In the face of competition, sperm cells travel faster when they move together in groups of an optimal size.
A team led by Heidi Fisher at Harvard University in Cambridge, Massachusetts, studied rodent sperm cells under a microscope, and used a mathematical model to analyse their swimming behaviour. They found that, in comparison to solitary sperm or those in larger groups, intermediate-sized aggregates of six or seven sperm (pictured) tend to migrate the fastest, by taking a more direct path.
Sperm cells from a sexually promiscuous species of deer mouse, Peromyscus maniculatus, were faster and more likely to form optimally sized clumps than were similarly shaped sperm from a monogomous sister species, Peromyscus polionotus.
The results show how sexual selection can shape the evolution of cooperation.