Under usual snow conditions throughout most of the range of the
white-tailed deer, healthy vigorous individuals can probably escape
most attacks by wolves. Observations by Mech (1966), Rutter and Pimlott
(1968), and Mech _et al._ (p. 1) indicate that a high percentage of
attempts by wolves to kill deer during winter are unsuccessful. This is
further implied by the figures of Pimlott _et al._ (1969) and Mech and
Frenzel (p. 35) showing that at least during winter wolves tend to kill
a disproportionate number of old deer as well as those with various
abnormalities and pathological conditions.
However, during a winter with extremely deep snow, the usual
relationships seem to change somewhat. Fewer deer are able to escape
wolves, and a surplus is killed. This means that some individuals not
vulnerable under the usual snow conditions become vulnerable during
extreme conditions. There are two main possible reasons for this, the
effect of the extreme weather conditions on the health and vigor of the
deer, and the physical effect of the snow on the escapability of the
deer.
In regard to the first possibility, there was limited evidence that
during February and March 1969 some fawns and yearlings in our study
area were losing their fat stores. Two of three yearlings, and both
fawns intact enough for examination during this period lacked back fat,
and the marrow in one of six fawn femurs was partly fat depleted.
Nevertheless, the third yearling inspected still had back fat, and a
3-1/2-year-old doe had heavy omental, renal, heart, and back fat during
the same period. Thus, although an abnormal decline in the physical
condition of some deer in the late winter might partly account for the
increased kill by wolves during February and March 1969, the effect of
snow on the escapability of the deer probably was also involved.
The key difference in snow conditions between the two periods--(1) the
winters of 1966-67, 1967-68, and December-January 1968-69, and (2)
February and March 1969--was the heavy, persisting accumulation of snow
during the latter period, combined with the increasing density of the
snow. As our observations show, this greatly hindered the movements of
deer fleeing from wolves.
Under more usual conditions, a running deer might sink through the snow
to the ground and thus obtain a firm footing from which to spring
again. In discussing wolf-caribou relations in snow, Kelsall (1968, p.
249) stated the following: "While caribou (_Rangifer tarandus_) will
sink into snow even deeper than wolves, their longer legs permit them
to run efficiently where a wolf will bog down. Nasimovich (1955)
considered that roe deer and sika deer could be taken by wolves when
snow was not more than 30 cm. (11.8 inches) in depth. At depths above
that their pursuit becomes difficult or fruitless."
However, it appears that when snow becomes extremely deep, wolves then
gain the advantage. With 22 to 48 inches or more of snow to plow
through, a deer would have trouble even touching a firm foundation.
According to Kelsall (1969), deer measure only 20 to 24 inches from
hoof tip to chest, with legs extended.
It is true that wolves stand even shorter than deer and so might be
expected to flounder even more. However, this is where another factor
becomes important, the "weight-load-on-track" or total weight per area
of track. As Kelsall (1969) has pointed out, the mean
weight-load-on-track for deer is extremely difficult to measure
directly, because the actual under-surface of the deer's foot slants
vertically, and a much greater area may be used to support an animal in
snow than on a hard surface. This probably explains the discrepancy
between Kelsall's measurements and work done by Verme (1968) in
Michigan. According to Kelsall, deer weight-load-on-track (hoof only)
varies between 431 and 1,124 gm./cm.^2. However, Verme stated that
his compaction gauge (with a weight load of about 211 gm./cm.^2,
described earlier in this paper) sank in virtually the same amount in
snow as did deer. Under the snow conditions in our study area, we found
that the same type of compaction gauge generally penetrated to a depth
within a half inch of that to which deer were sinking. On this basis,
it seems reasonable to suggest that a deer in snow is supported by more
of its foot than just the hoof, and that the actual
weight-load-on-track of deer in snow is about 211 gm./cm.^2.
For wolves, this measure varies from 89 to 103 gm./cm.^2 (Foromozov
1946). This means that for the same amount of force applied during
running, a wolf would have twice as much support as a deer. It also
means that in deep snow a walking wolf generally is much less
restricted than a walking deer. Late in February 1969, for example,
when deer were seriously limited in their ability to travel, wolves
were able to travel widely (Mech _et al._, p. 1).
Even though wolves have much greater support than deer, when running
they still sink into the snow almost as much as deer under most
conditions, probably because both run with such force that snow usually
offers little support. Nevertheless, with extremely deep snow, the
difference in support factor between wolves and deer could become
critical, and this is probably what happened during February and March
1969. With deer seriously restrained by the deep snow, even a slight
advantage in favor of the wolf could increase hunting success. A high
snow density during that period would accentuate this advantage. This
is because until the snow becomes dense enough to hold a running deer,
each increase in density would further the advantage of the wolf, which
would require only half the density to support it, while it would
hinder the deer.
One result of the extreme snow conditions of early 1969 was that deer
tended to gravitate to lakes, where snow was shallow and footing was
firm. Initially upon disturbance by human beings, and probably by
wolves, these deer usually headed inland, but it is apparent from a
number of kills examined that when pressed hard by wolves inland, deer
headed out onto lakes where possible. Apparently they could run there
with better footing. However, frozen lakes also provide wolves with
good running conditions, and even seem to give them an advantage
(Rutter and Pimlott 1968, Mech 1970), so many of these deer were killed
(fig. 11).
Stenlund (1955, p. 44) reported as follows on years of low snowfall,
the opposite condition, which demonstrated the same relationship
between snow depth and kills on lakes: "The winters of 1951-52 and
1952-53 were abnormally mild with little early snow. As a result, few
wolf-killed deer appeared on the lakes and most deer attempted to
outrun wolves in the woods."
[Illustration: _Figure 11.--On frozen lakes, wolves often seem to have
the advantage over deer, such as in this case where the wolf (center)
has just killed a deer and is trying to discourage a raven from joining
him in the feed. (Photo courtesy of L. D. Frenzel_.)]
Thus it appears that extreme snow conditions in our study area increase
the vulnerability of deer to wolf predation in three ways: (1) by
causing a decline in the health and nutritional state of some members
of the deer population; (2) by hindering the escapability of the deer;
and (3) by causing deer to congregate on frozen lakes where wolves have
the advantage in running.