A joints team formed from U.S. and Irish scientists discovered that some female sharks are able to reproduce without having sex. The team had studied a mysterious shark birth in Omaha and their findings are published in the British journal Biology Letters.

According to the scientists some female sharks can use a process known as parthenogenesis to reproduce asexually.

"The findings were really surprising because as far as anyone knew, all sharks reproduced only sexually by a male and female mating, requiring the embryo to get DNA from both parents for full development, just like in mammals," said marine biologist Paulo Prodohl of Queen's University of Belfast, Northern Ireland, a co-author of the report.

The team has investigated the birth of a baby shark at Omaha's Henry Doorly Zoo in December 2001. The tank the shark was born in contained only three female hammerhead sharks and none had contact with a male shark since before they reached sexual maturity. Analysis of its DNA found no trace of any chromosomal contribution from a male partner.

Asexual reproduction is common in some insect species, rarer in reptiles and fish, and has never been documented in mammals. Previous to these new findings the scientists explained the birth of the baby shark by prior matings coupled with long-term sperm storage by the females

"We have now demonstrated that sharks are actually able to use an alternative, previously unknown reproductive pathway, which is parthenogenesis," said Mahmood Shivji, one of the paper's authors and director of the Guy Harvey Research Institute.

Hammerheads are one of the most highly evolved types of shark. The wide separation of the eyes gives superior vision. The front of the head between a shark’s eyes is spanned by sense organs which detect changes in pressure and electrical field. In the case of hammerhead sharks, the width of the head makes these senses more acute than those of other sharks. The head also acts as a wing, improving maneuverability. Another attribute of higher evolution is that young hammerhead sharks are nurtured through a placenta inside the mother before birth.