Polar bears are the largest land carnivore, with males weighing up to 1500lbs (680kgs), and to get this big, they eat a high calorie diet consisting mostly of seal fat. Polar bears use the frozen sea ice as a platform for hunting their seal prey but in the summer months, when the vast majority of sea ice melts away, they are forced onto land into a period of fasting. A fasting polar bear can lose approximately 1kg of body weight per day, meaning a polar bear that is required to fast between mid-July to early November, could lose up to 120kg before the sea ice freezes once again.
For the bears in our care, they too experience seasonal weight fluctuations, typically ranging between 50-100lbs. Rather than strictly fasting our bears throughout the summer, we help them to lose weight (to avoid overheating) by reducing their calorie intake. We do this by reducing or removing the larger amounts of high calorie foods, such as seal fat and mackerel, and replacing them fruits and vegetables. As obligate carnivores, polar bears do not break down and metabolize fruits and vegetables in the same way that black or brown bears do. While they enjoy the taste and it helps to keep their bellies full, fruits and vegetable simply to do not contain the right nutrients to help polar bears maintain or gain weight.
Terrestrial food sources for wild bears include small grasses, berries, or seaweed. While these items may help to briefly relieve their hunger, just like fruit and vegetables, they do not contain the nutrients a polar bear needs. These items are typically also in short supply, just another reason why they are not a viable food source. Goose eggs, laid by snow geese who nest on the ground, do offer a valuable source of calories, but a single bear would need to eat approximately 88 eggs to gain the same amount of calories as eating one seal, according to researcher Dr. Robert Rockwell of the American Museum of Natural History. Rockwell has witnessed bears coming onto land and eating 200-300 nests of eggs (four eggs in each clutch), in a 96-hour period, potentially decimating that particular nesting ground.
Coming into September, the Animal Care Team here at The Habitat will be closely monitoring the changing temperatures, as well as the body condition and activity levels of the bears. These changes will help them to determine when they will begin to phase out the fruits and vegetables and increase their calorie intact, in preparation for the cold Cochrane winter ahead!
Above: Henry looking trim in the summer.
Below: Henry showing his pre-winter weight gain.
(Photo courtesy of Austin McIntosh- Animal Care Intern)