Sporty Sperm: A Stiff One Gets the Job Done More Quickly

A scientist who studies the phsyics of sperm “as a hobby” is challenging the current understanding of how sperm swim towards an egg. At the Society for Experimental Biology conference today Dr Christopher Lowe will present the results of his modelling of a sperm`s tail, suggesting we may need to re-think our assumptions of how sperm move through fluid.

Experimental studies of sperm have generated a fairly well established database of parameters on sperm movement. The frequency and wavelength of the tail movement is estimated at around 50 hertz down the tail. The low speed at which sperm swim is well known – perhaps suprisingly low given the urgency of the mission, but understandable because of the sheer force of the fluid it is moving in. “If you were a sperm it would be the equivalent of swimming in a liquid a thousand million times more viscous than air. There is not a substance known to man that is that viscous – even swimming in a pool of thick syrup would be easy going compared to the Olympic feats performed by sperm,” says Dr Lowe.

The fluids in which sperm swim are also well-characterised. Using both the sperm and fluid parameters Dr Lowe constructed a computer model which accurately recreated the shape and movement of the sperm`s tail as it swims towards the egg. The simulation also correctly reproduced the swimming speed. But to Dr Lowe`s surprise, he discovered a discrepancy between the computer model and the established theory, related to how stiff the sperm`s tail needs to be to counteract the resistance or drag of the surrounding fluid. “Either the tail is significantly stiffer when the sperm is swimming than previous experiments suggest, or the sperm is doing something very clever indeed to overcome the sticky forces exerted on it by the surrounding fluid. On the grounds that sperm, being on a kamikaze mission, are unlikely to be over-endowed in the brains department, I prefer the former explanation,” says Dr Lowe. He suggests the discrepancy arises because many of the previous studies have been performed on sperm parts or on dead sperm. His findings are wholly based on the simulation of a live sperm.