Abstract
Human marriage systems, characterized by long-term partnerships and extended windows of parental care, differ from the mating systems of pulsed or seasonally breeding non-human animals in which Bateman’s principles were originally tested. These features, paradigmatic of but not unique to humans, complicate the accurate measurement of mating success in evaluating Bateman’s three principles. Here, we unpack the concept of mating success into distinct components: number of partners, number of years partnered, the timing of partnerships, and the quality of partners. Drawing on longitudinal records of marriage and reproduction collected in a natural-fertility East African population over a 20-year period, we test and compare various models of the relationship between mating success and reproductive success (RS), and show that an accurate assessment of male and female reproductive behaviour requires consideration of all major components of mating success. Furthermore, we demonstrate that while Bateman’s third principle holds when mating success is defined in terms of years married, women’s fitness increases whereas men’s fitness decreases from an increase in the number of marriage partners, holding constant the total effective duration of marriages. We discuss these findings in terms of the distinct, sex-specific pathways through which RS can be optimized, and comment on the contribution of this approach to the broader study of sexual selection.
Highlights
As observed in most other mammalian populations [1], human groups are typically characterized by greater variance in male than female reproductive and mating success (Bateman’s 1st and 2nd principles [2]), and stronger effects of mating success on reproductive success (RS) in males relative to females (Bateman’s 3rd principle) [3,4,5]
In marriage systems characterized by both concurrent polygyny and serial monogamy, different individuals may have the same number of total spouses and yet a very different number of years married to these spouses, and different levels of total exposure time to fertile partners
Focusing on a population showing no evidence of demographic transition, we find support for Bateman’s three principles, insofar as: (i) men show more variation in RS than do women, (ii) men show more variation in mating success than do women— the effect only reaches high reliability upon accounting for marriage timing and spousal quality, and (iii) marital years are a stronger predictor of male than female RS
Summary
As observed in most other mammalian populations [1], human groups are typically characterized by greater variance in male than female reproductive and mating success (Bateman’s 1st and 2nd principles [2]), and stronger effects of mating success on reproductive success (RS) in males relative to females (Bateman’s 3rd principle) [3,4,5]. While populations exist where females do have agency—e.g. in societies with ‘informal polyandry’ [21] and societies where biological paternity is believed to be shared among the recent sexual partners of a given woman [17]—individual-level data revealing the RS consequences of variation in mating success therein are not available To address these limitations, we conducted a longitudinal demographic study of the Pimbwe of western Tanzania, where cultural norms allow both men and women to marry, divorce, and remarry with largely free agency [10]. In marriage systems characterized by both concurrent polygyny and serial monogamy, different individuals may have the same number of total spouses and yet a very different number of years married to these spouses, and different levels of total exposure time to fertile partners For this reason, the use of spouse number as a single measure of mating success can be problematic. Results are discussed in the electronic supplementary material, S4.1, S4.2 and S4.3
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