They won't win an Oscar for best performance in an underwater role, but four seals in Antarctica have made a scientific splash by filming themselves in action. Infrared cameras glued to the heads of Weddell seals have provided the first footage of how the animals hunt for fish under the ice. The sleek auteurs use stealth, backlighting of prey, and bursts of bubbles to a surprising degree, according to a report in tomorrow's Science.
Terrestrial biologists can sit in blinds and watch how birds or four-footed predators catch dinner. Not so for marine biologists, who face the barriers of water and ice. Divers and observers in submersibles have caught only fleeting glimpses of marine mammals foraging for prey. One enduring mystery has been the underwater exploits of seals, which can plunge hundreds of meters deep before coming up for air.
Now, a team led by marine biologist Randall Davis of Texas A&M University in Galveston has developed a tiny $20,000 camera that withstands the cold and pressure of the sea. Resting in a detachable cradle, the camera shines infrared light into the water around a seal's head. Other instruments record sound, depth, swim speed, and compass bearing, which together yield 3D maps of each dive. To make sure they got their pricey camera back, the team drilled a single hole in an unbroken ice sheet in McMurdo Sound to force the seals to return to that hole for air. About 57 hours of video taken from October through December 1997 showed seals quietly stalking large cod from below, using backlighting to spot them against the ice. On other occasions, seals expelled bubbles from their nostrils to startle and dislodge small fish hiding in icy crevices. "These behaviors have never been observed before," Davis notes.
Unpublished results from a second field season 2 months ago are even more exciting, Davis says, because the videos show seals catching and eating large numbers of Antarctic silverfish, their primary prey. Initial analyses show the seals didn't use backlighting to spot these fish. However, Davis thinks it's "very unlikely" that the seals use echolocation, as other researchers have speculated, because the microphones recorded few sounds.
The findings from the innovative camera represent the "tip of the iceberg in a revolution in our understanding of what marine mammals do," says marine biologist Burney Le Boeuf of the University of California, Santa Cruz. But the videos may not depict usual feeding behaviors, he says, because the team had moved the seals from their ordinary stomping grounds to a part of the sound with which the animals were presumably unfamiliar.