Like daily commuters, Adélie and emperor penguins are up at dawn, catching krill and fish in Antarctic waters, and back home to shore at dusk. Yet the food they prefer to dine on is easiest to catch after dark. Most researchers assumed that penguins had poor nighttime vision, which was why they stayed out of the water after dusk.
But in a new study, two marine ecologists argue that the penguins actually have no trouble seeing in the dark. Instead, they say, penguins head for shore at night because they cannot gauge the risk of being eaten by leopard seals or killer whales. Even their migration patterns, when they move from some of the Southern Ocean's most productive waters into those that are marginal, are likely shaped by the fear of predators. "They would rather be hungry" than dead, says the study's lead author, David Ainley, a marine ecologist at H. T. Harvey and Associates, an ecological consulting firm in Los Gatos, California.
To show that the penguins can see in the dark, Ainley and his colleague, Grant Ballard, a marine ecologist at PRBO Conservation Science, a conservation organization in Petaluma, California, outfitted 65 adult Adélie penguins with time-depth recorders. The devices, which register depth and light every second, were taped to the lower back, so that they caused the least amount of drag. Data collected on nearly 22,000 of the birds' foraging dives showed that most were hunting prey at 50 to 100 meters below the surface, where the water is quite dark—akin to early night. The birds also made a significant number of dives into deeper, darker waters, where they can forage successfully.
Although the two researchers did not collect similar data on emperor penguins, other scientists have shown that these birds dive even deeper, into waters more than 500 meters below the surface. "At that depth, it's absolutely black," Ainley says.
So why won't the penguins hunt at night? Ainley and Ballard note that leopard seals, which regularly kill both species of penguins, rest at midday, making it safer for penguins to hunt during this time. Even then, the penguins are cautious; they stay in the water only long enough to feed, and they're adept at remaining motionless when they're on thin ice and spot a leopard seal. At the Ross Island colony in Antarctica, Adélies that land at the far end of the island will even walk the 5 kilometers to reach their home rather than enter the water again and swim, which would get them back faster.
Killer whales may also be a problem. Although they have not been actually observed taking either Adélie or emperor penguins, cetacean researchers suspect that they do, because orcas have been seen killing and eating other penguin species in Antarctic and subantarctic waters. What's more, certain types of killer whales are prey specialists, feeding only on marine mammals and seabirds, and in the Antarctic these orcas are known to visit areas near emperor penguin colonies.
Fear of predators doesn't just affect the penguins' daily activities, however. It also influences the birds' migration patterns, Ainley and Ballard report this week in Polar Biology. Emperor penguin adults and chicks leave their colonies in the late Antarctic summer. But instead of heading to the closest and richest waters, they swim north to far less productive waters. During that journey, other researchers have noted, some 20% to 30% of juvenile emperors are killed.
"We don't have the evidence, but it is very likely killer whales are taking them," Ainley says. Similarly, the Adélie penguins migrate to northern areas in the Antarctic winter, presumably because they do not want to live in total darkness in the south. It's more difficult to spot predators during this period, Ainley says.
"They've provided a convincing argument for what look like very strange behaviors" on the part of the penguins, says Aaron Wirsing, a behavioral ecologist at the University of Washington, Seattle. "It's another good example of how widespread the ecology of fear is in nature," adds William Ripple, an ecologist at Oregon State University in Corvallis, who has studied the effects of fear on the elk population in Yellowstone National Park following the reintroduction of gray wolves. "Predators, and the fear they instill, are major shapers of ecosystems," he says.