Abstract
Sperm of some species form motile, coordinated groups as they migrate through the female reproductive tract to the site of fertilization. This collective motion is predicted to improve sperm swimming performance and therefore may be beneficial in a competitive context, but limited evidence supports this theory. Here we examine sperm aggregates across closely-related species of Peromyscus mice that naturally vary by mating system, and thus sperm competition intensity. We find that phylogenetic history predicts the likelihood that sperm will aggregate, and that relative testis size is negatively associated with variation in number of aggregated cells, suggesting that sperm competition has a stabilizing effect on sperm group size. Moreover, we show that aggregates are kinematically beneficial for some species but costly for others, and these differences are largely dependent on the orientation and composition of sperm within the groups. In addition, when we compared sperm of the two sister-species that aggregate most frequently, we find that sperm from the species that evolved under intense sperm competition forms aggregates with more efficient geometry more frequently than sperm from its monogamous congener. These results are consistent with the prediction that sperm aggregation evolved to improve motility in a competitive context; however, when monogamy evolved secondarily, relaxed sexual selection allowed for less efficient strategies to persist. Together, our findings in Peromyscus reveal that collective sperm behavior is likely to evolve rapidly and is shaped by changes in the selective regime.
Footnotes
Funding: This work was supported by a Eunice Kennedy Shriver National Institute of Child Health and Human Development K99/R00 Pathway to Independence Award to HSF [R00HD071972] and a National Science Foundation Postdoctoral Research Fellowship to KAH [1711817].