A humpback whale has travelled the longest distance recorded for any mammal, swimming across nearly a quarter of the globe between two breeding grounds.
The female whale's incredible journey began off the coast of Brazil and ended at Madagascar, almost 6,200 miles away. It was photographed there two years after first being identified by researchers on Abrolhos Bank, Brazil, in August 1999.
Scientists do not know whether the animal's epic trip was deliberate or the result of a navigational accident. Humpbacks are known for long distance migrations, regularly travelling 3,100 miles between their breeding and feeding grounds. But these journeys generally take them north and south, not east or west.
Such a long trip between breeding areas is unexpected in a female, as it is more normal for male mammals to wander in search of mates. The whale, known by staff the Antarctic Humpback Whale Catalogue (AHWC) as "No 1363", was recognised by its distinctive markings.
She was originally one of a pair observed for about an hour off the Brazilian coast. The animal then disappeared until being photographed on 21 September 2001 from a commercial "whale watch" tour vessel. Researchers led by Dr Peter Stevick of the AHWC, based in Maine, said the distance travelled by the whale was the "longest documented movement by a mammal".
Writing in the Royal Society's journal Biology Letters, he added: "Movement of an individual between breeding areas separated by approximately 90 longitudinal degrees, a continent, an ocean basin and nearly 10,000 kilometres illustrates the ability of humpback whales to range across large portions of the globe."
What led to the feat was uncertain. Humpbacks tended to be attached to particular breeding areas and long-distance movement between different sites us rare. This leads to restricted gene flow and relatively distinct breeding populations. Previously recorded movements between humpback whale breeding grounds had been made by males. Dr Stevick said extremely long travel by whales could result from exploring new habitats or a "navigational miscue".
He said: "Humpback whales may make large longitudinal movements while feeding at high latitudes, especially in changeable or unpredictable circumstances. Such movement may occur through tracking prey, exploring potential foraging sites or... through drift with the... current."