Polygynous fertility: sexual competition versus progeny

Multi-level sexual selection: individual and family-level selection for mating success in a historical human population

Precopulatory sexual selection is the association between fitness and traits associated with mate acquisition. Although sexual selection is generally recognized to be a powerful evolutionary force, most investigations are limited to characters belonging to individuals. A broader multilevel perspective acknowledges that individual fitness can be affected by aspects of mating success that are characters of groups, such as families. Parental mating success in polygynous or polyandrous human societies may exemplify traits under group-level sexual selection. Using fitness measures that account for age-structure, I measure multilevel selection for mate number over 55 years in a human population with declining rates of polygyny. Sexual selection had three components: individual-level selection for ever-mating (whether an individual mated) and individual- and family-level selection for polyandry and polygyny. Family- and individual-level selection for polygyny was equally strong, three times stronger than family-level selection for polyandry and more than an order of magnitude stronger than individual-level selection for polyandry. However, individual-level selection for polyandry and polygyny was more effective at explaining relative fitness variance than family-level selection. Selection for ever-mating was the most important source of sexual selection for fitness; variation for ever-mating explained 23% of relative fitness variance.

Reproductive preferences and contraceptive use: a comparison of monogamous and polygamous couples in northern Malawi

There is now widespread agreement on the importance of men's role in reproductive decision-making. Several studies have argued that fertility preferences and their translation into behaviour differ between polygamous and monogamous unions. Studies investigating the dominance of men's preferences over women's preferences, in cases of couple disagreement, found mixed evidence of the effect of polygamy. However, an often cited limitation of these studies has been the inability to link husband's intention with each of his wives in a polygamous union. By adding fertility-intention questions to an on-going Demographic Surveillance Site in Karonga District in northern Malawi the fertility preferences and contraceptive use of husbands and wives were investigated. An analysis of the relationship between the level of agreement and disagreement between husbands' and wives' fertility preferences was then performed to gain insight into the reproductive decision-making process of polygamous couples.

Mating system change reduces the strength of sexual selection in an American frontier population of the 19th century

Sexual selection, or competition among members of one sex for reproductive access to the other, is one of the strongest and fastest evolutionary processes. Comparative studies support the prediction that sexual selection is stronger in polygamous than in monogamous species. We report the first study of the effect on sexual selection of a change in mating system, from polygyny to monogamy, within a historical human population. Here we show that over the reproductive lifetimes of Utahns born between 1830 and 1894, socially induced reductions in the rate and degree of polygamy correspond to a 58% reduction in the strength of sexual selection. Polygyny conferred a strong advantage to male fitness as well as a weak disadvantage to female fitness. In contrast, mating with multiple males provided little benefit to females in this population. Polygamy benefitted males by increasing reproductive rates and by lengthening reproductive tenure. Each advantage contributed to roughly half of the increased total lifetime reproductive success. This study illustrates both the potency of sexual selection in polygynous human populations and the dramatic influence that short-term societal changes can have on evolutionary processes.

Does polygyny reduce fertility?

Polygyny can increase, decrease, or have no effect on fertility. Understanding how this can occur requires consideration of both the proximate determinants of fertility and the ultimate effects of polygyny as a female reproductive strategy. Several factors reduced the fertility of polygynous women in 19th century Utah, including marrying at an older age, marrying older men, and conflict between co-wives. Sterility did not explain the reduced number of children in polygynous women, nor is there evidence of a "dilution effect" from sharing a husband. If women could anticipate a reduction in their own fertility, why would they still choose polygyny? Evidence suggests that they chose it because the children of polygynous men had increased fertility, high enough to offset the low fertility of polygynous women themselves.