Condition indices of caribou (Rangifer tarandus caribou) found at
mortality sites in the North Columbia Mountains of British Columbia
For Columbia Mountains Caribou Research Project
Kelsey Furk1, B.Sc.
Michelle McLellan, B.Sc.
Robert Serrouya, M.Sc.
March 2008
1
Kelsey_furk@telus.net PO Box 2154, Revelstoke, BC.
Introduction ....................................................................................... 2
Study Area ........................................................................................ 4
Methods ............................................................................................ 4
Analysis........................................................................................................................... 5
Results .............................................................................................. 5
Pregnancy Rate ............................................................................................................... 5
Causes of Mortalities ...................................................................................................... 6
Condition and age of caribou found at mortality sites .................................................... 6
Discussion....................................................................................... 12
Introduction
Mountain caribou are an endangered ecotype of woodland caribou (Rangifer tarandus caribou)
occurring in the high snowfall, interior wetbelt of central and southeastern British Columbia. The
mountain caribou ecotype is defined by a winter diet of arboreal lichen (Stevenson and Hatler
1985). This ecotype was designated as ‘threatened’ by the Commmittee on the Status of
Endangered Wildlife in Canada (COSEWIC) in 2002 under schedule 1 of the Canadian federal
Species at Risk Act.
The distribution of mountain caribou has contracted and fragmented in the last century and some
populations are currently isolated (Wittmer et al. 2006), declining and at risk of extirpation (Hatter
2006). The decline of Mountain Caribou has been attributed to a variety of historical and recent
factors including habitat alteration, hunting (historic), disturbance by humans, and predation
(COSEWIC 2002). Predation has been identified as the proximate cause of decline in Mountain
Caribou (Wittmer et.al 2005b) and habitat alteration is identified as the ultimate cause of decline
across all scales (Wittmer et. al. 2006). Wittmer et al. (2005b) found rates of increase in mountain
caribou subpopulations were positively related to the density of caribou in suitable winter range
(old forests), suggesting mountain caribou are not currently food regulated. Summer forage is
expected to be as or more abundant than historic levels.
Mountain caribou avoid predators by occupying habitats distinct from alternate prey habitats (Seip
1991) and space out during seasons when predators are sympatric to their range (late spring,
summer and fall). They are closely associated with old-growth forests and in contrast to other
woodland caribou they migrate to high elevations in late winter to access arboreal lichens on
trees made available by a deep snowpack.
Even in predator regulated populations, food competition may still be important. As food
availability declines in safer habitats, prey may take increased (predation) risk to access food
(Ouellet et al 1997) presumably making substandard individuals disproportionably vulnerable to
predation by some predators (as in Wirsing et. al 2002). Murray (2002) found that snowshoe
hares in declining condition (but not necessarily those in existing poor condition) experienced
higher predation risk, implying they foraged in a more risk-prone manner. Parker and Luitich
(1985) found that caribou from the George River herd over 5 years old that were killed by wolves
had significantly lower percent marrow fat than non-wolf killed caribou. Mech et al. (1995) found
that poor nutrition predisposed caribou to being killed by wolves in Denali National Park.
Presumably the cost of anti-predator behaviour is reduced energy intake, ultimately resulting in a
reduction in survival or reproduction. However, the relationship between predation risk and
2
related tradeoffs (e.g., habitat choice), and population effects is complicated and poorly studied
especially in terrestrial mammals (Lima 1998).
Weather, disturbance, summer (Crete 1993) and winter range quality (Ouellet 1997), as well as
food quantity and accessibility may influence condition. Nutritional condition is the state of body
components for an individual that may influence current and/or future fitness (Harder and
Kirkpatrick 1996). Condition in turn affects future reproductive capacity by influencing fertility and
parturition rate, age at puberty, and survivorship of adults and calves.
In barrenland caribou the relationship between condition and reproductive success is well
documented. In barren-ground caribou fertility is related to autumn body condition (Thomas 1982;
Cameron et al. 1994, Russell et al. 1998). Thomas (1982) found pregnancy rates increased from
7-100% as percent marrow fat increased from 43-79% in late winter. Early embryonic mortality
(Russell et al. 1998) and reproductive pauses (Cameron et al. 1994) may result from poor
condition.
Birth date and early calf survival is influenced by condition during late pregnancy (Cameron et al.
1993, Adams 2003). Poor maternal condition during late gestation reduces the production of
viable calves (Skogland 1990) and may result in inadequate milk production (Crete and Huot
1993) and poor fattening in calves over summer likely impacts calf over-winter survival
(Adamczewski 1987). Adams et al. (2004) found that caribou neonate losses in Denali National
Park were inversely correlated with average birth mass.
When nutrition is limited, loss of calves to predation prior to the breeding season may influence
production in the following year (Adams and Dale 1998). In such a scenario pregnancy rates may
mask the effects of nutrition (Brown et al. 2007). Pregnancy rates for mountain caribou are
consistently high (92.4%) (Wittmer et al. 2005a) and Seip (1992) found no difference in calf
production between those that lost a calf the previous summer and those that did not.
Body condition is affected by both body fat and protein content (Gerhart 1996). However, the
dynamics of body protein are not as well known as the importance of body fat as a store of
energy (Barboza and Parker 2006). Body fat varies by sex (males are consistently fatter than
females) (Gerhart 1996), lactational status and by season (Allaye Chan-Mcleod 1999,
Adamczewski et al1987).
Fat is mobilized at different rates in different locations in the body. As animals decline in
condition, fat is believed to be mobilized first at subcutaneous fat depots, then viscera fat, and
finally marrow fat (Cederlund et al. 1989, in Cook et al.2005). Davis et al. (1987) found that as
condition declined, marrow fat was mobilized in proximal bones before distal bones in caribou.
Fat depots are likely mobilized concurrently so that one is not exhausted prior to initiation of
mobilization of the next fat depot (Stephenson 2002).
Determining composition of fat and protein directly typically involves destructive harvesting of
large numbers of animals (Allaye Chan-Mcleod 1999, Adamczewski 1987, Ouellet et al. 1997).
This is not feasible for the small and endangered population of Mountain Caribou. In this case, an
index of fat or protein that can be obtained from live animals during capture or dead animals at
mortality sites is required. A good index of nutritional status should be easy to obtain, sensitive to
changes in nutritional status, reproducible and a good predictor across the entire range of
nutritional condition, ages, sex and season
(Harder and Kirkpatrick 1996, Cook et al. 2005; Gerhart et al. 1996).
This report examines the condition of caribou found in the Columbia, Monashee and south
Cariboo Mountains of BC between 1992 and 2007. Femur fat of radio-collared caribou found at
mortality sites and of caribou reported dead from motor vehicle accidents and avalanches was
assessed. Other indices of body condition are analysed including pregnancy rates and juvenile
3
pregnancy rate (based on blood collected in March of each year) by population. Other potential
sources of information on body condition are identified.
Study Area
The study area falls in the Southern Interior Mountains ecosection (Demarchi 1996). Wells Gray
Provincial park form the Northwestern boundary of the study area and the Trans Canada Hwy
and Revelstoke National Park form the southern boundary of the study area. There are however
a few records in this dataset as far south as Nakusp, BC. The height of land of the Rocky
Mountains forms the eastern boundary of the study area around the Wood River, and Kinbasket
lake forms the eastern boundary between Tsar Creek and the Trans Canada highway. The study
area encompasses the Columbia Forest District, the southern half of the Headwaters Forest
District (south of Canoe River) and the northern part of the Okanagan Shuswap Forest District.
Within this study area caribou inhabit the following biogeoclimatic zones in the study area; the
Interior Cedar Hemlock Zone (ICH), the Engelmann Spruce – Subalpine Fir zone (ESSF) and the
Alpine Tundra Zone (AT).
Methods
Mortality sites were investigated after discovery of a mortality signal from a radio collar or on
reporting of non-radio collared mortality. Sites were investigated in order to determine the cause
of death. Clues such as the position of carcass, presence of predators and scavengers, signs of
struggle, bite wounds, presence of radio-collared predators, blood in snow and age of carcass
were used to make an assessment. Often it was difficult to determine if a carcass had been
predated or scavenged, especially in summer.
When a whole carcass was available a necropsy was performed on site or in a lab. A description
of thickness of rump fat and extent of visceral fats was recorded in most of these cases. It was
generally the same two staff members describing fat levels. A femur or other long bone was
collected from mortality sites if available. Prior to 2004 some of these bones were cut on site.
After 2004, all bones were collected whole. Bones were stored in plastic bags and frozen until
analysis, which occurred up to three years after the mortality (but more often within a year).
Rotten marrow was discarded before analysis. Generally only one bone was collected for each
individual. In the case that more than one bone per individual was analysed, the average value
was used for the sample value.
A hacksaw was used to cut out the middle section of long bones. This section was then cracked
with a hammer on a concrete floor. The amount of marrow sample ranged from 10-30g. Care
was taken to exclude bone fragments from the sample. Following this the procedure described in
Neiland (1970) was used. Samples were labelled and immediately placed in pre-weighed
aluminium dishes and weighed on a calibrated scale. They were then placed in an oven set at
approximately 60C. Samples were weighed every day thereafter until the weight of the sample
stabilized (2 or more days with less than .01g change). No correction was made for mineral
residue as reported in Neiland (1970). Percent fat was determined by dividing the wet weight by
the dry weight. The only non-femur long bone in our sample was a humerus and this sample was
regressed to the femur standard using the formula in Davis et al. (1987) (y=3.81+1.0X, where
y=femur percent marrow fat content and x=humerus percent marrow fat).
Sex of collared animals was known from capture records. Sex of un-collared animals was
recorded when possible. Teeth were collected when possible and age was determined by
cementum annuli (Mattson’s Lab, Milltown, Montana).
During March captures, blood was drawn for serum progesterone analysis. Samples were
analysed at the University of Saskatchewan. Interpretation was as follows; a sample with less
4
than 1 ng/mol was classed as not pregnant, anything above 2ng/mol was pregnant. None of the
samples fell between these two values.
Analysis
Each mortality record was categorized by season as in Wittmer et al. (2005a) (Table 1), mortality
agent, sex, caribou year (May 21st – May 20th), subpopulation and age. Fat values from males in
early winter were excluded from most of the analysis due to the likelihood of poor condition during
and immediately after the rut (get ref).
Causes of mortality were classified into the following categories for the purposes of analysing
marrow fat:
1. Accidents – where the mortality agent acts on a conceivably random selection of the ‘live’
population of caribou. Motor vehicle accidents, research related deaths, avalanches, hit
by a tree and suspected poaching,
2. Not Predation – where a carcass was definitely not predated, deaths were sometimes
accidents where the accident might conceivably be a result of poor condition such as
falling in tree wells, or broken legs,
3. Predation High to Moderate confidence – where there was considerable evidence of
predation,
4. Unknown- where there was no or limited evidence pointing to a specific mortality agent.
Percent marrow fat was split into values less than 70% (poor condition) and those above 70%
(not poor condition). Records that had a description of rump and visceral fat (but no marrow fat)
were assigned as ‘not poor’ if there was rump fat and visceral fat and ‘poor’ if there was no rump
fat.
ANOVA was used to test the null hypothesis that there was no difference in the mean percent
marrow fat of the groups formed by values of the independent variables (AGE (continuous),
SEASON and MORTALITY AGENT). The null hypothesis that SEASON and CONDITION CLASS
(poor or not poor) and MORTALITY AGENT and CONDITION CLASS were independent was
tested using contingency tables.
Table 1 Seasons were classified as in Wittmer (2005).
Season
Early Winter (EW)
Late Winter (LW)
Spring (SP)
Summer and calving (SU)
October 21st-January 11th
January 12th – April 23rd
April 24th-May 20th
May 21st -October 21st
Results
Pregnancy Rate
Serum progesterone levels for females classifed as not pregnant ranged from 0-0.8ng/ml and
levels for pregnant females ranged from 2.6-10.2ng/mol. 8 males were tested for pregnancy and
their progesterone levels ranged from 0-0.7ng/mol (all not pregnant).
Ages were known for 15 cases where pregnancy was tested. Single female yearlings tested for
pregnancy in 1992 (Duncan subpopulation) and 2004 (from the Columbia south subpopulation)
were both pregnant and 1 of 2 yearlings tested in 2006 (both from the Columbia North sub pop.)
were pregnant. There were 5 caribou captured at 34 months old tested for pregnancy, 4 were
pregnant, all of these were recorded in 1992. The oldest recorded pregnant caribou was 11.5
years old, but only one older caribou was tested for pregnancy and she was 14.5 years old. Table
5
4 summarizes pregnancy rates. Pregnancy rates before 1998 were previously reported by
Wittmer (2005).
Cause of Mortalities
112 caribou mortalities were recorded between March 1992 and September 2007 ( Table 2). All
but 14 of these mortalities were of radio-collared caribou. In total 195 caribou in the Columbia,
Okanagan Shuswap and Headwaters Forest District were monitored using radio-collars during
this period. Eleven of the samples were radio-collared caribou from another study (the Duncan
subpopulation). A total of 35 of these mortalities were confidently known to be due to predation
and Table 3 summarizes the predators involved at these sites.
Table 2. Cause of 112 mortalities recorded between 1992 and 2007 in the study area plus 11
mortalities recorded in from radio-collared caribou in the Duncan subpopulation.
Mortality Agent
number of mortalities
Avalanche
7
Vehicle accident
9
Other accident (not condition related)
6
Suspected poaching
1
Condition related
10
Unknown (not predation)
5
Predation (high/moderate confidence)
35
Predation (low confidence)
19
Unknown
20
Total
112
Table 3. Percent of total predation events by predator. Only events that were classified as predation
with a moderate to high level of confidence are included. Of the unknown cases, 1 was a suspected
wolf kill (2002) and 2 were suspected wolverine kills (1997,2003), but confidence in these assessments
was low.
Bear (Black and Grizzly)
Cougar
Wolf
Wolverine
Unknown
n
Pre 2000
20%(3)
27%(4)
7%(1)
33%(5)
13%(2)
Post 2000 Total
45%(9)
34%(12)
15%(3)
20% (7)
25% (5)
17%(6)
5%(1)
17%(6)
10%(2)
11%(4)
15
20
35
Condition and age of caribou found at mortality sites
Of the selection of dead caribou with a bone available for marrow fat analysis, 27 also had an
assessment of rump, and visceral fat. All samples with greater than 80% marrow fat that also had
an assessment of rump and visceral fat (n=5) reported ‘good condition’ characterized specifically
by the presence of rump fat and visceral fat. The remaining samples had less than 73% marrow
fat and 9 of 12 reported no rump fat and little visceral fat, 2 reported no rump fat but some
visceral fat and 1 reported both rump and visceral fat (marrow fat of this sample was 56%, it was
a male caught in an avalanche in January).
Mortality investigations yielded 47 long bones (1 humeri, 46 femurs) from 35 females, 7 males
and 5 samples where sex was unknown. The average percent marrow fat was 65.6% (range 890%, median 74%). Females averaged 68.8% (median 76.9%) marrow fat and males averaged
48.6% (median 72.5%) marrow fat.
6
Of the 7 samples from male caribou, 4 were collected in mid to late November when male caribou
might be expected to be in poor condition due to the rut. These four marrow fat samples (all had
less than 50% marrow fat) were excluded from most of the analysis. Of the remaining 43 long
bones analysed 16 (37%) had less than 70% marrow fat (Figure 1) and the average marrow fat
was 68.7%.
Age and Condition:
Ages were known from teeth cementum annuli analysis (Matson’s Lab, Miltown Montana) for 56
caribou and ages ranged from 1 to 16 years old (Figure 2). Ages were known for 30 of the 43
samples that also had a long bone available for marrow fat analysis. There did not appear to be a
relationship between percent marrow fat and age (Figure 3) (ANOVA p=.568) or between
condition class and age class (37.5% (n=8) of caribou 1-6 years, 36.4% (n=12) of caribou 7-11
years old, and 44.4%(n=9) of caribou 12-17 years old were in ‘poor’ condition).
Season and Condition:
No significant relationship (ANOVA (p=.585) was found between % marrow fat and season
(Figure 5) or condition class and season (2=1.53, p=.674). In early winter 58% (n=12) of samples
were in the ‘poor’ condition class, in late winter (n=10) and spring (n=6) 50% of samples were in
the ‘poor’ condition class and in summer 38% (n=24) of samples were in the ‘poor’ condition class
based on analysis of marrow fat and/or visceral and rump fat.
Mortality agent and Condition:
No significant relationship was found between percent marrow fat and mortality agent (ANOVA
p=.247) (Figure 6). However, when the number of samples in each condition class (‘poor’ or ‘not
poor’) was compared by mortality agent using a contingency table,and they were not found to be
independent (2=5.871, df=2, 0.10>p>0.05). When all ‘accidents’ and ‘not predation’ events were
grouped together and compared to ‘predation’ events by condition class, they were also not
independent (2=4.545, df=1, 0.05>p>0.25).
71% (n=14) of ‘not predation’ samples were in the poor condition class, 42% (n=19) of ‘predation
moderate/high confidence’ samples were in the poor condition class, 43% (n=7) of ‘unknown
mortality cause’ samples were in the poor condition class and 25%(n=12) of ‘accident’ samples
were in the poor condition class based on analysis of marrow fat and/or a visual assessment of
visceral and rump fat.
Marrow fat from caribou known to have been killed by grizzly or black bears averaged 74% (n=6,
values = 31%, 62%, 85%, 87%, 89%, 90%). Marrow fat from caribou known to have been killed
by cougars averaged 78% (n=6, values= 64%, 71%, 74%, 81%, 88%, 90%) in addition one
caribou with no marrow fat sample available had ‘very little to no rump fat’. Marrow fat from
caribou known to have been killed by wolverine averaged 67% (values = 46%, 73%, 83%).
Marrow fat from caribou known to have been killed by wolves averaged 55% (values = 20%, 59%
and 87% - however, this final sample (87%) was not confidently known to have been killed by
wolves).
Subpopulation and condition:
Most samples were collected from caribou in the Columbia North and Columbia South
subpopulations (Figure 7). Average percent marrow fat in the Columbia North subpopulation was
59.1% (n=16), 50% (n=16) of samples had less than 70% marrow fat and 50% of samples fell in
the ‘poor’ condition class (n=18). Samples from the Columbia South subpopulation had an
7
average of 77.8% (n=13) marrow fat, 30% (n=13) of samples had less than 70% marrow fat and
39% (n=18) of samples fell in the ‘poor’ condition class (based on an assessment of marrow
and/or rump fat).
Table 4. Pregnancy rates of adult females determined from serum progesterone by
subpopulation and year (number pregnant/number tested). If more than 5 total samples
were available in a year to the overall rate for the year is reported in the total column. * this
sample includes a (non-pregnant) female that lived at Bone Creek outside of delineated
subpopulations. **two juvenile females were tested and one was pregnant. CS= Columbia
south, CN=Columbia north, KIN=Kinbasket south, FB=frisby boulder, GH=groundhog,
MS=monashee south, WG=wells gray. Data from 1997 and prior is from Wittmer et. al
(2005).
CS
1984
1985
1986
1987
1988
1992
1993
1994
1995
1996
1997
2004
2005
2006
CN
KIN
.88 (7/8) 1.0 (2/2)
1.0 (3/3) .88 (7/8)
1.0 (2/2)
0.75 (3/4)
1.0 (1/1) .60 (3/5)*
1.0 (1/1) 1.0 (3/3)
.67(2/3) **
1.0 (1/1)
FB
GH
1.0 (2/2) 1.0 (1/1)
MS
0 (0/1)
WG
1.0 (12/12)
1.0 (6/6)
0.89 (24/27)
1.0 (4/4)
1.0 (1/1)
0.86 (12/14)
1.0 (4/4)
1.0 (4/4)
1.0 (5/5)
1.0 (6/6)
.80(4/5)
1.0 (2/2)
Total
1.0 (12/12)
1.0 (6/6)
.89 (24/27)
.91 (10/11)
.88 (22/25)
.91 (10/11)
.73 (8/11)
.90 (9/10)
0 10 20 30 40 50 60 70 80 90 100
MAROWFAT2
Figure 1. Density plot of percent femur marrow fat found in 48 caribou femurs collected at
caribou mortality investigation sights.
8
0
5
10
AGE
15
20
Figure 2. Distribution of ages of caribou found at mortality investigation sites where a
tooth was available for age analysis.
100
PERCENT MARROW FAT
90
80
70
60
50
40
30
20
10
0
0
5
10
AGE
15
20
Figure 3. Percent marrow fat vs. age based on long bones and teeth collected at 34
mortality investigation sites.
9
100
MARROW FAT PERCENT
90
80
70
60
50
40
30
20
10
0
90 92 94 96 98 00 02 04 06 08
19 19 19 19 19 20 20 20 20 20
CARIBOU YEAR
A
Figure 4. Percent marrow fat in femurs found at mortality sites by caribou year (May 24 thMay 23rd).
100
PERCENT MARROW FAT
90
80
70
60
50
40
30
20
10
0
EW
LW
SP
SEASON
SU
Figure 5. Box plot of percent marrow fat by season. EW=early winter, LW=late winter,
SP=Spring, SU= summer.
10
100
PERCENT MARROW FAT
90
80
70
60
50
40
30
20
10
0
nt
ide
c
Ac
d
t
No
e
Pr
M
H/
ed
r
P
Un
k
Mortality agent
Figure 6. Box plot of percent marrow fat by mortality cause. Not Pred= not predation, Pred
H/M= predation (high and moderate confidence) and Unk =unknown.
11
100
PERCENT MARROW FAT
90
80
70
60
50
40
30
20
10
0
r
u
n uld og sp ing ray
u
No So ca
h
ia bia Dun y-Bo und Nak end lls G
b
P
e
m um
sb Gro
u
i
l
W
l
r
F
Co Co
Subpopulation
Figure 7. Dot density plot of percent marrow fat by subpopulation (as defined in Wittmer
2005b)
Discussion
Mahoney and Virgl (2003) reported a mean pregnancy rate of 96% in a stable non-migratory
caribou population in Newfoundland. Of two juveniles tested in that population, neither were
pregnant. Rettie and Messier (1998) found an overall pregnancy rate for adult females from a
declining population of woodland caribou in Saskatchewan to be 94%, and 5/5 juveniles were
pregnant. Mountain caribou had an overall adult female pregnancy rate of 98% based on capture
data collected between 1984 and 1997, that includes data from this study area (Wittmer 2005a).
Stuart-Smith (1997) found that 18/21 (86%) adult females were pregnant in a population of
woodland caribou in Northeastern Alberta.
The pregnancy rate of caribou in our study area is consistent with these other woodland caribou
populations with the exception of 2004 when the pregnancy rate for adult females was 73%. The
presence of some pregnant yearlings suggests nutrition was at least adequate in the years it was
tested (Parker 1981). The combination of high pregnancy rates and some pregnant yearlings
suggests summer nutrition is good (Cameron 1994, Thomas 1982, Parker 1981). The cause for
the lower pregnancy rate in 2004 is unknown. However, populations remained stable in the two
subpopulations (Wells Gray and Columbia North) between 2004 and 2006 that made up the bulk
of the pregnancy samples (McLellan et al. 2006, Furk 2006).
The parturition rate and neonate survival rate of caribou in our study area is unknown. In Wells
Gray Park (which forms a portion of our study area) between 1984-1989, Seip (1992) found 57%
of females had calves immediately after the calving period and that calf survival through summer
(but not in early June) was related to the abundance of wolves. This is lower than that the
12
minimum parturition rate of 86% reported for a declining population of woodland caribou in
Saskatchewan (Rettie and Messier 1998), but higher than the average rate (42% between 19932000, with lots of variation year to year) reported for a sharply declining woodland caribou herd in
the southern Yukon where high neonatal losses to predation were identified as a major cause of
decline (Farnell and Gardner 2002). Gustine et al. (2006) found that 15 of 22 (68%) pregnant
females from a population of woodland caribou in northern BC were parturient. Parturition rate
and neonate survival is difficult to determine for mountain caribou due to difficult spotting
conditions, the behaviour of calving females which results in them being dispersed during calving
and inconsistent weather for flying. However, further efforts should be made to determine
parturition rate and the cause and timing of neonate mortality due to the declining recruitment (the
percent 10 month old calves in March) in the subpopulations that make up the southern part of
the study area (McLellan et al. 2006, Furk 2006).
Assessing body condition in ungulates would ideally involve taking a random sample of the live
population with collection of multiple condition indices over time and through different seasons.
This is not possible for our study area, where the population of caribou is small and endangered.
One major difficulty in viewing the results of this analysis is that the population surveyed is dead.
We do not have information on the condition of the live population in our study area although
experienced handlers report they are generally in good condition during March captures (B.N.
McLellan pers. comm.). The best approximation of a random sample of ‘live’ animals we have
from this data is caribou that were killed in random accidents, such as motor vehicle accidents
and avalanches. This assumes that condition is not related to the probability of being killed by a
vehicle or an avalanche or being hit by a tree.
The primary condition index collected at mortality sites was a femur for analysis of percent
marrow fat. Femur fat should be considered a one way index of condition, such that anything less
than full femur marrow fat indicates poor condition, but that full femur marrow fat doesn’t
necessarily indicate good condition (Mech and Delgiudice 1985). This is due to the late
mobilization of marrow fat relative to other body fat stores in concert with the relatively small
contribution of marrow fat to total body fat (Mech and Delgiudice 1985). In barren ground caribou
the analysis of femur fat as an index for condition only correlates with body fat below 9% or just
over 80% femur marrow fat (Chan-Mcleod et al. 1995). Chan-Mcleod (1995) found back fat was
not deposited until body fat exceeded 8% and marrow fat reached capacity. In elk Cook et al.
(2005) found it was only predictive below 6% body fat and Gustine et al. (2007) found pregnant
caribou had more than 6% body fat.
Based on this data, 70% femur marrow fat is the line below which caribou were considered to be
in poor condition. Caribou below this level are in sub-optimal condition since the majority of body
fat by this stage is depleted (also explained in Mech et al. 1995). Assessments of rump fat of
dead animals in the study area, give confidence to the assertion that caribou with less than 70%
marrow fat are in poor condition. Of samples with less than 73% marrow fat, 11 of 12 had no
rump fat, whereas all samples with more than 73% marrow fat reported at least some rump fat
(n=5).
There were no differences in average marrow fat values by season, mortality agent or age. I
expected to find caribou in poorest condition in late spring (Chan-Mcleod ). Most mortalities
occurred in summer (Wittmer et al. 2005a), and the sample size from mortalities in the rest of the
year may be too small to detect a difference.
Bears continue to be the primary predator of caribou and their relative contribution to predation
events has increased with wolves since the 1990’s. The decline of mountain caribou is thought to
be proximally caused by increased mortality from predation that is facilitated by habitat alteration
that supports an increasing number of alternative prey (Wittmer et al. 2005b). Subpopulations of
caribou in the south of the study area declined precipitously after 1997 and cougar predation has
been identified as a potential cause for this decline (McLellan et al.). The relative contribution of
cougars to caribou mortalities has decreased since 2000 and caribou populations have been
13
relatively stable since 2002 in this area (McLellan et al. 2006). Sample sizes of marrow fat values
by predator are too small to test for differences, however, two of three samples from wolf kills
were in notably poor condition, further samples should be sought to determine if wolves are
targeting vulnerable individuals (as has been found in some caribou populations (Parker and
Luitich 1985, Mech et al. 1995). Caribou killed by cougars and bears appeared to be in similar
condition to caribou killed in random accidents (our approximation of a live sample).
Although average marrow fat by mortality agent was not different, the frequency of caribou in
poor condition by mortality agent did differ. Caribou killed in random accidents were more likely to
be in good condition followed by caribou killed by predators and finally caribou that died from an
injury or condition. 25% of caribou that died in random accidents were in poor condition. 42%
percent of caribou killed by predators were in poor condition and 71% of caribou that died of a
cause other than predation (that wasn’t a random accident) were in poor condition. This suggests
caribou in poor condition are predisposed to predation or injury due to accidents. There were 10
samples from caribou that died from condition related problems. Some of these were found with
no apparent injuries, others had existing injuries. It is impossible to know if injury caused poor
condition or poor condition predisposed a caribou to injury in these cases. It is also conceivable
that those caribou that appeared to lay down and die, suffered a predisposing injury not obvious
to investigators.
Average marrow fat values of caribou killed in random accidents was 67% (n=20) which is
somewhat lower than many of the values reported for other populations of caribou (Table 5).
However, many of these examples are from barrenland caribou, as there are few studies
published with specific information on woodland caribou and marrow fat. This suggests a portion
of the live caribou in our study area are in less than ideal body condition. This conclusion should
be viewed with caution, since caribou killed in “random” accidents may be a poor approximation
of the live population.
Table 5. Marrow fat values from the literature.
Reference
Thomas 1982
Season
March April
Population
Peary Caribou
Thomas 1982
March April
1974
Peary Caribou
Thomas 1982
March April
1975, 1976
Peary Caribou
Parker 1981
April
George River
Herd
Denali Nat.
Park
Dale and Adams
1998
Comment
Pregnancy rates dropped precipitously
below 80% femur marrow fat
In 1974, after a winter of 62 and 68%
overwinter mortality in two populations,
femur marrow fat averaged 65%(n=5)
and 18% (n=12). Low to nil recruitment
in these years.
In one population, marrow fat averaged
88% (n=7) in 1975, 76%(n=26) in 1976
and pregnancy rate was 92%(n=12) and
73% (n=26). There was no calf
production in these years. Low to nil
recruitment in these years.
Average 88.6% (s=8.6, n=100) femur
marrow fat in an expanding population
Hunter killed caribou average percent
marrow fat = 90%, SD=0.048, N=52.
Wolf killed females average 67% (sd
.319, n=12), but two samples with <25%
marrow fat, had a strong impact on the
means.
14
Recommendations
Frequently, it is difficult to find intact femurs at mortality sites. Davis et al. (1987) found good
correlation between percent marrow fat in caribou femurs and percent marrow fat in paired tibia,
humerus, metatarsus, radius, metacarpus, and mandible fat. They suggest regressing other long
bones to a “femur standard” for comparative purposes. Any and all long bones as well as
mandibles should be collected at these sites and the percent marrow fat should be determined
and regressed to a “femur standard” as in Davis et al. (1987). Davis et al. (1987) did not examine
the effect of season on differences between marrow fat correlations in paired bones, so the effect
of season should be considered and investigated by analysing paired bones whenever feasible.
The type of bone along with the percent marrow fat value should be recorded.
Whenever possible an assessment of rump fat, marrow fat kidney fat and mass (Riney 1955),
and a general assessment of internal fat using a kistner score (Kistner 1980) should be made at
mortality sites as a combination of scores results in the best relation to percent fat across the
range of condition (Cook et al. 2005). At capture a combination an assessment of condition
should be made using the combination of a body condition score (Gerhart et al. 1996) and an
ultrasound rump fat measurement (Gustine et al. in press).
Parturition date should be estimated by looking at dates of clusters from existing GPS collar data
and compared to dates collected for other woodland caribou. The condition of calves is very
difficult to assess with minimum disturbance, however the parturition rate and the timing and
cause of neonate mortality needs to be determined for mountain caribou particularly in
subpopulations with low recruitment (those south of Wells Gray Park – Wittmer 2005a). This
could be accomplished using a combination of downloadable GPS collars on adult females, and
regular helicopter counts of collared animals. Collars with cameras might also help to determine
the timing of neonate mortality.
Acknowledgements
Funding for this research was provided by the BC Forest Science Program, the Okanagan
Innovative Forestry Society (project 4776005, telemetry to monitor mortality events), Federated
CooP (project 4772003, collar purchases for continued mortality monitoring) the Kamloops TSA
licensees and SIMPCW Development Corporation, and the B.C. Ministry of Forests and Range.
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