The hammerhead shark, captured off the Florida Keys, had been held with other females for three years when she had the pup, Paulo Prodohl of Queen's University Belfast in Northern Ireland and colleagues reported in the journal Biology Letters.

The pup was killed by a stingray soon after birth and the researchers studied its tissue. They found no evidence of male input.

Parthenogenesis occurs when a female egg cell develops into an embryo on its own, without sperm. It has been seen in a number of species, notably bees.

"Parthenogenesis has been documented in all major jawed vertebrate lineages except mammals" and sharks, the researchers wrote in their report.

"Reports of captive female sharks giving birth despite being held in the extended absence of males have generally been ascribed to prior matings coupled with long-term sperm storage by the females."

But the three female sharks "had been held in the absence of males for 3 years, since they were wild caught in the Florida Keys as immature animals less than 1 year old," the researchers wrote.

"At least 2 years away from the age of first maturity, it is improbable that they were capable of sexual activity and sperm storage prior to capture."