For bird sperm, size (not speed) matters most

"Long sperm win at sperm competition by fertilizing more eggs, even when short sperm are given a head-start," says Clair Bennison. (Credit: Cathy/Flickr)

Successful egg fertilization appears to be more than just a “race to the egg.” New research shows that sperm length is more influential than sperm speed for zebra finches.

Researchers carried out sperm competition experiments between pairs of males, where one male consistently produced long sperm and the other male always produced short sperm.


The experiments showed that more long sperm reached and fertilized eggs compared to short sperm. The advantage held true even when short sperm males were given a “head start” by being able to mate with the females first.

The findings, published in the Proceedings of the Royal Society B, demonstrate that when birds are in competition with other birds, fertilization success is at least partly dependent on the length of his sperm. The results also suggest that female birds have some say in the outcome.

“We know that in the zebra finch, long sperm swim faster than short sperm, so we might expect longer, faster swimming sperm to simply reach the egg first,” says Clair Bennison of the animal and plant sciences department at University of Sheffield.

“However, this reasoning does not explain why long sperm outcompete short sperm in our study. Long sperm win at sperm competition by fertilizing more eggs, even when short sperm are given a head-start.”

Lady’s choice?

Scientists allowed each pair of male zebra finches to mate with a female bird so that the long and short sperm from the males could compete to fertilize the female’s eggs.

Female birds store sperm inside their bodies for many days, and this is one way that the females themselves can influence the fertilization success of the males. It’s possible that long sperm are better at reaching and and staying inside these storage areas than short sperm. Long sperm may even be “preferred” by the female, by some unknown process.

“Our findings are important because they demonstrate for the first time in birds, using a controlled competitive scenario, that sperm length can influence the fertilization success of a particular male, Bennison says.

“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 that the chance of a male siring offspring may not be an outcome of a simple ‘race to the egg.'”

Scientists believe that a better understanding of how sperm length influences fertilization success in non-human animals such as the zebra finch may lead to new directions for investigation in human fertility research.

Researchers now plan to investigate if sperm storage duration in female birds varies according to the length of the male’s sperm, and the possible mechanisms responsible for this.

Source: University of Sheffield