This post was contributed by Pan-Pan Jiang.

Imagine the ultimate wingman. That rare male specimen so adept at the seek-and-pursue that his friends have only to be within spitting distance of him to reap the benefits.  As it turns out, the sperm of male mice have just such an evolutionary arsenal at their disposal: themselves.  And millions of them, at that.  In a paper published recently in Nature, Harvard researchers discovered that the sperm of deer mice (genus Peromyscus) form aggregates within the reproductive tract of females that move faster, and thus reach the elusive egg faster, than single sperm.  However, when they looked closer at the interaction between two different species they found that:

“…sperm aggregate more often with conspecific than heterospecific sperm, suggesting that individual sperm can discriminate on the basis of genetic relatedness.”

Cooler still is that the mating system of different species of Peromyscus also dictate aggregation patterns.  Sperm from a monogamous mouse clump indiscriminately with unrelated conspecific sperm.  However, sperm from a promiscuous mouse are more likely to clump with sperm from the same male, than with unrelated conspecific sperm.  The willy-nilly clumping of sperm from a monogamous mouse seems disadvantageous, since he ends up helping a stranger as often as he helps himself.  But this makes sense when you think about the selection pressures (or lack thereof) monogamy imposes: males don’t need to worry about unfaithful mates, therefore, they never evolved “self over other” sperm-clumping strategies.