www.bearbiology.com/fileadmin/tpl/Downloads/URSUS/Vol_7/Blanchard_Vol_7.pdf SIZE AND GROWTH PATTERNS OF THE YELLOWSTONE GRIZZLY BEAR:
Differences in size, weight, and growth patterns have been reported for several populations of grizzly bears in North America (Pearson 1975; Reynolds 1976, 1981; Ballard 1980; Glenn 1980; Spraker et al. 1981; Craighead and Mitchell 1982; Nagy et al. 1984). Nutrition has been suspected to be the major factor producing these differences in grizzly bears (Rausch 1963) and black bears (U. americanus) (Rogers et al. 1976, Beecham 1980).
The Yellowstone grizzly bear population was intensively studied from 1959 to 1970 by Frank and John Craighead (Craighead et al. 1974). During that period, major dumps were available to grizzly bears and provided a stable seasonal food source. Closure of those dumps in 1970 and 1971 eliminated that food supply for bears within Yellowstone National Park. Dumps serving communities adjacent to the Park were closed in 1982. Eliminating these food sources seriously affected the distribution and dynamics of the population (Knight and Eberhardt 1985). Effects of food supply changes on the size, weight, and growth patterns of bears from that population are reported here.
STUDY AREA
The study area is approximately 20,000 km2, encompassing Yellowstone National Park and adjacent portions of Idaho, Montana, and Wyoming. Climate,
physiography, and vegetative characteristics are described by Knight and Eberhardt (1985).
METHODS
Weights and Measurements
From 1975 to 1985, bears were captured in culvert traps, foot snares, or "free darted" with immobilizing agents administered with a dart gun as part of the Interagency Grizzly Bear Study Team effort (Blanchard 1985). Immobilizing agents used were Sernylan (phencyclidine hydrochloride) or M-99 (etorphine).
Ages were determined by examination of sectioned premolars for cementum annuli. Weights of live bears were determined with spring scales when possible;
otherwise weights were estimated by experienced field personnel.
Weights of grizzly bears were estimated before administering immobilizing drugs, whereas scale weights were obtained after immobilizing the bear.
Estimated and scale weights were recorded for 96 grizzly bears (53 males, 43 females). Paired t-tests indicated no statistical differences between estimated
and scale weights (P < 0.005), and comparison analyses showed a high correlation between the 2 groups (r = 0.95). Estimated and scale weights were therefore
combined for all further analyses. Weights of dead bears were obtained from necropsies performed by the Mont. Dep. of Fish, Wildl., and Parks.
Seven body measurements and 4 foot measurements were obtained from live bears (Fig. 1). All measurements were taken with a steel tape pulled snugly against the body and recorded along with sex and general body condition. Spring was April-June; summer, July-August; and fall, September-November.
"Dump bears" refers to those bears known to use reliable supplemental food from human sources, such as garbage dumps; "nondump bears" refers to those not known to use such food sources. Adult bears were 5 years and older unless otherwise stated. Methodology for calculating annual habitat quality indices is discussed by Mattson et al. (1986). Oneway analysis of variance, the Student's t-test, and multiple comparisons were used to statistically test differences in mean weights and measurements among sex and age groups. Analyses of annual and seasonal weight changes were performed on pooled samples.
RESULTS
Weight
Weights were recorded for 122 live grizzly bears immobilized 232 times, including 51 females and 71 males. Additional weights for 10 females and 19 males
were obtained at necropsies. Nearly 70% of the weights were recorded during July, August, and September (Table 1). The heaviest adult female weighed 194 kg on 2 August 1981 as a 10-year-old; the lightest weighed 91 kg on 5 July 1984 as a 14-year-old. The heaviest adult male weighed 325 kg on 17 August 1977 as a 16-year-old, and the lightest weighed 98 kg on 11 August 1977 as a 10-year-old. The heaviest male and female recorded 1959-70 were 500 and 204 kg, respectively (Craighead and Mitchell 1982).
Male grizzly bears were consistently heavier than females within all age classes beginning at age 2 (P = 0.001-0.159) (Table 2). Sexual dimorphism in weights was apparent for northern interior Canadian grizzly bears at age 2 (Pearson 1975) and at age 1 for southwestern Alaskan coastal brown bears (Glenn 1980). In general, males appeared to steadily gain weight annually until at least 15 years of age (Fig. 2). Mean annual rate of weight increase for males aged 4-15 years (5.6%) was markedly less than for males aged cub through 3 years (42.1%). One-way analysis of variance and multiple comparisons indicated mean weights of males 4 years and older belonged to 1 population. The mean weight of 5-year-old males was less than expected but could be explained by the small number of fall weights for this age class (1 of 9).
Annual weight patterns of females were less clear than those for males. Mean annual rate of weight increase for females aged cub through 3 years (36%), was also much greater than for females aged 4-13 years (5.2%). Females steadily increased in weight through age 13 (Fig. 2). At 6 years, mean weight dropped, partially due to a lack of fall weight samples.
Mean weights of subsequent age classes suggest this drop in weight may be real, probably a reflection of the mean age of 1st cub production of 6.15 years in this population (Knight and Eberhardt 1985) and the subsequent nutritional drain on still growing, lactating females. Multiple comparisons of mean age class weights indicated females aged 4-7 belonged to 1 population, and females aged 8-13 years belonged to another. Limited data indicated females declined in
mean weight after 13 years.
Females with cubs-of-the-year weighed an average
of 129 kg compared to 140 kg for females with yearlings
and 134 kg for lone adult females. These weight
differences were not statistically significant. Troyer
and Hensel (unpubl. data) found that the heaviest
female grizzly bears on Kodiak Island were usually
those without young.
Approximately 62% of the variation in male
weight could be explained by age using linear regression analysis, whereas only 48% of the variation in
female weight could be attributed to age. Correlations
between weight and age for females were higher for
cubs through 4 years (r2 = 0.67) compared to the
adult (5 + years) group (r2 = 0.05). Less difference
was observed for subadult and adult males (r2 = 0.75
and 0.21, respectively) reflecting the steady annual
weight increase for adult males.
Mean monthly weights by sex and age class indicated
adult grizzly bears lost weight from den emergence
through July, generally regaining emergence
weight by August (Fig. 3). Mean seasonal weight
gains for subadults 2-4 years old were less clear,
although limited data suggested emergence weights
were generally not regained until September.
Pooled samples indicated weaned yearlings steadily
lost weight July-September, whereas unweaned yearlings
gained weight during the same period (Fig. 3).
By September, weaned yearlings weighed an average
21.7 kg less than unweaned yearlings (N = 4). The
2-year-old age class demonstrated the lowest springto-fall
weight gain, with females gaining only 7% of
their final weight during the year and males only 2%
(Fig. 3). Low weight gains at this age probably reflected
the stress of weaning during late spring and subsequent dispersal from the maternal home range,
particularly for males (Knight et al. 1984).
Females exhibited greatest average spring-to-fall
weight gains as 3- and 4-year-olds (30% and 29%,
respectively). Males demonstrated greatest average
spring-to-fall weight gains as 4- and 5-year-olds (39% and 34%, respectively). Males aged 6 and older
gained an average 15% of their final weight springfall.
Greatest daily weight gain of 1.3 kg/day was
recorded for 2 3- year-old males, 1 between 3 September
and 20 September 1982, and 1 between 22
August and 13 September 1978. Lowest daily weight
gain of 0.1 kg/day was recorded for 2 adult females,
a 10-year-old with no young between 27 May and 25
August 1981, and a 6-year-old with cubs between 16
June and 5 July 1984. From 1 July to fall, males
gained an average 1.0 kg/day compared to 0.4 kg/
day for females (N = 11).
Adult male grizzly bears (4 + years) lost a greater
percent of body weight over winter than adult females
(5 + years) (18% and 8%, respectively). Kingsley et
al. (1986) also recorded that males in northern Canada
lost a greater percent of body weight over winter
compared to females. Weight losses may have been
greater than indicated for Yellowstone bears, because
only 2 bears were captured before 1 May and only
4 after 1 November.
Measurements
Body measurements were recorded for 127 male
and 100 female grizzly bears (Tables 3 and 4). The
largest adult male measured 241 cm long (measurement
A, Fig. 1), 117 cm at the shoulder (C), 95 cm
around the neck (D), and had a hind foot pad 170
mm wide (K) and 216 mm long (L). The largest
female was 193 cm long (A), 103 cm at the shoulder (C), 74 cm around the neck (D), and had a hind foot
135 mm wide (K) and 190 mm long (L).
Sexual dimorphism in body measurements was apparent
in cubs and became significant in 8 of 11
measurements by the yearling year (P < 0.170) and
in 10 of 11 by age 2 years. The 11th measurement,
neck circumference, became different by 3 years (P
= 0.003). Male grizzly bears on the central Alaska
peninsula were also significantly larger than females
at age 3 in 6 body dimensions recorded (Glenn 1980).
Measurements of adult males in this study were 8%-
17% greater than those of adult females, compared
to a 29% difference in weights. The greatest difference
was in neck circumference and the least in height
at the shoulder. Glenn (1980) reported a 19% difference
in mean total body size and 88% difference
in weights of males and females on the central Alaska
peninsula.
Females attained mean adult size in 5 of the 11
body dimensions by 4 years (contour body length,
height at the shoulder, neck circumference, head
length, and front pad width) and in all 11 by age 7
years. Males reached mean adult size in 7 of the 11
dimensions by 6 years (body length, girth, height at
the shoulder, neck circumference, head length, front
pad length, and rear pad width) and in all 11 by 9
years. Overlap in ranges of all measurements occurred
in every age class for both males and females.
Girth was the measurement most closely correlated
with weight for both male (r = 0.87) and female (r
= 0.91) grizzly bears. High correlations between
weight and girth have also been reported for grizzly
bears in 3 geographic areas of Canada (Nagy et al. 1984), Jasper National Park (Russell et al. 1979), and
the central Alaskan peninsula (Glenn 1980).
Head length and front pad width measurements
showed least variation for both males and females.
Both dimensions were relatively short and consequently
subject to less error in measurement and less
subject to extremes in fat deposition and loss. Greatest
variation was evident in total body length for both
sexes.
Effects of Food Availability on Weight
Adult grizzly bears feeding at garbage dumps
weighed more than bears relying on natural food
sources. The majority of weights of dump bears (17
of 18) were recorded July-September. Comparisons
with weights of nondump bears recorded during the
same months revealed 13 adult males feeding at
dumps weighed an average 222 kg compared to 184
kg for 29 adult males not known to use dumps (t =
2.26, P = 0.015). Fourteen adult females using dumps
weighed an average 142 kg compared to 130 kg for
nondump females (t = 1.26, P = 0.106). This dumpvs.-nondump
weight difference was apparent for females
with cubs and yearlings but not for lone adult
females (Table 5). The heaviest adult male (325 kg)
and female (194 kg) consistently foraged at the Cooke
City, Mont., dump during summer months for the
entire period they were monitored (3 and 7 years,
respectively). Grizzly bears feeding at dumps in Jasper
National Park, Alberta, also weighed more than
bears using natural foods only (Russell et al. 1979),
as did black bears in Minnesota (Rogers et al. 1976)Adult bears were heavier before 1970 when open
pit garbage dumps were available within Yellowstone
Park (Craighead and Mitchell 1982). During that
period, 33 adult males weighed an average 53 kg more
than adult males from this study. Males feeding on
garbage during this study weighed only 23 kg less
than during the earlier period, whereas nondump
males weighed 52 kg less. Weight differences were
not as great for adult females, with 72 adult females
from the earlier period weighing an average 17 kg
more than adult females from this study. Females
feeding on garbage during this study weighed 10 kg
less than during the earlier period, whereas nondump
females weighed 18 kg less. Differences of less than
20 kg were not considered significant. Four-year-old
females from the earlier period had reached 87% of
their mean adult weight compared to 86% for 4-yearold
females during this study. However, 4-year-old
males from the earlier period had attained only 62%
of their mean adult weight compared to 80% for 4-
year-old males during this study.
Mean annual weights of nondump adult females
were highly correlated with annual habitat quality
indices (Mattson et al. 1986) for Yellowstone Park
(r = 0.710; F = 4.19; 0.50 < P < 0.10) (Fig. 4).
Similar trends were not recorded for nondump adults
males (r = 0.330). Adult females foraging at dumps
were consistently heavier than nondump females, except
during 1980 when optimum seasonal natural
foods were available.
DISCUSSION
Yellowstone grizzly bears were smaller and
weighed less in this study than they did during 1959-
70, when major garbage dumps were available as a
food source within and adjacent to Yellowstone Park.
Natural adaptation to the loss of that stable, highenergy
food source was probably the major cause of
reduced body size, delayed sexual maturity in females,
and smaller litter sizes in the current population. One adult female (26) feeding at a dump
weighed 182 kg in September 1977, a very poor natural
food year when nondump females weighed a
mean of 120 kg. The dump was removed the following
year, and the same female was radio-monitored for
3 consecutive years during which she did not use
garbage as a food source. She was recaptured in July
1981 and weighed 91 kg, whereas dump females
weighed a mean 160 kg during that same year. Beecham
(1980) reported similar findings for 2 black
bear populations in Idaho. Seven species of berries
were available to 1 population, whereas only 1 species
was available to the 2nd, which was characterized by
smaller bears, smaller litter sizes, and females first
breeding 1 year later. Other researchers have suggested
that an additional factor influencing litter size
in Yellowstone has been a change in climate (Picton
and Knight 1986).
BEARS-THEIR BIOLOGY AND MANAGEMENT
Food with the highest nutrient values available to
Yellowstone grizzly bears during summer and fall
were the nuts of whitebark pine (Pinus albicaulis);
berries, primarily buffaloberry (Shepherdia canadensis)
and globe huckleberry (Vaccinium globulare); and
ungulates (Mattson et al., unpubl. data). Whitebark
pine had the highest value of all foods during the fall
but unfortunately was characterized by disjunct and
meager habitat and highly variable, unreliable productivity.
Berry-producing areas were typically specific
to limited habitat of also variable, unreliable
productivity. Use of ungulates was typically characterized by a
peak in spring, when bears primarily scavenged on
winter-killed animals and preyed on weakened ones.
Predation on newborn calves during June and on
rutting bulls during fall was fairly common. Recent
data indicate increased predation on prime condition
elk (Cervus elaphus nelsoni) during summer (Mattson
et al., unpubl. data). Adaptation to the use of this
reliable, high-energy food source during summer and
fall could compensate for the loss of garbage once
available during the same period. However, adult
females known to prey on elk during summer weighed
only 100 kg compared to the mean adult female
weight of 135 kg and the dump female weight of 142
kg. Adult females that were successful moose (Alces
alces) predators in south-central Alaska (Ballard et
al. 1980, Spraker et al. 1981) were similar in mean
weight (117 kg) to females that preyed on elk in
Yellowstone. Although the Alaskan females were relatively
small, they had 1 of the largest mean cub litter
sizes recorded (Table 6). Adult males in Yellowstone
known to prey on elk during the summer weighed
more than the nondump adult males (235 kg and 193
kg, respectively). Alaskan males preying on moose
were similarly heavier than expected (243 kg).
Comparisons of mean adult female weights and
productivity among 9 studies of North American
grizzly populations are listed in Table 6. A high correlation
between mean adult female weight and mean
cub litter size was apparent (r = 0.92) when the
south-central interior Alaskan population was omitted.
When that population was added to the regression,
the correlation became only moderate (r =
0.48). A small sample of 4 was used to calculate the
high litter size of 2.8 for that population. Limited
data indicated smaller adult females tended to produce
their 1st cubs at a later age. A moderately high
negative correlation existed between mean adult female weight and mean age at 1st cub production (r
= -0.52 for 5 populations).
During this study 9 females not known to use
garbage as a major food source had a mean reproductive
rate of 0.469 compared to 0.800 for 2 females
that relied on garbage as a food source (Knight et
al. 1986). Mean cycle length was 3.56 years and mean
litter size 1.92 cubs for the nondump females, compared
to 2.5 years and 2.17 cubs for the 2 females
feeding on garbage. In general, females with reliable,
high-value foods (meat, berries, and garbage) during
summer and fall tended to attain larger body sizes,
mature at an earlier age, and have larger cub litters
compared with females with relatively low-value
foods, such as roots.
