It takes more than one sperm to make a baby bird

"Our research shows that, in contrast to humans and other mammals, one sperm is not enough to ensure normal embryo development in birds," says Nicola Hemmings. (Credit: Mario Klingemann/Flickr)

Baby birds don’t develop normally unless multiple sperm penetrate the egg. A new study reveals a functional role for the “extra” sperm in the early stages of embryo development.

This is very different from humans and other mammals where the entry of more than one sperm into an egg is lethal.

Researchers also discovered female birds are able to regulate the number of sperm that make it to the egg, ensuring that sufficient sperm are available for fertilization—particularly when the numbers of inseminated sperm are limited.

The study, led by Nicola Hemmings from the University of Sheffield’s animal and plant sciences department, gives an insight into the biological significance of polyspermy—a major puzzle in reproductive biology.

It has been a long-standing question in the natural world whether the extra sperm that enter a bird’s egg have any role to play in fertilization or early embryo development.

The research shows that when very few sperm penetrate a bird’s egg, the embryo is unlikely to survive.

[This female shark stores sperm for 45 months]

“Our research shows that, in contrast to humans and other mammals, one sperm is not enough to ensure normal embryo development in birds,” says Hemmings.

“When just a single sperm enters the bird egg, fertilization may occur normally, but the resulting embryo will probably die at a very early stage. This is surprising because when more than one sperm enters the human or mammalian egg—a process we call polyspermy—the egg is destroyed.

“Polyspermy has generally been considered to be bad for reproduction, but our results suggest that, in certain animal groups, polyspermy may in fact be necessary.”

The research appears in the Proceedings of the Royal Society B.

Source: University of Sheffield