BioEd Online

On one of the Earth's most remote islands, mice have learned, and are apparently teaching each other, how to attack and kill bird chicks that are 200 times their size.

Far from exulting in the cleverness of mice, the researchers who discovered this want to eradicate the rodents from the island in order to save endangered albatrosses.

Biologists on Gough Island, a speck in the Atlantic between the southern tips of Africa and South America, first learned of the problem when they found that tristan albatrosses (Diomedea dabbenena) were losing their chicks at an extremely high rate: up to 80% were dying.

Researchers suspected that house mice, which were accidentally introduced to the island, might be the culprits. So husband-and-wife team Ross Wanless and Andrea Angel spent a year on the island videotaping birds' nests and collecting data.

Pick on someone your own size

The videos confirm that mice are taking on the chicks, biting them over and over until they die from loss of blood or infection. Wanless, an invasive-species biologist from the Percy FitzPatrick Institute of African Ornithology at the University of Cape Town, South Africa, vividly recalls watching the first videos. "It was carnage. Chicks half alive, with massive gaping wounds and guts hanging out."

The mice are able to defeat the much larger birds by biting the same spot over and over ( see video). They take advantage of the fact that the birds, which have evolved in an area that has been without land predators for millions of years, have no defensive response against such attacks.

The results were presented this week at the annual meeting for the Society for Conservation Biology in Brasília, Brazil.

Trained killers

Wanless was surprised by the results. Such behaviour is unprecedented in mice, he says. And, oddly, the attacks only take place on some of the island's peaks, despite the fact that the mice live everywhere on the island.

The research duo chose two sites for further inspection that had radically different death rates for chicks. They found the same vegetation, altitude, slope, numbers of mice and albatross nests at each site. But one group of mice attacked chicks and the other did not. From this the team infers that the attack is a learned behaviour.

The transmission of learned skills from one generation to the next is a relatively rare phenomenon, and not one seen in mice in the wild before. The researchers note that it is particularly surprising in this case because only a few mice from each brood would be expected to live through a winter.

Wanless and Angel are now determined to save the albatrosses by removing the mice. But they warn that similar attacks might be threatening other bird species.

"This is probably not unique to Gough," says Wanless. "It is just that nobody has looked."