothers provided more direct childcare, financial expenditures, and had more emotionally close relationships with grandchildren than did step-grandmothers. Biological grandfathers provided less direct care and had less emotionally close relationships than step-grandfathers but did not exhibit differences in financial expenditures. Biological grandmothers provided more direct care, financial investment, and were more emotionally close to the referential grandchild than were biological grandfathers. Step-grandfathers were more emotionally close and more often played with grandchildren than step-grandmothers. These findings partially support kin selection theory. We discuss the relevance of factors such as competing demands on grandmothers' investment in biological and step-grandchildren and grandfathering serving in part as mating effort. Sex differences in biological grandparenting also mirror those in parenting. We suggest directions for future research, including on grandfathers, particularly in patrilineal societies.

The influence of gender on inheritance of exceptional longevity

While the search for genetic contributors to exceptional longevity has yielded candidates, gender differences in inheritance have generally not been considered. The aim of this study was to investigate gender specific differences in the inheritance of exceptional longevity. Using a standardized questionnaire, we assessed the parental ages of death of Ashkenazi Jews with exceptional longevity and their spouses without exceptional longevity, who served as controls (n=1,114). Mothers of centenarian males and females had significantly longer lifespans compared to the mothers of non‐ centenarians, 79.0 ± 13.4 vs. 73.0 ± 16.3 years, p<0.01 and 75.7 ± 15.8 vs. 70.5 ± 18.0 years, p=0.02, respectively. There was also a trend toward longer lifespan among the fathers of centenarian men compared to the lifespan of fathers of non‐ centenarian men, 73.5 ± 17.0 vs. 69.5 ±15.0 years, p=0.07. The lifespan did not differ between the fathers of centenarian and non‐centenarian daughters. Logistic regression models revealed that the odds of being a centenarian for the female and male offspring increased by 21% and 31%, respectively, for every additional 10 years of life achieved by the mother (p<0.05). These findings support a gender‐specific inheritance pattern of human longevity and may help focus the search for longevity genes.

Human kin detection

Natural selection has favored the evolution of behaviors that benefit not only one's genes, but also their copies in genetically related individuals. These behaviors include optimal outbreeding (choosing a mate that is neither too closely related, nor too distant), nepotism (helping kin), and spite (hurting non-kin at a personal cost), and all require some form of kin detection or kin recognition. Yet, kinship cannot be assessed directly; human kin detection relies on heuristic cues that take into account individuals' context (whether they were reared by our mother, or grew up in our home, or were given birth by our spouse), appearance (whether they smell or look like us), and ability to arouse certain feelings (whether we feel emotionally close to them). The uncertainties of kin detection, along with its dependence on social information, create ample opportunities for the evolution of deception and self-deception. For example, babies carry no unequivocal stamp of their biological father, but across cultures they are passionately claimed to resemble their mother's spouse; to the same effect, 'neutral' observers are greatly influenced by belief in relatedness when judging resemblance between strangers. Still, paternity uncertainty profoundly shapes human relationships, reducing not only the investment contributed by paternal versus maternal kin, but also prosocial behavior between individuals who are related through one or more males rather than females alone. Because of its relevance to racial discrimination and political preferences, the evolutionary pressure to prefer kin to non-kin has a manifold influence on society at large.

Sexually antagonistic zygotic drive: a new form of genetic conflict between the sex chromosomes

Sisters and brothers are completely unrelated with respect to the sex chromosomes they inherit from their heterogametic parent. This has the potential to result in a previously unappreciated form of genetic conflict between the sex chromosomes, called sexually antagonistic zygotic drive (SA-ZD). SA-ZD can arise whenever brothers and sisters compete over limited resources or there is brother-sister mating coupled with inbreeding depression. Although theory predicts that SA-ZD should be common and influence important evolutionary processes, there is little empirical evidence for its existence. Here we discuss the current understanding of SA-ZD, why it would be expected to elude empirical detection when present, and how it relates to other forms of genetic conflict.

Testing evolutionary theories of discriminative grandparental investment

This study tests two evolutionary hypotheses on grandparental investments differentiated by the child's sex: the paternity uncertainty hypothesis and the Trivers–Willard hypothesis. Data are from two culturally different countries: the Dutch Longitudinal Aging Study Amsterdam (n=2375) and the Chinese Anhui Survey (n=4026). In the Netherlands, grandparental investments are biased towards daughters' children, which is in accordance with the paternity uncertainty hypothesis. But in China, grandparental investments are biased towards sons' children, which is in conflict with the paternity uncertainty hypothesis. This study found no support for the Trivers–Willard hypothesis. These results raise doubts over the relevance of paternity uncertainty as an explanation of a grandparental investment bias towards daughters' children that is often found in Western populations. The results suggest that discriminative grandparental investments are better understood as the outcome of cultural prescriptions and economic motives.