Abstracts (first author)
Dissecting the Bateman gradient: what it really tells us about sex roles
At the heart of the recent debate on sex roles lies the interpretation of the Bateman gradient, the slope of the regression of reproductive success (number of offspring, T) over mating success (number of mates, M). Typically, males are considered to have a steeper Bateman gradient, thus to undergo stronger sexual selection than females. Recent work challenges this paradigm by questioning the measurement and interpretation of Bateman gradients. In this study we combine an experimental approach with multivariate analyses to resolve the significance of male and female Bateman gradients in the red junglefowl, Gallus gallus. First, the male Bateman gradient is measured by deducing mating success from paternity of the offspring, without accounting for matings that fail to result in fertilisation. We demonstrated that inferring M from offspring parentage leads to a 50% overestimate of the male Bateman gradient compared to a regression using fine-grain mating behaviour. Second, while the male Bateman gradient is concerned with the causal relationship between M and T, we show that variation in other components of male reproductive success, namely female fecundity and paternity share, also cause the Bateman gradient to be overestimated of a further 69%. Finally, there is growing appreciation that female Bateman gradients can be steeper than originally thought. We show that females display a positive Bateman gradient, suggesting that they benefit by mating with multiple males. However, an experimental test shows no evidence that productivity increases with number of mates in females, suggesting that the female Bateman gradient emerges as spurious consequence of males preferentially mating with more fecund females. Our results demonstrate that the mechanisms underpinning Bateman gradients are more complex than currently appreciated, and that understanding the causal relationship of M and T and how it defines sex roles requires an integrated experimental approach.