Picking Up Mates at the White Shark Cafe

Nov. 7, 2009— -- Great whites aren't all alike. Even though the sharks travel all over the Pacific Ocean to hunt, they tend to mate with others from the same area, forming genetically distinct groups.

That's what local great whites revealed to Barbara Block of Stanford University in California and her colleagues.

The team headed out into the Pacific to find the sharks, which they lured to the surface using a silhouette of a seal. They then used a pole to attach two different tags to the sharks and took a sneaky biopsy at the same time.

GPS tags were used to track the long-distance movements of the creatures, allowing the team to follow their migration during the colder months from coastal areas to the deep ocean.

The other tags gave off sonic "pings" that were picked up by sensors moored in coastal areas, providing more precise location fixes than the satellite measurements, so that the team could tell if the sharks returned to the same areas.

Block noted that the sharks tended to follow a predictable, if mysterious route every year.

"They go to an area we call 'the white shark café'," she says. Why this is such a cool place to hang out isn't clear, however.

The tags revealed that the females weave in and out of the males, and because there doesn't seem to be much in the way of food at the café 2000 kilometres off the California coast, Block thinks they may be mating.

The tags also confirmed the team's theory that the sharks return to specific coastal sites in the summer. "These animals aren't just wandering aimlessly through the sea," says Block. "They seem to maintain neighbourhoods."

Australian cousins

The tissue samples taken from the sharks showed that their mitochondrial DNA, which is inherited from the mother, was significantly different from that of great whites from the south-west Pacific.

The group reckon that great white sharks originated around Australia and New Zealand and formed a separate group on the other side of the Pacific around 200,000 years ago, during the late Pleistocene epoch.