Voss: Bears 101

For Aspen residents, seeing a black bear is hardly unusual. They wander through our neighborhoods, climb trees, cross roads and occasionally end up in our homes or restaurants. 

This year, those encounters have been particularly noticeable. The few bears that have taken a liking to the serviceberries in my yard have me looking both ways before opening my door. 

But before we think about the increased bear activity as a drought problem, a trash problem or a safety problem, it is worth remembering that they are wild animals with an extraordinary biology, and they have been part of this landscape far longer than Aspen has been a town.

As a small animal veterinarian, wild bears don’t exactly fall under my scope of practice. Nonetheless, I find them fascinating and think it’s worth learning more about Aspen’s most notorious neighborhood guests. So, let’s consider this a little Bear 101.

First, they are not all black. Colorado has only one species of bear: the American black bear, Ursus americanus. Despite the name, black bears can be black, brown, cinnamon, blonde or even honey-colored. They are also Colorado’s largest carnivore, weighing in on average between 175 to 275 pounds.

They are remarkably adaptive animals, which is one reason they have managed to persist as human development has expanded throughout their habitat. From 2005 through 2010, a collaborative study involving Colorado State University, the U.S. Department of Agriculture’s National Wildlife Research Center and the Colorado Division of Wildlife tracked black bears in Aspen to better understand what was drawing them into town. Researchers followed 50 bears and found that when natural food sources were scarce, bears expanded their use of urban areas and became increasingly resourceful at finding food around people. When natural food became more abundant, their use of human-dominated areas decreased. In other words, bears don’t necessarily want to live in our backyards. They are remarkably good at adjusting their behavior to whatever the landscape gives them.

Let’s go back to the beginning of a bear’s life: the den. Black bear reproduction is unique. Bears mate during the summer, but the fertilized embryo does not immediately begin developing. Instead, bears use a process called delayed implantation, in which embryonic development pauses for several months before the embryo implants in the uterus later in the year, if the sow has sufficient fat reserves. This allows the mother to time the birth of her cubs with her winter denning period. Once implantation occurs, fetal development is surprisingly rapid, only taking about six to eight weeks. The cubs are born in the middle of winter, while still tucked away in the den.

A typical litter contains one to three cubs. The newborns weigh less than a pound and are blind, toothless and completely dependent on their mother during this stage.

By the time the family emerges from the den in spring, the cubs have grown to roughly 10 to 15 pounds. Increased daylight and warming temperatures, along with availability of food, help cue the bears to emerge from the den. 

The cubs then have a lot to learn. Black bears are opportunistic omnivores, so young bears need to learn from their mother where to forage, where to find shelter and how to navigate their home range. Their diet can include grasses and other vegetation, fruits, nuts, insects, carrion and small animals. Exactly what is on the menu depends heavily on the season and what is available.

Aside from a mother with her cubs, bears are mostly solitary animals. Adult males typically live alone, and females spend much of their time alone except when raising young. Bears may tolerate one another around particularly valuable food sources, but they do not form social groups like other species do. Usually baby bears will leave their mother during their second summer together, when the mother goes into estrus and prepares to mate again.

While we have all seen bears out and about, it’s much less common to find one sleeping. Black bears den in a variety of protected places, including rock shelters, beneath roots or hollowed trees. Their primary goal is to stay protected from the elements. Once they tuck in for the winter, a remarkable physiologic process begins. 

Hibernation is much more complicated than a winter nap. We call it hibernation, although bear hibernation differs from the deep hibernation seen in many small mammals in that they can still wake up if disturbed. A bear’s metabolic rate falls, its heart rate and breathing slow and body temperature decreases. The animal stops eating and drinking and can go months without urinating or defecating. 

Bears can lose as much as 30% of their body weight during this period, which helps explain why they spend so much time preparing for winter. That preparation is called hyperphagia. In the late summer and early fall, bears dramatically increase their food intake and spend much of their time foraging for calorie-dense foods. 

As a veterinarian, one of the most remarkable aspects of bear hibernation is what doesn’t happen. If a dog or cat became profoundly inactive and stopped eating and drinking for several months, we would expect to see significant muscle wasting, bone loss and serious metabolic consequences.

Bears have evolved a different metabolic strategy. During hibernation, their bodies dramatically suppress metabolism while preserving much of their muscle and bone. Research has shown that black bears experience changes in bone turnover and calcium metabolism that help protect their skeleton during months of inactivity and nutritional deprivation.

Bears are remarkably specialized survivors and understanding their biology makes me appreciate sharing our backyards with them even more. 

Dr. Kelly Voss is a veterinarian at Aspen Animal Hospital, where she practices small animal medicine and surgery. She has a background in emergency and critical care and is passionate about bringing high-quality veterinary care to the mountains. Have a question for the column? Email her at dr.kellyvoss@gmail.com.