By Jeremy Nicholson, Regional Wildlife Biologist
Idaho Department of Fish and Game monitors a number of collared grizzly bears. One of these bears is a seven year old female, #728. This particular sow gave birth to her first litter of three cubs in 2015. She was spotted on an observation flight in September with her three offspring, now yearlings. Based on GPS data, she entered her den in early November. It is likely she entered the den with all of her young from last year. That is quite the feat for a female bear, particularly if it was her first litter.
Pregnant females are usually the first bears to enter a den, followed by females with young, solitary females, and finally, males. The timing of den entrance varies from year to year according to weather and food availability. Female bears enter their den in October and November, whereas males enter their den by November to December.
Bears remain in their den for four to six months without feeding or drinking. To make this possible, bears need significant fat reserves in place to begin their slumber.
From late July to den entrance, a bear needs to increase its body weight by roughly 30 percent. Bears spend up to 20 hours per day feeding, and consume 20,000 calories in a single day to pack on the pounds. Gaining girth is especially important for pregnant females. Expectant mothers need at least 20 percent body fat to have a good chance of successfully producing cubs.
Bears in the Yellowstone Ecosystem excavate their own dens. Many times a hole is dug under a fallen tree, root ball, or into the side of a slope to provide overhead cover. Dens are found on moderately steep slopes, most often between 30 degrees and 60 degrees at elevations above 7,500 feet. Research has shown that bears seek a new den location each year. However, dens used for consecutive years by the same bear and other bears have been observed.
Bear #728 did not reuse her den from last year. She settled within a couple miles of her previous den, near Reas Peak in the Caribou-Targhee National Forest. If events unfold as expected, she will push her offspring away soon after they emerge from their den in the spring. She will breed again in the summer. Come next winter, a new litter of cubs may fill her den.