Grizzly bears, Ursus arctos horriblils, are
a subspecies of the brown bear. This being so, not all brown bears are
grizzlies, but all grizzlies are brown bears.
This is not to say that the color of all grizzly bears
is brown. Natural variation in pellage occurs, and colors
can range from black to blonde to brown (shown in above photo). This variation makes the distinction between
and grizzly bears difficult for some people, but, unlike their
cousins, grizzly bears have a hump between their shoulder blades. This is easily noted in the latter two photos above, and it is this
characteristic that serves as the easiest way to differentiate
between the two species when observed from a distance.
Brown bears are distributed throughout
the holarctic regions of North America, Asia, and Europe.
It has long been believed that the bears first evolved either
in Europe or Asia, migrating to Alaska during the Pleistocene
(Kurten 1968). After the
retreat of the ice sheets, the bears were able to expand their
range south into what are currently Canada, the contiguous United
States, and northern Mexico. Molecular data support this hypothesis, as
phylogenetic trees made from both nuclear (DNA obtained from the
nucleus of a cell that is inherited from both parents) and mitochondrial
DNA (obtained from the mitochondria of a cell that is maternally
inherited) put Eurasian bears at the base, followed by three clades
(groups) of bears in Alaska, and one in the lower 48.
Interestingly, these genetically based trees also show
that polar bears (Ursus maritimus)
are closely related to one of the Alaskan clades, specifically
those bears located in the ABC (Admiralty, Baranof, Chichagof)
Islands in the SE part of the state.
This means that polar bears most likely evolved from brown
bears after they crossed the Bearing Straight.
Diet and adaptations
Grizzly bears, contrary to the belief of many,
are not man-eating monsters.
In fact, grizzly bears are omnivores, and much of their
of plants, berries, roots and seeds, as well as moths, carrion,
and cutthroat trout
Examination of the teeth
of grizzly bears clearly demonstrates this, as in addition to
their carnassial pair, a set of teeth generally designed for consuming
flesh, grizzly bears have well defined bunodont molars (short
cusped) that are common in most omnivores, including humans.
Humans and grizzly bears
Being reclusive animals, it is thought by many that bears prefer
areas with low human densities.
This is most likely due to the fact that the bears have
found humans to be inhospitable neighbors (though some are trying
to remedy this). Humans are the primary cause of mortality
for the bears, in large due to fear and other such views. This has only been compounded by the fact that
bears are huge, males ranging from 300-800 pounds, depending
upon the quality of their habitat.
Excessive human induced mortality has resulted in ESA
listing of grizzly bears, as well as a recent
concern about their genetic
Like most carnivores, rates of fecundity for grizzly bears
are low. This is compounded
by the fact that it takes several years for bears to reach sexual
maturity, as they generally live to be about 20 years of age. Mating generally occurs between themonths of May and July, and females
implantation of the blastocyst
(fertilized egg) until late summer or autumn.
Gestation from the time of insemination until parturation
(birth) is usually between 180 - 250 days, with the bears giving
birth in winter while still in their dens (Feldhammer 1999).
|Grizzly bears do not undergo true hibernation? Each winter, bears undergo torpor, which
is similar to hibernation, in that the animal’s metabolic
rate is slowed and body temperature drops.
Due to the fact that a bear’s body does not reach
ambient temperature, it is said to undergo carnivorian
lethargy, or “false hibernation.” This is not unique to bears, though.
Nothing larger that marmots, which are a bit
bigger than house cats, are capable of true hibernation
due to the energy requirements for raising the body
temperature to a functioning level (Feldhammer 1999).