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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