The patriarch hypothesis is a hypothesis that explains the occurrence of menopause in human females and how a long post-fertile period (up to one third of a females life-span) could confer an evolutionary advantage. It is an alternative theory to the grandmother hypothesis which tends to ignore male benefits of continued spermatogenesis and their roles in assistance.
The patriarch hypothesis incorporates these neglected areas. It suggests selection pressure on male longevity extended the female lifespan; whose adjustment of life history has been constrained by the size of the ovaries – resulting in human females surviving beyond the age at which they can reproduce. With an extension of the post-reproductive female life stage, they could enhance their inclusive fitness by giving kin assistance. This way, with no choice in the timing of fertility termination, females are optimising an essentially bad situation.
Frank Marlowe first put forward the patriarch hypothesis. He postulates that if women survive beyond an age at which they can reproduce and men continue spermatogenesis, then old males can benefit greatly if they can copulate with younger females. It is theorised that increased use of tools and weapons compensates for the decline in natural fighting ability with age. This serves to produce a more stable male hierarchy, where attainment of high social status and reproductive access is less reliant on physical strength.
With such a scenario older males are able to retain a competitive ability with younger males, thereby asserting a selection pressure on extending longevity in males that could retain social status. Higher ranking males may also be a more attractive mate choice. One mechanism that could extend the lifespan is delaying the age at maturity. Offspring with a slower life history would exhibit a protracted period of dependence. If depletion of oocytes occurs at age 50, females should selectively counter this as it reduces their fecundity.
Recruitment of help from kin and husbands may compensate by enabling females to reduce birth intervals by weaning offspring at an earlier age. In addition, by passing on longevity to her sons, a female would stand to gain inclusive fitness.
The patriarch hypothesis rests on three assumptions:
Longevity is a central determinant of the grandmother hypothesis; selection for greater longevity in males, as suggested by the patriarch hypothesis, could extend female lifespan, provided such a gene is not on the Y chromosome. Males have much to gain from late reproduction, even if they die shortly after conception. Females that found their longevity extended, were constrained by the difficulties of increasing their follicular reserves and thus could enhance their inclusive fitness by giving kin assistance.