Simone Immler

Hooking up.
The sperm of several rodent species have scythe-shaped heads, which help them attach to other sperm.

Sperm Find Strength in Numbers

You don't have to be a complete organism to take part in Darwinian evolution: Even sperm engage in the survival of the fittest. A new study indicates that the sperm of certain rodent species have evolved hook-shaped heads, apparently to beat each other to the egg. Sperm with better hooks are able to attach to more of their brethren, allowing them to form fast-moving chains that leave their rivals behind.

For years, biologists have puzzled at the strange shape of rodent sperm. As opposed to the sperm of most other mammals, which have paddle-shaped heads, the sperm heads of many rat and mouse species are curved like scythes. About 10 years ago, scientists studying the European woodmouse discovered that these hooks allow groups of up to 100 sperm to attach to each other, and that these "sperm trains" moved faster than sperm swimming alone.

Curious if there were any evolutionary forces at play, evolutionary biologist Simone Immler of the University of Sheffield in the United Kingdom and colleagues studied the sperm of 37 rodent species, including the Norway rat and the house mouse. As in the European woodmouse, the team found that--in most of the species studied--sperm hooked into an entourage moved faster than loners did. What's more, species with larger testes--and thus greater quantities of sperm per ejaculate--tended to have sperm with sharper hooks. That may be because more sperm equals more competition between sperm to reach the egg, the team speculates in the 24 January issue of Public Library of Science ONE. Because each sperm has a slightly different genetic make-up, those who reach the egg first pass their superior hooks to the next generation.

"It's a really interesting study," says Rhonda Snook, an evolutionary biologist of the University of Sheffield, who was not affiliated with the paper. The findings address the long-held question about the unique shape of rodent sperm, she says, as well as the role competition has played in hook formation.

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