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Increasing loss of spring sea ice taxes polar bear metabolism

American Association for the Advancement of Science

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IMAGE: Polar bear wearing a GPS video-camera collar on the sea ice of the Beaufort Sea. This material relates to a paper that appeared in the 2 February 2018 issue of... view more 

Credit: Anthony Pagano, USGS

Tracking polar bears during the spring - their prime hunting season, when sea ice conditions should be ideal - reveals that in recent years, many bears are expending notably more energy than they are consuming. The results highlight how these animals' energy requirements are rising in tandem with a changing climate. Polar bears rely almost exclusively on a fat-rich diet of seals, which are most efficiently hunted from the surface of sea ice. However, the abundance of sea ice across the Arctic is decreasing at a rate of 14% per decade, which is likely reducing polar bears' access to their prey. In April of 2014, 2015, and 2016, Anthony Pagano and colleagues aimed to better understand the energy expenditure of polar bears during this critical season by capturing nine females on the sea ice of the Beaufort Sea. They measured the metabolic rates of each bear by analyzing blood and urine samples upon capture and then again at recapture, after 8 to 11 days. As well, the bears were equipped with GPS collars that also collect video records of activity during daylight (see polar bear "point-of-view" videos). The data suggest that polar bear metabolism is 1.6 times higher than previously thought. What's more, four of the bears lost 10% or more of their body mass over the 8- to 11-day period, with an average loss of 1% per day (one bear lost not only her fat reserves, but lean muscle as well) The authors note that this is four times the mass lost per day that has been observed in fasting polar bears on land. Increasingly fragmented sea ice will put an even greater imbalance on the energy expenditure to consumption ratio for these animals, the authors say. John P. Whiteman provides more context in a related Perspective.

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