Post by warsaw on Dec 18, 2012 10:50:50 GMT -9
Hal W. Reynolds
C. Cormack Gates
Randal D. Glaholt
C N. American bison, plains bison, prairie bison, bison,
buffalo, wood bison, or woodland bison
S N. Bison bison
S. B. b. bison, B. b. athabascae
The bison is a member of the family Bovidae, to which domestic
cattle, muskox (Ovibos moschatus), sheep, and goats belong. Both
sexes of bovids possess true horns, which are never shed. These horns
are composed of a bony core and a hard, outer sheath of epidermis. Fossil
bovids date to the Lower Miocene (Feldhamer et al. 1999). Bovids are
grazers primarily and browsers secondarily, and they possess a fourchambered,
ruminating stomach. They inhabit major grassland, shrubland,
forest, and tundra ecosystems. They feed by biting off forage with
forward-projecting incisors, which are present only in the lower jaw.
The genus Bison is characterized by a short, broad forehead with a
narrowed muzzle and pointed nasal bones (Hall 1981). Bison are particularly
noted for a massive head, short neck, a high hump at the shoulders,
and short, curved, rounded horns, which exhibit annual growth patterns
(Soper 1964). The moderate-length tail is haired, with a terminal tassel
of long hair (Banfield 1974). There is a distinct beard formed by long,
woolly hair on the chin. The hair on the head, neck, and shoulders is
brownish black. Body pelage is generally brown, varying moderately
with season to light brown. Although technically a misnomer, the popular
name buffalo has been used interchangeably for North American
bison since early European explorers first encountered the species on
the continent. True buffalo, while they are bovids, belong to genera
distinctly different from bison and do not possess the shoulder hump
characteristic of bison. The African cape buffalo (Syncerus caffer) is
native only to Africa, whereas the water buffalo (Bubalus bubalis) is
native to Asia.
Plains Bison. The American bison ranged throughout much of North
America (Fig. 48.1). They were formerly widespread from western
and northern Canada across the United States and into northern Mexico
(Meagher 1986). Ernest Thompson Seton estimated that 75 million
were in North America before White settlers arrived (Dary 1989). However,
McHugh (1972) estimated that 30 million bisonwas the maximum
number that available range could support. Although millions of bison
once roamed this region, few free-ranging herds of North American
bison remain. Descriptions of historical distribution patterns for plains
bison are provided by Allen (1876), Hornaday (1889), Skinner and
Kaisen (1947), Roe (1970), and Meagher (1986). The original distribution
of the American bison included most of central North America,
from northernMexico to Great Slave Lake and fromWashington to the
Rocky Mountain states (Banfield 1974).
In Canada, the northeastern boundary for the original range of
plains bison is roughly outlined by a line extending from north-central
Saskatchewan in a southeastward direction to the southern shore of the
Great Lakes. The northern boundary for bison in central Canada is also
approximated by this line (Fig. 48.1).
In the United States, the eastern boundary was that of the Allegheny
Mountains extending south through the states of Maryland,
Virginia, North Carolina, and South Carolina. The southern boundary
for plains bison extended from Alabama across southern Mississippi
and Louisiana and continued westward along the southeastern coast of
Texas and into Mexico. It was in southeastern Texas near present-day
Houston that the American bison was first seen by a European, Cabeza
de Vaca, in 1530 (Hornaday 1889).
Thewestern boundary of North American plains bison distribution
generally extended northward from north-centralMexico, merging with
the original range for wood bison in northeastern British Columbia
near the central western border of Alberta, Canada (Fig. 48.1). Here,
the western and northern limits of plains bison range approximated the
boundary of the ecotone between grassland, aspen parkland, and boreal
forest habitat. To the north of this interface is boreal forest habitat,
which is the beginning of the original range of wood bison.
Wood Bison. Based on additional information since 1987, the original
range of wood bison in North America has been expanded to include a
large area northwest ofwhatwas previously designated “historic range”
(Gates et al. 2001b), including much of Alaska, theYukon, and thewestern
portion of the Northwest Territories (Fig. 48.1). Oral narratives
obtained from aboriginal people and radiometric data of bison bones
from archaeological and paleontological locations (van Zyll de Jong
1986; Harington 1990) indicate that bison were present in the Yukon
and Alaska during the last few hundred years and persisted in small
numbers into the early twentieth century (Lotenberg 1996; Stephenson
et al. 2001), similar to areas in northeastern British Columbia and the
southwestern Northwest Territories (Gates et al. 1992a). In view of
the historical documentation and physical evidence demonstrating that
wood bison inhabited this region for several thousand years, the area
properly constitutes “historic” range. Therefore, in the absence of objective
and biologically meaningful criteria, the dichotomy between
“historic” and “prehistoric” range is not useful for the purposes of
conservation and recovery, and the continuum of history should be recognized
to avoid this artificial distinction (Stephenson et al. 2001). The
descriptive term “original range” better represents this concept (Gates
et al. 2001b).
The original range of wood bison included most of the boreal regions
of northern Alberta, northeastern British Columbia, a small portion
of northwestern Saskatchewan, the western Northwest Territories,
most of the Yukon, and much of Alaska (Fig. 48.1) (Harington 1977;
van Zyll de Jong 1986; Wood Bison Recovery Team 1987; Guthrie
1990; Gates et al. 2001b; Stephenson et al. 2001). Populations persisted
in the area south of Great Slave Lake in Canada, but wood bison
were extirpated in other parts of their range. The coniferous forests and
aspen (Populus) parkland with interspersed grass and sedge (Carex sp.)
meadows and prairies, typical of this area, constituted the main habitat
for the wood bison throughout this expansive region.
DESCRIPTION
Plains Bison. The unmistakable appearance of plains bison is characterized
by a massive, heavy head with a short, broad nasal area. The
large head appears to be carried low because of the high shoulder hump
and massive forequarters (Pattie and Fisher 1999). A short, thick neck
and a high shoulder hump leave the impression that the forequarters
are out of proportion to the much smaller appearing hindquarters. The
hindquarters of plains bison are lighter than the forequarters, a disproportion
that is further accented by differing pelages between front and
rear. Pelage is long over the forehead, neck, hump, and forequarters,
but short over the rear and tail (Meagher 1986). There is a tufted tail of
moderate length (Banfield 1974). The short, round, black horns rise laterally
from the side of the head and curve inward over the head. Horns
of the female are more slender and tend to curve inward to a greater
degree than those of the male. The eyes are located anterolaterally on
the head, and the ears are nearly invisible as they are buried under the
long thick pelage of the head (Meagher 1986). Plains bison have rather
short legs and large, rounded hooves,which leave tracks similar to those
of domestic cattle. Sexual dimorphism is evident among adults, with
females being smaller and slighter. In general, however, females resemble
males in color, body configuration, and presence of permanent
horns. Males have larger, more evenly curving horns, with bases that
are buried in head hair; a larger hump and thicker neck; longer pelage
on the forehead, chin (beard), ventral mane, and chaps of the forelegs;
and a better defined, demarcated cape of longer hair on the forequarters
ending abruptly with shorter hair on the flanks and rear (Meagher
1986).
Wood Bison. Wood bison possess the same general characteristics as
plains bison except for some minor differences in body morphology,
general conformation, pelage, and skeletal measurements. For example,
wood bison have larger horn cores and exhibit differences in other cranial
elements. Karsten (1975) reported that wood bison possess denser
fur than plains bison, they are larger and heavier (within similar age and
sex classes as verified by greater mass), more elongated in the forequarters,
and darker in color, and they have a squarish hump with a more
gently sloping back contour than the plains bison. Geist and Karsten
(1977) described howthewood bison bull and cowdiffer significantly in
external morphological characteristics from their prairie counterparts.
There is less sexual dimorphism in wood bison compared with plains
bison in respect to body size, horn structure, pelage characteristics, and
body proportions.Asummary of these differing characteristics follows.
1. Hair on top of the head, around the horns, in the beard, and in the
midventral neck area is significantly shorter and less dense in wood
bison bulls than in their plains counterparts of the same age. Plains
bison generally exhibit a dense, woolly bonnet of hair between the
horns, whereas in wood bison, the dark forelock of hair tends to
hang in strands over the forehead. Thus, the head of the wood bison
appears smaller, the horns longer, and the ears more noticeable. The
beard of the wood bison is thinner and more pointed (Fig. 48.2),
and the long, full throat mane, which extends from the beard to the
brisket on the plains bison, is rudimentary or absent in the wood
bison. The head and neck of the wood bison generally are darker in
color than those of the plains bison.
2. Long hair in the area of the “chaps” on the front legs is well developed,
forming a skirt on the plains bison, but is reduced or absent on
the wood bison (Fig. 48.2). This most striking difference in pelage
between wood and plains bison partly accounts for the more massive
appearance of the plains bison in the front quarters.
3. The “robe” or cape of the shoulders, hump, and neck region of the
plains bison is more distinct and lighter (golden) colored than that
of the wood bison. The well-demarcated cape of the plains bison is
composed of longer hair, which forms an obvious boundary with the
rest of the body fur just posterior to the shoulders and is generally
lighter in color than that of the wood bison. There is no clear cape
demarcation in the wood bison (Fig. 48.2) and the hair is usually
darker than that of the plains bison.
4. The tail of the wood bison is usually longer and more heavily haired
than that of the plains bison.
Size and Weight. Bison are the largest native terrestrial mammal in
North America. Plains bison are smaller than wood bison, based on
body mass. However, bison weights and measurements differ considerably
by age and sex among different localities (Table 48.1). In studying
the relationship of weight to chest girth, Kelsall et al. (1978) discovered
that males were 9.1% heavier than females of equal chest girth in an
approximately linear relationship. However, the maximum amount of
variation for weight explained by chest girth in a study of live male
and female bison in Badlands National Park, South Dakota, was only
64% and 33%, respectively (Berger and Peacock 1988). Although chest
girth may be a good measure for approximating relative weight within
a population, such information is difficult to collect, therefore, an alternative
method for estimating weights is to rely on differences in head
length or width or body length (Berger and Peacock 1988).
McHugh (1958) reported that plains bison bulls approach maximum
size by 5–6 years of age, with small yearly increments for a few
years thereafter. Banfield (1974) stated that male plains bison reached
adult size at 6 years of age, whereas females attained maximum size
at about 4 years. Weight data for plains and wood bison herds at Elk
Island National Park, Alberta, have been sporadically collected over a
30-year period since 1962 and annually since the early 1980s providing
data by which growth could be compared between the two subspecies
and between males and females (Fig. 48.3). Male bison continued to
grow until they were 8–10 years of age (Fig. 48.3A), whereas females
reached mature weight at between 5 and 6 years (Fig. 48.3B). Wood
bison females became markedly heavier than plains bison females after
3 years of age; male weights diverged after about 6 years, when wood
bison males became markedly heavier than their plains counterparts,
based on mean weights by age category.
During the routine, midwinter weighing of bison at Elk Island National
Park, mean weights of wood bison were greater than those for
plains bison (Table 48.1) for each sex and age category (Olson 2002).
The asymptotic mean weight for mature plains bison males was 739 ±
10.0 kg, which was attained at age 8–9 years (Fig. 48.3A). The greatest
overall mean weight for plains bison males of 769 kg was reached at
13 years of age. The asymptotic mature mean weight for wood bison
males was 880 ± 15.1 kg and this was attained at age 8 years, similar
to the plains bison (Fig. 48.3A). The greatest overall mean weight for
wood bison males of 910 kg was reached at age 13 years. The asymptotic
mean weight for mature plains bison females was 440 ± 2.1 kg
and this was attained at 6 years (Fig. 48.3B). The greatest overall mean
weight for plains bison females of 454 kgwas reached at 10 years of age.
The asymptotic mean weight for mature wood bison females was 540±
5.7 kg, exactly 100 kg greater than that for plains bison, and this was attained
at 7 years of age (Fig. 48.3B). The maximum overall meanweight
for wood bison females of 567 kgwas reached at 12 years of age, 2 years
older than when plains bison females reached their maximum weight.
A wide range of intrapopulation and interpopulation variability in
body weight and growth occurs in bison (Berger and Peacock 1988).
Although some studies have implicated age, sex, and season as variables
(Halloran 1961; Lott 1979; Rutberg 1983; Olson 2002), other
factors such as population density, nutrition, weather, reproductive effort,
and inbreeding should be considered as possible influences at the
individual and the population levels (Berger and Peacock 1988). Food
as a limiting factor may lead to smaller animals in poorer condition.
On Santa Catalina Island, California, where bison forage is limited, the
largest plains bison mature cow weighed only 410 kg and was lighter
than the smallest (427 kg) of 18 mature cows studied by Rutberg (1983)
in the National Bison Range, Montana (Lott and Galland 1987). The
average cow in the National Bison Range weighed 482 kg, compared to
only 362 kg for an average Santa Catalina cow, a difference attributed
to poorer nutrition on Santa Catalina Island (Lott and Galland 1987).
MORTALITY
Predation. A few circumstances have suggested occasional predation
by the grizzly bear (Ursus arctos) on bison calves and adults in Yellowstone
National Park,Wyoming (McHugh 1958; Meagher 1973). In
the past, predation on bison by wolves in Yellowstone has not been a
problem, as evidenced by the long survival time of injured and solitary
animals and by the fact that wolves were rare and never observed in
packs (Meagher 1973). However, with the reintroduction of 31 Canadianwolves
intoYellowstone National Park in 1995–1996, this situation
is expected to change with an increasing number of wolf–bison interactions
and kills made during spring (Smith et al. 2000). Between April
1995 and March 1999, 14 bison kills by wolves were documented;
however, the first wolf-killed bison was not observed until 25 months
after the release (Smith et al. 2000). Furthermore, all kills were made
during late winter when bison were more vulnerable because of poor
condition, injuries, or young age (Smith et al. 2000).
Wolf predation on bison is an important mortality factor in Wood
Buffalo National Park, Alberta, (Fuller 1961, 1966; Oosenbrug and
Carbyn 1982; Carbyn and Trottier 1987, 1988; Carbyn et al. 1993,
1998; Carbyn 1997). Fuller (1961) concluded that bison form the staple
diet of Wood Buffalo National Park wolves during summer and
winter. He found that 80% of the summer wolf scats examined contained
bison hair, and in an early-winter sample of 59 wolf stomachs,
65% of the contents was bison (Fuller 1966). During winter 1978/79 in
Wood Buffalo National Park, a study of radio-collaredwolves indicated
that bison were their major prey item. The Hornaday River wolf pack,
composed of 10 members, killed an average of one bison every 7.8 days
from 12 February to 31 March 1979. This equaled an estimated consumption
rate of 5.3 kg/wolf/day and an estimated winter predation of
19 bison or 9.9% of the study area population (Oosenbrug and Carbyn
1982). Similar consumption rates were observed in the Slave River
Lowlands, Northwest Territories, where wolves killed 13 adult female
bison, 2 adult males, and 4 calves during 33 pack-weeks of effort (Van
Camp 1987).
In Wood Buffalo National Park during the 1950s, wolves selectively
preyed on calves or old animals, but predationwas not considered
to be detrimental to the bison population (Fuller 1961, 1966). However,
during the 1978–1979 wolf study in the park, a higher proportion of
bison killed by wolves were adult males (Oosenbrug and Carbyn 1982),
whereas in the Slave River Lowlands, cows and calves were killed more
often (Van Camp 1987). However, packs of wolves in other areas of
Wood Buffalo National Park preferentially attacked bison herds with
calves (Carbyn and Trottier 1987, 1988). Similarly, wolves associated
with the Mackenzie wood bison population exhibited a preference for
bison calves (Gates and Larter 1990). Recently reintroduced wolves in
Yellowstone National Park showed a decided preference for bison that
were more vulnerable because of poor condition, injury, or young age
(Smith et al. 2000). Prey species, such as bison, are more vulnerable in
winter and represent a greater amount of resource per unit effort at that
time than do smaller prey species.
In the Slave River Lowlands, free-ranging bison herds numbered
at least 2000 animals in 1971, but had declined to an estimated 750 by
March 1977.Wolf numbers in the region were estimated to be between
64 and 76 during winter 1976/77 (VanCamp1987). During this time, six
packs ofwolveswere observed operating in the Lowlands, ofwhich four
were radio-marked. During 1975–1977, bison was the most important
of six major prey types of wolves in this region and represented 88%
of prey weight of wolf diets in winter (Van Camp 1987). Based on
estimates for 22 weeks of winter in 1976/77, wolf predation accounted
for approximately 31% of the adult and subadult mortality and about
27% of the calf mortality. When combined with hunting mortality, it
accounted for at least 70% of the adult and subadult bison lost that
year (Van Camp 1987). Wolf predation was exerting a major role in
the continued decline of the Slave River Lowlands bison population. A
wolf control program selectively removed 72 wolves from the region
during the winters of 1977–1979, but the bison population continued
to decline (Van Camp 1987).
In conclusion, wolf predation is a significant limiting factor for
some bison populations (Van Camp 1987; Carbyn and Trottier 1988;
Carbyn et al. 1993, 1998) and may regulate diseased herds at lowdensity
(Gates 1993). The “disease–predation” hypothesis described by Gates
(1993) and Joly and Messier (2001b) suggests that tuberculosis and
brucellosis reduce bison survival and reproduction, thereby increasing
vulnerability of some animals to predation and causing populations to
decline to chronically low densities where predation by wolves can be
regulatory. In contrast, disease-free bison populations may be regulated
at a high-density equilibrium where interspecific food competition can
be regulatory, despite the presence of wolves.
Hunting. During regulated seasons from 1968 to 1977, sport hunting
of bisonwas permitted in the Slave River Lowlands north of Fort Smith,
Northwest Territories. The reported number of bison kills from license
www.wildlifeaccidents.ca/docs/bisonchap48%20p-1009-1060reynoldsgatesglaholt290803.pdf
C. Cormack Gates
Randal D. Glaholt
C N. American bison, plains bison, prairie bison, bison,
buffalo, wood bison, or woodland bison
S N. Bison bison
S. B. b. bison, B. b. athabascae
The bison is a member of the family Bovidae, to which domestic
cattle, muskox (Ovibos moschatus), sheep, and goats belong. Both
sexes of bovids possess true horns, which are never shed. These horns
are composed of a bony core and a hard, outer sheath of epidermis. Fossil
bovids date to the Lower Miocene (Feldhamer et al. 1999). Bovids are
grazers primarily and browsers secondarily, and they possess a fourchambered,
ruminating stomach. They inhabit major grassland, shrubland,
forest, and tundra ecosystems. They feed by biting off forage with
forward-projecting incisors, which are present only in the lower jaw.
The genus Bison is characterized by a short, broad forehead with a
narrowed muzzle and pointed nasal bones (Hall 1981). Bison are particularly
noted for a massive head, short neck, a high hump at the shoulders,
and short, curved, rounded horns, which exhibit annual growth patterns
(Soper 1964). The moderate-length tail is haired, with a terminal tassel
of long hair (Banfield 1974). There is a distinct beard formed by long,
woolly hair on the chin. The hair on the head, neck, and shoulders is
brownish black. Body pelage is generally brown, varying moderately
with season to light brown. Although technically a misnomer, the popular
name buffalo has been used interchangeably for North American
bison since early European explorers first encountered the species on
the continent. True buffalo, while they are bovids, belong to genera
distinctly different from bison and do not possess the shoulder hump
characteristic of bison. The African cape buffalo (Syncerus caffer) is
native only to Africa, whereas the water buffalo (Bubalus bubalis) is
native to Asia.
Plains Bison. The American bison ranged throughout much of North
America (Fig. 48.1). They were formerly widespread from western
and northern Canada across the United States and into northern Mexico
(Meagher 1986). Ernest Thompson Seton estimated that 75 million
were in North America before White settlers arrived (Dary 1989). However,
McHugh (1972) estimated that 30 million bisonwas the maximum
number that available range could support. Although millions of bison
once roamed this region, few free-ranging herds of North American
bison remain. Descriptions of historical distribution patterns for plains
bison are provided by Allen (1876), Hornaday (1889), Skinner and
Kaisen (1947), Roe (1970), and Meagher (1986). The original distribution
of the American bison included most of central North America,
from northernMexico to Great Slave Lake and fromWashington to the
Rocky Mountain states (Banfield 1974).
In Canada, the northeastern boundary for the original range of
plains bison is roughly outlined by a line extending from north-central
Saskatchewan in a southeastward direction to the southern shore of the
Great Lakes. The northern boundary for bison in central Canada is also
approximated by this line (Fig. 48.1).
In the United States, the eastern boundary was that of the Allegheny
Mountains extending south through the states of Maryland,
Virginia, North Carolina, and South Carolina. The southern boundary
for plains bison extended from Alabama across southern Mississippi
and Louisiana and continued westward along the southeastern coast of
Texas and into Mexico. It was in southeastern Texas near present-day
Houston that the American bison was first seen by a European, Cabeza
de Vaca, in 1530 (Hornaday 1889).
Thewestern boundary of North American plains bison distribution
generally extended northward from north-centralMexico, merging with
the original range for wood bison in northeastern British Columbia
near the central western border of Alberta, Canada (Fig. 48.1). Here,
the western and northern limits of plains bison range approximated the
boundary of the ecotone between grassland, aspen parkland, and boreal
forest habitat. To the north of this interface is boreal forest habitat,
which is the beginning of the original range of wood bison.
Wood Bison. Based on additional information since 1987, the original
range of wood bison in North America has been expanded to include a
large area northwest ofwhatwas previously designated “historic range”
(Gates et al. 2001b), including much of Alaska, theYukon, and thewestern
portion of the Northwest Territories (Fig. 48.1). Oral narratives
obtained from aboriginal people and radiometric data of bison bones
from archaeological and paleontological locations (van Zyll de Jong
1986; Harington 1990) indicate that bison were present in the Yukon
and Alaska during the last few hundred years and persisted in small
numbers into the early twentieth century (Lotenberg 1996; Stephenson
et al. 2001), similar to areas in northeastern British Columbia and the
southwestern Northwest Territories (Gates et al. 1992a). In view of
the historical documentation and physical evidence demonstrating that
wood bison inhabited this region for several thousand years, the area
properly constitutes “historic” range. Therefore, in the absence of objective
and biologically meaningful criteria, the dichotomy between
“historic” and “prehistoric” range is not useful for the purposes of
conservation and recovery, and the continuum of history should be recognized
to avoid this artificial distinction (Stephenson et al. 2001). The
descriptive term “original range” better represents this concept (Gates
et al. 2001b).
The original range of wood bison included most of the boreal regions
of northern Alberta, northeastern British Columbia, a small portion
of northwestern Saskatchewan, the western Northwest Territories,
most of the Yukon, and much of Alaska (Fig. 48.1) (Harington 1977;
van Zyll de Jong 1986; Wood Bison Recovery Team 1987; Guthrie
1990; Gates et al. 2001b; Stephenson et al. 2001). Populations persisted
in the area south of Great Slave Lake in Canada, but wood bison
were extirpated in other parts of their range. The coniferous forests and
aspen (Populus) parkland with interspersed grass and sedge (Carex sp.)
meadows and prairies, typical of this area, constituted the main habitat
for the wood bison throughout this expansive region.
DESCRIPTION
Plains Bison. The unmistakable appearance of plains bison is characterized
by a massive, heavy head with a short, broad nasal area. The
large head appears to be carried low because of the high shoulder hump
and massive forequarters (Pattie and Fisher 1999). A short, thick neck
and a high shoulder hump leave the impression that the forequarters
are out of proportion to the much smaller appearing hindquarters. The
hindquarters of plains bison are lighter than the forequarters, a disproportion
that is further accented by differing pelages between front and
rear. Pelage is long over the forehead, neck, hump, and forequarters,
but short over the rear and tail (Meagher 1986). There is a tufted tail of
moderate length (Banfield 1974). The short, round, black horns rise laterally
from the side of the head and curve inward over the head. Horns
of the female are more slender and tend to curve inward to a greater
degree than those of the male. The eyes are located anterolaterally on
the head, and the ears are nearly invisible as they are buried under the
long thick pelage of the head (Meagher 1986). Plains bison have rather
short legs and large, rounded hooves,which leave tracks similar to those
of domestic cattle. Sexual dimorphism is evident among adults, with
females being smaller and slighter. In general, however, females resemble
males in color, body configuration, and presence of permanent
horns. Males have larger, more evenly curving horns, with bases that
are buried in head hair; a larger hump and thicker neck; longer pelage
on the forehead, chin (beard), ventral mane, and chaps of the forelegs;
and a better defined, demarcated cape of longer hair on the forequarters
ending abruptly with shorter hair on the flanks and rear (Meagher
1986).
Wood Bison. Wood bison possess the same general characteristics as
plains bison except for some minor differences in body morphology,
general conformation, pelage, and skeletal measurements. For example,
wood bison have larger horn cores and exhibit differences in other cranial
elements. Karsten (1975) reported that wood bison possess denser
fur than plains bison, they are larger and heavier (within similar age and
sex classes as verified by greater mass), more elongated in the forequarters,
and darker in color, and they have a squarish hump with a more
gently sloping back contour than the plains bison. Geist and Karsten
(1977) described howthewood bison bull and cowdiffer significantly in
external morphological characteristics from their prairie counterparts.
There is less sexual dimorphism in wood bison compared with plains
bison in respect to body size, horn structure, pelage characteristics, and
body proportions.Asummary of these differing characteristics follows.
1. Hair on top of the head, around the horns, in the beard, and in the
midventral neck area is significantly shorter and less dense in wood
bison bulls than in their plains counterparts of the same age. Plains
bison generally exhibit a dense, woolly bonnet of hair between the
horns, whereas in wood bison, the dark forelock of hair tends to
hang in strands over the forehead. Thus, the head of the wood bison
appears smaller, the horns longer, and the ears more noticeable. The
beard of the wood bison is thinner and more pointed (Fig. 48.2),
and the long, full throat mane, which extends from the beard to the
brisket on the plains bison, is rudimentary or absent in the wood
bison. The head and neck of the wood bison generally are darker in
color than those of the plains bison.
2. Long hair in the area of the “chaps” on the front legs is well developed,
forming a skirt on the plains bison, but is reduced or absent on
the wood bison (Fig. 48.2). This most striking difference in pelage
between wood and plains bison partly accounts for the more massive
appearance of the plains bison in the front quarters.
3. The “robe” or cape of the shoulders, hump, and neck region of the
plains bison is more distinct and lighter (golden) colored than that
of the wood bison. The well-demarcated cape of the plains bison is
composed of longer hair, which forms an obvious boundary with the
rest of the body fur just posterior to the shoulders and is generally
lighter in color than that of the wood bison. There is no clear cape
demarcation in the wood bison (Fig. 48.2) and the hair is usually
darker than that of the plains bison.
4. The tail of the wood bison is usually longer and more heavily haired
than that of the plains bison.
Size and Weight. Bison are the largest native terrestrial mammal in
North America. Plains bison are smaller than wood bison, based on
body mass. However, bison weights and measurements differ considerably
by age and sex among different localities (Table 48.1). In studying
the relationship of weight to chest girth, Kelsall et al. (1978) discovered
that males were 9.1% heavier than females of equal chest girth in an
approximately linear relationship. However, the maximum amount of
variation for weight explained by chest girth in a study of live male
and female bison in Badlands National Park, South Dakota, was only
64% and 33%, respectively (Berger and Peacock 1988). Although chest
girth may be a good measure for approximating relative weight within
a population, such information is difficult to collect, therefore, an alternative
method for estimating weights is to rely on differences in head
length or width or body length (Berger and Peacock 1988).
McHugh (1958) reported that plains bison bulls approach maximum
size by 5–6 years of age, with small yearly increments for a few
years thereafter. Banfield (1974) stated that male plains bison reached
adult size at 6 years of age, whereas females attained maximum size
at about 4 years. Weight data for plains and wood bison herds at Elk
Island National Park, Alberta, have been sporadically collected over a
30-year period since 1962 and annually since the early 1980s providing
data by which growth could be compared between the two subspecies
and between males and females (Fig. 48.3). Male bison continued to
grow until they were 8–10 years of age (Fig. 48.3A), whereas females
reached mature weight at between 5 and 6 years (Fig. 48.3B). Wood
bison females became markedly heavier than plains bison females after
3 years of age; male weights diverged after about 6 years, when wood
bison males became markedly heavier than their plains counterparts,
based on mean weights by age category.
During the routine, midwinter weighing of bison at Elk Island National
Park, mean weights of wood bison were greater than those for
plains bison (Table 48.1) for each sex and age category (Olson 2002).
The asymptotic mean weight for mature plains bison males was 739 ±
10.0 kg, which was attained at age 8–9 years (Fig. 48.3A). The greatest
overall mean weight for plains bison males of 769 kg was reached at
13 years of age. The asymptotic mature mean weight for wood bison
males was 880 ± 15.1 kg and this was attained at age 8 years, similar
to the plains bison (Fig. 48.3A). The greatest overall mean weight for
wood bison males of 910 kg was reached at age 13 years. The asymptotic
mean weight for mature plains bison females was 440 ± 2.1 kg
and this was attained at 6 years (Fig. 48.3B). The greatest overall mean
weight for plains bison females of 454 kgwas reached at 10 years of age.
The asymptotic mean weight for mature wood bison females was 540±
5.7 kg, exactly 100 kg greater than that for plains bison, and this was attained
at 7 years of age (Fig. 48.3B). The maximum overall meanweight
for wood bison females of 567 kgwas reached at 12 years of age, 2 years
older than when plains bison females reached their maximum weight.
A wide range of intrapopulation and interpopulation variability in
body weight and growth occurs in bison (Berger and Peacock 1988).
Although some studies have implicated age, sex, and season as variables
(Halloran 1961; Lott 1979; Rutberg 1983; Olson 2002), other
factors such as population density, nutrition, weather, reproductive effort,
and inbreeding should be considered as possible influences at the
individual and the population levels (Berger and Peacock 1988). Food
as a limiting factor may lead to smaller animals in poorer condition.
On Santa Catalina Island, California, where bison forage is limited, the
largest plains bison mature cow weighed only 410 kg and was lighter
than the smallest (427 kg) of 18 mature cows studied by Rutberg (1983)
in the National Bison Range, Montana (Lott and Galland 1987). The
average cow in the National Bison Range weighed 482 kg, compared to
only 362 kg for an average Santa Catalina cow, a difference attributed
to poorer nutrition on Santa Catalina Island (Lott and Galland 1987).
MORTALITY
Predation. A few circumstances have suggested occasional predation
by the grizzly bear (Ursus arctos) on bison calves and adults in Yellowstone
National Park,Wyoming (McHugh 1958; Meagher 1973). In
the past, predation on bison by wolves in Yellowstone has not been a
problem, as evidenced by the long survival time of injured and solitary
animals and by the fact that wolves were rare and never observed in
packs (Meagher 1973). However, with the reintroduction of 31 Canadianwolves
intoYellowstone National Park in 1995–1996, this situation
is expected to change with an increasing number of wolf–bison interactions
and kills made during spring (Smith et al. 2000). Between April
1995 and March 1999, 14 bison kills by wolves were documented;
however, the first wolf-killed bison was not observed until 25 months
after the release (Smith et al. 2000). Furthermore, all kills were made
during late winter when bison were more vulnerable because of poor
condition, injuries, or young age (Smith et al. 2000).
Wolf predation on bison is an important mortality factor in Wood
Buffalo National Park, Alberta, (Fuller 1961, 1966; Oosenbrug and
Carbyn 1982; Carbyn and Trottier 1987, 1988; Carbyn et al. 1993,
1998; Carbyn 1997). Fuller (1961) concluded that bison form the staple
diet of Wood Buffalo National Park wolves during summer and
winter. He found that 80% of the summer wolf scats examined contained
bison hair, and in an early-winter sample of 59 wolf stomachs,
65% of the contents was bison (Fuller 1966). During winter 1978/79 in
Wood Buffalo National Park, a study of radio-collaredwolves indicated
that bison were their major prey item. The Hornaday River wolf pack,
composed of 10 members, killed an average of one bison every 7.8 days
from 12 February to 31 March 1979. This equaled an estimated consumption
rate of 5.3 kg/wolf/day and an estimated winter predation of
19 bison or 9.9% of the study area population (Oosenbrug and Carbyn
1982). Similar consumption rates were observed in the Slave River
Lowlands, Northwest Territories, where wolves killed 13 adult female
bison, 2 adult males, and 4 calves during 33 pack-weeks of effort (Van
Camp 1987).
In Wood Buffalo National Park during the 1950s, wolves selectively
preyed on calves or old animals, but predationwas not considered
to be detrimental to the bison population (Fuller 1961, 1966). However,
during the 1978–1979 wolf study in the park, a higher proportion of
bison killed by wolves were adult males (Oosenbrug and Carbyn 1982),
whereas in the Slave River Lowlands, cows and calves were killed more
often (Van Camp 1987). However, packs of wolves in other areas of
Wood Buffalo National Park preferentially attacked bison herds with
calves (Carbyn and Trottier 1987, 1988). Similarly, wolves associated
with the Mackenzie wood bison population exhibited a preference for
bison calves (Gates and Larter 1990). Recently reintroduced wolves in
Yellowstone National Park showed a decided preference for bison that
were more vulnerable because of poor condition, injury, or young age
(Smith et al. 2000). Prey species, such as bison, are more vulnerable in
winter and represent a greater amount of resource per unit effort at that
time than do smaller prey species.
In the Slave River Lowlands, free-ranging bison herds numbered
at least 2000 animals in 1971, but had declined to an estimated 750 by
March 1977.Wolf numbers in the region were estimated to be between
64 and 76 during winter 1976/77 (VanCamp1987). During this time, six
packs ofwolveswere observed operating in the Lowlands, ofwhich four
were radio-marked. During 1975–1977, bison was the most important
of six major prey types of wolves in this region and represented 88%
of prey weight of wolf diets in winter (Van Camp 1987). Based on
estimates for 22 weeks of winter in 1976/77, wolf predation accounted
for approximately 31% of the adult and subadult mortality and about
27% of the calf mortality. When combined with hunting mortality, it
accounted for at least 70% of the adult and subadult bison lost that
year (Van Camp 1987). Wolf predation was exerting a major role in
the continued decline of the Slave River Lowlands bison population. A
wolf control program selectively removed 72 wolves from the region
during the winters of 1977–1979, but the bison population continued
to decline (Van Camp 1987).
In conclusion, wolf predation is a significant limiting factor for
some bison populations (Van Camp 1987; Carbyn and Trottier 1988;
Carbyn et al. 1993, 1998) and may regulate diseased herds at lowdensity
(Gates 1993). The “disease–predation” hypothesis described by Gates
(1993) and Joly and Messier (2001b) suggests that tuberculosis and
brucellosis reduce bison survival and reproduction, thereby increasing
vulnerability of some animals to predation and causing populations to
decline to chronically low densities where predation by wolves can be
regulatory. In contrast, disease-free bison populations may be regulated
at a high-density equilibrium where interspecific food competition can
be regulatory, despite the presence of wolves.
Hunting. During regulated seasons from 1968 to 1977, sport hunting
of bisonwas permitted in the Slave River Lowlands north of Fort Smith,
Northwest Territories. The reported number of bison kills from license
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