Long sperm swim faster, fertilise more eggs

Even when short sperm is given a head-start, long sperm wins the competition comfortably by fertilising more eggs, a study has observed.

According to scientists from University of Sheffield, the chance of a male fathering an offspring may not be a simple race of his sperm to the egg but is influenced by the length of sperm.

"The results demonstrate for the first time in birds that sperm length can influence the fertilisation success of a particular male," said Clair Bennison from the university's department of animal and plant sciences.

The results also add to the body of evidence suggesting that the final outcome of sperm competition may be partly dependent on the female and the chance of a male siring an offspring may not be an outcome of a simple "race to the egg".

For the research, using captive zebra finches, the team carried out sperm competition experiments between pairs of males, where one male consistently produced long sperm and the other male produced short sperm.

These experiments showed more long sperm reached and fertilised the eggs compared to short sperm.

The long sperm advantage was evident even when the short sperm males mated with the females first and were effectively given a "head start".

Scientists believe that a better understanding of how sperm length influences fertilisation success in non-human creatures such as the zebra finch may point us in new directions for investigation in human fertility research.

The paper is forthcoming in in the journal Proceedings of the Royal Society B.