Hammerhead sharks can give birth without mating, study shows

May 23, 2007

According to marine biologists participating in a study between the United States and Ireland, DNA samples taken from a hammerhead shark, born in 2001, located in Omaha Nebraska at the Henry Doorly Zoo, show that the shark's mother performed a "virgin birth," giving birth without mating.

The mother shark was in a tank with three other hammerheads, all female, and the baby shark was also born in the same tank. Tests on the DNA from the baby shark show that there was no "chromosomal contribution" of a male shark present in the blood, something that is required in order for mating to have taken place.

"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 one author of the study from Queen's University located in Belfast, Ireland, Paulo Prodahl.

It was previously thought by researchers that shark had stored sperm to later use as a reproduction procedure, but researchers said that it would have been incredible for the shark to store it for three years. Six months of storage is what researchers would call normal.

"We didn't have a male and had never had a male, and of course, the question was, how did this happen? There were really only two possibilities. One was virgin birth, and the other one was a delayed insemination," said director of the zoo, Lee Simmons.

Researchers say that the study could answer the mystery of why many zoos have said that sharks were giving birth with no males present, but that the event is not necessarily a good thing.

"[This study] may have solved a general mystery about shark reproduction. [The study says sharks can] switch from a sexual to a non-sexual mode of reproduction. [This may not be a good thing because] reduced genetic diversity may occur in the sharks," said another author of the study, Mahmood Shivji, who is located in Dania Beach, Florida at the Guy Harvey Research Institute.

Employees who work at the zoo along with Simmons are glad the mystery is solved saying, "one of the good things about good science is that your skeptics have to eat crow."