Polar bears (Ursus maritimus), increasingly forced on shore due to sea ice loss, may be eating land-based foods, but any nutritional gains are limited to a few individuals and likely cannot compensate for lost opportunities to consume their traditional prey, says a new study led by Dr Karyn Rode of the U.S. Geological Survey’s Alaska Science Center in Anchorage.
Willy (Wilhelm) the polar bear at the North Carolina Zoo in Asheboro. Image credit: John / CC BY 2.0.
Several previous studies have suggested that polar bears consume the highest lipid diet of any species, which provides all essential nutrients and is ideal for maximizing fat deposition and minimizing energetic requirements.
Potential foods found on the land are dominated by high-protein, low-fat animals and vegetation.
Polar bears are not physiologically suited to digest plants, and it would be difficult for them to ingest the volumes that would be required to support their large body size.
“Although some polar bears may eat land-based foods, there is no evidence the behavior is widespread. In the regions where terrestrial feeding by polar bears has been documented, polar bear body condition and survival rates have declined,” said Dr Rode, the first author of the paper published in the journal Frontiers in Ecology and the Environment.
Dr Rode and her colleagues from the Polar Bears International and Washington State University noted that over much of the polar bear’s range, terrestrial habitats are already occupied by grizzly bears (Ursus arctos spp.).
Those grizzly bears occur at low densities and are some of the smallest of their species due to low food quality and availability. They are a potential competitor as polar bears displaced from their sea ice habitats increasingly use the same land habitats as the grizzly bears.
“The smaller size and low population density of grizzly bears in the Arctic provides a clear indication of the nutritional limitations of onshore habitats for supporting large bodied polar bears in meaningful numbers,” Dr Rode said.
“Grizzly bears and polar bears are likely to increasingly interact and potentially compete for terrestrial resources.”
Dr Rode and co-authors found that fewer than 30 individual polar bears have been observed consuming bird eggs from any one population, which typically range from 900 to 2,000 individuals.
“There has been a fair bit of publicity about polar bears consuming bird eggs. However, this behavior is not yet common, and is unlikely to have population-level impacts on trends in body condition and survival,” Dr Rode said.
“The reports of terrestrial feeding by polar bears provide important insights into the ecology of bears on land,” she said.
_____
Karyn D Rode et al. 2015. Can polar bears use terrestrial foods to offset lost ice-based hunting opportunities? Frontiers in Ecology and the Environment 13: 138–145; doi: 10.1890/140202
