(Related news: "Hibernating Animals Suffer Dangerous Wakeup Calls Due to Warming" [February 2, 2007].)
But for now the researchers aren't sure why the cod employ this biological trick.
"Annual sea temperatures in the Antarctic vary from around 1 degree Celsius [33.8 degrees Fahrenheit] in summer to around -1.8 degrees Celsius [28.7 degrees Fahrenheit] in winter," Fraser said.
"This change isn't enough to explain the changes in metabolic rate we see in these fish."
And, he said, "warming up winter-acclimatized fish by two degrees [Celsius] in the laboratory doesn't increase their heart rates."
Lack of prey also doesn't appear to be a motivating factor.
"There still seems to be plenty of food available, but the fish adopt a strategy which doesn't involve going to look for it," Fraser said.
"Perhaps they are visual predators, and in the [dark] winter they basically can't see their food."
The team is also unsure what triggers the fish to start their winter slowdown.
"Like many other hibernating animals, they may be responding to changes in day length" study co-author Campbell said.
In the summer the Antarctic Peninsula—where this study took place—is bathed in 24-hour daylight. But during the winter there are only three hours of dusky light every day, around midday. (Explore an interactive map of Antarctica.)
Arthur DeVries is a marine biologist at the University of Illinois, Urbana-Champaign, who was not involved with the new research.
"This study is a very interesting observation, especially from an ecological point of view," DeVries said.
Further studies on the cod could show whether they also alter their body chemistry to survive so long without much food—which would offer proof that their hibernation is a survival strategy.
"It would be interesting to see whether the activities of [the fish's] enzymes involved in energy production are also depressed during this period of dormancy," he said.
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