Killer whales migrate, study finds, but why?

Oct 25, 2011 by Marlowe Hood
Some killer whales, a study published Wednesday shows for the first time, wander nearly 10,000 kilometres (6,200 miles) from Antarctica's Southern Ocean into tropical waters -- but not to feed or breed.

Some killer whales, a study published Wednesday shows for the first time, wander nearly 10,000 kilometres (6,200 miles) from Antarctica's Southern Ocean into tropical waters -- but not to feed or breed.

Rather, these fearsome predators at the apex of the traverse the sea at top speed -- slowing as they reach warmer climes -- to exfoliate, the study speculates.

They are driven, in other words, by the urge or need to make their skin all shiny and new.

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, John Durban and Robert Pitman of the US fitted a dozen so-called "type B" killer whales off the west coast of the with .

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 (15 to 50 feet).

"Type B" orcas inhabit the inshore waters of Antarctica near pack ice, the better to feed on seals and penguins. Type A killer whales prefer open water and a diet of , and the smaller, fish-eating type C is most common in the eastern Antarctic.

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/hr (six mph), across the southwest Atlantic east of the to the subtropical waters off the coasts of Uruguay and southern Brazil.

The study, published in the British Royal Society's journal Biology Letters, provides the first direct evidence of long-distance migration by killer whales.

But why they do it remains something of a mystery.

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 (5,840 mile) 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.

Which is where skin comes into the picture.

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 (28.6 degree Fahrenheit) may be dangerous, even lethal.

Surface temperatures at the killer whales' tropical destinations, by contrast, were a balmy 20.9 to 24.2 C (69.6 to 75.6 F).

"We hypothesise that these migrations were thermally motivated," the authors conclude.

(Orcinus orca) are the most widely distributed cetacean -- and perhaps mammal species -- in the world.

Explore further: A step into the unmown creates a 'win-win' for wildlife and humans

add to favorites email to friend print save as pdf

Related Stories

Blainville’s beaked whales go silent at the surface

Jul 27, 2011

A new study published in the journal Marine Mammal Science revealed how Blainville’s beaked whales go completely silent in an apparent stealth mode when they near the surface in an effort to avoid predators.

Marine scientists monitor longest mammal migration

Apr 10, 2007

Marine scientists recently published a research paper in the science journal, Biology Letters, that found humpback whales migrate over 5,100 miles from Central America to their feeding grounds off Antarctica; a record distan ...

Recommended for you

Research helps steer mites from bees

2 hours ago

A Simon Fraser University chemistry professor has found a way to sway mites from their damaging effects on bees that care and feed the all-important queen bee.

Bird brains more precise than humans'

2 hours ago

(Phys.org) —Birds have been found to display superior judgement of their body width compared to humans, in research to help design autonomous aircraft navigation systems.

User comments : 2

Adjust slider to filter visible comments by rank

Display comments: newest first

MrVibrating
5 / 5 (2) Oct 25, 2011
...so not so much a migration as a holiday then..!
Cynical1
not rated yet Oct 26, 2011
It's why I would migrate to the tropics for a week or so...