Wildlife Climatology The Expedition Peoples Of The North The Global Issues The "Making Of"

Wildlife

Climatology

The Expedition

Peoples Of The North

The Global Issues

The "Making Of"

Catherine Giroulby Catherine Giroul, Internet Communications Co-ordinator

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What’s the hot topic aboard SEDNA these days? The polar bear and its status as a marine or land mammal! The subject comes up often, especially with all the polar bears we’ve been sighting. We put the question to our Web readers, and their responses were as follows: 67% believe polar bears are land mammals, 17% say marine, 11% say both and 5% don’t know. Among the crew, however, the two options are neck and neck: 8 of us say land, 8 say marine. You can see why the question has led to many heated and lengthy discussions.

Polar bears are strong swimmers. Our underwater cinematographer caught this one in action near Walrus Island, a tiny island some 25 miles from the coast!Before we can make any headway in the debate, we must agree upon just what a marine mammal is. We can define mammals as warm-blooded, vertebrate animals with teats to suckle their young, hair covering their bodies, and lungs for breathing. According to The National Audubon Society Guide to Marine Mammals of the World, published in 2002, marine mammals are defined as mammals that live in the sea for part or all of their lives. This category includes cetaceans (whales and dolphins), pinnipeds (seals, sea lions, and walruses), sirenians (dugongs and manatees), marine mustelidae (otters) and polar bears.

Many of you (and some of us!) are surprised to learn that scientists consider polar bears to be marine mammals. The polar bear does spend most of its time in a marine habitat, although the water in question – pack ice – is in frozen form. Despite the classification in the National Audubon Society book, a recent and reliable source, debate continues aboard our vessel. Interestingly, we’re not the only ones who are divided on the subject: in the United States, the polar bear is considered a marine mammal in the eyes of the law, but in Canada, it’s considered a land mammal. Nonetheless, in terms of its ecology, i.e. the relation between the animal and its environment, the polar bear is clearly a fully-fledged inhabitant of the marine ecosystem. Dr. Ian Stirling, who has studied polar bears in the Hudson Bay area for over 20 years, points this out in his recent work on polar bears.

Polar bearOne reason for all the wrangling is that the taxonomy of marine mammals is not set in stone. Marine mammals once lived on land and evolved towards the marine environment at different points in time to exploit a new ecological niche (habitat and source of food). Cetaceans are the mammals best adapted to marine life, as their transformation began some 55 to 60 million years ago. Pinnipeds appear to have been meat-eating land mammals resembling wolves 25 million years ago. Sirenians are believed to have descended from the same ancestors as elephants some 50 million years ago. Otters moved into the marine environment 5 to 10 million years ago. The polar bear, on the other hand, is a much more recent convert: it is believed to have begun its transition to the marine environment nearly 250,000 years ago, which in evolutionary terms is the blink of an eye.

Recent DNA data shows that the polar bear is more closely linked to the brown bear than to any other species of bear in the world. The close relation between the two species is demonstrated by the fact that their hybrid is fertile, whereas we know that two different species cannot reproduce and give birth to a fertile animal. The polar bear is believed to have evolved rapidly from a population of brown bears isolated in the upper latitudes, where it exploited the resources of the Arctic and its pack ice. It is believed to have adapted to this hostile environment by acquiring a white coat (in fact the hairs are translucent) that lets heat from the sun through, black skin that facilitates heat retention, a thick layer of fat that protects it from the cold and helps it to float, small ears that prevent heat loss, webbed front feet, etc. On an evolutionary scale, the polar bear changed quickly, but because of its resemblance to the land bear, its gait, the fact that it is plantigrade, and its dependence on land for reproduction, some people still question its recent classification as a marine mammal.

And so, the debate continues aboard SEDNA IV, with much arguing back and forth… and much laughter!

Is the polar bear a marine mammal or a land mammal?

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