Does the Size of a Sperm Really Matter?

by Tanya Thomas on Nov 26 2008 9:51 AM

A long trusted medical notion said that the Darwinian concept of “survival of the fittest” applied at the earliest levels of human reproduction- selection of the sperm. However, a new study from University of Sheffield has contested this notion of whether the length of a sperm's tail determines how fast it can swim.

The researchers claim that longer tails don't always provide a speed advantage. The speed at which a sperm swims is a key factor in its capability to fertilize an egg.

The research team including Stuart Humphries, from the University of Sheffield, and collaborators from the University of Western Australia has critically evaluated the evidence linking sperm shape to swimming speed.

They research team found a longer tail does allow a sperm to generate more thrust but the drag created by a sperm's head is often enough to counteract any such gains.

"It seems clear that some assumptions regarding the physics of sperm locomotion have hampered our progress in understanding the processes mediating sperm competition," said Humphries.

"It is commonly believed that selection for increased sperm performance will favor the evolution of longer, and therefore faster swimming, sperm.

"In fact, the relative lengths of a sperm's constituent parts, rather than their absolute lengths, are likely to be the target of selection," he added.

"We suggest that, irrespective of whether tail length, total length or head length is used, and any attempts to correlate a single measure of length to speed are likely to be futile.

"We argue that accounting for the balance between drag from the head and thrust from the tail will allow us to extend our understanding of the link between sperm form and function," he added.

The research is published in the open access journal BMC Evolutionary Biology.