EFFECTS OF FOOD SUPPLY, PREDATION, CANNIBALISM, PARASITES, AND
OTHER HEALTH PROBLEMS ON BLACK BEAR POPULATIONS
Lynn L. Rogers
Intraspecific Aggression and Cannibalism
The role of resident adult black bears in aggressively
preventing immigration into their ranges has been well documented
in long-term population and behavior studies (Kemp 1976, Rogers
1977, Young and Ruff 1982). Resident adult males repel
immigrating subadult males, reducing future competition for mates
and reducing immediate competition for food, with potential
benefits extending to offspring and pregnant mates. Females in
Minnesota occasionally chase or fight females that encroach upon
their territories (Rogers 1977), and an immigrant subadult male
ran from a territorial female that did not give chase (Rogers,
unpublished data). How often these potential immigrants are
killed rather than evicted is unknown. Thorough studies to
determine causes of death among dispersing subadult males have
not been conducted. The only report I found of a free-ranging
subadult male of dispersal age (2 or 3 years of age) being killed
by another bear is that of LeCount (1982). In that case, a
subadult male with bear bite wounds on its head was captured in
June and monitored until Infection from the wounds killed the
bear the following winter.
Whether or not immigrants are killed directly, aggression by
residents toward them conceivably could prevent their access to
preferred food patches and could force them into suboptimal
habitat (Rogers 1976, Bunnell and Tait 1981). This, together with
the high mobility of dispersing males, may explain the
disproportionately high number of subadult males that become
nuisances in residential areas (Rogers et al. 1976) and the high
number of these bears that are shot (Rogers 1976, Bunnell and
Tait 1981). In Minnesota, eartag returns showed that at least 8
(24%) of 33 males that were born in the study area and studied
there until the age of dispersal were shot outside that area as
dispersing 2- or 3-year-olds. How much these losses of subadult
males eventually affected population density is unknown. To
properly assess the effect on population density, the losses must
be considered in relation to any gains in reproductive success
and cub survival among resident bears-as a result of decreased
competition for food. This will require additional study.
Several authors have suggested that black bear populations
that are not limited by gunshot might be limited by large bears
killing smaller ones (Kemp 1972, 1976, Lindzey and Meslow 1977,
Bunnell and Tait 1981, LeCount 1982). Thorough study of a
population in which human-related mortality is not the usual
cause of adult deaths has not been conducted due in part to
inaccessibility of such populations, so this idea has not been
tested. Indications from studies of black bear
social organization are that both males and females conceivably-
could increase their fitness through prudent cannibalism (Rogers
1977). Benefits of killing genetically unrelated bears include
food value of carcasses and reduction of competition for food,
mates, and space. Costs are expenditures of energy and incurring
injuries that lead to reduced reproductive success. Considering
risk. of injury, cost:benefit ratios from cannibalism probably
are most favorable when bears kill nonkin that are too small or
in too vulnerable a situation to inflict significant injury.
Numerous researchers have reported that bears behaving abnormally
because they are drugged or in traps are attacked (Kemp 1976,
Rogers 1977, Beecham 1979, Johnson and Pelton 1980). In natural
situations, too, reported victims of cannibalism have been either
very young bears or denning bears. Deaths of 6 cubs, 2 yearling
males, a yearling female, and 2 adult denning females have been
documented as follows:
1. A mother with cubs killed and ate a cub from another
litter in Yellowstone National Park in late summer 1930
(Arnold 1930).
2. A male climbed a tree, killed a cub, returned to the
ground, and ate it in Yellowstone National Park on 14
July 1959 (M. Hornocker, personal communication, 1974).
3. One of two radio-collared cubs accompanying their mother
was killed by another bear in Arizona (LeCount, personal
communication, 1982).
4. A cub was killed during or after a fight between two
adults in Yosemite National Park on 17 July 1955
(Hartesveldt 1955).
5. A radio-collared yearling female that was traveling with
its mother in Arizona was killed by an adult female or a
subadult male according to the track size of the attacker
(LeCount, personal communication, 1982).
6. A radio-collared yearling male was killed and partially
eaten by a 5-year-old male in Alberta, Canada, in early
fall 1976 (Tietje, Pelchat, and Ruff, personal
communication, 1982).
7. A radio-collared yearling was killed by a male in
Alberta, Canada, in 1977 (Ruff and Kemp 1980).
8. A large bear killed and ate a mother and two cubs at a
den in the upper peninsula of Michigan in mid-April 1963
(D. Wenzel, unpublished report on file at the Michigan
Department of Natural Resources Headquarters at Crystal
Falls, Michigan).
9. A 16-year-old radio-collared female was dug out of a den,
killed, and partially eaten by a bear in early
October 1976 in Alberta, Canada. Evidence suggested the
predator was a male (Teitje, Pelchat, and Ruff, personal
communication, 1982).
Additional evidence of attempted or actual cannibalism
include reports that a male attempted to catch a cub in
Yellowstone National Park (Barnes and Bray 1967); a bear scat
collected in May 1973 on Long Island, Washington, contained
remains of a cub (Lindzey and Meslow 1977) ; and a 5-year-old
radio-collared male was dug out of its den and wounded by another
bear in Alberta in mid-October 1976 (Teitje, Pelchat, and Ruff,
personal communication, 1982).
Despite these observations, indications are that cannibalism
is rare in black bear populations studied to date. Documentation
is infrequent even in garbage dumps and national parks where
black bears are concentrated and highly visible. No radio-
collared yearling, subadult, or adult was killed during studies
in Minnesota, although undetected cannibalism of noninstrumented
cubs could have occurred. Whether cannibalism is a major cause
of mortality in any black bear population and whether such deaths
follow any pattern other than opportunistic predation remain to
be determined. Telemetry technology has now advanced to the
point that instrumenting cubs and radio-tracking dispersing
subadults are feasible. Thorough studies of mortality in these
bears would significantly increase our knowledge of black bear
biology and behavior.
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