The measure and significance of Bateman's principles

Abstract

Bateman's principles explain sex roles and sexual dimorphism through sex-specific variance in mating success, reproductive success and their relationships within sexes (Bateman gradients). Empirical tests of these principles, however, have come under intense scrutiny. Here, we experimentally show that in replicate groups of red junglefowl, Gallus gallus, mating and reproductive successes were more variable in males than in females, resulting in a steeper male Bateman gradient, consistent with Bateman's principles. However, we use novel quantitative techniques to reveal that current methods typically overestimate Bateman's principles because they (i) infer mating success indirectly from offspring parentage, and thus miss matings that fail to result in fertilization, and (ii) measure Bateman gradients through the univariate regression of reproductive over mating success, without considering the substantial influence of other components of male reproductive success, namely female fecundity and paternity share. We also find a significant female Bateman gradient but show that this likely emerges as spurious consequences of male preference for fecund females, emphasizing the need for experimental approaches to establish the causal relationship between reproductive and mating success. While providing qualitative support for Bateman's principles, our study demonstrates how current approaches can generate a misleading view of sex differences and roles.

Keywords: Bateman principles; Gallus gallus; polyandry; sex roles; sexual dimorphism; sexual selection.

Figure 1.

(a) Male (solid squares and line) and female (empty diamonds, dashed line) Bateman gradients based on MS_gen. (b) The quadratic relationship between female MS_gen and reproductive success over a MS_gen range of 0–3, vertical bars represent CI. The function shows a maximum for MS_gen = 2. The vertical dashed line shows (approximately) mean female MS_gen across groups, the shaded section around the line represents SE around the mean (2.13 ± 0.16).

Figure 2.

Average number of eggs produced by females mated three times with a single male (monoandry) or once with each of three males (polyandry). (Online version in colour.)