The Sea’s Astronomers

In a matter of weeks, one of the planet’s great migrations will begin as humpback whales set off from cool polar feeding grounds and swim towards tropical waters. With trajectories that can exceed 5,000 miles in a single direction, their voyage is the longest-known of any mammal’s on Earth.

Astoundingly, the largest humpback tracking effort ever undertaken reveals that, despite traversing enormous distances, these ocean behemoths navigate arrow-straight paths.

In 2003, a group of researchers led by Travis Horton and Richard Holdaway of the University of Canterbury, New Zealand, fitted seven South Atlantic humpback whales with radio tags. In 2005, they placed another nine on whales in the South Pacific. Each tag was eight to 12 inches long, embedded with a battery-operated transponder, and designed to fall off of the animal after a period of up to seven months.

Over the course of eight years, the team monitored the migratory movements of the whales using data collected from polar-orbiting satellites. They found that although the whales’ courses took them across vast distances of open ocean, the humpbacks never deviated by more than five percent from their linear paths. In over half of the trajectories logged by the study, the whales strayed by only one percent or less.

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

These findings are particularly remarkable considering the strength of the sea-surface currents through which the humpbacks traveled, undeflected. In one case, some even passed through a rare tropical storm off of the coast of Brazil. Using drifter-buoys placed along the expected migration corridors, the research team discovered that even if the sea currents rerouted a whale’s heading by as much as 25 percent, the humpbacks would compensate and reestablish a nearly impeccable path.

The longest voyage monitored by the team was completed by a whale that swam 1,386 miles over 28 days and strayed only 0.4 degrees from its course.

“Humpback whales are going across some of the most turbulent waters in the world, yet they keep going straight,” said Horton. “They’re orienting with something outside of themselves, not something internal.”

Many animals, from loggerhead turtles to pigeons to tiger sharks, navigate long distances by attuning themselves to Earth’s magnetic field. In other instances, species of honeybees and fish will take cues from the sun’s passage across the sky.

It seems unlikely, however, that humpback whales rely on either of these methods alone to determine their course. The migratory trails mapped in the New Zealand study revealed that each whale passed through many varying undulations of the Earth’s magnetic field. Furthermore, whales followed different headings even though they saw the sun from similar angles, suggesting that solar positioning is not an essential factor in their navigation.

“These whales are clearly using something more sophisticated to migrate than anything we’ve surmised,” said John Calambokidis from the Cascadia Research Collective.

Although Horton recognizes that whales could use the sun and magnetic field in conjunction with each other, he remains unconvinced that even a combination of these two methods offers a sufficient explanation for the humpbacks’ extraordinarily precise migrations. Instead, he proposes that they may orient themselves by means of a different external mechanism: the position of the moon and the stars.

Yet, given that the bulk of migratory research has historically focused on birds and land animals, the question of just how animals navigate underwater remains enigmatic.

“We have a reasonable handle now on what they are doing,” Horton said, “but very little information on how they are doing it.”

He hopes to expand upon his team’s initial findings by gathering more data and exploring alternate mechanisms of migratory orientation.

Featured Photo courtesy of Chensiyuan.