Patriarch hypothesis

The Patriarch Hypothesis is a theory to explain the occurrence of menopause in human females and how a long post-fertile period (up to one third of a females life-span)^[1] 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.

See also Grandmother hypothesis

The Patriarch Hypothesis

Frank Marlowe first put forward the patriarch hypothesis.^[2] He postulates that if women survive beyond an age at which they can reproduce and men continue spermatogenesis, then old males stand to 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.^[3] 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.^[2]

Evidence for

The patriarch hypothesis rests on three assumptions:

• Older males must reproduce.

It is clear that older males do reproduce, as the oldest verified paternity is 94 years, 35 years beyond the oldest documented birth attributed to females.^[4]

• The allele for slowing life history and extending longevity is not on the Y chromosome.

To date a ‘longevity’ gene(s) is still elusive. However, INK4a/ARF situated on the human chromosome 9p21 does appear to act as a tumour suppressor^[5] therefore extending longevity.^[6]

• Increasing the ovarian follicular reserve is difficult.

There are few explanations on density restrictive mechanisms other than physical size. NOS3 has been proposed as a candidate gene for the regulation and timing of reproductive functions, such as menopause,^[7] although it is unclear why timing has not adjusted with longevity. More importantly there is a lack of understanding why 70-99.9% of mammalian follicles are subjected to atresia. Future analysis of the differential expression of genes of the bcl-2 family may hold the key.^[8]

Conclusion

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.^[9] 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.

References

1. ↑ Harman, S.M. and Talbert, G.B. (1985). Reproductive aging. In: Handbook of the Biology of Aging, by C. E. Finch and L. Hayflick (eds). pp 457-510. Van Nostrand Reinhold.
2. ↑ 2.0 2.1 Marlowe, F. (1999). Male care and mating effort among Hadza foragers. Behavioral Ecology and Sociobiology. 46(1): 57-64.
3. ↑ Marlowe, F. (2000). The Patriarch Hypothesis: An Alternative Explanation of Menopause. Human Nature. 11(1): 27-42.
4. ↑ Seymour, F.I., Duffy, C., and Koerner, A. (1935). A case of authenticated fertility in a man of 94. Journal of the American Medical Association. 105: 1423-1424
5. ↑ Sharpless, N.E. (2004a). Ink4a/Arf Links Senescence and Aging. Experimental Gerontology. In Press. – available via www.sciencedirect.com.
6. ↑ Sharpless, N.E. (2004b). Ink4a/Arf: a Multifunctional Tumour Suppressor Locus. Mutation Research. In Press – cited in Sharpless, N.E. (2004a). Ink4a/Arf Links Senescence and Aging. Experimental Gerontology. In Press – available via www.sciencedirect.com.
7. ↑ Tempfer, C., Moreno, R.M., O’Brien, W.E. and Gregg, A.R. (2000). Genetic Contributions of the Endothelial Nitric Oxide Synthase Gene to Ovulation and Menopause in a Mouse Model. Fertility and Sterility. 73(5): 1025-1031.
8. ↑ Hsueh, A.J.W., Billing, H. and Tsafriri, A. (1994). Ovarian Follicle Atresia: A Hormonally Controlled Apoptotic Process. Endocrine Reviews. 15(6): 707-725.
9. ↑ Hudson, M. (2005). Why do females (especially human females) survive beyond an age at which they can reproduce? Sussex University Press. Brighton