Our data strongly indicate that in northeastern Minnesota wolves prey
much more heavily on the older members of the deer population, at least
during winter (fig. 7). Substantial vulnerability to wolves seems to
begin at about the age of 5 years (fig. 13), because the percentage of
wolf-killed deer in each year class increases from 9 percent for
4-1/2-year-old animals to 15 percent for 5-1/2-year-olds (table 4).
Indeed, 48 percent of the wolf-kills were aged 5-1/2 and over, which
compares favorably with the Ontario figure of 58 percent for these age
classes (Pimlott _et al._ 1969).
[Illustration: _Figure 13.--Relative rates of predation on deer of
various ages, based on comparisons of the ages of wolf-killed deer with
those of a theoretical population (dashed line) and those of the
hunter-killed population. See figure 7._]
These figures assume added significance when compared with a sample of
deer killed by hunters in the same general area (fig. 1). Only 10
percent of the hunter-killed deer were 5-1/2 years old or older, and
the percent killed in each year class dropped off suddenly from 13
percent aged 4-1/2 to 6 percent aged 5-1/2. If the age structure of the
hunter-kill sample is reasonably representative of the age structure of
the population at large, the wolf-kill data show that wolf predation in
our study area during winter has a definite selective effect on the
deer population.
There is no direct way of knowing that the age structure of the
hunter-killed deer represents the age structure of the deer population
at large. However, sampling hunter-kills is the most practical means
available for gaining an index to the age structure of the existing
herd. Further, there are three indirect pieces of evidence indicating
that the hunter-kill sample represents the actual age structure of the
population, just as Maguire and Severinghaus (1954) found in New York.
First, our sample has the basic theoretical form expected of a stable
deer herd; i.e., the youngest year class contained the most members,
and each older cohort included fewer (fig. 7). Second, the age
structure of our sample has the same form as most other deer age
structures from widely diverse areas, (Ontario, Pimlott _et al._ 1969;
southern Minnesota, Erickson _et al._ 1961; Massachusetts, Shaw 1951).
Third, there is no reason to believe that in our area rifle hunting is
especially selective for any particular age classes. In talking with
large numbers of hunters, we have learned that most shoot at any and
all deer they happen to see.
Even if the age structure of the hunter-kill sample did not approximate
that of the actual herd, the comparison of the wolf-kill with the
theoretical population dictates the same conclusion: the rate of kill
of older deer by wolves was several times greater than that of younger
deer, excluding fawns (fig. 13). In any case, if the actual deer
population in our study area had an age structure similar to that of
our sample of wolf-kills (which would be the only age structure that
would contradict our conclusion), its numbers would be declining by
orders of magnitude each year, and there would now be only a remnant
population. Such obviously is not the case.
The only other question that might arise from a comparison of the age
structure of our wolf-killed deer with that of the hunter-killed deer
concerns the area from which each sample was taken. Fifty of our
wolf-kills came from a region almost inaccessible to hunters (fig. 1).
However, the other 92 came from the same general area as the
hunter-kills. Nevertheless, there was no statistically significant
difference in age structure between the wolf-kills from the wilderness
versus those from the hunted area (table 1). This fact also suggests
that the human hunting in the area is relatively light and has little
effect on the age structure of the deer population in the area.
Wolves may also be taking a disproportionately high number of fawns,
although our data do not show this. Nevertheless, there may be a bias
against fawns in our method. It is not unusual to discover the remains
of a wolf-killed deer so completely eaten that there is no indication
left of the animal's age. Because fawns often are only about half the
size of adult deer, and their skeletons have not yet completely
ossified, the chances are better that fawns will be more completely
eaten. Pimlott _et al._ (1969) also recognized this possible bias,
although their data did indicate that wolves were killing a higher
percentage of fawns than occurred in the population.
Our study does support the other conclusion of Pimlott _et al._ (1969),
based on a study of 331 kills, that wolf predation on deer during
winter shows a definite selection for older animals. It does not agree
with the tentative conclusion of Stenlund (1955) that wolves in the
Superior National Forest do not prey disproportionately on old deer.
However, Stenlund's conclusion was based on 36 kills and on the
assumption that only deer at least 7 years old were "old." Deer 5 years
old and older composed 33 percent of Stenlund's sample, a figure
considerably higher than the 10 percent in these age classes in our
hunter-kill sample (table 4). Thus Stenlund's data do not contradict
our conclusion.
The age of 5 years seems to be the beginning of the period of
vulnerability for adult deer. Although 5 years might not seem
especially old, there are two aspects of significance concerning deer
of this age and older. First, they are in the second half of the life
span for most members of the species, and their alertness and ability
to bolt quickly away might be expected to decline. It is of interest in
this regard that Klein and Olson (1960, p. 87) believed 5 years of age
to be "the upper limit of physiological efficiency" of black-tailed
deer (_Odocoileus hemionus_) in Alaska. Second, up to the age of at
least 4-1/2 years, and perhaps beyond, the apparent
weight-load-on-track of deer increases with age (Kelsall 1969). Thus
older deer would sink farther into the snow than younger ones, and
their escape might be slowed and hindered more. For further discussion
of the effect of snow on the vulnerability of deer, see Mech _et al._
(p. 51).