|
Photo by Ben Augustine |
Female black
bears typically reach sexual maturity at three years of age, depending
upon their physical condition. Biologically, male bears may reach
maturity at three to four years of age, but their ability to breed is
often limited due to the presence of older, more mature males. The
breeding season occurs from June to mid-July and males will breed with
as many females as possible. Births occur in January while the female
is denning. Females give birth to
one to five cubs, with an average litter size of two. Litters are
produced every other year and cubs generally weigh only 8–16 ounces at
birth.
While
pregnancy lasts approximately 220 days, embryonic development does not
continue throughout the gestation period. In fact, bears possess a
unique reproductive adaptation called “delayed implantation” that allows
births to occur when females are generally in prime physical condition.
After females are bred in June, the fertilized eggs remain in a state of
arrested development and are not implanted in the uterus until the
fall. Once implanted, embryonic development resumes and growth only
occurs during the remaining 10 weeks of pregnancy. Consequently, cubs
are born in late January while still in the den and weigh only ounces at
birth.
The advantage
of this reproductive strategy is that females are able to devote all
energy reserves towards milk production at a time when natural foods are
practically nonexistent. In years of poor acorn production, however,
females may not be able to store adequate fat reserves and reproduction
can be significantly decreased. Generally, however, cubs that weighed
only ounces at birth will emerge from their dens weighing 5–10 pounds
and will be physically prepared to meet their new world.
Offspring
remain with their mother for the entire year after birth and generally
throughout the second denning season. After spring emergence, the
mother will run off the yearling bears as she comes into estrus and
prepares for the new breeding season. This is actually for the benefit
for her young as adult males will often kill smaller bears during the
breeding season. Yearlings can weigh anywhere from 20–100 pounds,
depending upon food availability, as they strike out to establish their
own new territory.
|
