Female birds select sperm "super swimmers"

Sperm with shorter heads and longer tails are better at fertilising bird eggs and the "specific design" of a sperm is more important than its total length to reach the egg, according to a research published today that negates the previously-held notion.
According to the new research by the University of Sheffield in the UK, sperm with specific "looks" are selected to fertilise bird eggs, an advance which may produce clues to understanding human fertility.
The research found that fewer than one per cent of inseminated sperm reach the egg and a distinctive subset of sperm are selected inside the female for fertilisation.

"Previously it was thought that longer sperm were always more likely to reach the egg. However our study shows that the specific design of sperm is more important than total length alone," said Nicola Hemmings from the Department of Animal and Plant Sciences.
"Long tails act as propellers for sperm, but beyond a certain length they seem to become less efficient. Shorter heads are also important as they have less surface area to create drag, allowing sperm to swim faster. The tiny subset of "super swimmers" with these traits are better able to reach the egg," Hemmings said.
Examining zebra finches, scientists found that these "super swimmers" tend to have shorter heads with longer tails and are more similar to each other than other inseminated sperm.

Scientists believe that these traits help the sperm swim faster through the vagina. The bird then stores these sperm before giving them the chance to fertilise her egg.
The research is published todayin the Royal Society journal Biology Letters.
Scientists believe that a better understanding of how the shape of sperm and size influences fertilisation success in animals such as the zebra may point us in new directions for investigation in human fertility research.