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Published on: Jan. 18, 2012

Inside A Bear's Den

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dens, three bears denned in rock caves, three in root wads/brushpiles and one each in a tree cavity and a ground nest. Interior den size varied but typically had little extra space and conformed to the size of each bear. A couple of bears did excavate their dens in order to gain more space; both were females with cubs. Den aspects had no consistent patterns. However, for bedding material, all bears consistently used grass, small twigs and leaves.

Depth of the bears’ beds typically ranged from 3 to 5 inches and elevations of dens ranged from 1,100 to 1,580 feet. Finally, all dens were in similar habitat structure consisting of an abundance of downed trees and moderate hardwood regeneration.

Bears den during winter months because their primary food sources—nuts, insects and berries—are scarce, and it would be impossible to find enough to eat. Biologists prefer to say that bears enter a “winter lethargy” and don’t consider their sleep to be a true hibernation. During our March den visits we found that some bears appeared lethargic while others were wide awake.

In the months prior to entering their dens, bears can gain up to 40 pounds of fat per week. Several days before denning, a bear will eat a great deal of roughage, including grass, leaves and small twigs in order to form a fecal plug that stays in place until after den emergence. A bear’s body goes through several changes once it is denned. Its heartbeat can drop from 55 beats per minute to 10 beats per minute, and the bear’s body temperature will drop from 5 to 9 degrees below normal. Bears don’t eat or drink during their winter sleep, and adults can lose 10–40 percent of their body weight. When bears emerge they are hungry and lean.

Pregnant females give birth to cubs (usually two) during winter denning. Cubs are born in January or February, often while the mother is sleeping. They weigh about 8 ounces, but they grow quickly and will weigh around 6 pounds when they leave the den.

No one knows why some bears choose den sites such as hollow trees or rock caves while others den in a brush pile or open woods. Research has demonstrated that cub survival is higher when bears den in trees or protected areas. Bears likely investigate several potential den sites throughout summer and will sometimes move from one den site to another if disturbed. Individual bears don’t usually reuse dens but sometimes their adult offspring will use the den in which they were born. For more pictures of bear dens and Missouri black bears visit the bear project website at

My internship opened my eyes to the work and procedures required in the conservation field. It has given me a real-world example of how the scientific method is used to answer a question. I’ve also, in the words of Jeff, become a “junior bear expert,” since the beginning of my internship. I am thrilled to have had the opportunity to contribute to our state’s growing body of information on black bears, which are once again calling Missouri home.

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