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Authors: Kieran Mulvaney
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seals that had never had cause to fear predators, and perhaps then graduating to adult ringed seals at their breathing holes. Over time, natural selection presumably favored those bears with lighter coats for camouflage, with teeth better adapted for tearing at meat, and with other physical adaptations such as larger feet for easier swimming between ice floes.
It all happened in the blink of an evolutionary eye. When the last Ice Age began, there were no polar bears. By the time the most recent glacial period ended, polar bears as we know them had become established. The other descendants of the original, isolated brown bear stock that gave rise to them presumably either were reabsorbed into the broader grizzly gene pool or died out. Except, that is, for those on the Alexander Archipelago.
When ice advanced southward into what are now more temperate latitudes, the islands of the archipelago pierced the glacial shroud, acting as what biologists call a refugium, a safe haven where wildlife populations were able to endure. Those populations were isolated by the surrounding ice and then, when the ice retreated, by the passages of water between the islands and the mainland. Which is how it came to be that the archipelago boasts, for example, its own subspecies of dusky shrew and northern flying squirrel, even an Alexander Archipelago wolfâand, of course, the ABC Islands' brown bears, which today contain, deep within their cells, echoes of a distant past and the birth of a new species.
There is not an abundance of polar bear fossilsâanimals that live on sea ice tend to die on sea ice, and animals that die on sea ice tend to sink to the bottom of the ocean when the ice meltsâbut those that do exist suggest that early polar bears were larger even than the ones that wander the Arctic today. They were, in the words of one researcher, "gigantic." *
(Given the greater size of those early bears, a 1971 study that concluded average skull size was larger in polar bears in the Chukchi Sea between Siberia and Alaska was cited as evidence that this must have been where the species originated. But although the origination location may be correct, the notion that there is a gradient in skull size almost certainly is not. The study was based on skulls housed in museums around the world, but the sources of those skulls were not uniform; it is believed that many of those from the Chukchi Sea, for example, may have been donated by trophy hunters, who were of course adept at, and focused on, hunting larger bears.)
But if modern polar bears are smaller than their predecessors, they are still the largest bear species in the world. The largest male weighed in Hudson Bay was a thirteen-year-old that tipped the scales at almost 1,450 pounds; the average weight for mature male bears in that population is approximately 1,100 pounds. Steven Amstrup observes that the heaviest bear he and his colleagues have weighed in the Beaufort Sea area of Alaska was roughly 1,350 pounds but that some tranquilized animals were so enormous that they could not be lifted up with a weighing tripod, or even by helicopter. Doug DeMaster and Ian Stirling have estimated that some weigh as much as 1,760 pounds. Females are significantly smaller, and their maximum weights rarely exceed 880 pounds even when they are at their most obese, when they have gorged following mating and prior to denning.
A mature male may measure over nine feet from the tip of his nose to his stubby tail, stand five and a half feet at the shoulder when on all fours, and reach thirteen feet into the air when standing on his hind legs. Polar bears are not only the largest bears in the world today, they are very nearly the largest that have ever existed; only their ancient ancestors and the extinct giant short-faced bear were larger.
Size, however, isn't everything. There is more to being a polar bear than being big.
***
It all begins with the head.
If you had never before seen a polar
It all happened in the blink of an evolutionary eye. When the last Ice Age began, there were no polar bears. By the time the most recent glacial period ended, polar bears as we know them had become established. The other descendants of the original, isolated brown bear stock that gave rise to them presumably either were reabsorbed into the broader grizzly gene pool or died out. Except, that is, for those on the Alexander Archipelago.
When ice advanced southward into what are now more temperate latitudes, the islands of the archipelago pierced the glacial shroud, acting as what biologists call a refugium, a safe haven where wildlife populations were able to endure. Those populations were isolated by the surrounding ice and then, when the ice retreated, by the passages of water between the islands and the mainland. Which is how it came to be that the archipelago boasts, for example, its own subspecies of dusky shrew and northern flying squirrel, even an Alexander Archipelago wolfâand, of course, the ABC Islands' brown bears, which today contain, deep within their cells, echoes of a distant past and the birth of a new species.
There is not an abundance of polar bear fossilsâanimals that live on sea ice tend to die on sea ice, and animals that die on sea ice tend to sink to the bottom of the ocean when the ice meltsâbut those that do exist suggest that early polar bears were larger even than the ones that wander the Arctic today. They were, in the words of one researcher, "gigantic." *
(Given the greater size of those early bears, a 1971 study that concluded average skull size was larger in polar bears in the Chukchi Sea between Siberia and Alaska was cited as evidence that this must have been where the species originated. But although the origination location may be correct, the notion that there is a gradient in skull size almost certainly is not. The study was based on skulls housed in museums around the world, but the sources of those skulls were not uniform; it is believed that many of those from the Chukchi Sea, for example, may have been donated by trophy hunters, who were of course adept at, and focused on, hunting larger bears.)
But if modern polar bears are smaller than their predecessors, they are still the largest bear species in the world. The largest male weighed in Hudson Bay was a thirteen-year-old that tipped the scales at almost 1,450 pounds; the average weight for mature male bears in that population is approximately 1,100 pounds. Steven Amstrup observes that the heaviest bear he and his colleagues have weighed in the Beaufort Sea area of Alaska was roughly 1,350 pounds but that some tranquilized animals were so enormous that they could not be lifted up with a weighing tripod, or even by helicopter. Doug DeMaster and Ian Stirling have estimated that some weigh as much as 1,760 pounds. Females are significantly smaller, and their maximum weights rarely exceed 880 pounds even when they are at their most obese, when they have gorged following mating and prior to denning.
A mature male may measure over nine feet from the tip of his nose to his stubby tail, stand five and a half feet at the shoulder when on all fours, and reach thirteen feet into the air when standing on his hind legs. Polar bears are not only the largest bears in the world today, they are very nearly the largest that have ever existed; only their ancient ancestors and the extinct giant short-faced bear were larger.
Size, however, isn't everything. There is more to being a polar bear than being big.
***
It all begins with the head.
If you had never before seen a polar
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