Long haul

Abstract

Researchers have tracked the world's longest mammalian migration from Antarctica to the tropics. Nigel Williams reports. Researchers have tracked the world's longest mammalian migration from Antarctica to the tropics. Nigel Williams reports. Birds do it, fish do it and even some insects do it: seasonal, continent-scale migrations. Mammals appear much less adapted to move over such great distances, although whales undertake substantial annual journeys. Now new research suggests that the humpback whale of the Eastern Pacific travels further than any previously documented mammal — moving up and down the south-east Pacific. Researchers have shown that humpback whales travelling between grounds off the west coast of central America and feeding grounds in Antarctica clocked up more than 8,000 kilometres on one leg of the journey — the longest recorded for any individual mammal. The researchers believe the whales are heading for warmer waters in which to give birth to their calves. Mark Simmonds, director of science at the Whale and Dolphin Conservation Society, who was not involved in the work, said: “It has this extraordinary lifestyle with its breeding grounds in one place and its feeding grounds far away.” Kristin Rasmussen at the Cascadia Research Collective in Olympia, Washington, and her colleagues made daily excursions in small boats off the coast of Central America to observe the whales between June and October. They took photographs of the underside of the animals' tails so that they could be identified at the other end of their journey. “Just like humans have unique fingerprints, whales have unique tail markings,” said Rasmussen. During the Antarctic summer, the team made similar observations and looked for whales which had been spotted in the tropical waters. Seven animals were photographed in both locations and a mother and calf pair were seen in Antarctic waters after they had earlier been spotted off Costa Rica, having travelled 8,000 kilometres. Another individual was seen in different locations also 8,000 kilometres apart. The study is reported in Biology Letters (published online). The study ends the controversy over which whale species travels the furthest. The grey whale's migration from Mexico to the Arctic is also an impressive voyage. Blue whales — the largest animals on the planet — also migrate thousands of miles, with the North Atlantic population moving from Arctic feeding grounds to waters off the eastern coast of the US. But none of these animals has been shown to travel as far as humpbacks. The humpback whales studied cross the equator on their journey from the Antarctic to reach their preferred breeding grounds. To work out why they go so far, the team looked at satellite measures of sea surface temperatures for the wintering grounds of 24 humpback whale populations around the world. In all cases, the animals were opting for waters between 24 and 25°C. The whales in this study would have had to continue north of the Equator to find water this warm. Rasmussen believes that a high enough water temperature is crucial for them to breed. “It's likely that being in warm water is somehow beneficial to the calf”, she says. The researchers sighted a total of 207 whales off Central America in 2001–2004 winter surveys. Mother–calf pairs, groups of competing males and singing males were all recorded. Forty-one whales were individually identified photographically: of these, seven were also photographed off the Antarctic peninsula. Three of these whales were seen within the same year: one off Costa Rica 262 days after it was seen in Antarctica and two — a mother and calf pair — off Antarctica 161 days after being seen off Costa Rica. The researchers calculated that the distances between these two sites ranged from 8300 kilometres to 8460 kilometres. The researchers also found that the wintering areas off Central America were the chosen favourite locations for northern humpback whales migrating from the Arctic. “Eastern North and South pacific populations share genetic traits indicating a trans-equatorial exchange, probably off Central America,” the authors write. “Our analysis shows that worldwide humpback whale wintering areas are found in warm coastal waters irrespective of latitude,” the authors write. “In the ongoing debate on the reasons for migration, this result supports previous ideas linking temperature at the wintering area to energetic strategies.” The authors suggest that, as in terrestrial mammals, energy conserved during offspring development can be devoted to growth, leading to larger size and increased reproductive success in adulthood.