What is the black bear reproductive cycle?

reproductive_cycle.jpgBlack bear females typically produce cubs every two years once they become mature. The 2-year reproductive cycle is genetically timed to fit the annual cycle of plant growth and fruiting of the region.

In northeastern Minnesota, black bears mate in late May or June. Sperm fertilizes microscopic eggs in the uterus, and each egg quickly develops into a tiny ball of cells called a blastocyst. Black bears have delayed implantation, which means the blastocysts suspend further development until they implant in the uterine wall in November. After implantation, the blastocysts develop rapidly and become cubs that are born in mid to late January.

Mother and cubs remain together for 16-17 months until May or June of the following year. Then, the family members separate, the mother mates again, and the 2-year cycle repeats.


Cub Survival in Litters of 1,2 3, or 4

What size litter contributes the highest number of surviving cubs to the next generation? In northeastern Minnesota, it is litters of three.

The litter size (3) that contributed the highest number of surviving cubs (2.45) to the next generation was also the most common litter size; 22 (63%) of 35 litters were 3.

Litters of 1 or 2 were usually from first-time mothers. Subsequent litters from these mothers were typically 3.

Litters of 4 showed a higher death rate. Only half those cubs survived a year or more.

Litters of 5 are rare but have been reported from Minnesota, Pennsylvania, New Jersey, and New England. A litter of 6 was reported from Pennsylvania.  


Black Bear Litter Size

black_bear_w_5_cubs_nh_june_2007.jpgHow many cubs can black bears have? The record is six in Pennsylvania. Most litters are two or three. Two is most common in western North America. Three is most common in Minnesota and eastward in North America where food tends to be more abundant than in the west.

In New Hampshire, Tom Sears saw a litter of five shortly after the family emerged from a den in April 2007. He photographed them 6 weeks later when the cubs were about 5 months old.

black_bear_w_5_cubs_nh_4-25-08.jpgAll five cubs survived their first year, which is unusual for such a large litter. They likely received supplemental food, considering the predictability of their movements to allow such photos.


Can malnourished black bears reproduce?

can_malnourished_reproduce.jpg Malnourished black bear females do not successfully reproduce. Their offspring are absorbed in utero, are stillborn, or starve from lack of milk.

In northeastern Minnesota, black bear females reproduce best if they weigh over 176 pounds in the fall. Twenty-eight (93%) of 30 pregnant females weighing over 176 pounds produced surviving cubs, but 16 weighing less than 148 pounds produced no cubs. Of 8 weighing between 148 and 176, only 4 produced cubs, and 43% of their cubs died.

The usual interval between litters is 2 years. But when natural foods are scarce, it sometimes takes 3 or 4 years for mothers to build up enough body reserves to produce the next litter.