title: The dynamics of sperm cooperation in a competitive environment.
abstract: Sperm cooperation has evolved in a variety of taxa and is often considered a response to sperm competition, yet the benefit of this form of collective movement remains unclear. Here, we use fine-scale imaging and a minimal mathematical model to study sperm aggregation in the rodent genus Peromyscus. We demonstrate that as the number of sperm cells in an aggregate increase, the group moves with more persistent linearity but without increasing speed. This benefit, however, is offset in larger aggregates as the geometry of the group forces sperm to swim against one another. The result is a non-monotonic relationship between aggregate size and average velocity with both a theoretically predicted and empirically observed optimum of six to seven sperm per aggregate. To understand the role of sexual selection in driving these sperm group dynamics, we compared two sister-species with divergent mating systems. We find that sperm of Peromyscus maniculatus (highly promiscuous), which have evolved under intense competition, form optimal-sized aggregates more often than sperm of Peromyscus polionotus (strictly monogamous), which lack competition. Our combined mathematical and experimental study of coordinated sperm movement reveals the importance of geometry, motion and group size on sperm velocity and suggests how these physical variables interact with evolutionary selective pressures to regulate cooperation in competitive environments.
authors: Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, MA 02138, USA Howard Hughes Medical Institute, Department of Molecular and Cellular Biology, Museum of Comparative Zoology, Harvard University, Cambridge, MA 02138, USA.
authors: Heidi S
authors: HS
authors: Fisher
authors: School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138, USA SISSA, International School for Advanced Studies, Trieste, Italy.
authors: Luca
authors: L
authors: Giomi
authors: Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, MA 02138, USA Howard Hughes Medical Institute, Department of Molecular and Cellular Biology, Museum of Comparative Zoology, Harvard University, Cambridge, MA 02138, USA hoekstra@oeb.harvard.edu.
authors: Hopi E
authors: HE
authors: Hoekstra
authors: Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, MA 02138, USA School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138, USA Department of Physics, Kavli Institute for Nanobio Science and Technology, Wyss Institute for Bioinspired Engineering, Harvard University, Cambridge, MA 02138, USA lm@seas.harvard.edu.
authors: L
authors: L
authors: Mahadevan
mesh_terms: D000818:Animals; D002449:Cell Aggregation; D005306:Fertilization / Q000502:physiology; D008297:Male; D008962:Models, Theoretical; D010542:Peromyscus / Q000502:physiology*; D012726:Sexual Behavior, Animal / Q000502:physiology*; D013045:Species Specificity; D013081:Sperm Motility / Q000502:physiology; D013094:Spermatozoa / Q000502:physiology*
keywords: collective motion; mathematical model; sexual selection; sperm aggregation; sperm competition; sperm motility