Black bears begin hibernation in late October or early November. Females with cubs tend to be the first to enter dens with adult males being the last. They enter dens not because it is cold, or snowing, but as an adaptation to deal with dwindling food supplies. An abundant food supply may cause a later hibernation date but a bad food year will not cause a early hibernation date.
Bears will hibernate in excavated dens, under trees, in rock caves or in hollowed-out trees and often will rake leaves, twigs and other plant material into dens as bedding.
During hibernation, the bears body temperature drops about five degrees, its heart rate drops from 40 to 70 beats per minute to 8 to 12 beats per minute, and its metabolism lowers by half. Incredibly, they do not eat, drink, urinate or defecate for the five to six months spent hibernating. They can easily be awakened in dens and will move around on occasion or even venture outside for brief periods.
Hibernating bears have the ability to reuse protein byproducts and live off of stored fat reserves. Even so, male bears may lose between 15% to 30% of their body weight during hibernation, while lactating females may lose up to 40%. Despite adding three to four pounds of fat daily in the fall, bears suffer no heart-related illness. While hibernating, they avoid bone loss by recycling calcium back into their bones, they lose fat but not muscle mass and somehow their muscles don’t atrophy.
Black bears emerge from their dens starting in mid-March with males emerging earlier than females by about two weeks. Females with new cubs typically are the last to exit dens. Weather or elevation of den sites does not seem to effect emergence dates from year to year.
Black bear cubs are born in the den in late January and weigh between 8 and 12 ounces. On average, two cubs are born, although three is not unusual. Newborn cubs are barely the size of squirrels, only about 1/300th to 1/500th the size of their mother. They are born blind, toothless and covered with very fine hair. The cubs do not hibernate, feeding instead on milk from their mothers that contains 33 percent fat. Watch a YouTube video of a crying baby bear.
Cubs grow quickly. They will weigh 10 to 15 pounds when they exit dens and are about the size of a volleyball. Only half of all cubs survive the first six months and one in three will die before its first birthday. In good food years cubs will grow to 45 pounds their first summer and may weigh 300 pounds just two years later.
Cubs stay with their mother their first year and a half (they re-enter dens with their mother and siblings in the fall of their first year) and learn how to protect themselves, locate food and climb trees. Much of what an adult bear knows it has learned from its mother in this formative time. By their second spring they will be self-reliant and will separate from their mother by early summer.
Black bears reproduce slowly. Males are capable of breeding when they are three years old, whereas females do not start producing cubs typically until they are five years old and only give birth every other year. Mating occurs in June. Females carry a fertilized egg in their womb for months and delayed implantation allows the mother a way out if food is scarce. If she has not accumulated enough fat reserves by hibernation, she will reabsorb the fertilized egg rather than continue development of a fetus.
Reproduction and population is determined in large part by the availability and quality of natural foods. In periods of poor natural foods fewer cubs are born and there is an increase in cub mortality.
Delayed Implantation: After mating in June, the fertilized egg develops into a tiny ball of cells or blastocyst, at which time development stops and the blastocyst remains unattached in the uterus. If all goes well, the blastocyst implants in the uterine wall in late November and roughly two months later cubs are born. If the female fails to attain sufficient fat reserves or weight, the blastocyst will not implant and pregnancy is terminated.
Banner photo by Anne Schrier
Mother and cub photo courtesy: Tom Beck
Photo of cubs courtesy: Jennapher Teunnissen van Manen