Relocated Sea Snakes Cross Seas to Go Home

John Pickrell
for National Geographic News
October 1, 2002
Plans to restore some locally extinct populations of sea snakes by relocating the reptiles from neighboring islands may be doomed to fail, researchers report. Like homing pigeons, some types of amphibious snakes have an unstoppable urge to return home.

The Laticaudidae family of sea snakes can be found around islands and reefs in the Pacific Ocean and are not endangered. However, in some areas, snakes such as the yellow-lipped sea krait, Laticauda colubrina, have been heavily exploited by the international leather industry. Over-harvesting has caused local extinctions on some Japanese and Filipino islands.

But an attempt to move the sea kraits living on one island in Fiji to another failed because the relocated sea snakes kept returning to their original home, scientists report in the October issue of the journal Conservation Biology.

Tourism and Sea Snakes

Laticaudid sea snakes are not true sea snakes. Though they forage in the ocean for moray and conger eels, they return to shore to digest prey, shed skins, mate, and lay eggs. True sea snakes—species in the family Hydrophiidae—never go ashore, and give birth to live young out at sea.

Both groups of amphibious reptiles must return to the ocean's surface to breathe air. Scientists believe that historically the groups invaded the ocean separately, evolving adaptations to marine life independently.

Conservation biologists Sohan Shetty at Nanyang Technological University in Singapore and Richard Shine at the University of Sydney in Australia were studying populations of yellow-lipped sea kraits on the rocky, uninhabited island of Mabualau when they were approached by resort owners from the nearby isle of Toberua.

The resort owners wanted Toberua to be sea snake-free, but didn't want to kill the animals.

Despite the fact that they tote venom powerful enough to kill a man with one bite, the snakes have a docile nature and are highly tolerant of people, said Shetty. However, though they'd never attacked anyone, the snakes were affecting tourism on Toberua.

"The snakes would surface at night, scare some of the guests, and then seek shelter in the vegetation," said Shetty. Some travel agents refused to promote the island because of the problem, he said.

Together, the researchers and resort owners came up with a plan to translocate sea snakes from one island to the other.

In 1998 and 1999 the resort owners captured a total of 328 yellow-lipped sea kraits on Toberua. The snakes were sent to Shetty, who marked and released them on Mabualau. At the same time, Shetty captured, marked, and released 674 sea kraits he found on Mabualau.

Shetty gradually came to realize that many of the snakes marked as having come from Toberua were being sent to Mabualau a second time. The researchers found that most snakes were returning home within a month or so.

"In the end, it turned out that the snakes [originally] marked on Mabualau Island never showed up in the Toberua catch," said Shetty. "None of the Toberua snakes were found on Mabualau either."

This is "pretty strong evidence that translocation isn't going to work over small distances," said Robert Reed, a reptile ecologist at the University of Georgia's Savannah River Ecology Laboratory. "For some reason, these guys really like to go home."

Natural Navigators

Homing behavior is known to occur in other reptiles, such as crocodiles and sea turtles.

"Some turtles migrate 3,000 to 4,000 miles (4,800 to 6,400 kilometers) between nesting beaches and foraging sites," said Reed. "Some types of snake can relocate winter dens [year after year] even if their summer range is several miles away."

There are two types of homing behavior. Some animals are able to find their way home by memorizing landmarks and plotting a mental map of the local area.

Homing behavior seen in turtles, whales, and homing pigeons involves some innate ability to pinpoint a location irrespective of where the animal is. Researchers believe that these animals may be using the positions of stars or the Earth's magnetic field to point them in the right direction.

"We can't tell at this stage which of these two types of homing behavior the sea kraits are displaying," said Reed. However, they range quite widely over these reefs, so it may well be that they know their way around, he said.

It's possible that snakes learn how to recognize home at some early stage in their lives, said Reed. If this is true, he suggested, it might be possible to translocate eggs, instead and circumvent the problem.

Of course, this won't help Toberua's resort owners, who may just have to learn to live alongside the sea kraits.

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