Terrestrial Foraging | the-polar-bear

Feeding Habits based on Age and Gender

Terrestrial diet of summer bears includes: berries, seaweed, grass, carrion, eggs, sedges, rodents, birds, other vegetation and food wastes (Practical Dictionary of Siberia and the North, 2005). However there have been documented cases of polar bears hunting caribou and readily hunting birds (Derocher, Stirling and Andriashek, 1993.
Frequency of terrestrial feeding varies across age classes and sexes. Adult male bears prefer to feed on calorie rich blubber throughout the winter season and rest during the summer seasons. More energy is exhausted on foraging for low calorie vegetation then remaining in a resting state for such a large mammal.
Cubs, yearlings and sub-adults have substantially less mass and experience catching prey then an adult male bear, so berries and other vegetative sources are a viable terrestrial food source for growing bears, or to at least reduce weight loss (Derocher, Stirling and Andriashek, 1993, p. 253).
Feeding on berries in adult females can be linked to the required energy stores needed for successful reproduction (Derocher, Stirling and Andriashek, 1993, p. 252) and survival of the newborn cubs (Derocher, Lunn and Stirling, 2004, p. 165; Derocher, Stirling and Andriashek, 1993, p. 253).
Similarly, summer scavenging of reindeer carcasses in Svalbard by females with cubs may be a crucial aspect of cub survival in the family’s first spring (Derocher, Wiig and Bangjord, 2000, p. 677).

Terrestrial Diet and Foraging Habits

There is a distinct difference in diet composition between island and mainland polar bear populations.
This is due to their access of varying frequency and abundance of food sources in open ocean and coastal ecosystems; Determined by the amount of avian remains found in bear scat piles, compared to grasses and marine algae. Figure 1 depicts this comparison of mainland (coastal Manitoba and Ontario, Canada) bears to island bears (North Twin Island in James Bay, Canada) (Russell, 1971 p. 68-70).
Birds have a higher content of fat and protein compared to the carbohydrates found in vegetation. Therefore shore birds provide benefits to polar bear health by storing fats and proteins for the future, as well as providing an immediate effect; The island bears had a better overall well-being when entering winter season (Russell, 1971).

Predation on Ungulates

Polar bears have been known to consume Arctic ungulates (Rangifer tarandus); The North American wild caribou, and eastern hemisphere domesticated reindeer.

Predation on Reindeer:

In the Svalbard sub-population of polar bears, there have been several documented instances of a bear feeding on fresh reindeer carcasses. Observations of polar bears attempting to hunt reindeer in Svalbard have always resulted in failure.
The reindeer has an advantage of climate conditions in Svalbard of minimal snow, and complimented with its own agility, to avoid polar bear encounters.
Most observed cases of bears feeding on reindeer are of scavenged remains: Death related to seasonal “climatic conditions that restrict access to food during the winter/spring” (Derocher, Wiig, Bangjord, 2000, p. 677).
Polar bears stalk reindeer similar to seals underneath ice, using the stealth provided by snow and prowess of a surprise attack. Without snow, the effectiveness of a short dash to the prey is lost by increased visibility of the bear and reduced movement of the reindeer.
As reviewed and docutmented by Derocher, Wiig, and Bangjord (p. 676), tracks of a female bear suggest a successfully captured reindeer as it was resting in a nearby, shallow bed.
This is similar to other observed cases of polar bears feeding on reindeer in the area, while lying down or sleeping. Some bears have benefited from predation on reindeer in Svalbard and “the observations suggest plasticity in foraging behavior of polar bears.” (2000, p. 678).

Predation on Caribou:

Polar bears in the Western Hudson Bay sub-population have been witnessed chasing caribou but with unsuccessful results.
It was noted by Brook and Richardson (2002 p. 194) that the bear utilized the wind direction and vegetation cover to reach within a 20 meter proximity to a herd of 120 grazing caribou, before making a dash lasting approximately 20 seconds, towards the caribou in which case they ultimately avoided the bear.
A more recent study conducted by Dr. Robert Rockwell between 2005-2008 in Wapusk National Park, sampled 362 bear scat piles (2008) which contained more caribou and geese remains than the two previous years (Wapusk National Park 2007/2008 p. 22).

Island Populations compared to Mainland Populations

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References

Practical Dictionary of Siberia and the North. 2005.

Derocher, Stirling and Andriashek. September 1993. Terrestrial Foraging by Polar Bears during the Ice-Free Period in the Western Hudson Bay. Arctic Vol 46:3 p. 251-254.

Derocher, Andrew E., Lunn, Nicholas J. and Stirling, Ian. 2004. Polar Bears in a Warming Climate.

Derocher, Andrew E., Wiig, O. and Bangjord, G. 2000. The predation of Svalbard reindeer by polar bears

Russell, Richard H. (1971). Summer and Autumn Food Habits of Island and Mainland Populations of Polar Bears - A Comparative Study. University of Alberta: Department of Zoology.

Brook, Ryan K. and Richardson, Evan S. (2002). Observations of Polar Bear Predatory Behaviour toward Caribou. Arctic.

Richard Russell, 1971.