11/11/2010 1:20:27 PM
Titans Tussling on the Tundra
Walk into any gift shop in Churchill, Manitoba, and you're guaranteed to find a postcard of two male polar bears wrestling in the snow. This ritualistic behavior occurs every fall along the coast of Hudson Bay as adult male bears congregate and wait for the sea ice to re-form.
Male bears will often form aggregations of two to 10 individuals along the coast. These bachelor groups spend much of their day laying in the kelp along the shores of Hudson Bay. But as the temperature begins to drop in late-October and early-November, adult male bears begin to match wits in what is a very important evolutionary game of king of the castle.
When it comes to being a male polar bear, the rule of thumb is bigger is better. Out on the sea ice larger male bears are able to steal seal kills from smaller bears but are also more competitive in the polar bear mating system. Over evolutionary time adult male polar bears have evolved to be about twice the size of adult female polar bears through a process that Charles Darwin referred to as sexual selection. Sexual selection in its simplest form is the process by which a trait, such as body size, arises as a result of competition between individuals of the same sex (in this case competition between male bears to gain access to females during the breeding season).
The mating system of polar bears is believed to be extremely competitive as females keep their cubs for two years, which means that only one in every three females is ready to breed each spring. Field observations of adult male bears during the mating season have revealed that these male bears enter into intense physical battles to gain access to mates, often resulting in large cuts and broken canines.
So what does this all have to do with polar bears wrestling in the snow? Researchers believe that male bears enter into these playful games of king of the castle to see where they are in the pecking order amongst other adult males. Being able to size up an opponent during the breeding season is important, as taking on a much larger and stronger male bear could prove costly. (Adult male polar bears in western Hudson Bay can weigh in excess of 1400 lbs. and are capable of pushing their canines through the skull of another bear.) Despite the magnificent power of these giant bears, these fall tussles on the tundra rarely result in any blood being drawn or any serious injuries.
Photo Credits: Top, ©Daniel J. Cox/Natural Exposures; Middle, ©Dr. Tom Smith; Bottom, ©Shari Burnett.