Dave Garshelis has spent the last 22 years as a bear project leader for the Minnesota Department of Natural Resources. He has directed a study of the ecology and population dynamics of American black bears and specializes in the development of methods for estimating population size and monitoring population trends. In a quest to apply his knowledge to serve species with greater conservation needs, Dave has worked with students in Asia and South America to study four other species of bears, mainly in low-density populations. He is presently Co-chair of the IUCN Bear Specialist Group, which focuses on conservation of all the world’s bears. In addition, he collaborates with researchers in the medical field on aspects of bear hibernation, specifically related to potential human benefits.
Dave had a great deal to share with us about hibernation. Beginning with these fun facts:
- The coldest hibernator is the Arctic ground squirrel. Parts of their body can get down to a temperature of 26°F. Somehow they’re able to do this without incurring any kind cell damage.
- The longest hibernating mammal is the Columbian ground squirrel, which goes into hibernation in late July and wakes up in late April. That’s nine months…75% of the year. The Richardson’s ground squirrel, a close second, goes into hibernation around the first of July and comes out in the middle of March, about 8½ months. These species can be found in the Plains states and provinces.
- Polar bears don’t actually hibernate, except for pregnant females. For polar bears, the time of greatest food abundance is opposite that of the other bears: during the winter, when everything is frozen, so they have access to seals. But in some places like southern Hudson Bay in Manitoba, ice melts during the late spring/early summer and the bears are forced on shore, where they have basically nothing to eat for a few months. Some of the bears will eat dandelions and a few berries, but others apparently eat nothing at all. They are very lethargic and their blood chemistry becomes similar to hibernating bears, so the condition has been referred to as “walking hibernation.” Pregnant females go from a state of walking hibernation to actual hibernation in a den. So they are fasting from late July till March – eight months. During this period, they give birth to cubs, nurse their cubs and eventually have to walk back to the ice to find their first seal meal. The duration of this incredible fasting period has been increasing, with global warming causing less Arctic ice.
- The most perfect hibernator would have to go to the bear. They go the longest without eating, drinking, urinating or defecating. Other species will either have a cache of food, or, like bats, will wake up and drink a little bit of water, or they’ll urinate and defecate sometime during the winter period. Bears don’t do any of those things.
- People often say, “I wish I was a bear so I could sleep for six months.” But recent interesting findings indicate that the hibernators are actually sleep deprived. Their brain does not get the same benefits from hibernation as from sleep. All hibernators have to wake up periodically during the hibernation period.
Sometimes it’s three or four times a day – where they wake up and shiver – and their brain waves suddenly change. It’s not clear exactly why they have to do this, but one of the things that’s been noticed is that when they “wake up”, they actually catch a quick nap because they’re so sleep deprived.
How did you get into this type of work?
After I finished my PhD I was looking for a university job or a research job with an agency. Having previously studied bears for my Masters degree, working for the Minnesota Department of Natural Resources as a bear research biologists sounded like a pretty good job. The hibernation part of it comes with the territory, as far as bears are concerned. It’s a time that we can always find them reliably and can safely check on their health and adjust the fit of the radio collar that we put on them. As we’re doing that, then we may as well also learn about hibernation.
What do you think is the greatest misconception among the general public about hibernation/seasonal slumber?
One very common misconception is that hibernation is caused by cold. That may be true to an extent for cold-blooded animals, but for mammals, hibernation is a response to a scarcity of food. In temperate environments, fruits and insects disappear in the winter, so animals that subsist on these foods must either go somewhere else or hibernate. Secondly, many people have the impression that hibernation is similar to a very deep sleep or coma. People are often surprised, for example, that we have to drug hibernating bears to pull them out of their dens to make physiological measurements and take blood samples. They think that the animals are so sleepy that we could just do that while they’re sleeping, but that’s not the case. Smaller animals, such as ground squirrels, are in a much deeper state, where they cannot be aroused very quickly, but bears are very easily aroused when they’re in hibernation.
What are some of the other animals, like bears, that are very easily aroused?
Bears are the only ones that are considered hibernators that are easily aroused, but there are other animals like skunks and raccoons, for example, that are also easily aroused, but are not considered to be truly hibernating.
I’ve heard some say that bears are not considered to be true hibernators, but you say they are?
It’s sort of a semantic thing –“what is hibernation?” Bear biologists have now agreed that, based on a number of criteria, bears should be considered true hibernators. In fact, they are exceptional hibernators in that they don’t have to eat, drink, urinate or defecate for six or seven months. The main reason that people have considered bears not to be true hibernators is that their temperature doesn’t drop much. Their normal summer body temperature is kind of like ours, around 99° Fahrenheit, and in hibernation it only drops to around 92°F on average, or at the extreme, about 88°F. In other hibernating mammals, body temperatures get down around 40°F, and sometimes even cooler than that. The debate concerns the semantics of what should be considered hibernation.
Going back to those misconceptions…some of our readers wondered if animals can come out of hibernation if temperature increases and then go back into it when the temperature decreases, but based on what you’ve said, I’m assuming they would stay “asleep” through temperature changes.
That’s right, with some caveats. It’s possible that the hibernaculum could get wet from very warm temperatures and a lot of melting snow. The animals could then be forced out because they’re uncomfortable. In some cases, it may get so bad that they’ll have to find a new den. Sometimes it refreezes and they go back in.
Another question our readers asked is about the tolerances various species have for interruption of a hibernation cycle. I’m guessing that could vary widely.
It does vary widely among species, and I’m really just a bear expert, so can’t address this across the range of hibernators. But the whole situation with hibernation is that the animal is going a long period of time without eating or drinking. If there is a disturbance – and the disturbance can be people or very unusual weather conditions – then the animal could lose a lot of its energy stores.
That could be problematic, particularly for smaller species or individuals that don’t have large fat reserves. Bears normally don’t starve even if they are disturbed in their den or have to move to a second den. They have enough fat to cover such contingencies. In fact, they often even have a “back-up” den in mind in case they have to move.
What do you find most fascinating about hibernation?
I think it’s remarkable that not only can animals go for so many months without food and water – but that the situation occurs at the coldest time of the year, when the costs of thermoregulation are the highest. For bears, it’s even more remarkable that in the middle of this hibernation period, they give birth and must also nurse their cubs for several months using only the fat stores they put on in the fall. After they give birth, they eat the placenta, they lick off their cubs and then go back into a hibernating state. When the cubs get hungry, they cry and the mother will change her body position so that the cubs can nurse.
Do the cubs hibernate?
No. The cubs are fully awake. Black bear cubs stay with their mother for 1½ years, and brown bear cubs stay with their mother for, in some cases, 2½ or 3½ years. They den with their mother again, and will all hibernate as a family. Their only non hibernating winter is when they are newborn cubs.
In general, what are a hibernating animal’s greatest needs – before, during and after hibernation?
Both before and after the greatest need has to be food. The critical thing is to put on enough fat to make it through the hibernation period, and afterward, to regain the body mass that was lost during the hibernation period. Some animals double their weight before hibernation, then lose it all. An average bear might lose about 25% of its weight and a female that gives birth to cubs might lose around 40% of its weight during the hibernation period. All of the weight that is going to be lost has to be put on in advance, in the form of fat.
Before hibernation the animal also needs to find a good place to den. Bats, for example, have very critical tolerances for certain temperatures and moisture levels in their hibernaculum. There are certain caves that attract a lot of them because they’re just the right conditions. Then, as conditions of a certain cave change over time because something happens – say, forest conditions change right outside the entrance which affects what’s happening inside the cave – that would change whether the bats continue to use the cave in the future.
If the conditions did change during hibernation, would the bats just die…never wake up?
Yes, they do die. For many smaller species, there is mortality suffered during hibernation. For bears, that seems not to be the case. For bears, the greatest risk of mortality occurs right after they wake up. They might be in a very bad situation from not having put on enough weight prior to hibernation, and then when they wake up, and their metabolism increases, there’s not enough food around and they starve.
So for bears, the greatest threat is after hibernation?
Yes, they seem to be able to make it through hibernation, almost no matter what. Then, when they arouse in early spring and start burning calories at a much higher rate, they need to find food, but there is often not much to eat at that time of year. Bears that are one or two years old, with little body mass, are most susceptible to starve.
Are other, smaller animals more at risk of predation immediately following hibernation because they’re sleepy and slow?
Not so much after, but during: some animals, like ground squirrels, are vulnerable to predation when they’re in their hibernaculum. They attempt to close it off, but something like a badger can dig it out during the middle of the winter, and the hibernating squirrel is completely vulnerable.
A great deal of your work has been focused on the study of American black bears. Have you been able to apply your knowledge to serve more severely threatened species?
I’ve certainly tried to. I’ve learned a lot of methods for studying bears here in Minnesota that can be applied to many other situations around the world.
Specifically, I’ve learned to appreciate how hard (and expensive) it is to track trends in the number of bears or to estimate the number of bears in a population with any certainty. I’ve tried to get at this issue of assessing population trends through simpler means like extracting local knowledge through interviews and looking for evidence of bear use of habitats just from the sign that they leave in the forest.
I know you just returned from China. Has any of the work you’ve done there, in other parts of Asia, in South America or in any other part of the world shed insight on your work in the U.S.?
I’ve discovered what may seem like a curious paradox regarding hunting and the status of bear populations. Hunting is entirely illegal in most parts of Asia and South America, but bear populations there are generally declining due to illegal hunting – poaching. In North America, where there’s an infrastructure of people in enforcement, management, research – and even hunters themselves – built around a system of legal hunting, bear populations are almost consistently increasing. It might appear quite paradoxical that we have increasing populations in areas where there is legal hunting and decreasing populations in areas where hunting is forbidden. I’ve learned now why that is the case. In China, for example, where people in a local village know about a poacher, they consider it to be not their business to inform enforcement personnel because they’re not that concerned about the resource. Here in North America, if somebody hears about a poacher, the first thing they’re going to do is call the authorities. There are even ways for people to call anonymously. We value our wildlife more, and a large part of that value stems from the fact that there are a lot of people that like to hunt. We end up with agencies like the one that I work for, the Minnesota Department of Natural Resources, whose sole responsibility is to manage and maintain these animals. That whole infrastructure isn’t there when you don’t have legal hunting.
What are you working on right now? Anything that can be related to ecological restoration and conservation planning?
We’ve worked for more than 20 years trying to learn about bears’ use of different sorts of habitats, using VHF and GPS radio collars. We’re now about to start a new project in Minnesota looking at bear use of habitat at the edge of their range. The population is expanding both numerically and geographically, with males pushing into new areas ahead of females. What we want to study is what limits this expansion. Is it roads, lack of forest, people killing bears in cornfields? Is it a habitat issue, or is it an issue of people’s tolerance of bears? We’re hoping to start this study in 2006. Hopefully in two years, we will have some pretty good information about what’s going on at the edge of the range. It’s something that really hasn’t been looked at before.
Is there any type of research you think might help ecological restoration, conservation planning and regenerative design practitioners and clients to begin thinking more about hibernation habitat in their work?
For bears, the biggest ecological issue related to hibernation is probably logging activity near den sites, which could cause den abandonment, extreme loss of energy, and abandonment of newborn cubs by females. A lot of this logging takes place specifically in the winter in lowland sites, which in summer are too wet to get into. It’s an important issue, but I don’t know how it can be resolved. The best advice I can give is that if people actually see a bear abandon a den, particularly if there are cubs there, try to arrest activity in the area for a few days, and often it will come back and settle down. Females with cubs may come back and carry their cubs to another site as long as the disturbance doesn’t continue. But if the disturbance continues then the mother won’t come back.
Another thing practitioners can do if they are affecting an area is to be aware of things that make good den sites. For example, large, hollow trees, which are not very common and certainly not of any economic value, if left standing make ideal winter den sites.
What about things like that for other animals?
Lots of work has been done related to improving and maintaining caves or abandoned mines for bat hibernation. People have specifically tried to make sure that the temperature and humidity conditions are right, and have gated the entrance to keep people out. Because there are a number of species of bats that are threatened or endangered, protection of hibernacula has really become quite a big issue.
Logging is an issue affecting another hibernating species we’re featuring in Leaf Litter – the Vancouver Island marmot. How does logging affect conditions during the critical time when the animal wakes up and has to find food?
For black bears, logging is often a positive thing because it enhances growth of the kinds of food they eat early in the spring, like grasses and succulent kinds of vegetation, due to increased light. Mature forests tend to be darker and take longer to warm up. On the other hand, bears do need cover, so very large clear-cuts will deter their use. This is specifically important around wet areas, which are ideal habitats for bears in the spring. We think of protecting small ponds for fish, herps or waterfowl, but those areas are also important for bears because the edges of those ponds are places that have the best bear foods in the spring.
Many of our readers asked if there are species that hibernate in urban and suburban areas.
Bears are certainly one. Pennsylvania is notable for bears hibernating under people’s decks and in culverts under roads. Initially this alarmed people, but after awhile, it became almost a thing of pride to have a bear hibernating really close to your house. There are also species that are not true hibernators, but also look for a den in the winter — like skunks and raccoons – that are notable for often being very close to people. Bats are sometimes under the eaves of houses. You can also try to entice bats to over-winter near your house by making a hibernaculum. There are designs for those sorts of things available on the Internet. Most designs are just adequate for summer roosting, but there are some that are actually good enough for a bat to hibernate in.
Other than building a bat hibernaculum or leaving hollow trees alone, can you suggest any simple things our readers can do in their own communities to help, or at least not hinder hibernating animals?
We have gotten many reports of people who have stumbled across a bear den in the winter. This may be a once in a lifetime event. Certainly enjoy the experience, but keep in mind that disturbance to the hibernating animal can have severe consequences, like abandonment of its den. The animal may appear to be asleep, but its heart rate often increases radically when there is noise outside, especially a snowmobile or loud voices.
Although we’re focused on hibernation as it pertains to ecological restoration, I can’t help but ask…what do you see as some of the potential human benefits from the study of hibernation?
There’s a lot of interest in this. I’ll just mention a few things:
- How is it that bears do not lose their muscle mass and are able to live solely off of fat? –– Answers to this have obvious implications for treating obesity.
- How is it that bears can avoid putting any weight on their bones (they don’t stand up during the whole hibernation period) yet they don’t lose any bone mass? The answer appears related to certain hormones. This has implications for people who are bedridden or in wheel chairs or even astronauts. People who, even for just a few days or weeks, don’t stand up start to lose bone mass very quickly.
- How is it that bears are able to heal wounds while they’re in hibernation? They’re not actually taking in any foods, yet if they have any wounds when they go into hibernation, they are totally healed when they come out. This has implications, for example, for burn patients.
- How are bears able to maintain very high cholesterol levels – particularly in the winter when their levels are in the range of 300-600 – yet they don’t suffer any heart disease? Their good cholesterol, their HDL, seems to be just like normal human HDL, so that’s not the answer.
Has anything been discovered and applied to humans yet, or is all of this still being studied?
Probably the thing that is most near to being applied to humans is a benefit from the chemical, called an opioid, that induces hibernation. Not only bears, but all mammalian hibernators apparently have this sort of chemical that triggers hibernation. If human muscles are treated with these opioids, they are much more able to withstand periods without oxygen. So, in open heart surgery, for example, when the heart is stopped and without oxygen, damage occurs to the heart muscle. By pre-treating the heart by bathing it in hibernation opioids, this damage can be reduced. This is one of the areas closest to having practical applications for people, although it’s not actually in the operating room yet.