Arctic birds: Rock ptarmigans stay fit when fat
Experiments using miniature treadmills have revealed how an Arctic-dwelling bird is superbly adapted for life in extreme conditions.
The rock ptarmigan - a chicken-like bird that lives year-round on the Arctic ice and tundra - is just as fit when it has fattened up for the winter.
The birds, also known as snow chickens, can double in weight.
Scientists presented the results at the Society for Experimental Biology's annual conference in Salzburg, Austria.
The mini-treadmill set-up allowed the researchers to monitor the birds as they ran, and to measure how much oxygen they consumed.
Lead researcher John Lees from the University of Manchester explained that he was interested in the "energetic cost of movement, because it's fundamental to survival in the Arctic".
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He and his colleagues hope to understand more about how species that live in the far north will be affected as the temperature of their habitat increases.
Mr Lees and his colleague Dr Jonathan Codd worked on their Arctic project in Svalbard in the far north of Norway. With the help of their Norwegian colleagues, the scientists set up a "little gym" for their ptarmigans, and kept the room at a chilly 6C to make sure the birds were comfortable.
"They're natural born athletes," Mr Lees told BBC Nature.
"As soon as you put them on the treadmill, they start to run."
By measuring how much oxygen the birds consumed and how much carbon dioxide they produced, he and team were able work out exactly how much energy the birds used as they ran.
The researcher said he was "amazed" by the results.
"In the winter (when the birds are significantly fatter) they actually use exactly the same amount of energy when they run as in the summer.
"That's like us running with a backpack that weighs about a third of our bodyweight.
"We would use up much more energy with all that extra weight, but the birds are carrying all this fat for free."
The fat stores build up around the bird's chest and act as a combined food store and winter jacket - insulating a diminutive rock ptarmigan from temperatures as low as -40C (-40F).
The combination of these fat stores and thick, white winter plumage makes the birds' appear very different in the summer compared to the winter.
"They're chubby little things in the winter," said Mr Lees.
He explained that studying how Arctic animals survive gives the earliest gives indications of the effects of global warming on wildlife, because their adaptations to the cold are so extreme.
He added: "They're just extreme examples of how all species are adapted to their environments, so we can learn so much from studying them."