Females prefer cooperative males even when cooperative behavior is unobserved: evidence from the mound-building mouse, Mus spicilegus

Theoretical and empirical studies in humans suggest that cooperative behaviors may act as signals during mate choice. However, cooperation is not always observable by potential partners before mate choice. To address whether cooperative phenotypes are preferred based on cues different from cooperative behaviors per se, we designed an experimental paradigm using wild-born mound-building mice (Mus spicilegus), a species with biparental care. In this species, females cannot observe male cooperative behaviors: mate choice occurs in the spring, whereas mounds are cooperatively built in the fall. We first assessed the variation in mound building investment and identified males exhibiting high and low amounts of cooperation. Second, we presented these males to females during two-way choice tests. As offspring survival relies on mound protection, we hypothesized that mound building could be a form of paternal care and assessed whether cooperative males were more involved in offspring attendance using pup-retrieval experiments. Our results indicate that females were more attracted to highly cooperative males over less cooperative, even when they did not observe them build. This finding suggests that female mate choice is influenced either by cues of cooperativeness different than cooperative behaviors or by preferences for traits associated with cooperativeness. Moreover, male offspring attendance was negatively correlated with cooperativeness, suggesting the potential existence of two alternative paternal strategies in offspring care (mound building versus offspring attendance). Overall, our findings support the existence of preference for cooperative phenotypes in a non-human species and suggest that sexual selection might be involved in the evolution of cooperation.