Weddell seals dive hundreds of meters beneath the sea ice surrounding Antarctica. Having caught their prey, or not, they need to get back to the air fast. Even their remarkable physiology can't go without oxygen forever. However, when diving under think ice finding a hole to surface at is a matter of life and death.
The mystery of how they do this may soon be resolved. The favored theory is that they're using the Earth's magnetic field to guide them, and marine biologists have finally worked out how to test this.
Migratory birds and pigeons are known to use the planetary magnetic field on scales orders of magnitude larger. However, if proved this will be the first time such a capacity has been identified in a marine mammal, potentially offering insight into mass whale strandings.
"These animals are doing a remarkable amount of exercise all while on breath hold," says Professor Terrie Williams of the University of California, Santa Cruz. "The reason a seal wants to be efficient is that they have a limited amount of oxygen onboard. The trick is conserving that 'scuba tank' on a dive."
Williams has been working on the idea that Weddell seals use magnetic field lines to orientate themselves since the late 1990s. Dr Lee Fuiman of the University of Texas, has collaborated with Williams over that time, and describes their reasoning from the first time they tracked the seals' movements. "The animal always found its way back. It's like he knew exactly where the hole was," Fuiman said. "I couldn't figure out how they would do that. How did they know where they were by the time they turned around?"
In summer the seals might look for light streaming through holes in the ice, but the seals also dive during the long Antarctic winter when the sun never rises.
Credit: Peter Rejcek, NSF. Weddell seals are cute but lazy on land. However, under water they become remarkable athletes.
Magnetic fields was an obvious theory, so to test it the team mapped the magnetic field in different parts of McMurdo Sound. Filed lines are not uniform, owing to the influence of metal deposits. The team reasoned that, if their theory was right, as the seals moved into areas with different magnetic fields they should change their behavior in some way.
With funding from the National Science Foundation the team have attached infrared video recorders and sensors to the seals' heads and placed them in areas with different magnetic fields. While promising, the data they have collected so far does not provide a sufficient sample size to confirm the magnetic theory with certainty.
Next southern winter the team are heading back to McMurdo to gain a larger sample size, with the study conducted in darkness so they can be sure the seals aren't “cheating” by using sunlight as might have been the case in the studies done so far. Directional hydrophones will test the possibility of acoustic clues. It is anticipated three years of research will be required to firmly establish the theory.
Credit: Story by Peter Rejcek/Produced by Ralph Maestas/Additional footage provided by Randall Davis