A study, by San Diego Zoo Global conservationists, released this week (Sept. 12, 2016) is shedding new light on how scientists evaluate polar bear diet and weight loss during their fasting season. On average, a polar bear loses up to 30 percent of its total body mass while fasting during the open-water season. Although some scientists previously believed land-based foods could supplement the bears' nutritional needs until the sea ice returns, a new study published in the scientific journal Physiological and Biochemical Zoology has revealed that access to terrestrial food is not sufficient to reduce the rate of body mass loss for fasting polar bears. The study--undertaken by Manitoba Sustainable Development, the University of Alberta, and Environment and Climate Change Canada--weighed polar bears that were detained in the Polar Bear Holding Facility in Churchill, Manitoba, Canada from 2009 to 2014. Polar bears were kept in this facility as part of the Polar Bear Alert Program, which aims to reduce conflict between humans and polar bears around the town of Churchill. To prevent habituation, polar bears are not fed while in the facility, which allowed for a controlled measure of their weight loss. On average, polar bears lost 2.2 pounds (1 kilogram) of mass per day--exactly the same amount as free-ranging bears measured during the ice-free season on the coastline of Hudson Bay. Scientists reported that even with land-based food opportunities, polar bears lost the same amount of weight.

"Some studies have suggested that polar bears could adapt to land-based foods to offset the missing calories during a shortened hunting period on the ice," said Nicholas Pilfold, Ph. D., lead author of the study and a postdoctoral associate in Applied Animal Ecology at San Diego Zoo Global. "Yet, our results contradict this, as unfed polar bears in our study lost mass at the same rate as free-ranging bears that had access to land-based food."

Researchers also estimated starvation timelines for adult males and subadults, and found that subadults were more likely to starve before their adult counterparts. "Subadult polar bears have lower fat stores, and the added energy demands associated with growth," said Pilfold, "Future reductions to on-ice hunting opportunities due to sea ice loss will affect the younger polar bears first--especially given that these bears are less-experienced hunters."