Bateman's principle is reversed in a cooperatively breeding bird

Abstract

Bateman's principle is not only used to explain sex differences in mating behaviour, but also to determine which sex has the greater opportunity for sexual selection. It predicts that the relationship between the number of mates and the number of offspring produced should be stronger for males than for females. Yet, it is unclear whether Bateman's principle holds in cooperatively breeding systems where the strength of selection on traits used in intrasexual competition is high in both sexes. We tested Bateman's principle in the cooperatively breeding superb starling (Lamprotornis superbus), finding that only females showed a significant, positive Bateman gradient. We also found that the opportunity for selection was on average higher in females, but that its strength and direction oscillated through time. These data are consistent with the hypothesis that sexual selection underlies the female trait elaboration observed in superb starlings and other cooperative breeders. Even though the Bateman gradient was steeper for females than for males, the year-to-year oscillation in the strength and direction of the opportunity for selection likely explains why cooperative breeders do not exhibit sexual role reversal. Thus, Bateman's principle may not hold in cooperative breeders where both sexes appear to be under mutually strong sexual selection.

Keywords: Bateman gradient; Bateman's principle; cooperative breeding; sexual selection.

Figure 1.

Bateman gradients showing the relationship between the number of mates and the actual reproductive success across years for (a) females and (b) males. The size of the circle represents the frequency of each observation (range = 1–9 and 1–12 for females and males, respectively). Although one female breeder sired 52 offspring over the study period, 32 more than the next highest female breeder, excluding her from the analysis did not qualitatively change the results.

Figure 2.

Annual differences in the opportunity for selection, ΔI (I_m − I_f), for (a) total offspring hatched and (b) total offspring fledged. ΔI was on average less than zero for both datasets, indicating a greater opportunity for selection in females in most years.