Southern Whale Migration Overlapping Northern Whale Breeding Grounds

Kelly Hearn
for National Geographic News

April 20, 2007

Humpback whales, known for their haunting songs and arduous migrations, are the ocean's great wanderers.

The whales undertake the world's longest mammalian migration, but their motivations are poorly understood.

Scientists have assumed that the animals are driven by a mix of reproductive needs and a desire for warm water, which saves energy and thus helps calves grow larger.

Now, data from whales wintering off the Pacific coast of Central America is backing up assumptions that the marine mammals go on the move mostly in search of warmer waters (see photos of humpback whales).

What's more, the study suggests that cold-water patterns are forcing southern humpback whales into the breeding grounds of their northern cousins for the first time, raising the possibility of genetic mingling.

"We have shown that, at the basin scale, humpback whales are choosing their wintering areas based on warm water temperatures," said Kristin Rasmussen, a study co-author and researcher at Cascadia Research Collective.

"While this had always been the assumption, it had never before been quantified globally."

Changing patterns of sea-surface temperatures could lead different whale populations to mix and mingle, potentially altering the whales' gene pool and even leading to changes in a given population's song patterns.

(Related news: "Humpback Whale Calls Are Love Songs, Biologist Suggests" [September 15, 2006].)

Whale Crossing

Humpback whales are divided into Northern and Southern Hemisphere populations.

Both groups of whales feed in one place and breed in another, sometimes crossing thousands of ocean miles in between.

Rasmussen and colleagues documented seven individuals from Southern Hemisphere populations for their study, which appeared April 3 in the online journal Biology Letters.

Their subjects included a mother and calf that swam nearly 5,160 miles (8,300 kilometers) from feeding areas off Antarctica up to wintering areas off Central America's Pacific Coast.

Surprisingly, the whales went so far north that they crossed into breeding areas used by Northern Hemisphere populations.

"Whales were observed as far north as 11 degrees North of the Equator off Costa Rica, in an area also used by a [northern] population during the opposite winter season, resulting in unique spatial overlap between Northern and Southern Hemisphere populations," the team wrote in their report (map of Central America).

The researchers suggest that cold-water patterns were at play.

"Such a northerly wintering area is coincident with the development of an equatorial tongue of cold water in the eastern South Pacific, a pattern that is repeated in the eastern South Atlantic," the team wrote.

Rasmussen said the trend is not linked to global warming and that the territorial overlap is unlikely to cause resource competition among the different populations.

"This is a case where whales from two different populations use one area to breed, but they do not occupy the area at the same time of the year, so there is no direct competition for resources," she said.

Phillip Clapham, a whale expert at the National Oceanic and Atmospheric Administration, said the study is significant for showing links between water temperature and humpback calving grounds.

He also cited the potential for populations to overlap with genetic consequences.

"The migration across the Equator and far up into northern waters makes it likely that there is some genetic exchange between the two hemispheric populations of humpback whales," he said.

"Whales from the south may well overlap, and perhaps mate, with those from the Northern Hemisphere at the tail ends of their respective breeding seasons, thus promoting gene flow across the Equator."

He also raised the possibility that the humpback whales' songs—which are unique to a population—might change when exposed to another population in a common breeding ground.

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