Reproductive conflict and the separation of reproductive generations in humans

Abstract

An enduring puzzle of human life history is why women cease reproduction midway through life. Selection can favor postreproductive survival because older females can help their offspring to reproduce. But the kin-selected fitness gains of helping appear insufficient to outweigh the potential benefits of continued reproduction. Why then do women cease reproduction in the first place? Here, we suggest that early reproductive cessation in humans is the outcome of reproductive competition between generations, and we present a simple candidate model of how this competition will be resolved. We show that among primates exhibiting a postreproductive life span, humans exhibit an extraordinarily low degree of reproductive overlap between generations. The rapid senescence of the human female reproductive system coincides with the age at which, in natural fertility populations, women are expected to encounter reproductive competition from breeding females of the next generation. Several lines of evidence suggest that in ancestral hominids, this younger generation typically comprised immigrant females. In these circumstances, relatedness asymmetries within families are predicted to give younger females a decisive advantage in reproductive conflict with older females. A model incorporating both the costs of reproductive competition and the benefits of grandmothering can account for the timing of reproductive cessation in humans and so offers an improved understanding of the evolution of menopause.fertility ͉ grandmother hypothesis ͉ human evolution ͉ life history ͉ menopause

Does grandparental help mediate the relationship between kin presence and fertility?

“…For instance, a woman of reproductive age may provide help to her kin, acting as helper-at-the-nest (Kramer 2005); resulting in close kin more effectively reproducing, but the woman herself suffering a reduction in fertility. This may occur because of reproductive competition between generations (Cant and Johnstone 2008;Moya and Sear 2014).…”

Grandma plays favourites: X-chromosome relatedness and sex-specific childhood mortality

“…The grandmother's invaluable role in caretaking led to higher fertility in her daughter, and thus provided a strong inclusive fitness advantage to those grandmothers who survived longer. In recent years, the grandmother hypothesis has also been interpreted to include all tactics a grandmother could use to increase the fitness of her grandchildren, as opposed to Hawkes' original hypothesis that focused on dietary provisioning (Cant & Johnstone 2008). Among our species, alloparents play an important role in assisting mothers with childcare, and so the participation of a grandmother in child-rearing could make a serious impact on a child's likelihood of survivorship (Hrdy 2009).…”

Genomic imprinting and the evolutionary psychology of human kinship

“…What determines who becomes the helper at whose nest? Cant and Johnstone (61) have argued that when a young woman moves into the extended family of her husband, her mother-in-law is predisposed to become the helper at the younger woman's nest because the older women is related to the younger's offspring (and therefore has a genetic incentive to help), whereas the younger woman is unrelated to the older's offspring (and therefore has no genetic incentive to help). Perhaps a similar dynamic can play out after divorce between a younger woman and her own mother.…”