You may believe that male competition finishes when the winner copulates with the females. But there are cases when even sperm cells must compete.

This is the case when the females are promiscuous and sperm from different males is found in the womb of the female at a given moment.

Sperm competition has produced in rodents a bizarre sperm collaboration and explains the strange morphology of their sperm cells. Most mammals have paddle-shaped headed sperm cells but the sperm heads of many rat and mouse species are curved like scythes, forming a hook.

10 years ago, researchers found that in the case of the European woodmouse, these hooks permitted a sperm cell to attach to more of its brethren, allowing them to form groups of up to 100 sperms, and that these "sperm trains" were faster and stronger than sperm swimming alone, increasing their chance of fertilizing the egg.

Only the highly specialized design of rat and mouse sperm permits this form of sperm cooperation.

New research by evolutionary biologist Dr Simone Immler and colleagues from the Department of Animal and Plant Sciences, University of Sheffield in the United Kingdom, investigated the sperm of 37 rodent species, including the Norway rat and house mouse.
The researchers found that, like in the case of the European woodmouse, in the promiscuous rat and mice, individual sperm cooperates with one another in order to out-compete sperm of rival males and the larger the testes (thus the amounts of ejaculated sperm), the sharper the hooks, because more sperm means more competition between sperm to reach the egg.

"It was previously believed that sperm not only competed against rival males but that they also competed against each other in order to fertilize the female egg. However, this research shows that when the pressure from rival males is high, individual sperm will cooperate with one another to ensure that at least one of their siblings successfully reaches the female egg", said Immler.

As each sperm cell carries a slightly different genetic information, those who reach the egg first transmit their superior hooks to the next generation. "The findings address the long-held question about the unique shape of rodent sperm as well as the role competition has played in hook formation", said Rhonda Snook, an evolutionary biologist of the University of Sheffield.