An eight-year project that tracked humpback whale migrations by satellite shows the huge mammals follow uncannily straight paths for weeks at a time.

The results suggest a single migratory mechanism isn't responsible. Instead, humpbacks may use a combination of the sun's position, Earth's magnetism and even star maps to guide their 10,000-mile journeys.

“Humpback whales are going across some of most turbulent waters in the world, yet they keep going straight," said environmental scientist Travis Horton of the University of Canterbury, whose team will publish their findings April 20 in Biology Letters. "They're orienting with something outside of themselves, not something internal."

Humpback whales feed during the summer near polar oceans and migrate to warm tropical oceans for the winter, where they mate and calves are born. A one-way trip can last upwards of 5,000 miles, making the cetaceans one of the farthest-migrating animals on Earth. (One was tracked migrating 6,200 miles).

To better understand humpback migrations, Horton's colleagues embedded satellite tags in seven South Atlantic and nine South Pacific whales from 2003 through 2010.

Each tag contained a battery-operated transponder that beamed its location to the researchers. The tags lasted from four weeks to seven months before falling out; altogether, they provided one of the most detailed sets of long-term migratory data for humpbacks ever collected.

“You can’t stick a large whale in a box like you can with a bird to study its migratory behavior. This is why detailed field data on whales is so important,” said research biologist John Calambokidis of the Cascadia Research Collective, who wasn’t involved in the work.

The researchers found that, despite surface currents, storms and other distractions, the humpbacks never deviated more than about 5 degrees from their migratory courses.

In about half the segments mapped by the researchers, humpbacks deviated by one degree or less.

“When we first starting seeing data, we thought, ‘Wow, these are really, really straight paths,’” said marine biologist Alex Zerbini of the National Oceanic and Atmospheric Administration, a co-author of the study who led the satellite-tracking effort. “We immediately wondered how they accomplish that.”

Decades of research on long-range animal migrations has identified geomagnetic and sun-tracking mechanisms, but that work focuses primarily on birds. Humpbacks don’t seem to rely on either method alone.

Earth's magnetism varies too widely to explain the whales’ arrow-straight patterns, and solar navigation requires frames of reference that water doesn't often provide. "The open ocean is an endless horizon of blue," Horton said.

Horton suspects humpbacks rely on both mechanisms, and perhaps the position of the moon or stars. His team is preparing to submit a second study on reference frames in marine mammals, birds, fish and reptiles. After publishing that work, Horton hopes to further investigate the humpbacks' abilities.

Calambokidis suggested a fourth mechanism for steering: long-distance songs that can carry for hundreds or thousands of miles underwater, and may provide navigational cues or help migrating whales coordinate their movements.

"These whales are clearly using something more sophisticated to migrate than anything we’ve surmised,” said Calambokidis. "I’m really looking forward to seeing what this team does next.”

Image: 1) A humpback whale swimming underwater (gwoodford/Flickr). 2) Satellite maps of nine humpback whales' migratory patterns in the South Atlantic. Triangles represent migrations in 2008 while circles represent migrations in 2009 (Biology Letters) 3) Using a long fiberglass pole, researchers tag a humpback whale for satellite tracking (Paul Hilton).

Citation: “Straight as an arrow: humpback whales swim constant course tracks during long-distance migration.” Travis W. Horton, Richard N. Holdaway, Alexandre N. Zerbini, Nan Hauser, Claire Garrigue, Artur Andriolo and Phillip J. Clapham. Biology Letters. Published online April 20, 2011. DOI: 10.1098/rsbl.2011.0279

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