Zinc induces cooperative group formation in sperm from the deer mouse, Peromyscus maniculatus.

Sperm cooperation can improve fertilization success in species with intense sperm competition, but the mechanisms that assemble cooperative groups remain unclear. We tested whether extracellular zinc, an abundant component of the epididymal and seminal environment, promotes sperm clustering in the deer mouse, Peromyscus maniculatus. Zinc increased the proportion of sperm in groups, and these groups persisted after washout, suggesting that zinc induces relatively stable sperm-sperm interactions. Chelation reduced clustering, indicating that these interactions remain reversible. Deer mouse sperm also accumulated on zinc-conjugated beads, whereas sperm from the non-clustering house mouse, Mus musculus, did not, consistent with a species-specific, zinc-dependent adhesive property. Together, these findings identify extracellular zinc as a candidate signal that promotes cooperative sperm clustering and suggest that extracellular zinc may provide a physiological cue that enables cooperative sperm behavior in species under strong sperm competition.