Bateman gradients in a promiscuous mating system

The Bateman gradient is increasingly used to measure sexual selection and characterize mating systems. In a landmark paper, Arnold and Duvall (Am Nat 143:317–348, 1994) formulated predictions about the relationships between sex-specific Bateman gradients and the major types of mating system. In promiscuous species, gradients are expected to be strong and similar in both sexes. Current support for this prediction however remains equivocal as reported male gradients are almost constantly steeper than female gradients. Here, we estimated Bateman gradients in a wild population of Eastern chipmunks (Tamias striatus) over two reproductive seasons characterized by extreme levels of promiscuity and unbiased operational sex ratios. We found significant and positive Bateman gradients for both sexes. The gradients were not different among sexes suggesting that the strength of sexual selection was similar for males and females. The opportunity for selection was also particularly strong for a promiscuous species and not different among sexes. Our results thus support the predicted Bateman gradients for a promiscuous mating system.