Scientists have observed killer whales making an epic 10,000 km journey from icy Antarctic waters into tropical waters near Brazil, where they can safely shed and regenerate a layer of skin.
Credit: Wikimedia/U.S. National Oceanic and Atmospheric Administration
PARIS: Using satellite transmitters, scientists have observed killer whales making a 10,000 km migration from Antarctica into tropical waters, where they can safely shed and regenerate a layer of skin.
Published this week in the British Royal Society's journal Biology Letters, the study provides the first direct evidence of long-distance migration by killer whales (Orcinus orca), which travelled at top-speed, slowing only as they hit warmer waters.
"Our tagged whales followed the most direct path to the nearest warm waters north of the subtropical convergence, with a gradual slowing of swim speed in progressively warmer water," noted authors and biologists John Durban and Robert Pitman from the U.S. National Marine Fisheries Service.
But rather than making the journey for food, or to reproduce, the authors speculate that these fearsome predators - at the apex of the marine food chain - are driven by an urge to exfoliate.
"We suggest that these movements may represent periodic maintenance migrations, with warmer waters
allowing skin regeneration without the high cost of heat loss," they said.
Evidence for migration
Despite our intense fascination with seal-chomping orcas, next to nothing was known about their long-haul movements, or whether they migrate at all.
To find out more, Durban and Pitman fitted a dozen so-called "type B" killer whales off the west coast of the Antarctic Peninsula with satellite transmitters.
In January 2009, the scientists used bolt-shooting crossbows to attached tags to the five-tonne mammals' dorsal fins from a distance of five to 15 metres.
'Type B' orcas inhabit the inshore waters of Antarctica near pack ice, which is the preferential for feeding on seals and penguins. Type A killer whales prefer open water and a diet of minke whales, and the smaller, fish-eating type C is most common in the eastern Antarctic.
Remarkable voyage
Half the satellite tags stopped working after three weeks, but the remaining six revealed a remarkable and unexpected wanderlust over the following two years.
"Our tagged whales followed the most direct path to the nearest warm waters north of the subtropical convergence, with a gradual slowing of swim speed in progressively warmer water," the authors note.
The whales made a beeline, cruising at up to 10 km/hour across the southwest Atlantic east of the Falkland Islands to the subtropical waters off the coasts of Uruguay and southern Brazil.
The reason behind the migration is not fully understood, but scientists have ruled out a couple of 'usual suspects' for animal migration.
The speed and duration of the voyages, undertaken individually, did not leave enough time for prolonged foraging, and would have been too demanding for a new-born calf. "Remarkably, one whale returned to Antarctica after completing a 9,400 kilometre trip in just 42 days," the study said.
The varied departure dates, between early February and late April, also suggested these expeditions were not annual migrations for feeding or breeding.
The skin hypothesis
Durban and Pitman suspect that killer whales move into warmer waters in order to shed a layer - along with an encrustation of single-celled algae called diatoms - without freezing to death.
Orcas are the smallest cetaceans - a group including whales and dolphins - which live for extended periods in subzero Antarctic waters. Replacing and repairing outer skin in waters where the surface temperature is minus 1.9 degree Celsius may be dangerous, even lethal.
Surface temperatures at the killer whales' tropical destinations, by contrast, were a balmy 20.9 to 24.2 degree Celsius. "We hypothesise that these migrations were thermally motivated," the authors conclude.
Mammal species.
"Killer whales are the most widely distributed cetacean - and perhaps mammal species - in the world."
Given the distribution of your readership, you might want to fact check that. :)
-- Erik Baard, New York City