Female sharks don't need males to reproduce, study finds

DUBLIN -- Female sharks can fertilize their own eggs and give birth without sperm from males, according to a new study of the asexual reproduction of a hammerhead in a US zoo.

The joint Northern Ireland-US research, published today in the Royal Society's Biology Letter journal, analyzed the DNA of a shark born in 2001 in the Henry Doorly Zoo in Omaha . The shark was born in a tank with three potential mothers, none of whom had had contact with a male hammerhead for at least three years.

Analysis of the baby shark's DNA found no trace of any chromosomal contribution from a male partner. Shark specialists said this was the first confirmed case in a shark of parthenogenesis, a word derived from Greek that means "virgin birth."

Asexual reproduction is common in some insect species, more rare in reptiles and fish, and has never been documented in mammals. The list of animals documented as capable of the feat has grown along with the numbers being raised in captivity -- but until now, sharks were not considered a likely candidate.

Before the study, many shark specialists had presumed that the Nebraska birth involved a female shark's well-documented ability to store sperm for a lengthy period of time. Doing this for six months is common, but three years would be exceptional, they said.

The lack of any paternal DNA in the baby shark ruled out this possibility.

"We were all very skeptical about these reports, about the possibility of a so-called virgin birth in a shark, because sharks have this unusual ability to store sperm for months if not years. So this finding is new and definitely unexpected," said Bob Hueter, director of the Center for Shark Research at Mote Marine Laboratory in Sarasota, Fla., who wasn't involved in the project.

A co-author of the report, Mahmood Shivji of the Guy Harvey Research Institute in Dania Beach, Fla., said the finding explained the growing reports of mysterious male-free shark births in captivity.