“Natural History”
transcription of presentation by Steve Amstrup
February 5, 2004, San Diego
I'll talk specifically about Natural History phenomena that might relate to husbandry issues that many of you face with captive animals. So, I'll be talking about natural history features that you might contrast with some of the situations you have in zoos. I'll also be talking about how polar bears are different from some of the other members of the bear family. Also going to add some aspects of our research activities in recent years, because they add some depth and breadth to these other topics.
So what is a polar bear? The polar bear is the largest non-aquatic predator on the globe. Large adult male stands almost 5 feet high at the shoulder on all fours, can be almost 12 feet long, feet the size of platters, 45 or 50 inch neck. But like all members of the bear family, they start out small and vulnerable. Polar bears have a low reproductive rate and long reproductive cycle. They breed in the spring (March and April), but they have delayed implantation, so development is arrested after a few hundred cells until October or November, at which point implantation occurs. Birth is thought to occur – in the Alaska region at least – around the first of the year. The cubs stay in the maternal den until springtime (March, April) and then they remain with the female through their first, second years of life. They’re finally weaned at close to 3 years of age, at which point they leave their mother and their mother is free to breed again. If a female is successful in bringing her brood through the weaning stage, she can breed no more often than every 3 years. So that’s a pretty low reproductive rate.
Polar bear cubs grow rapidly. They emerge from dens weighing around 30 pounds, with males typically a little heavier than females. By the fall of the year, females weigh about 150 pounds and males a bit more. As spring yearlings, females weigh about 180 pounds and males heavier. By fall, they are substantially heavier.
In the Beaufort Sea region, growth patterns are perhaps a bit different than they are in other regions. In the Hudson Bay area, for example, when the females come ashore in the summertime they don’t have anything to eat. The ice is gone, the main food sources are gone, so they don’t hit their peak weights in the fall. Typically, they’re at their peak weights at the time they come ashore. In the Beaufort Sea, it’s a much different situation.
As two-year-olds, they continue to grow and become pretty substantial creatures by this time in their life. One important thing to point out: Females generally stop growing in the 400-500 pound range, whereas males continue to grow pretty much throughout their life. Females hit a certain weight that gives them enough mass to reproduce, and then once they hit that reproductive age, they pretty much stop growing. Males continue to grow pretty much throughout their lives, or at least until the age when they stop being competitive, which is around the mid-20s.
Distribution of polar bears is closely tied to the distribution of sea ice in the Northern Hemisphere. Pretty much wherever there’s persistent pack ice for a major portion of the year we find polar bears. In Alaska, the ice penetrates down into the Bering Sea, often as far south as the Aleutian Chain. But when the ice retreats in the summer, most of the polar bears go with it. This is in contrast to the situation in the Hudson Bay, where the bay is covered with ice through the wintertime but then disappears entirely during the summer. As it disappears, drifting to the south, the bears are trying to stay on it as long as they can. As it melts entirely, the bears get off on the southern southwestern shore of Hudson Bay where Churchill is, and that’s where bears tend to congregate. The situation in Alaska is quite a lot different. This slide shows the distribution of ice in autumn 2002. The ice had retreated that year about as far north as it ever does and the vast majority of polar bears – indicated by these yellow dots – were on that ice. At that time, we had 3 radio-collared bears on the beach. Much different than Hudson Bay.
Polar bears are very much at home on this sea ice. They can swim very well. Sometimes swimming in the Arctic requires breaking a little ice. When it’s too thin to walk on but too thick to plow through with their chest, they can actually dive under the water and swim up to 30 yards or so holding their breath. They’ll poke their heads up and breathe. I’ve seen them cross leads that are a mile or so wide, and every 30 yards or so they’ll poke their head up to get a breath. Pretty impressive creatures.
The ice is where they’ll find their mates, and sometimes where they’ll enter maternal dens. Females with young select habitats that provide greater security for their young. Of course, the sea ice is also where polar bears find most of their nutrition.
Polar bears are probably the most mobile of all quadrupeds. We know this from radio telemetry studies that we’ve been doing recently. First, I thought I’d tell you about how we catch polar bears. Because of their mobility and low density, they’re very hard to catch. It’s very difficult to follow them on foot. It’s even more difficult to sneak up on them and give them an injection. We’ve come to depend entirely upon the use of helicopters to capture them. We hover helicopters over them at close distance so that we can shoot them with a projectile syringe. Sometimes they take more than one injection and after a few minutes they have difficulty standing up or following a straight line. Within about 5 minutes they lay down and fall asleep and at that point we move in and begin our work. We take a number of measurements, tag their ears, tattoo lips, take weight. The main thing I want to talk about is attaching radio transmitters to polar bears.
The advent of Effective Satellite Radio Telemetry Research has really been a boon to understanding the distribution of polar bears. As I said, they are highly mobile, and this slide shows an example of a bear we followed for two years in an area over 300,000 square kilometers. We have had some animals with activity area up to 600,000 square kilometers, about the size of the state of Montana. So, they’re extremely mobile animals. But they’re also highly individualistic. Some animals are less mobile and we tend to call them “homebodies” for lack of a better term. For example, this bear in the Southern Beaufort Sea area occupied a much smaller area, far less than 100,000 square kilometers, over 2 years. So there’s a lot of individual variation and we never really would have understood that without Satellite Radio Telemetry. One of the things we can do with this data, which is a whole bunch of dots on a map, is depict their activity areas (or home ranges) with mathematical models that actually surround the cloud of points that represent their distribution. This slide shows one of those very mobile polar bears versus the activity area of one of those homebodies. Here’s another: a very mobile bear versus a homebody. We’ve taken a look at the distribution of all of the bears, we’ve clustered them together, and determined that in this part of the world there are 3 different populations, or stocks, of bears. There’s the Chukchi Sea stock, the South Beaufort Sea stock, and the Northern Beaufort Sea stock. One of the problems with this type of view of distribution of animals is this tremendous area of overlap of the three populations. What good is it to describe three different stocks if they overlap so extensively? That, of course, is a considerable problem for wildlife managers who might need to allocate harvest to different villages or to understand the impact of an oil field development might have on a particular stock. So we’ve been developing a way to make these sorts of radiotelemetry data – which are always sort of a backward view of where bears have been, a retrospective view – more appropriate for real-time management decisions. One of the things we’ve been able to do is divide the study into a series of grid squares, and have been able to determine the relative probability of a bear in a particular grid cell being from a particular population. If you look from a bigger perspective, those grid cells, each with its own individual relative probability, convert into contour boundaries. So you can look at any place along the coast and say what the probability is of a bear being from one of those populations. This is very useful for harvest management and other things like that.
One of the ways that polar bears differ from other members of the bear family is in maternal denning. In polar bears, only pregnant females enter dens for the wintertime. With brown bears and black bears, the whole population dens over the winter because the things that they normally eat are unavailable. Hibernation is a foraging strategy. In the case of polar bears, they eat seals that are available year-round. Pregnant female polar bears need to den in order to provide a secure environment for their newborn cubs. So denning in polar bears is purely a reproductive strategy. Another difference is that polar bears over most of their range den just in snow. Whereas black and brown bears might tunnel under trees or dig into the ground or occupy a hollow log, polar bears are surrounded just by snow and ice. They dig a snow cave in the fall of the year and subsequent storms cover them over.
Up until a few years ago, it was thought that Alaska didn’t have much in the way of polar bear denning. In fact in1980 there was debate between Russian and Canadian research scientists about who really “owned” Alaskan polar bears. At that time there had only been about 30 dens documented in northern Alaska. We turned again to radio telemetry to try and figure out where polar bears in Alaska were denning. By putting radio collars on pregnant females and tracking them to their dens, we were able to document that in fact Alaska has quite a few dens. Many dens were out on the drifting pack ice, which was kind of an unusual and unexpected discovery. From 1981-1991 – the first decade of our efforts to document – we found about 140 dens and within the last decade there appeared to be fewer bears denning on the pack ice and perhaps more denning in the westerly portion. Not clear why yet. One thing is for sure, though, we have determined that we have polar bears denning in Alaska in adequate numbers to support the population.
We were interested in making the information about denning useful to management. It wasn’t clear to us that describing where bears denned in the past would help predict where bears might den in the future. So we went out looking at the dens – after the bears had left, of course – crawled in, made measurements, etc. We visited the den sites after the snow left to see what the habitat looked like beneath the snow. We noted in many cases that there was evidence that a bear had denned there long after the bear had left. Here’s hair left on a gravel bank, after a bear had denned there the previous winter. We found out that in Alaska, where the coastline is very flat, there’s relatively little in the way of topographic relief and what relief there is is these bank habitats that occur mainly along coastlines, barrier islands, stream channels, and occasionally dry lake beds. Those are the places that polar bears choose to den. In the wintertime, these banks get filled up almost entirely with snow, which is why they’re attractive to polar bears. There’s enough snow for them to construct a snow cave and create a den.
We worked with a cartographer to develop a map showing the distribution of highly probable polar bear denning habitat across a big chunk of northern Alaska. We’re in the process of extending this both to the east and west right now. Having identified the places where polar bears prefer to den, we thought well, now, if we can figure out how to search those areas to see if there was a bear in a particular piece of that habitat, then we’d be in a really good position from a management standpoint, because we’d really be able to protect polar bear dens. We began to test whether we could see dens with forward-looking infrared device. We hung it on the nose of a helicopter, and it creates an image that looks like this. Light colors are warmer sea ice and the very dark areas are the big deep snow drifts. Those are the places we’d expect to find polar bear dens. On infrared, you could see a light spot in the middle of a dark area – that’s the signature of a polar bear den as seen through infrared. Here’s a den that’s actually open – that’s why it’s so bright – the bear had left the day before we came. But there was still enough latent heat in that den that it looked like a flashlight as viewed through the infrared. Some dens never appeared on infrared, or sometimes appeared and sometimes didn’t. We determined several important factors for having dens show up, like temperature dew point, sunlight, airborne moisture. The sun would raise the heat on the surface just enough so that it would erase the signature. Basically, we just couldn’t do it when sunlight was shining on the snow. We made some recommendations in a paper that’s coming out in the April edition of BioScience, talking about how this technique might be used for protecting polar bear denning sites in the future. In this slide, we see an example of what we hope to prevent. These people obviously didn’t consult our map, but hopefully they will in future. This is the sort of thing that’s totally avoidable, so hopefully this den map will help out.
Diet is another way that polar bears differ from most other members of the bear family. Whereas black bears and brown bears are omnivores, the polar bear is strictly a carnivore and a predator. In Alaska, a main source of food is the ring seal. Polar bears also eat bearded seals and any other marine mammal they can catch. They catch their prey by sneaking up on them, hauling them out at leads or at breathing holes. There’s been a sort of evolutionary war going on between polar bears and ring seals, where polar bears try every way they can to become more effective hunters of ring seals and ring seals meanwhile are trying not to be polar bear food. One thing that you see is that in the spring, when seals are hauling out to let their fur grow (they need to be warmed by the sun), they punctuate their basking by lifting their heads up to look around for polar bears, while also lifting their tails so that they can immediately smack down and push into the water if necessary. Another way that polar bears find seals in springtime is by searching for their undersnow lairs (below the snow, above the sea ice). Polar bears have become very adept at finding these caves.
Predatory lifestyle has its risks. There’s always a chance for injury. Jaw injuries are probably fatal to polar bears. Here’s a large male that we caught in the fall that should have weighed around 1300 pounds and he was just emaciated. We couldn’t figure out why until we opened his mouth to give him a lip tattoo and discovered that his maxilla was broken all the way through. He apparently had lunged at a seal, maybe hit his jaw on a block of ice, or maybe it happened while fighting. In the brown bear and black bear literature, there’s plenty of evidence of bears breaking their jaws, having them heal (perhaps at odd angles), and them surviving the injury just fine. But with polar bears, the particle size of their food is such that without the ability of those canine teeth to penetrate down on the prey, the ability to forage is seriously compromised. We’ve never found evidence of healed jaw injuries in polar bears. So it’s a good supposition that jaw injuries can be fatal to polar bears.
I want to take a minute now to look into the future for polar bears. We’ve made a lot of management progress dealing with many things, but one of the things that we really don’t have much control over are the changes presently occurring in the sea ice habitat. We’ve seen a loss in the last 25 years of over a million square kilometers of sea ice in the Northern Hemisphere and people are predicting that this trajectory is going to continue. In northern Alaska, that has meant longer periods when the ice has pulled away and periods where the ice has pulled away farther than ever before. So you have situations where the waters near shore, which are most productive, are covered by ice for shorter periods of time. We’re seeing far more polar bears in northern Alaska and this may have something to do with the retreating sea ice. We’ve also seen that because there are several places in northern Alaska where local native peoples harvest bowhead whales and butcher them right on the beach, there are a lot of whale remains right on the beach. We’ve got a little study going on right now to try and understand the significance of whale remains in the diet of polar bears. We’re also looking at how we can model sea ice use by polar bears to determine the kinds of ice that they prefer. If we can predict what the ice is going to look like in the future, perhaps we can predict what the polar bear distribution is going to look like.
One final comment. You’ll remember that I said that the ice in Hudson Bay has always retreated entirely. Well, in the last couple of decades it’s been melting a little bit sooner – 2 to 2.5 weeks erarlier, on average. It’s hypothesized that that’s why females have been coming ashore at lighter weight and having smaller cubs. One thing we’ll be watching for are similar changes in Alaskan waters. So far we haven’t seen them. We may in fact be seeing some transient benefits to the warming phenomenon, but we know that if the sea ice continues to retreat, the benefits will quickly dissipate.
Question: Is there still debate about whether polar bears are induced ovulators? Answer: I don’t know for sure, but I’ve long assumed that they are. Don’t know whether the data has been developed extensively enough to answer that conclusively.
Question: Is there sexual variance of "homebodies" vs. mobile bears? Do male bears tend to range farther than female bears?
Answer: All radio telemetry is from females, only because of the thick neck of the male, so we don’t know the answer to that question. One small study that Amstrup published in 1999 described movements of males. It was a very small sample size, but their movements didn’t appear to be significantly different from females. Most mark and recapture information has suggested very little difference in movement patterns between males and females, which is very different from most terrestrial carnivores. But the ecosystem of the sea ice is also very different from land. Age doesn’t seem to have anything to do with it either. More just individual idiosyncrasies.
Question: What about females with cubs? Answer: Less mobile the first year compared to later. Bears with young cubs show a high level of activity but low level of mobility. Anyone with small children understands that.
Question: Breeding season is March-May, but does that include the time when males are squaring off? It’s something we see in the captive population – males squaring off long before they’re showing any interest in the females, and that happens prior to March.
Answer: Most researchers are not out there to know whether males are squaring off long before actual breeding. We’re usually out there in March at the easiest because that’s the earliest we can get enough light and enough good sea ice. And we’re usually off the ice by May. So, we’re not out there earlier to know whether this is a pre-breeding behavior. However, the only time we see males fighting is when females are around. Otherwise, males avoid each other.
Question: Are we attributing change in denning distribution to global climate change phenomenon? Answer: We haven’t studied data thoroughly enough to ascribe an explanation yet. I wouldn’t be surprised if changes in the conditions of the sea ice has made it less suitable for denning, but don’t know for sure yet.
Question: Are there legal protections in place to know that once you have the data, companies will have to comply with protecting denning areas? Are there regulations to prevent interference with polar bear dens?
Answer: That is the hope of this information, but we’ll get an answer from Scott Schliebe.
Question: Are the three populations that you listed genetically distinct?
Answer: No. Across the polar basin there is some genetic structure among polar bear populations. In the polar basin, there’s very little genetic structure. We’ve done some limited surveys that show a few genotypes that are more common in some areas than others, but not statistically common. There are no geographic boundaries. The populations may be discrete from a management standpoint, but they aren’t discrete from a genetic standpoint.
Back to Natural History
So what is a polar bear? The polar bear is the largest non-aquatic predator on the globe. Large adult male stands almost 5 feet high at the shoulder on all fours, can be almost 12 feet long, feet the size of platters, 45 or 50 inch neck. But like all members of the bear family, they start out small and vulnerable. Polar bears have a low reproductive rate and long reproductive cycle. They breed in the spring (March and April), but they have delayed implantation, so development is arrested after a few hundred cells until October or November, at which point implantation occurs. Birth is thought to occur – in the Alaska region at least – around the first of the year. The cubs stay in the maternal den until springtime (March, April) and then they remain with the female through their first, second years of life. They’re finally weaned at close to 3 years of age, at which point they leave their mother and their mother is free to breed again. If a female is successful in bringing her brood through the weaning stage, she can breed no more often than every 3 years. So that’s a pretty low reproductive rate.
Polar bear cubs grow rapidly. They emerge from dens weighing around 30 pounds, with males typically a little heavier than females. By the fall of the year, females weigh about 150 pounds and males a bit more. As spring yearlings, females weigh about 180 pounds and males heavier. By fall, they are substantially heavier.
In the Beaufort Sea region, growth patterns are perhaps a bit different than they are in other regions. In the Hudson Bay area, for example, when the females come ashore in the summertime they don’t have anything to eat. The ice is gone, the main food sources are gone, so they don’t hit their peak weights in the fall. Typically, they’re at their peak weights at the time they come ashore. In the Beaufort Sea, it’s a much different situation.
As two-year-olds, they continue to grow and become pretty substantial creatures by this time in their life. One important thing to point out: Females generally stop growing in the 400-500 pound range, whereas males continue to grow pretty much throughout their life. Females hit a certain weight that gives them enough mass to reproduce, and then once they hit that reproductive age, they pretty much stop growing. Males continue to grow pretty much throughout their lives, or at least until the age when they stop being competitive, which is around the mid-20s.
Distribution of polar bears is closely tied to the distribution of sea ice in the Northern Hemisphere. Pretty much wherever there’s persistent pack ice for a major portion of the year we find polar bears. In Alaska, the ice penetrates down into the Bering Sea, often as far south as the Aleutian Chain. But when the ice retreats in the summer, most of the polar bears go with it. This is in contrast to the situation in the Hudson Bay, where the bay is covered with ice through the wintertime but then disappears entirely during the summer. As it disappears, drifting to the south, the bears are trying to stay on it as long as they can. As it melts entirely, the bears get off on the southern southwestern shore of Hudson Bay where Churchill is, and that’s where bears tend to congregate. The situation in Alaska is quite a lot different. This slide shows the distribution of ice in autumn 2002. The ice had retreated that year about as far north as it ever does and the vast majority of polar bears – indicated by these yellow dots – were on that ice. At that time, we had 3 radio-collared bears on the beach. Much different than Hudson Bay.
Polar bears are very much at home on this sea ice. They can swim very well. Sometimes swimming in the Arctic requires breaking a little ice. When it’s too thin to walk on but too thick to plow through with their chest, they can actually dive under the water and swim up to 30 yards or so holding their breath. They’ll poke their heads up and breathe. I’ve seen them cross leads that are a mile or so wide, and every 30 yards or so they’ll poke their head up to get a breath. Pretty impressive creatures.
The ice is where they’ll find their mates, and sometimes where they’ll enter maternal dens. Females with young select habitats that provide greater security for their young. Of course, the sea ice is also where polar bears find most of their nutrition.
Polar bears are probably the most mobile of all quadrupeds. We know this from radio telemetry studies that we’ve been doing recently. First, I thought I’d tell you about how we catch polar bears. Because of their mobility and low density, they’re very hard to catch. It’s very difficult to follow them on foot. It’s even more difficult to sneak up on them and give them an injection. We’ve come to depend entirely upon the use of helicopters to capture them. We hover helicopters over them at close distance so that we can shoot them with a projectile syringe. Sometimes they take more than one injection and after a few minutes they have difficulty standing up or following a straight line. Within about 5 minutes they lay down and fall asleep and at that point we move in and begin our work. We take a number of measurements, tag their ears, tattoo lips, take weight. The main thing I want to talk about is attaching radio transmitters to polar bears.
The advent of Effective Satellite Radio Telemetry Research has really been a boon to understanding the distribution of polar bears. As I said, they are highly mobile, and this slide shows an example of a bear we followed for two years in an area over 300,000 square kilometers. We have had some animals with activity area up to 600,000 square kilometers, about the size of the state of Montana. So, they’re extremely mobile animals. But they’re also highly individualistic. Some animals are less mobile and we tend to call them “homebodies” for lack of a better term. For example, this bear in the Southern Beaufort Sea area occupied a much smaller area, far less than 100,000 square kilometers, over 2 years. So there’s a lot of individual variation and we never really would have understood that without Satellite Radio Telemetry. One of the things we can do with this data, which is a whole bunch of dots on a map, is depict their activity areas (or home ranges) with mathematical models that actually surround the cloud of points that represent their distribution. This slide shows one of those very mobile polar bears versus the activity area of one of those homebodies. Here’s another: a very mobile bear versus a homebody. We’ve taken a look at the distribution of all of the bears, we’ve clustered them together, and determined that in this part of the world there are 3 different populations, or stocks, of bears. There’s the Chukchi Sea stock, the South Beaufort Sea stock, and the Northern Beaufort Sea stock. One of the problems with this type of view of distribution of animals is this tremendous area of overlap of the three populations. What good is it to describe three different stocks if they overlap so extensively? That, of course, is a considerable problem for wildlife managers who might need to allocate harvest to different villages or to understand the impact of an oil field development might have on a particular stock. So we’ve been developing a way to make these sorts of radiotelemetry data – which are always sort of a backward view of where bears have been, a retrospective view – more appropriate for real-time management decisions. One of the things we’ve been able to do is divide the study into a series of grid squares, and have been able to determine the relative probability of a bear in a particular grid cell being from a particular population. If you look from a bigger perspective, those grid cells, each with its own individual relative probability, convert into contour boundaries. So you can look at any place along the coast and say what the probability is of a bear being from one of those populations. This is very useful for harvest management and other things like that.
One of the ways that polar bears differ from other members of the bear family is in maternal denning. In polar bears, only pregnant females enter dens for the wintertime. With brown bears and black bears, the whole population dens over the winter because the things that they normally eat are unavailable. Hibernation is a foraging strategy. In the case of polar bears, they eat seals that are available year-round. Pregnant female polar bears need to den in order to provide a secure environment for their newborn cubs. So denning in polar bears is purely a reproductive strategy. Another difference is that polar bears over most of their range den just in snow. Whereas black and brown bears might tunnel under trees or dig into the ground or occupy a hollow log, polar bears are surrounded just by snow and ice. They dig a snow cave in the fall of the year and subsequent storms cover them over.
Up until a few years ago, it was thought that Alaska didn’t have much in the way of polar bear denning. In fact in1980 there was debate between Russian and Canadian research scientists about who really “owned” Alaskan polar bears. At that time there had only been about 30 dens documented in northern Alaska. We turned again to radio telemetry to try and figure out where polar bears in Alaska were denning. By putting radio collars on pregnant females and tracking them to their dens, we were able to document that in fact Alaska has quite a few dens. Many dens were out on the drifting pack ice, which was kind of an unusual and unexpected discovery. From 1981-1991 – the first decade of our efforts to document – we found about 140 dens and within the last decade there appeared to be fewer bears denning on the pack ice and perhaps more denning in the westerly portion. Not clear why yet. One thing is for sure, though, we have determined that we have polar bears denning in Alaska in adequate numbers to support the population.
We were interested in making the information about denning useful to management. It wasn’t clear to us that describing where bears denned in the past would help predict where bears might den in the future. So we went out looking at the dens – after the bears had left, of course – crawled in, made measurements, etc. We visited the den sites after the snow left to see what the habitat looked like beneath the snow. We noted in many cases that there was evidence that a bear had denned there long after the bear had left. Here’s hair left on a gravel bank, after a bear had denned there the previous winter. We found out that in Alaska, where the coastline is very flat, there’s relatively little in the way of topographic relief and what relief there is is these bank habitats that occur mainly along coastlines, barrier islands, stream channels, and occasionally dry lake beds. Those are the places that polar bears choose to den. In the wintertime, these banks get filled up almost entirely with snow, which is why they’re attractive to polar bears. There’s enough snow for them to construct a snow cave and create a den.
We worked with a cartographer to develop a map showing the distribution of highly probable polar bear denning habitat across a big chunk of northern Alaska. We’re in the process of extending this both to the east and west right now. Having identified the places where polar bears prefer to den, we thought well, now, if we can figure out how to search those areas to see if there was a bear in a particular piece of that habitat, then we’d be in a really good position from a management standpoint, because we’d really be able to protect polar bear dens. We began to test whether we could see dens with forward-looking infrared device. We hung it on the nose of a helicopter, and it creates an image that looks like this. Light colors are warmer sea ice and the very dark areas are the big deep snow drifts. Those are the places we’d expect to find polar bear dens. On infrared, you could see a light spot in the middle of a dark area – that’s the signature of a polar bear den as seen through infrared. Here’s a den that’s actually open – that’s why it’s so bright – the bear had left the day before we came. But there was still enough latent heat in that den that it looked like a flashlight as viewed through the infrared. Some dens never appeared on infrared, or sometimes appeared and sometimes didn’t. We determined several important factors for having dens show up, like temperature dew point, sunlight, airborne moisture. The sun would raise the heat on the surface just enough so that it would erase the signature. Basically, we just couldn’t do it when sunlight was shining on the snow. We made some recommendations in a paper that’s coming out in the April edition of BioScience, talking about how this technique might be used for protecting polar bear denning sites in the future. In this slide, we see an example of what we hope to prevent. These people obviously didn’t consult our map, but hopefully they will in future. This is the sort of thing that’s totally avoidable, so hopefully this den map will help out.
Diet is another way that polar bears differ from most other members of the bear family. Whereas black bears and brown bears are omnivores, the polar bear is strictly a carnivore and a predator. In Alaska, a main source of food is the ring seal. Polar bears also eat bearded seals and any other marine mammal they can catch. They catch their prey by sneaking up on them, hauling them out at leads or at breathing holes. There’s been a sort of evolutionary war going on between polar bears and ring seals, where polar bears try every way they can to become more effective hunters of ring seals and ring seals meanwhile are trying not to be polar bear food. One thing that you see is that in the spring, when seals are hauling out to let their fur grow (they need to be warmed by the sun), they punctuate their basking by lifting their heads up to look around for polar bears, while also lifting their tails so that they can immediately smack down and push into the water if necessary. Another way that polar bears find seals in springtime is by searching for their undersnow lairs (below the snow, above the sea ice). Polar bears have become very adept at finding these caves.
Predatory lifestyle has its risks. There’s always a chance for injury. Jaw injuries are probably fatal to polar bears. Here’s a large male that we caught in the fall that should have weighed around 1300 pounds and he was just emaciated. We couldn’t figure out why until we opened his mouth to give him a lip tattoo and discovered that his maxilla was broken all the way through. He apparently had lunged at a seal, maybe hit his jaw on a block of ice, or maybe it happened while fighting. In the brown bear and black bear literature, there’s plenty of evidence of bears breaking their jaws, having them heal (perhaps at odd angles), and them surviving the injury just fine. But with polar bears, the particle size of their food is such that without the ability of those canine teeth to penetrate down on the prey, the ability to forage is seriously compromised. We’ve never found evidence of healed jaw injuries in polar bears. So it’s a good supposition that jaw injuries can be fatal to polar bears.
I want to take a minute now to look into the future for polar bears. We’ve made a lot of management progress dealing with many things, but one of the things that we really don’t have much control over are the changes presently occurring in the sea ice habitat. We’ve seen a loss in the last 25 years of over a million square kilometers of sea ice in the Northern Hemisphere and people are predicting that this trajectory is going to continue. In northern Alaska, that has meant longer periods when the ice has pulled away and periods where the ice has pulled away farther than ever before. So you have situations where the waters near shore, which are most productive, are covered by ice for shorter periods of time. We’re seeing far more polar bears in northern Alaska and this may have something to do with the retreating sea ice. We’ve also seen that because there are several places in northern Alaska where local native peoples harvest bowhead whales and butcher them right on the beach, there are a lot of whale remains right on the beach. We’ve got a little study going on right now to try and understand the significance of whale remains in the diet of polar bears. We’re also looking at how we can model sea ice use by polar bears to determine the kinds of ice that they prefer. If we can predict what the ice is going to look like in the future, perhaps we can predict what the polar bear distribution is going to look like.
One final comment. You’ll remember that I said that the ice in Hudson Bay has always retreated entirely. Well, in the last couple of decades it’s been melting a little bit sooner – 2 to 2.5 weeks erarlier, on average. It’s hypothesized that that’s why females have been coming ashore at lighter weight and having smaller cubs. One thing we’ll be watching for are similar changes in Alaskan waters. So far we haven’t seen them. We may in fact be seeing some transient benefits to the warming phenomenon, but we know that if the sea ice continues to retreat, the benefits will quickly dissipate.
Question: Is there still debate about whether polar bears are induced ovulators? Answer: I don’t know for sure, but I’ve long assumed that they are. Don’t know whether the data has been developed extensively enough to answer that conclusively.
Question: Is there sexual variance of "homebodies" vs. mobile bears? Do male bears tend to range farther than female bears?
Answer: All radio telemetry is from females, only because of the thick neck of the male, so we don’t know the answer to that question. One small study that Amstrup published in 1999 described movements of males. It was a very small sample size, but their movements didn’t appear to be significantly different from females. Most mark and recapture information has suggested very little difference in movement patterns between males and females, which is very different from most terrestrial carnivores. But the ecosystem of the sea ice is also very different from land. Age doesn’t seem to have anything to do with it either. More just individual idiosyncrasies.
Question: What about females with cubs? Answer: Less mobile the first year compared to later. Bears with young cubs show a high level of activity but low level of mobility. Anyone with small children understands that.
Question: Breeding season is March-May, but does that include the time when males are squaring off? It’s something we see in the captive population – males squaring off long before they’re showing any interest in the females, and that happens prior to March.
Answer: Most researchers are not out there to know whether males are squaring off long before actual breeding. We’re usually out there in March at the easiest because that’s the earliest we can get enough light and enough good sea ice. And we’re usually off the ice by May. So, we’re not out there earlier to know whether this is a pre-breeding behavior. However, the only time we see males fighting is when females are around. Otherwise, males avoid each other.
Question: Are we attributing change in denning distribution to global climate change phenomenon? Answer: We haven’t studied data thoroughly enough to ascribe an explanation yet. I wouldn’t be surprised if changes in the conditions of the sea ice has made it less suitable for denning, but don’t know for sure yet.
Question: Are there legal protections in place to know that once you have the data, companies will have to comply with protecting denning areas? Are there regulations to prevent interference with polar bear dens?
Answer: That is the hope of this information, but we’ll get an answer from Scott Schliebe.
Question: Are the three populations that you listed genetically distinct?
Answer: No. Across the polar basin there is some genetic structure among polar bear populations. In the polar basin, there’s very little genetic structure. We’ve done some limited surveys that show a few genotypes that are more common in some areas than others, but not statistically common. There are no geographic boundaries. The populations may be discrete from a management standpoint, but they aren’t discrete from a genetic standpoint.
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