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Selectivity of wolf predation on red deer in the Bieszczady Mountains, Poland

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A pattern of wolf Canis lupus Linnaeus, 1758 predation on red deer Cervus elaphus Linnaeus, 1758 was studied in Bieszczady Mountains in 1991–2002. In total 324 remains of red deer > 4 months old, killed by wolves throughout the year, were found. The sex, age and bone marrow fat content of wolf kills were compared with the same characteristics within the free living red deer population. The overall contribution of calves killed by wolves (24%) in October-May was higher than in the population (17%), and decreased from autumn to spring. Adult males were more vulnerable to wolf predation than adult females: stags constituted 62% and hinds 38% of adult red deer killed by wolves, whereas in the population, the percentages were 37 and 63%, respectively. Stags killed by wolves were younger (bar x = 4.1 years old) than hinds (bar x = 8.9 years old). Wolves killed more > 8 years old hinds and < 5 years old stags than available in the population. In wolf kills, the average fat content in femur marrow was higher among hinds (84.9%) than stags (69.3%) and calves (66.1%). Only 8% of hinds had < 70% femur marrow fat content, whereas 40% of calves and 38% of stags had marrow fat values below that level. Marrow fat content showed seasonal variation and was the lowest in March among all sex-age classes. The monthly share of stags in all kills, and hinds in hind-calf part of the sample was negatively correlated with their monthly average bone marrow fat content, and monthly share of calves was positively correlated with monthly average bone marrow fat content of adults. The segregation of social units (hind-calf and stag groups), except during the rutting season, and the low fat reserves of males from midwinter until spring contribute to the high overall incidence of calves and adult males and the relatively low incidence of adult females among wolf kills. Key words: Canis lupus, Cervus elaphus, predation pattern, Carpathians
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Selectivity of wolf predation on red deer
in the Bieszczady Mountains, Poland
Wojciech ŒMIETANA
Œmietana W. 2005. Selectivity of wolf predation on red deer in the Bieszczady
Mountains, Poland. Acta Theriologica 50: 277–288.
A pattern of wolf Canis lupus Linnaeus, 1758 predation on red deer Cervus elaphus
Linnaeus, 1758 was studied in Bieszczady Mountains in 1991–2002. In total 324
remains of red deer > 4 months old, killed by wolves throughout the year, were found.
The sex, age and bone marrow fat content of wolf kills were compared with the same
characteristics within the free living red deer population. The overall contribution of
calves killed by wolves (24%) in October–May was higher than in the population
(17%), and decreased from autumn to spring. Adult males were more vulnerable to
wolf predation than adult females: stags constituted 62% and hinds 38% of adult red
deer killed by wolves, whereas in the population, the percentages were 37 and 63%,
respectively. Stags killed by wolves were younger (
x
= 4.1 years old) than hinds (
x
=
8.9 years old). Wolves killed more > 8 years old hinds and < 5 years old stags than
available in the population. In wolf kills, the average fat content in femur marrow was
higher among hinds (84.9%) than stags (69.3%) and calves (66.1%). Only 8% of hinds
had < 70% femur marrow fat content, whereas 40% of calves and 38% of stags had
marrow fat values below that level. Marrow fat content showed seasonal variation
and was the lowest in March among all sex-age classes. The monthly share of stags in
all kills, and hinds in hind-calf part of the sample was negatively correlated with their
monthly average bone marrow fat content, and monthly share of calves was positively
correlated with monthly average bone marrow fat content of adults. The segregation
of social units (hind-calf and stag groups), except during the rutting season, and the
low fat reserves of males from midwinter until spring contribute to the high overall
incidence of calves and adult males and the relatively low incidence of adult females
among wolf kills.
Institute of Nature Conservation, Polish Academy of Sciences, Mickiewicza 33, 31-120
Kraków, Poland, e-mail: wojsmietana@go2.pl
Key words: Canis lupus, Cervus elaphus, predation pattern, Carpathians
Introduction
Wolves Canis lupus Linnaeus, 1758 inhabit wide areas of northern hemisphere
where they coexist with many different species of ungulates (Mech 1970, Bibikov
1985, Okarma 1995). Selectivity of wolf predation on moose Alces alces, caribou
Rangifer tarandus caribou, white-tailed deer Odocoileus virginianus, elk Cervus
elaphus canadensis and Dall sheep Ovis dalli during winter has been frequently
studied in North America (eg Murie 1944, Mech 1966, Mech and Frenzel 1971,
Peterson 1977, Fritts and Mech 1981, Carbyn 1983, Ballard et al. 1987, Huggard
1993, Mech et al. 1995, Hayes et al. 2000, Smith et al. 2004). In contrast, only a few
[288]
Acta Theriologica 50 (2): 277–288, 2005.
PL ISSN 0001–7051
studies have focused on the selectivity of wolf predation within a given ungulate
species in Europe (Okarma 1984, Okarma 1991, Bobek et al. 1992a, Okarma et al.
1995, Gula 2004).
In central Europe the main prey species of wolves is red deer Cervus elaphus
Linnaeus, 1758 (Leœniewicz and Perzanowski 1989, Œmietana and Klimek 1993,
Jêdrzejewski et al. 1992, 2000, 2002). In the Carpathian Mountains, research on
wolf predation on red deer describes age, sex and bone marrow fat content of kills
found only during winter (Okarma 1984, Okarma 1991, Bobek et al. 1992a, Gula
2004). Moreover no comparison of these characteristics was made with the free
living population, which is necessary for the determination of predator selectivity.
The aim of this study was to determine selectivity of wolf predation on red deer
(> 4 months old) in respect to age, sex and body condition in different seasons in
the Carpathian Mountains. I attempted to explain the mechanisms of seasonally
varying predation pattern in relation to the body condition of prey and social
structure of the red deer population.
Study area
The study was conducted in the Bieszczady Mountains (a range within the Carpathians) within
Bieszczady National Park and adjacent three hunting districts; 49°00’–18’N 22°25’–54’E. The total
study area amounted to about 600 km
2
. Bieszczady National Park (ca 300 km
2
) lies in the highest
parts of the Bieszczady Mountains, with the highest peak – 1346 m a.s.l. The lowest point of the
study area lies at 420 m a.s.l. in the San River valley northwest of the National Park. Long, parallel
mountain ridges build up of Carpathian flysch (Zarzycki and G³owaciñski 1986) are a characteristic
feature of the area.
About 80% of the study area is covered by forest. According to Michalik (1993) 75% of the tree
stand is natural. Within this area botanists distinguish 9 forest plant associations, with Carpathian
beech forest Dentario glandulose-fagetum as the most common (Michalik 1993). Beech Fagus sil-
vatica and in some areas fir Abies alba are the dominant tree species. Other important trees are grey
alder Alnus incana, sycamore Acer pseudoplatanus and spruce Picea abies mainly introduced by man
(Zarzycki 1963). Meadows with blue berries Vaccinium myrtillus, grass and sedge associations and,
in some places, green alder Alnus viridis and mountain-ash Sorbus aucuparia scrub (Zarzycki and
G³owaciñski 1986) cover the zone above 1150 m a.s.l. The average annual air temperature is 4.9°C
and during especially severe winters can drop below – 30°C. Annual precipitation is 800–1200 mm.
Snow cover persists for 90–120 days in the valleys and for 140–150 days on the mountain tops. Snow
depth might exceed 1.5 m, although usually reaches 40–80 cm (Michna and Paczos 1972). The
relatively low human density (6 persons/km
2
), dominance of forested areas, and geographical location
makes the Bieszczady Mountains and its surroundings one of the most valuable ranges for large
mammal and avian predators in Poland. Apart from wolves, there are also other large carnivores,
such as the lynx Lynx and the brown bear Ursus arctos. There are five ungulate species in the
Bieszczady Mountains: red deer, roe deer Capreolus capreolus, wild boar Sus scrofa, European bison
Bison bonasus and moose Alces alces. During the winter, most of the red deer migrate from higher
mountain elevations to the wintering ranges, lying northwest from the National Park (Œmietana and
Wajda 1995).
During the study, red deer, which were the most numerous ungulates, were exploited within the
hunting districts every year and inside the National Park they were controlled by yearly culling until
1995. The area was occupied by 3 to 5 wolf packs (10–33 individuals) every year (Œmietana and
Wajda 1997, W. Œmietana, unpubl.). Until 1998 wolves were hunted outside the National Park, with
a closed season from 1 April to 31 July. Since April 1998, wolves have been fully protected
throughout the study area although they are poached frequently.
278 W. Œmietana
Material and methods
Examination of red deer killed by wolves
The study area was regularly searched for carcasses of ungulates throughout the year by the
author (150–200 days/year), 6–8 park guards and foresters during their routine inspections, and
volunteers (100–200 person-day/year) from September 1991 until April 2002. Kills were mostly found
by observing raven Corvus corax activity and during snow tracking of wolves. In situ cause of death,
sex, age class (juvenile or > 1 year old) and accurate or approximate date of death was determined.
Kills, for which the cause of death was identified as a wolf predation, by obvious evidence of tracks
and chase and/or struggle or canine slash marks on the killed animal, were used in this analysis. The
mandibles and femurs were removed from the bodies for further examinations in the laboratory. The
mandible was used for age determination on the basis of tooth eruption pattern (only individuals up
to second year of life) (Lochman 1987), cementum annuli (Mitchell 1967, Godawa 1989), and tooth-
-wear (Lochman 1987). To determine bone marrow fat content, which may reflect physical condition,
a 5–20 g sample from mandible and femur was extracted. Fat content was determined by Neiland’s
method (1970).
Age-sex structure and physical condition of red deer population
Sex and age structure of the free living red deer was determined by direct observations and by
analysis of individuals killed by hunters. Observed red deer were classified as: calves (individuals < 1
year old), hinds (females > 1 year old) or stags (males > 1 year old). Moreover, on the basis of size and
shape of the body and by the look of the antlers (Lochman 1987) observed stags were classified to
four groups 1) yearlings, 2) stags from third to fifth year of life, 3) stags from sixth to tenth year of
life and 4) stags above ten years old. In practice, hunters killed hinds randomly. Therefore, in order
to determine the age structure of the free living hinds, 94 mandibles of harvested individuals were
collected. Their age was determined using the same method as for red deer killed by wolves. Age and
bone marrow fat content of hunted stags could not be used for comparisons because of non random
harvest of them. To assess the selection of particular age classes of adult red deer by wolves, Ivlev’s
selectivity index D (modified by Jacobs 1974) was used according to the formula: D = (r – p) / (r + p –
2rp), where r is the proportion of a given age class in wolf kills and p is its proportion in the free
living population. Ivlev’s selectivity index ranges from –1 (total avoidance) through 0 (no selection) to
+1 (maximum positive selection). The bone marrow fat content of 27 calves and 94 hinds harvested
by hunters in the study area was determined. Because sampling of bone marrow from femur decreases
trade value of hunted individuals only bone marrow from mandibles was available for analyses.
Results
Age and sex structure
Among 324 red deer > 4 months old killed by wolves, 69 were calves, 96 hinds
and 159 were stags. A strong bias against finding dead juvenile red deer
prevented evaluation of their share in wolf kills before October; we found only one
wolf-killed calf < 4 months old. Due to the usually high consumption of bodies,
only 18 calves were sexed: 9 females and 7 males.
The share of a given red deer class in kills varied by months. To test changes of
these proportions, I combined consecutive months with similar percentage
contributions of red deer classes. From October until May, the overall share of
calves in wolf kills (24%) was higher than in the free living population (17%) (c
2
=
7.78, df = 1, p = 0.005) (Table 1). The proportion of calves in hind-calf part of the
kill sample (0.47) was also higher than their proportion in hind-calf segment of
Wolf predation on red deer 279
the population (0.23) (c
2
= 37.88, df = 1, p < 0.001) (Table 1). Wolves tended to kill
more calves than available in the population in October–December and in
January–February, but the difference was significant only for October–December
sample (Table 1). If only hind-calf part of the samples were compared, differences
were significant for both periods (Table 1). In March–May the share of calves in
kills was not significantly different from their share in the population neither for
total samples nor for their hind-calf segments (Table 1).
In general, adult males were more vulnerable to wolf predation than adult
females. Stags and hinds constituted 62 and 38% of adult red deer killed by
wolves, respectively, whereas the share of stags and hinds in the adult segment of
the free living population was 37 and 63%, respectively (c
2
= 64.61, df = 1, p <
280 W. Œmietana
Table 1. Share of calves in all red deer (calves and adults) killed by wolves and in the
hind-calf (calves and hinds) segment of the sample in comparison to the structure of the
free living population in the Bieszczady Mountains from 1991 through 2002.
a
Yates’
corrected c
2
was used because of small sample size of calves in the kill sample.
Period
Wolf kills Population
c
2
p
n
Share of
calves (%)
n
Share of
calves (%)
Calves and adults
October–December 98 34 608 20 8.61 0.003
January–February 124 23 551 17 2.27 0.071
March–May 70 10 830 15 1.16
a
0.281
Calves and hinds
October–December 67 49 466 27 14.45 < 0.001
January–February 53 55 389 23 23.14 < 0.001
March–May 28 25 583 21 0.08
a
0.780
Table 2. Share of females and males in adult red deer killed by wolves in comparison to the
structure of the free living population in the Bieszczady Mountains from 1991 through
2002.
Period
Number of
adults of
both sexes
Share
of stags
(%)
Share
of hinds
(%)
c
2
p
Wolf kills
June–September 32 47 53 1.35 0.245
October–December 65 48 52 3.18 0.074
January–February 95 75 25 56.12 < 0.001
March–May 63 67 33 23.37 < 0.001
Population
January–December
3319 37 63
0.001) (Table 2). Wolves tended to kill more stags than available in adult segment
of the population year round but the differences were significant only for
January–February and March–May samples (Table 2).
The age distribution of adult red deer killed by wolves differed apparently
between sexes (Kolmogorov-Smirnov test: D = –0.556, p < 0.001) (Fig. 1). On
average, stags killed by wolves were younger (n = 66,
x
= 4.1) than hinds (n = 61,
x
= 8.9). Among hinds killed by wolves, individuals older than 8 years prevailed
(62%) and among stags most individuals were below 5 years old (76%). The age
structure of hinds killed by wolves and by hunters also differed significantly (D =
0.378, p < 0.001) (Fig. 1). The age of 94 hinds killed by hunters averaged 6.0 years
Wolf predation on red deer 281
30
25
20
15
10
5
0
20
15
10
5
0
+1
0
-1
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9
10 11 12 13 14 15 16
10 11 12 13 14 15 16
1 2 3 4 5 6 7 8 9
10 11 12 13 14 15
1 2 3 4 5 6 7 8 9
10 11 12 13 14
1 2 5 10
1 2 5 10
Hinds Stags
Killed by wolves
Killed by wolves
n = 574
n = 61, x = 8.9
-
n = 94, x = 6.0
-
n = 66, x = 4.1
-
Killed by hunters
Observed live
Age in years
Selectivity
index Ivlev's D
Percent of individuals
Fig. 1. Age structure of adult (> 1 year old) female and male red deer killed by wolves, killed by
hunters and observed live, in the Bieszczady Mountains from 1991 through 2002, and Ivlev’s
selectivity index for wolf predation on each age class of red deer.
and individuals below 8 years old dominated (75%). Calculated Ivlev’s selectivity
index was negative for hinds below 8 years old and varied from –0.20 to –0.51 (Fig.
1). Among older hinds Ivlev’s index was positive, varying from 0.22 to 1.00 and
was increasing with their age. Generally, hinds below 8 years old were less
vulnerable to wolf predation than older ones. The age structure of stags killed by
wolves also contrasted with the age structure of the free living males (D = –0.191,
p < 0.05) (Fig. 1). Selectivity index was the lowest for 5–10 years old stags (D =
–0.47) and the highest for 2–5 years old ones (D = 0.23). Generally, stags below 5
years old were more vulnerable to wolf predation than older ones.
Condition of red deer killed by wolves and hunters
The average fat content in femur marrow of red deer killed by wolves was the
highest in hinds (n = 48,
x
= 84.9%) followed by stags (n = 53,
x
= 69.3%) and calves
(n = 35,
x
= 66.1%) (Fig. 2). A significant difference in femur marrow fat content
was found between calves and hinds (Mann-Whitney U-test: U = 536, p = 0.005)
and between hinds and stags (U = 828, p = 0.003) but not between stags and calves
(U = 919, p = 0.942). Only 8% of hinds had femur marrow fat content below 70%,
whereas 40% of calves and 38% of stags had marrow fat values below that level
(Fig. 2). Femur marrow fat content was similar between young (< 8 years) (n = 19)
and old (> 8 years) (n = 28) hinds (U = 236, p = 0.515) and between young (< 5
years) (n = 25) and old (> 5 years) (n = 10) stags (U = 119, p = 0.827), averaging
87.0, 83.2, 67.5, and 64.4%, respectively.
Mandible marrow fat content in calves killed by wolves between November and
February (n = 26,
x
= 50.8%) was significantly lower than in calves killed by
hunters during the same period (n = 27,
x
= 62.4%) (Mann-Whitney U-test: U =
282 W. Œmietana
Fig. 2. Femur marrow fat content of red deer killed by wolves in the Bieszczady Mountains from 1991
through 2002. Each point represents one individual plotted against the date when killed. Line shows
monthly changes of average femur marrow fat content. Broken line indicates no data for at least one
month. Horizontal line indicates critical value (70%) of femur marrow fat content.
100
80
60
40
20
0
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Jan
Feb
Mar
Apr
May
Oct
Nov
Dec
Jan
Feb
Mar
Apr
May
Calves Hinds Stags
Months
Femur marrow fat content (%)
n = 35, x = 66.1%
-
n = 48, x = 84.9%
-
n = 53, x = 69.3%
-
195, p < 0.001). In November–February, there was no significant difference in
bone marrow fat values (U = 1508, p = 0.337) between hinds killed by wolves (n =
36;
x
= 70.9%) and those killed by hunters (n = 94,
x
= 69.6%).
The average femur marrow fat content of all age-sex classes of red deer killed
by wolves was maintained at high level (77.5–94.6%) during summer, autumn and
early winter (Fig. 2). Then, it decreased rapidly among calves and stags in
February and among hinds in March. The lowest monthly average fat content
in femur marrow for all age-sex classes was recorded in March (Fig. 2). In
February–April 86% of calves (n = 14), in March–April 50% of hinds (n = 8) and in
February–May 75% of stags (n = 20) had femur marrow fat content below 70%,
which contrasts with the remaining part of the year when only 10% of calves (n =
21) (Fisher exact test: p < 0.001), no hind (n = 40) (p < 0.001) and 18% of stags (n =
33) (p < 0.001) had such a low level of the fat content.
From October until May, the monthly share of stags in kills tended to decrease
with their growing average fat content in femur marrow (n = 7, r
s
= –0.750, p =
0.052). The monthly proportions of hinds in the total number of wolf kills in the
same period did not correlate with their average femur marrow fat content (n = 7,
r
s
= –0.500, p = 0.253), but there was a strong correlation if only hind-calf part of
the monthly kill samples were considered (n = 7, r
s
= –0.928, p = 0.003). No
correlation was found between monthly proportions of calves among kills and
their femur marrow fat levels neither for the whole samples (n = 6, r
s
= 0.486, p =
0329) nor for hind-calf sub-samples (n = 6, r
s
= 0.257, p = 0.623), but there was a
strong correlation between monthly proportions of calves in kills and femur
marrow fat levels of adults (n = 8, r
s
= 0.833, p = 0.010).
Discussion
This study documents that vulnerability of red deer to wolf predation in the
Bieszczady Mountains depends on age, sex and body condition and varies according
to the time of year. Calves were important prey from October until February. The
high vulnerability of red deer calves to wolf predation during winter was described
also in earlier studies from the same mountain range by Okarma (1991) and Bobek
et al. (1992a). In Bia³owie¿a Primeval Forest, Okarma et al. (1995) found slightly
higher share of calves in kills than in the free living population. Gula (2004)
claimed that winter wolf predation on red deer in the Bieszczady Mountains is not
selective for calves, although in his study calves composed 29% of wolf-killed deer or
42% if only hind-calf part of the sample is considered (which is similar to my
results). The researcher compared the kill sample with the data provided by the
State Forest Administration. According to this data there were 72 calves per 100
hinds in winter red deer population, which seems to be an unusually high ratio
(see Bobek et al. 1992b), and is 2.4 times higher than my estimations. In my study
the contribution of calves among red deer killed by wolves was decreasing from
autumn to spring. This phenomenon was a result of an increase in the contribution
Wolf predation on red deer 283
of adults among kills. The lack of correlation between average bone fat levels and
monthly proportions of calves in kills suggests that body condition was not a
primarily factor predisposing them to wolf predation. Juvenile ungulates being
generally slower, weaker (less dangerous to attack) and less experienced with
predators than adults, are expected to be usually more vulnerable prey (Mech
1970). My study suggests that such situation persists until the body condition of
adults decrease substantially in late winter and in spring. The overall dispro-
portionately higher occurrence of juveniles among wolf kills in my study well
corresponds with the most of North American studies on wolf predation on white-
-tailed deer, elk and moose (eg Mech 1966, Peterson 1977, Fritts and Mech 1981,
Huggard 1993, Mech et al. 1995 , Hayes et al. 2000, Smith et al. 2004).
Higher natural mortality of males than females resulting in shorter life span
should be expected in highly sex dimorphic species like red deer (Clutton-Brock et
al. 1982). Contrary to my study, earlier studies conducted in the Carpathians
(Okarma 1984, 1991) and in Bia³owie¿a Primeval Forest (Okarma et al. 1995)
reported that hinds are more vulnerable to wolf predation than stags. Gula (2004)
found no selection neither for hinds nor for stags, and Bobek et al. (1992a) reported
slight selection towards stags. However, the sex proportion in the sample of adult
red deer killed by wolves could depend on the area from where the material was
collected, as female and male groups of red deer occupy separate large ranges most
of the year (except the rutting season). Sex ratio among kills could also vary
between years and depends on winter severity. In my study area both female and
male groups were present, material was collected over a long time (11 years) and, in
contrast to the study of Okarma (1984, 1991), Bobek et al. (1992a) and Gula (2004),
red deer killed by wolves were found year round and sex proportion of the local
population was well documented. In my opinion, sex segregation of red deer, except
the rutting season (September–October), and the difference in body condition
between sexes explains the high contribution of stags and low contribution of hinds
to the total number of adults killed by wolves. Hinds living mostly in female
groups are obviously less vulnerable (except during March–May) than calves
accompanying them, which is not the case in stags living mostly in male groups.
Moreover, stags killed by wolves had, in general, lower bone marrow fat content
than hinds. This difference was especially clear from mid-winter until spring, when
the share of stags in kills was the highest. Monthly proportions of stags among kills
tended to increase with decreasing their bone fat levels, which suggests that body
condition was a very important factor predisposing them to wolf predation. Mech
and Peterson (2003) generalized that adult males of ungulates are most vulnerable
immediately before, during and after the rut, but such selection pattern was not
found in my study. During the rut, adult males are less alert and, in some systems
could become more vulnerable prey class, but on the other hand, during this period,
they are in relatively good condition, equipped with weapons (antlers) and aggres-
sive. My results support the opinion of Clutton-Brock (1985) that because of larger
size, fast and prolonged growth and lower fat reserves, stags are more likely to die
than hinds during periods of food shortage and harsh weather. Higher vulnerability
284 W. Œmietana
to wolf predation of elk males than females, during winter were reported by
Huggard (1993) and Smith et al. (2004). Also in areas where wolves were absent,
adult males of Cervus elaphus occurred proportionally more frequently than adult
females among dead individuals found during winter (Houston 1978, Clutton-Brock
et al. 1982). I found that hinds are the most vulnerable to wolf predation in the end
of winter and in spring, but taken by wolves only proportionally to their availability
in the population. The strong correlation between the share of hinds in hind-calf
sub-sample of kills and their bone fat levels indicate that their body condition was a
very important factor causing they vulnerability to wolf predation.
In cervids, marrow fat makes up only 2–3% of total body fat reserves
(McCullough and Ullrey 1983) and fat level in femur marrow, at more than
70–85%, depending on species and age, indicate only that the total body fat of a
given individual is above 5% (Adamczewski et al. 1987, Torbit et al. 1988, Holand
1992). Fat content in femur marrow below this threshold indicates not only very
low total fat stores, but also substantial loss of protein and total body mass (Torbit
et al. 1985, Delguidice et al. 1990). Thus, as remarked by Mech et al. (1995), any
level below the threshold of bone marrow fat content indicates not only very low
fat reserves but also considerable loss of skeletal muscle mass and vigour of the
animal. Considering this, I believe that in my study at least 38% (percent of
individuals with femur marrow fat content below 70%) of total number of stags,
40% of calves, and 8% of hinds killed by wolves were in poor condition. Positive
selection of calves with low bone marrow fat content was found in my study, but
unfortunately I was not able to compare bone marrow fat levels of stags killed by
wolves with the same characteristic of stags from the free living population.
However, this does not disprove my assertion that large number of stags killed by
wolves was in poor condition.
Wolf selectivity for old animals has been recorded in numerous studies (eg
Murie 1944, Mech 1966, Fritts and Mech 1981, Mech et al. 1995, Hayes et al.
2000). In my study such a selectivity is true only in case of hinds. Among stags
wolves killed mostly 1–5 year old individuals. Such stags are usually the youngest
members of male groups or (mostly yearlings) still members of female groups, and
normally do not take part in reproduction (Clutton-Brock et al. 1982). Red deer
males less than 5 years old grow intensively, and, as such, they do not deposit
large fat reserves (Dziêcio³owski et al. 1996). As members of male groups, these
individuals have the lowest rank in the hierarchy and graze on the edge of the
herd (Appleby 1980, 1982). Probably this reduces their body condition (poor food)
and exposes them to wolf attacks. Stags between 5 and 10 years of age are at their
optimum, they are the most active males during rut and they copulate with a
number of hinds (Clluton-Brock 1985). Although during the rutting season their
fat stores substantially decline (Bobek et al. 1990, Dziêcio³owski et al. 1996), this
age group stags were less vulnerable to wolf predation than younger males in my
study. It must be considered, however, that the age structure of stags killed by
wolves and those in the free living population were obtained by different methods.
Although in reality, selective pressure on young and fully matured stags might
Wolf predation on red deer 285
differ slightly from my results, it is clear that young males dominated among
stags killed by wolves. In contrast to my results, North American studies on
natural mortality of elk report dominance of >5 year old males both among
individuals killed by wolves (Huggard 1993) and dead due to other natural
mortality factors (Houston 1978, Smith 1998). The results obtained by Mysterud
et al. (2003) suggest that, when prime-aged males of reindeer Rangifer tarandus
are absent or rare, immature ones are more active during rut and as a consequence
lose more body mass. Therefore, it might be possible, that high vulnerability of
immature, 1–5-year-old males to wolf predation during winter and spring could be
an outcome of the heavy harvest of males for trophies, which took part outside the
National Park during my study (W. Œmietana unpubl.).
In summary, my study shows that wolves attacking female groups of red deer
usually killed calves and old hinds, while attacking male groups they usually killed
their youngest members. Wolves killed proportionally more calves from fall until
midwinter, and hinds proportionally to their availability in hind-calf groups in the
end of winter and in spring. In case of male groups, wolves were more likely to find
vulnerable individuals from midwinter until spring and, in consequence killed more
stags during this period. High vulnerability of adults from midwinter until spring
was caused by their poor body condition. Segregation of social units (hind-calf and
stag groups), except the rutting season, and low fat reserves of males from
midwinter until spring contribute to the high overall incidence of calves and adult
males and the relatively low incidence of adult females among wolf kills.
Acknowledgements: This work was financed by the Institute of Nature Conservation PAS budget
with aid of the Ecovolunteer Program, Biosphere Expeditions, and Wolvenfonds organisations.
Numerous volunteers and field staff of Bieszczady National Park helped in the fieldwork. I’m
especially grateful to K. Nienhuys, J. Wajda, D. Badstuber, W. Wiœniewski, T. Kwolek, W. Pietrasz
and W. Kalinowski. I want also to thank W. Wojciechowski (director of BNP), T. Winnicki, W. Holly
and L. Bekier who provided logistical help, R. Cosijn from Ecovolunteer Program and M. Hammer
from Biosphere Expeditions who organized teams of volunteers, and D. Rees who corrected the text.
Drs R. O. Peterson, W. Jêdrzejewski, and an anonymous referee provided helpful comments and
suggestions on an earlier draft.
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288 W. Œmietana
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