Last week I was captivated by this photo, taken by Paul Nicklen and posted on National Geographic’s instagram, of two polar bears (Ursus maritimus) in a mock battle.
I was intrigued that polar bears have fake battles for breeding rights, and I decided to find out more. In my search, I came across this review by Dr. Kevin Fitzgerald in the November 2013 issue of Topics in Companion Animal Medicine that explained all I needed to know about polar bear battles (and more)! It isn’t a research article per se, but in it, Fitzgerald explains a lot about polar bear ecology that has been uncovered by people such as the Inuit, early explorers, and research biologists (and he gives a compelling case as to why polar bears need to be protected from the potentially catastrophic effects of global warming). I’d like to share some of that information with you.
Polar bears have a really unique system of reproduction. In fact, Fitzgerald states, “perhaps nothing about them is as remarkable as their unique reproduction”! Polar bear mating occurs between April and May out on the sea ice, and the females give birth in December or January in dens insulated by snow. Between pregnancy and birth, females must store up a lot of fat in their bodies in order to survive the harsh winter and provide for their offspring. The eggs are fertilized by mid-May, but they do not attach to the uterine wall until August or September. If the females do not reach a certain body weight by this time, the eggs get reabsorbed into the body. This system of delayed implantation helps the polar bear mothers have healthier babies by allowing them to carry their offspring only if they have built up enough fat reserves. When the cubs are born, between 1-3 per litter, they are very small, weighing only 1 lb each. (For comparison, adult females weigh 350-550 lb, while males weigh a whopping 750-1400 lb.)
So what does all this have to do with pretend battles?
Look at the very cute picture above of a mother bear and her cub. That cub was tiny when it was born, but look how big it is now! Cubs stay with their mothers until around age 2-2.5, when they are finally weaned. During that time, mothers cannot have more young. This means that female polar bears can only have babies every three years. This also means that when an adult male wants to breed, only 1/3 of adult females are available for breeding; in other words, there are 3 adult males for every female. Most mammals have more available females than males, but this is not the case for polar bears.
This gender disparity causes intense competition among the males. It’s also why polar bears show a lot of sexual dimorphism (males look different than females). Males are 2-3 times bigger than females because the largest males have a better chance of fighting off other males in order to mate.
In order to prepare themselves for the competition they will face as adults, younger polar bear males engage in play-fighting. These mock battles usually last for less than 5 minutes. The opponents bite and wrestle each other, but they do not intend to seriously injure each other. Instead, mock battles help the bears develop the ability to judge the strength of their opponent during a real fight someday, when all-important breeding rights are at stake.
Unfortunately, global warming could create major problems for the 20,000 polar bears currently in existence, and having such a low reproductive rate doesn’t help. As polar bears wander farther and expend more energy to find food, cannibalism of cubs by males could increase and may already be on the rise. But it’s not too late. As Fitzgerald says, “We must demonstrate the courage necessary to implement the policies and lifestyle changes required to ensure sustainable ecosystems worldwide.”
If you have access to Fitzgerald’s review, “Polar Bears: the Fate of an Icon”, I highly recommend reading the entire article. Unfortunately, I could not find a free version online, but you can read the lengthy abstract here.
Title image: more polar bears in the Arctic National Wildlife Refuge. Source: U.S. Fish and Wildlife Service Headquarters / Flickr
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Fitzgerald KT (2013). Polar bears: the fate of an icon Topics in Companion Animal Medicine, 28 (4), 135-42 PMID: 24331553