The Secret Life of Bog Bears

It’s January which means the newest generation of black bear cubs have arrived! For this post, I examine the world of black bears in Maine and how they use bogs for denning and foraging.

Excluding the densely populated coastal, southern and Downeast regions of the state, Maine has over 26,000 square miles of suitable bear habitat. That accounts for more than 90% of the land. Consisting of mostly secondary growth conifer-deciduous forest, these areas provide an ample variety of food including berries, nuts, fruits, and carrion. Within this densely forested and largely uninhabited (by humans) region, are many wetlands and bogs. Over 7,800 square miles of the state is covered by freshwater wetlands (source), peatlands making up 1,350 square miles (source). With much of the state being so wet, it is unsurprising that some bear populations have taken to wetlands such as bogs to be their year-round home base. One population in the Alton Bog, an approximately 2,300-acre fen bisected by I-95 near Alton, Maine has been studied for decades. State Biologist Randy Cross and dozens of field technicians from the University of Maine, Orono have been collecting data on Maine bears for over 45 years making this the longest running black bear demography study in the United States. At the height of the program, 30 bog bears had been fitted with VHF radio collars.

Winter in Alton: The semi-frozen Alton Bog is pictured to the left. A sign at the Alton Municipal Offices at the right displays the current temperature of 29 degrees Fahrenheit on January 4th, 2021.

Black bears are managed as a game species by the Maine Department of Inland Fisheries and Wildlife. The state’s bear management program is primarily funded through hunting permits which allows biologists to afford the expenses needed to study the population. By understanding bear demographics, biologists are able to make informed decisions on how many bears can be harvested each season. This process starts by fitting bear cubs with radio collars in March when they are about a year and three months old. So far, the program has trapped and monitored over 3,000 bears. Having identified this many bears allows biologists to keep track of health, mortality rates, and significant events that occur within the population. For example, bears typically live for about 30 years in the wild, but Cross’s team found that one female they had been tracking had given birth at 26 years old! Because of this research we also know that the black bear population in Maine is stable, even increasing from 23,000 individuals in 2004 to 36,000 in 2015, with a significant subset spending ample time in bogs.

Bogs serve as both a food source and shelter for foraging and denning bears. Bogs are some of the first areas to sprout grasses and herbaceous plants like impatiens and jack-in-the-pulpit in the spring which provide much needed calories after months of not eating. Later in the summer, blueberries and huckleberries produce sugary fruits, perfect for replenishing body fat lost during the winter. In the fall, cranberries are available for consumption which help bears bulk up for the winter. Of course, not all bear food that will sufficiently create enough fat stores can be found in bogs. Female bears, called sows, have a home range of around one square mile and will search for hazelnuts, beechnuts, and acorns in upland forests in the fall in preparation to enter the winter den. Contrary to popular belief, bears are not actually true hibernators, rather they go into a state of torpor. Animals that either hibernate or go into torpor both lower their body temperatures and heart rates while they are sleeping. Animals in torpor, however, can wake up easily and run away from predators if they feel threatened, or in the case of a female bear, to give birth. Once bear cubs are born in January, the sow will typically go back to sleep with her newborns safely nestled in the den against her for warmth. The cubs will remain tucked away for three months until March at which point they will have grown from 8 ounces to 4 pounds (source).

Field technicians with the Maine Department of Inland Fisheries and Wildlife Bear Program fit a female yearling with a radio collar.

Left: Mitchell Jackman preparing two newborn cubs to be tagged. Their mother was fitted with a VHF radio collar by Randy Cross’s team as a yearling which is how they knew where her den was. Right: Three newborn cubs receive ear tags.

In terms of shelter, bog bears may take an unconventional approach compared to their upland-dwelling kin. Many subtle elevation changes in the landscape known as microtopography are present throughout older, undisturbed bogs, creating dry islands. According to researchers studying the Alton Bog population, some bears take advantage of this by constructing nests of sticks on top of the heath out in the open. Others will den in tree cavities, under root balls or in white cedar buttresses. Adult bears don’t seem to mind these wetter spots due to their torpor-induced lowered body temperature, thick fur, and high body fat content keeping them warm. However, this strategy may be fatal for newborn cubs, so females may move dens to reduce cub mortality. In contrast, the cool temperature of the water helps to cool hot bears in the summer. In addition to being in close proximity to an abundance of food, denning in bogs may be a human-avoidance strategy. Bears are hunted August through November and may have learned that they are safer from humans and bear dogs in these expansive wetlands.

Well, there you have it! Just one more reason why bogs are the coolest habitats around. Thanks for reading!