Stiff competition: size matters

Are longer sperm more successful than shorter sperm in the race for life? This is one of the questions Dr. Matthew Gage (University of East Anglia) will be addressing on Wednesday 31st March 2004 at the annual SEB meeting in Edinburgh (29th March – 2nd April 2004). Dr. Gage will present evidence that sperm speed, size and shape all matter when it comes to male success in sperm competition.

Sperm competition is a widespread phenomenon that occurs when sperm of more than one male compete to fertilize a female’s egg. Even scientists suspect that size matters, claiming that longer sperm can achieve greater swimming velocity than shorter sperm and therefore get to the egg faster. However, recent research by Dr. Gage on Atlantic salmon suggests that longer sperm do not necessarily do better. Instead, the crucial determinant of male success is simply relative sperm velocity. “For salmon at least, sperm competition really is a race where timing is everything” says Dr. Gage. His research on field crickets also indicates that bigger isn’t necessarily better, since decreased sperm size and increased sperm number independently predict male fertilization success. “Rather than invest in a small number of monster sized sperm, male field crickets are better off producing astronomical numbers of miniaturised sperm” explains Dr. Gage.

However, it’s not just about quantity, quality matters too! More recently, Dr. Gage has investigated the relationship between inbreeding and sperm quality. “Inbred rabbits have higher numbers of abnormal sperm than rabbits with access to a larger number of potential mates” says Dr. Gage “which provides some of the first evidence that inbreeding leads to lower quality sperm”.