Size Matters, Not Speed, For Bird Sperm

Fertilization has always been seen as a race to the egg, where only the fittest and fastest sperm will be ever be able to naturally lead to new life. However, a new study has revealed that in the case of birds, this may not be the case. Instead, the longest and biggest avian sperm seems to get eggs "cooking," so to speak.

The study, recently published in the journal Proceedings of the Royal Society B, details how avian fertilization in zebra finches appears heavily dependent on sperm length and size, and this is a factor that may apply to most bird species.

"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," researcher Clair Bennison explained in a recent statement through the 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."

So how could anyone determine this in the first place? According to the study, Bennison and her colleagues let male finches boasting short sperm and finches boasting long sperm both have a chance to mate with a single female finch. These feathered females can store sperm inside their bodies for many days in order to increase their chances of a successful fertilization. They then ran tests to determine which sperm "won" in the end.

Interestingly, the team also found that larger sperm may last longer stored away compared to their shorter counterparts, giving them even more of an edge in the fertilization game.

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

Experts believe that better understanding fertilization drivers in animals can help in rare species management and breeding programs, and also may lead to new direction for investigation in human fertility research.

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