Snow measurements for each winter are shown in figures 1 through 3. The
winter of 1968-69 was the most extreme of the three in terms of
accumulated snow, and was generally regarded as having one of the
heaviest snowfalls and accumulations on record for the study area. Snow
depth on the level near Isabella reached 3.9 feet at one time, and from
January 3 to April 4 it exceeded 2.4 feet. The highest snow level
reached during 1966-67 was 2.4 feet, and the highest level reached
during 1967-68 was 1.4 feet. In the vicinity of Ely, some 30 miles from
Isabella, the 1968-69 peak accumulation was 39 inches, the highest
accumulation since 1948-49 when records were first kept.[36] Thus we
consider the winters of 1966-67 and 1967-68 to be within the normal
range for the study area, and the 1968-69 winter as being most unusual
(fig. 4).
[Illustration: _Figure 1.--Snow depth and penetrability by deer and
wolves near Isabella, Minnesota, 1966-67._]
[Illustration: _Figure 2.--Snow depth and penetrability by deer and
wolves near Isabella, Minnesota, 1967-68._]
[Illustration: _Figure 3.--Snow depth and penetrability by deer and
wolves near Isabella, Minnesota, 1968-69._]
The snow penetrability in 1966-67 remained high throughout January,
February, and March. During the following winter, penetrability
fluctuated more, but even at its greatest, it was relatively
unimportant to deer because the total snow depth was so low. During
1968-69, however, penetrability was a very important aspect of snow
condition. It was so high during late January and early February, when
snow accumulation was also at its peak, that a walking deer would be
expected to sink in 2.5 to 3.5 feet. Snow penetrability then decreased
through February and March to a point where a walking deer would sink
in approximately 0.6 foot on March 21. However, because snow
accumulation remained so high through February and March, the lower
penetrability during late February and March still afforded no relief
to running deer, because they must exert forces several times as great
as when walking. On the contrary, the low penetrability (which is an
indirect measure of density) could be expected to hinder a running deer
in deep snow, for it would cause much more resistance.
[Illustration: _Figure 4.--During the winter of 1968-69, the snow was
unusually deep in the study area. (Photo courtesy of L. D. Frenzel.)_]
Deer movements, like snow conditions, varied greatly during the three
winters of the study. During the first two winters, deer were generally
found singly and in groups of two to six, often around the shores of
lakes but also scattered about inland. In late January and February
1967, running deer were observed sinking deeply into snow, but their
movements still did not seem to be hindered, no doubt because of the
high penetrability (low density) of the snow that year (fig. 1).
However, during late January, February, and March of 1969 the deer were
much more concentrated, mostly in conifer swamps, along
southwest-facing slopes, or on lakes. Although groups of two or three
animals could be found in scattered inland "pockets" throughout the
winter, groups of five or six were not uncommon on lakes during
January. The tendency to concentrate continued to increase, and on
February 6, as many as 11 deer were observed on one lake; by March 13,
group size had increased to as high as 22 deer in the same area.
Throughout February and March, heavy concentrations of deer tracks
covered most wilderness lakes, further evidencing much greater use of
shorelines than had occurred in the two previous winters (fig. 5).
No doubt deer tended to concentrate on lakes because travel inland
became so difficult. On January 28, two deer were seen plowing through
snow up to their necks. Although the snow began settling in February,
and the penetrability decreased, by late February running deer still
plunged chest-deep and had to hesitate at every bound. These conditions
persisted until about March 26, by which time a surface crust strong
enough to hold a running deer had formed.
[Illustration: _Figure 5.--Under unusually deep snow conditions, deer
used lake shores heavily. (Photo courtesy of L. D. Mech.)_]
In considering wolf mobility in snow, two types of movement must be
recognized: the trot used during general travel, and the bounding used
while chasing prey. The trot is an easy gait of about 5 m.p.h. on firm
footing (Mech 1970), and can be continued for hours at a time. During
periods of deep snow and high penetrability, most wolf travel is on
frozen waterways, roads, snowmobile trails, and animal trails,
including the wolves' own pathways, which become well packed with
frequent use (fig. 6, 7A, B). Such travel was observed during each of
the three winters of this study.
[Illustration: _Figure 6.--Wolves travel single file in deep snow.
(Photo courtesy of L. D. Mech.)_]
[Illustration: _Figure 7.--(A) A single wolf must break his own trail
through the snow. (Photo courtesy of L. D. Frenzel.) (B) Regular use by
a pack keeps trails open. (Photo courtesy of L. D. Mech.)_]
The second type of wolf movement affected by snow is the leaping and
bounding associated with chasing prey. The shallower angle of the
wolf's bound (fig. 8) (compared with that of the deer) often causes the
wolf to flounder in snow that presents little hinderance to deer (Mech
1970). Such was the case in January and February 1967 in our study
area. During 1967-68 no observations of wolves chasing deer were made
by the authors, but reports by other field workers indicated that
running conditions were similar to those of 1967.
[Illustration: _Figure 8.--Wolves run at a shallow angle, thus
hindering them in deep snow. (Photo courtesy of D. H. Pimlott.)_]
During the winter of 1968-69, wolves also bogged down a great deal in
snow when chasing deer. However, after January 1969 the snow was so
deep that deer were floundering even more than wolves in many cases.
The fact that wolves could run in the trail broken by deer probably
also gave the wolves an advantage under the conditions that severely
restricted deer movements.
The above observations of snow conditions, deer movements, and wolf
movements during the three winters of the study are in accord with
observations made on the differences in the ability of the wolves to
capture deer during the same period. Two indices support the conclusion
that wolves had a much easier time catching deer during February and
March 1969 than earlier in the winter and in the two previous winters:
(1) the degree of utilization of wolf-killed deer, and (2) the kill
rate of radiotagged wolves.
During the winters of 1966-67 and 1967-68, and in December and early
January 1968-69, most wolf-killed deer found had been thoroughly eaten,
and the bones--if present at all--were well chewed and scattered at
each kill (fig. 9). All skin and flesh from the skull were eaten, and
the mandible was usually separated from the skull. During late February
and early March 1967, few fresh kills were even found, and wolves were
returning several times to old kills that had been cleaned up many days
before.
[Illustration: _Figure 9.--Usually the remains of a wolf kill are well
chewed and scattered before the wolves abandon them. (Photo courtesy of
L. D. Mech.)_]
[Illustration: _Figure 10.--During a period of especially deep snow,
wolves abandoned many kills before pulling apart the skeletons. (Photo
courtesy of L. D. Mech.)_]
However, in late January 1969 a substantial change began taking place.
The skeletons of most kills found were almost intact, the flesh having
been eaten from around the bones (fig. 10). Appreciably more skin was
usually left on the carcass, especially on the side lying on the snow,
and the neck and head were generally intact. This was true even of
fawns, which in the past often were almost completely consumed.
In several cases, only about half of the flesh had been eaten from the
carcasses. On February 2, 1969, four deer recently killed by wolves
were found along a 1-1/2-mile stretch of Birch Lake and nearby
Polaris Lake (Minnesota-Ontario border). One large doe was completely
uneaten and remained so for at least 24 hours after discovery from the
air. Further, one fawn had only a few pounds of flesh eaten, a yearling
doe was half eaten, and another fawn was about 75 percent eaten.
Hazardous landing conditions during this period severely limited the
number of carcasses that could be examined from the ground, but on
February 6 a yearling doe was discovered that had only about 5 to 10
pounds of flesh eaten, and on February 8 an adult doe was found that
was completely intact except for wounds.
In past winters some kills had been located that had been only partly
eaten, but in each case the carcasses were soon revisited and cleaned
up (Mech 1970). This was often not the case in 1969. For the rest of
the winter most of the deer killed by wolves in our study area were not
as completely consumed as in previous winters. Pimlott _et al._ (1969)
found a similar relationship between the severity of the winter and
the degree to which wolf-killed deer were utilized.
Correlated with the above information was the kill history of our
radiotagged wolves (Mech _et al._, p. 1). From December 1968 through
January 1969 No. 1051 had killed three or possibly four deer, and
generally had spent 6 or 7 days feeding on each. However, throughout
most of February this animal visited a new deer carcass (which
presumably he killed) every 3 days, and he spent only 1 or 2 days at
each. In two cases two new carcasses were found in the immediate
vicinity of this animal during the same day, and in each case the wolf
spent only 1 day in the area. A second wolf (1053) which had spent most
of December and January scavenging on the remains of both deer and
moose (_Alces alces_) that had died long before, made her first known
kill of a deer on January 31, 1969. The kill rate of the other three
radiotagged wolves also increased, although the data for them are less
complete. The average kill rate for all radiotagged wolves and their
associates was one deer per wolf per 16 to 20 days before February 1,
and one per 8 to 12 days after February 1 (see Mech _et al._, p. 1).