Discuss as:

Polar bear cannibalism: More to come in a warmer Arctic?

Jessica Robertson / USGS

Sea ice is critical habiat for polar bears, which use it as platforms from which to hunt seals.

Adult male polar bears preying on cubs and even females has long been observed, but recent sightings off Norway suggest that cannibalism may happen more often as warming temperatures reduce Arctic sea ice, a leading polar bear scientist says in a new report.

The conclusion is based on images and observations made of three separate cannibalism incidents in summer and early fall while the adult polar bears were on sea ice, which polar bears use as platforms to hunt seals.

Most previous cannibalism observations have involved polar bears on the shoreline and later in the year.

"The three observations we describe are different from most other reports of infanticide and cannibalism in polar bears because they took place between midsummer and early autumn, while some bears of all age and sex classes were still actively using the remaining sea ice as a platform from which to hunt," wrote Ian Stirling, a polar bear expert with Canada's environmental agency, and Jenny E. Ross, a photographer who captured images of an incident on July 21, 2010.

Dec. 10, 2009: Tourists visiting Churchill, Canada, were shocked to see an adult polar bear eating a cub. CBC's Mychaylo Prystupa reports wildlife experts say bear cannibalism is becoming more common due to changes in the food supply.

Because few earlier observations of polar bears on summer sea ice have been made, such cannibalism "may be relatively normal and possibly occurs more frequently than has previously been thought," the authors wrote in the study published in the December issue of the peer-reviewed journal Arctic.

"However, as the climate continues to warm and sea ice continues to break up and melt at earlier dates, thus making seals less available earlier in the summer, the frequency of intraspecific predation and cannibalism may increase," they added. "Similarly, as more ships go farther into the disintegrating ice to view and photograph polar bears, it is likely that similar instances of intraspecific predation and cannibalism on the sea ice will be reported more frequently."

Arctic 'not cooling as well as it used to'

Arctic sea ice follows a natural cycle of growing in winter and shrinking in summer, but in the last decade the shrinkage has spread.

Last September, "sea ice covering the Arctic Ocean fell to the second-lowest extent in the satellite record, which began in 1979," the National Snow and Ice Data Center reported last October.

Read more content from msnbc.com and NBC News: