Chapter III: Part 3

As an additional test of the degree to which the age structure of the wolf-killed deer might differ from that of the actual population, we compared our wolf-kill age structure with the age structure of a hypothetical deer population. This was considered advisable just in case the hunter-kill data were poorly representative of the age structure of the actual deer herd. Several hypothetical age structures were constructed and compared according to advice from Downing.[27] In all cases, the comparisons produced the same basic results as the tests with the hunter-killed sample. An example of one comparison is given in figure 7.

A further result obtained by aging the wolf-killed deer pertained to the young individuals killed. The deciduous first incisors of fawns and the deciduous premolars of yearlings are usually replaced with permanent teeth by December (Severinghaus 1949). Of 24 wolf-killed fawns examined, however, three (13 percent) taken during January, February, and March had not yet replaced their deciduous first incisors. Of the 13 yearlings found during this same period, nine (70 percent) had failed to replace their deciduous premolars, and two (15 percent) had just replaced them (one deer killed in February and one killed in March).

FOOTNOTES:

[27] _R. L. Downing. Personal correspondence to L. D. Mech, October 2, 1969_.

_Table 4.--Age and sex distribution of deer killed by wolves and hunters in northeastern Minnesota_

-------:-------------------------------:-------------------------------
: Wolf-killed deer : Hunter-killed deer
Age :---------------------------:---:---------------------------:---
(years): Number of: : : Number of: :
:Males Females Unknown Total: % :Males Females Unknown Total: %
-------:---------------------------:---:---------------------------:---
Fawns 9 13 2 24 17 54 54 4 112 26
1+ 5 7 1 13 9 63 26 1 90 21
2+ 3 8 5 16 11 42 19 2 63 15
3+ 2 4 2 8 6 47 16 1 64 15
4+ 6 3 4 13 9 32 22 1 55 13
5+ 12 9 -- 21 15 15 12 1 28 6
6+ 9 2 1 12 8 3 -- -- 3 --
7+ 12 4 -- 16 11 7 4 -- 11 3
8+ 4 2 -- 6 4 5 1 -- 6 1
9+ 4 2 -- 6 4 1 -- -- 1 --
10+ -- 3 -- 3 2 -- -- -- -- --
11+ -- 1 -- 1 } -- -- -- -- --
12+ -- -- -- -- } 4 -- -- -- -- --
13+ -- 1 -- 1 } -- -- -- -- --
14+ -- 2 -- 2 } -- -- -- -- --

Total 66 61 15 142 100 269 154 10 433 100 -----------------------------------------------------------------------

Mandibles from the 142 wolf-killed deer and 259 hunter-killed deer were examined closely for abnormal dentition (table 5, figs. 8-10) (Mech _et al._ 1970) and pathological conditions (table 6), and the lower limbs of 75 wolf-kills and 126 hunter-kills were also checked for abnormalities and pathology (table 7, fig. 11). Statistical comparison showed that the incidence of each condition was significantly higher in the sample from wolf-killed deer (table 8).

Jaw necrosis found in our specimens was similar to that described by Murie (1944) for Dall sheep and Mech (1966a) for moose. Generally animals with this condition are old, and ours were no exception.

_Table 5.--Abnormalities in the mandibular dentition of deer from the Superior National Forest, Minnesota_

--------:---:-------:--------:-------:-------------------------------- Specimen: : :Cause of:Side of: number :Sex:Age[28]: death :jaw[29]: Abnormality --------:---:-------:--------:-------:-------------------------------- _Years_

M-8 F 3+ Wolves Right P_1 present (fig. 9)
Left Normal; no P_1 present outside
or inside jaw

M-31 F _17 mon._ Wolves Both Deciduous P_1 present (fig. 8)
and permanent P_1 present inside
left ramus; right side not
examined internally

M-45 M _4+_ Wolves Right P_2 rotated 90 deg.
Left P_2 absent

M-52 M 4+ Wolves Right P_2 absent
Left Normal

M-96 F _2+_ Hunters Right 2 permanent P_4s present; both
crooked in orientation (fig. 10)
Left P_2 diagonal; P_3 normal; P_4 below
gumline, pointed posteriorly and
wedged against M_1; appears to have
pushed out original P_4 (fig. 10)

M-117 M 5+ Hunters Right Third column of M_3 reduced

M-191 M 4+ Wolves Right Third column of M_3 absent
although rudimentary root present
Left Third column of M_3 much reduced,
peg-like, and almost separate

M-225 -- 4+ Wolves Right P_2 absent
Left P_2 situated diagonally

M-234 F 5+ Wolves Right Third column of M_3 reduced

M-254 M 2+ Hunters Right P_2 slightly crooked in orientation
Left P_2 slanting posteriorly and
crowding P_3

M-272 M 5+ Hunters Right Third column of M_3 reduced,
peg-like, and almost separate
Left Third column of M_3 peg-like and
separated from second column by
4 mm.

M-296 F 5+ Wolves Right Normal
Left Extra permanent P_4 crowding
original P_4; much like M-96

M-369 M 3+ Hunters Right Permanent P_2 still not emerged
but appears to be wedged against
root of P_3
--------:---:-------:--------:-------:--------------------------------

FOOTNOTES:

[28] Based on incisor sectioning method of Gilbert (1966) except that _underlined_ figures are based on tooth replacement or wear (Severinghaus 1949).

[29] Where only one side is listed, the other was not available.

_Table 6.--Pathological conditions in the lower jaws of deer killed by wolves or hunters[30]_

--------:---:------:--------:-----------:----------------------------- Specimen: : :Cause of:Approximate: number :Sex: Age : death : date of : Condition : : : : death : --------:---:------:--------:-----------:----------------------------- _Years_

M-70 M 6-1/2 Wolves Feb. 1968 Lump in left side of mandible
near M_1 and M_2

M-192 M 7-1/2 Wolves Jan. 1969 Large lump in left diastema
apparently from healed fracture

M-206 M 8-1/2 Wolves Jan. 1969 Light necrosis around base
of teeth

M-218 M 3-1/2 Wolves Feb. 1969 Large lump in left diastema
apparently from healed fracture

M-228 F 11-1/2 Wolves Mar. 1969 Heavy necrosis around molars
and extending into bone; half
of each M_3 destroyed, both
roots and crown

M-236 F 14-1/2 Wolves Feb. 1969 Light necrosis around base
of teeth

M-402 F 10-1/2 Hunters Nov. 1968 Heavy necrosis and lumps on both sides of mandible --------:---:------:--------:-----------:-----------------------------

FOOTNOTES:

[30] Not including dental abnormalities, which are described in table 5.

The following organs were excised from wolf-killed deer and examined grossly in the field for parasites and abnormalities (fig. 12): lungs (six animals, normal); heart (seven animals, normal); liver (four animals, one small unidentified tapeworm cyst). Twin fetuses were found in each of two adult does examined.

Twelve deer were checked for body fat in one or all of the following areas: back (subcutaneous), kidneys, heart, omenta. Of these animals, seven had large amounts of fat, but five were almost depleted of fat from these stores. These five were all killed in February or March 1969; three were fawns, and two were yearlings that had not yet shed their deciduous premolars.

Of 69 animals examined for femur marrow condition, two had fat-depleted marrow. One was a fawn killed in March 1969 that had not shed its deciduous first incisors, and the other was a 5-1/2-year-old buck killed in February 1966.

A fawn and a yearling that had died in February 1969 from unknown causes also had fat-depleted, marrow. These animals might have been killed by wolves, for wolves had fed on them. However, they could have died from malnutrition and been eaten as carrion.

_Table 7.--Pathological conditions in the lower limbs of deer killed by wolves or hunters_

--------:---:-----:------:-------------------------------------------- : : :Cause : Specimen:Sex: Age : of : Condition number : : :death : --------:---:-----:------:-------------------------------------------- _Years_

M-28 M 5-1/2 Wolves Right hind foot: "Old healed ankylosis of
the pastern joint ... a spontaneously healed
bacterial arthritis with the destroyed joint
cavity filled in by solid bone. This deer
probably had defective gait"[31] (fig. 14).

M-29 F 5-1/2 Wolves Front foot: "A 3x4x5 cm. fibrous mass in the
subcutis about the digital flexor tendon on
the volar surface of the metacarpus. The
surface was denuded, ulcerated, and
superficially infected by surface bacteria....
Probably did detract from the animal's speed
of flight"[31] (fig. 15).

M-37 F 7-1/2 Wolves Hind foot: "Probable that the lesion was at
one time an active bacterial bone marrow
infection that had eventually fistulated to
the skin.... Regional tendons and their
sheaths were also present among this
inflammation and scarring, and it would be
fair to assume that the animal's agility was
impaired to some extent."[31]

M-115 M 4-1/2 Hunter Right front hoof: Broken at tip.

M-196 F 4-1/2 Wolves Left front foot: "Two severe transverse
lacerations on the volar surface. Each was
approximately 4 cm. in length. One was
located at the margin of the heel, and the
other was located several cm. proximad. The
more proximal wound had severed the flexon
tendons, and the consequent uselessness of
the limb was suggested by the splayed toes,
the unmarred hoof wall and unworn soles"[32]
(fig. 16).

M-227 M 9-1/2 Wolves Left hind leg: "A diffuse swelling of the
distal metatarsal bone, the surface of which
was studded with small osteophytic spicules.
The major flexor and extensor tendons were
forced to assume a convex course over the
summits of the dorsal and plantar surfaces
of the defect, but the tendon sheaths were
clean and the normal wear on soles of the
involved toes suggested that functional
deficit and pain were probably minimal ...
quite certainly a callus from previous
fracture"[32] (fig. 17).
--------:---:-----:------:--------------------------------------------
1
FOOTNOTES:

[31] D. M. Barnes. Personal correspondence to L. D. Mech, April 11, 1967.

[32] D. M. Barnes. Undated laboratory report transmitted to L. D. Mech in 1969.

DISCUSSION AND CONCLUSIONS

It has been established that wolves hunting Dall sheep (Murie 1944), caribou (Crisler 1956), moose (Mech 1966a), and other species usually have a low percentage of success. In the case of a pack of 15 wolves hunting moose on Isle Royale during winter, only 4.6 percent of all the moose detected by the pack were killed; considering only the moose that the wolves caught up to or held at bay, the kill rate was 7.6 percent (Mech 1966a).

What little evidence there is about wolves hunting deer indicates that the success rate is also low with this prey species, at least in winter. The senior author has now observed a total of 14 deer being chased by wolves in northeastern Minnesota, mostly by packs of five, seven or eight wolves (Mech 1966b, and see Mech _et al._, p. 1). In only one case (6.7 percent) did the wolves (a pair) succeed in catching their prey.

Low hunting success rates imply that the circumstances influencing hunts are seldom favorable enough, or the prey animals encountered are seldom vulnerable enough for the wolves to succeed. When the evidence cited earlier that most wolf-killed animals are inferior members of their populations is considered, the most cogent explanation for the low hunting success of wolves is that relatively few prey animals are vulnerable.

_Table 8.--Incidence of various abnormalities and pathological conditions in wolf-killed deer compared with that in hunter-killed deer_

#: _Number_ %: _Percent_

---------------------:----------------:----------------:-------------
: Wolf-kills : Hunter-kills :
:------:---------:------:---------: Level of
Condition : Deer : Deer : Deer : Deer : significance
: in : with : in : with :
:sample:condition:sample:condition:
---------------------:------:---------:------:---------:-------------
# # % # # % %

Dental abnormalities 142 8 5.6 259 5 1.9 [34]90

Jaw necrosis, lumps, 142 6 4.2 259 1 0.4 [34]95
or fractures[33]

Pathology of lower 75 5 6.7 126 1 0.8 95 limbs ---------------------:------:---------:------:---------:-------------

FOOTNOTES:

[33] Two mandibles from wolf-killed deer had large lumps from healed fractures in the region of the diastemas.

[34] If all dental and jaw abnormalities are pooled, the difference between the incidence in the wolf-kill sample (9.8 percent) and that in the hunter-kill (2.3 percent) is significant at the 99 percent level.

Age Structure

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).

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).

Sex Ratio

Statistical tests comparing a number of subsamples of both wolf-killed deer and hunter-killed deer showed a series of significantly different sex ratios (tables 1-3). The ratio of males to females in the fawn cohort of the hunter-kill, which is probably the most representative of the actual fawn sex ratio, was even (table 2). With wolf-kills, however, a significantly higher percentage of females was taken in the fawn subsample (59 percent) than in the adult subsample (46 percent). These results compare favorably with those of Stenlund (1955), who found that from 1948 to 1953 in the same area as the present study 68 percent of 19 sexable fawn wolf-kills were females and 44 percent of 63 sexable adult wolf-kills were females.

If the sex ratio of fawns began even, and more females than males were killed by wolves, then a higher proportion of males would be left in the adult population, unless some other mortality factor kills more male fawns. Thus it is not surprising that in the wilderness area, where little or no hunting is done, the sex ratio of wolf-kills in the adult cohort is significantly heavy toward males (71 percent: 29 percent). This was also true of the wolf-kills in Algonquin Provincial Park, where males made up 57 percent of the total sexable wolf-kill (Pimlott _et al._ 1969). The latter figure may even have been higher if calculated for adults alone, for a preponderance of female fawns in the Algonquin Park data (such as occurred in our and Stenlund's samples) would tend to obscure the preponderance of males in the adult sample.

The adult subsample of hunter-kills also contained a higher percentage of males (66 percent : 34 percent). Although this might also reflect the influence of wolf predation on female fawns, it probably is more a result of the greater movement of bucks during the hunting season, which overlaps with the rutting season. Even the sex ratio of adult deer killed in wolf-free areas shows a preponderance of males (Erickson _et al._ 1961).

However, it appears that the higher harvest of bucks by human hunters does markedly affect the sex ratio of the deer population in the hunted area, for the wolf-kill of adults in that area contained a significantly higher percentage of does (56 percent) than did the wolf-kill of adults in the wilderness area (29 percent).

Evidently the hunter harvest is not heavy enough to affect the age structure of the deer population to any marked degree, for no significant difference in age structure was found between the wolf-kill in the hunted area and that in the wilderness area (table 1). This does not conflict with the conclusion that hunting affects the sex ratio of the deer herd, because it would take much less to influence a population characteristic having two classes (sex) than one having 14 (age).

One additional difference in the sex ratio was found between two other subsamples of the wolf-kill--that is, the wolf-kill before and after an unusually high snow accumulation, which reached its peak about February 1, 1969 (table 1). Of a total of 77 animals killed before this snow condition occurred (including those from previous years), 38 percent were females. Of 44 animals killed after the heavy accumulation, 57 percent were females. One possible explanation for this is that females may normally be less vulnerable to wolf predation, for Kelsall (1969) has shown that they probably have a lighter weight-load-on-track than males. Thus when snow conditions changed greatly, making deer generally much more vulnerable to wolves (see Mech _et al._, p. 35), a preponderance of does suddenly might have become available. There is some evidence that does may be generally less vulnerable under most conditions, for all seven of our wolf-killed deer over 10 years old were females, and the oldest was over 14.

Condition of Wolf-Killed Deer

Because the data show that wolves in our study area tend to kill a disproportionate number of older deer, it is not surprising to discover that wolves also tend to capture a disproportionate number of individuals with abnormalities and pathological conditions (table 8). The explanation for such selection is obvious in regard to the abnormalities of the lower limbs (figs. 14-17): deer with injured or abnormal limbs simply cannot run as fast or as agilely as normal animals (table 7). Our observations show that deer usually depend on their alertness and speed to escape approaching wolves (Mech 1966b, Mech _et al._, p. 1). Any trait or condition that tended to interfere with either alertness or speed would decrease an individual's chance of escape.

It is more difficult to explain how dental abnormalities or pathological conditions of the mandible (figs. 8-10) would predispose an individual to wolf predation. However, in the case of dental abnormalities the genetic or environmental conditions that caused the abnormality might also have caused some other trait that increased the animal's vulnerability. Or the abnormal condition itself may have caused a further, more critical, disruption of the animal's physiology or behavior, which in turn predisposed it to wolf predation.

The finding of several wolf-kills with poor fat stores could indicate that primary or secondary malnutrition was a factor in the animals' deaths. However, it would take a statistical comparison between the fat stores of the deer at large and those of the wolf-kills to establish this.

The discovery that 13 percent of the fawns and 84 percent of the yearlings killed during January, February, and March had not yet shed their deciduous incisors and premolars, respectively, also fits well with the rest of our information. Evidently some unusual factor had caused the delay in tooth development and replacement. One possibility is that the animals were born in August or September, much later than normal. Although most deer in Minnesota are born in May and June, there are records of births in July and August. In addition, a fetus 181 to 200 days old was found in a doe killed on September 26 (Erickson _et al._ 1961).

An alternate explanation for the delay in tooth replacement is that the animals were suffering from malnutrition or nutrient deficiency. Severinghaus[35] has evidence that yearling bucks that have not replaced their deciduous premolars during November, and thus are aged at 17 months (Severinghaus 1949), generally have shorter, narrower antlers and fewer points than 18-and 19-month-old individuals. Degree of antler development in turn is considered related to nutritional state (Latham 1950). Thus it is reasonable to conclude that animals behind in tooth development and replacement, whether this is caused by age or diet, are physiologically inferior.

Most of the abnormal conditions discussed above pertain to the skeletal parts of wolf-kills. If the soft parts of a large number of kills could be examined thoroughly, one might discover a much higher incidence of diseases and other pathological conditions.

In conclusion, our data on both age and condition of wolf-killed deer show that at least during winter, wolves in our study area usually do not kill just any deer they discover, although they do try to. Evidently, most deer can usually escape wolf predation. The most frequent exceptions are those 5-1/2 years old and older, those born late, those suffering from poor nutrition, those with abnormalities or pathological conditions, and possibly fawns.

The above conclusions parallel those of Murie (1944), Crisler (1956), Mech (1966a), and Pimlott _et al._ (1969) for wolves preying on Dall sheep, caribou, moose, and deer respectively, and further substantiate the claim by Mech (1970) that they can be extended to wolves preying on most, if not all, species of large mammals under most conditions. It is also apparent from the data presented above that deer over 5 years of age and those with abnormalities of the jaw or lower limbs represent such a small percentage of the total population that they are seldom taken by human hunters. In this respect, competition between timber wolves and human hunters appears to be minimal in the study area.

FOOTNOTES:

[35] _C. W. Severinghaus. Unpublished data_.

SUMMARY

White-tailed deer (_Odocoileus virginianus_) killed by wolves (_Canis lupus_) during winter in a relatively unhunted wilderness area and in an immediately adjacent hunted area of Minnesota were compared with deer killed by hunters in the same general area, and with a hypothetical population. Deer killed by wolves were significantly older. Statistical comparisons also showed the following: (1) hunters generally killed an even sex ratio of fawns, and a disproportionate number of adult bucks, (2) wolves took a higher percentage of female fawns than female adults, a disproportionate number of bucks in the wilderness area, and a higher percentage of does in the hunted area. The latter fact evidently reflects the higher hunter success on males in the hunted area. Significantly higher incidences of abnormalities and pathological conditions of both mandibles and lower limbs were found in wolf-killed deer than in hunter-killed deer, and these conditions are described. It is concluded that wolf predation on white-tailed deer in the study area during winter generally is selective in that it tends to remove members of the prey population that are old, debilitated, or abnormal. Apparently these classes of deer represent such a small percentage of the population that they are seldom taken by human hunters.

ACKNOWLEDGMENTS

This study was supported by Macalester College, the New York Zoological Society, the Minnesota Department of Conservation, the USDA Forest Service, and the U.S. Bureau of Sport Fisheries and Wildlife. Pilots Robert Hodge, Pat Magie, John Winship, Jack Burgess, Don Murray, and Walt Neumann aided substantially in obtaining jaws from wolf-killed deer. Students from the Macalester College Biology Department and personnel of the USDA Forest Service and the Minnesota Department of Conservation helped secure mandibles from both wolf-killed and hunter-killed deer. The interest of Mr. John E. Peninger and of many deer hunters in contributing the jaws is also greatly acknowledged.

Mr. David W. Kuehn sectioned the incisors of the deer jaws and determined their ages. Dr. Donald M. Barnes of the University of Minnesota Veterinary Diagnostic Laboratory examined the abnormal lower limbs, described their pathology, and provided photos of specimens used herein.

Mr. Wallace C. Dayton and Miss Elizabeth Dayton and the Quetico-Superior Foundation, all of Minneapolis, financed Mech during the preparation of this paper.

The following individuals read the manuscript and offered many helpful suggestions: Mr. R. L. Downing, Mr. C. W. Severinghaus, Mr. J. M. Peek, Dr. C. T. Cushwa, Mr. M. H. Stenlund, and Dr. R. R. Ream.

LITERATURE CITED

Crisler, Lois. 1956. Observations of wolves hunting caribou. J. Mammal. 37: 337-346.

Downie, N. M., and Heath, R. W. 1959. Basic statistical methods. 289 p. New York: Harper and Bros.

Erickson, A. B., Gunvalson, V. E., Stenlund, M. H., Burcalow, D. W., and Blankenship, L. H. 1961. The white-tailed deer of Minnesota. Minn. Dep. Conserv. Tech. Bull. 5, 64 p.

Gilbert, F. F. 1966. Aging white-tailed deer by annuli in the cementum of the first incisor. J. Wildl. Manage. 30: 200-202.

Kelsall, J. P. 1969. Structural adaptations of moose and deer for snow. J. Mammal. 50: 302-310.

Klein, D. R., and Olson, S. T. 1960. Natural mortality patterns of deer in southeast Alaska. J. Wildl. Manage. 24: 80-88.

Kuehn, D. W. 1970. An evaluation of the wear method as a criterion for aging white-tailed deer. M.S. Thesis., Univ. Minn.

Latham, R. M. 1950. Pennsylvania's deer problem. Penn. Game News, Spec. Issue 1. (Cited from: Allen, D. L. 1962. Our Wildlife Legacy.)

Maguire, H. F., and Severinghaus, C. W. 1954. Wariness as an influence on age composition of white-tailed deer killed by hunters. N. Y. Fish and Game J. 1: 98-109.

Mech, L. D. 1966a. The wolves of Isle Royale. U.S. Nat. Park Serv. Fauna Ser. 7, 210 p.

Mech, L. D. 1966b. Hunting behavior of timber wolves in Minnesota. J. Mammal. 47: 347-348.

Mech, L. D. 1970. The wolf: the ecology and behavior of an endangered species. 389 p. New York: Natural History Press, Doubleday.

Mech, L. D., Frenzel, L. D., Jr., Karns, P. D., and Kuehn, D. W. 1970. Mandibular dental anomalies in white-tailed deer from Minnesota. J. Mammal. 51: 804-806.

Murie, A. 1944. The wolves of Mount McKinley. U.S. Nat. Park Serv. Fauna Ser. 5, 238 p.

Pimlott, D. H., Shannon, J. A., and Kolenosky, G. B. 1969. The ecology of the timber wolf in Algonquin Provincial Park. Ont. Dep. Lands and Forests Res. Rep. (Wildl.) 87, 92 p.

Ryel, L. A., Fay, L. D., and Van Etten, R. C. 1961. Validity of age determination in Michigan deer. Mich. Acad. Sci., Art, and Letters 46: 289-316.

Severinghaus, C. W. 1949. Tooth development and wear as criteria of age in white-tailed deer. J. Wildl. Manage. 13: 195-216.

Severinghaus, C. W. 1955. P. R. Rep. W-28-R-9: Job 1A, April 13, 1955.

Shaw, S. P. 1951. The effect of insufficient harvests on an island deer herd. N.E. Wildl. Conf. (Mimeo).

Siegel, S. 1956. Non-parametric statistics for the behavioral sciences. 312 p. New York: McGraw-Hill.

Stenlund, M. H. 1955. A field study of the timber wolf (_Canis lupus_) on the Superior National Forest, Minnesota. Minn. Dep. Conserv. Tech. Bull. 4, 55 p.

THE EFFECT OF SNOW CONDITIONS ON THE VULNERABILITY OF WHITE-TAILED DEER TO WOLF PREDATION

L. David Mech, L. D. Frenzel, Jr., and P. D. Karns

Wolves (_Canis lupus_) and deer (_Odocoileus virginianus_) having evolved together, no doubt have become adapted to contending with each other's physical abilities. Thus it is not surprising to learn that deer which succumb to wolf predation are generally weaker, older, or abnormal compared with the total deer population (Pimlott _et al._ 1969, also see Mech and Frenzel, p. 35).

However, the structural and behavioral adaptations of both species must have evolved under environmental conditions that are average or usual; otherwise, an adjustment of wolf to deer populations, and vice versa, could not have been maintained over long periods. This implies that extreme or unusual conditions might sometimes occur, to which either the wolf or the deer is poorly adapted.

One of the most important environmental factors that can influence the interactions of wolves and deer is snow. The total fall, depth on the ground, and the density are all aspects of snow that may vary considerably and affect the ability of wolves to capture deer. Recent studies of wolves and deer in northeastern Minnesota (see Mech _et al._, p. 1, also Mech and Frenzel, p. 35) afforded us opportunities to investigate the relationships between snow and the interactions of wolves and deer.

METHODS

Two principal methods of study were used in this investigation. The first involved recording the snow depth and support quality ("penetrability") in feet and tenths of feet (Verme 1968). Snow measurements were taken during the winters of 1966-67, 1967-68, and 1968-69, in which large differences in snow conditions existed. Ten such measurements were made weekly near Isabella, Minnesota, in an open aspen (_Populus tremuloides_) stand away from influences that might have caused drifting or other unusual snow conditions; the measurements were averaged. Penetrability was determined with Verme's snow-compaction gauge--a 3-foot piece of 1-1/8-inch (outside diameter) copper tube filled with lead to total 3 pounds, which gives a weight per area of 211 gm./cm.^2. To obtain a measurement, the pipe is held vertically with its lower end just flush with the snow, and then is released. The depth to which it sinks is considered the penetrability of the snowpack by a walking deer.

Although the snow conditions measured at Isabella are not representative of the entire study area, year-to-year comparison in the Isabella area should also apply generally throughout the region.

The second technique used in this study was observing the movements of wolves and deer. This was usually done from low-flying aircraft, and was facilitated by the use of radiotracking, as described by Mech _et al._ (p. 1). Close inspection of wolf-killed deer was made from the ground (Mech and Frenzel, p. 35).

RESULTS AND OBSERVATIONS

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).

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.

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.

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.

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.

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.

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).

FOOTNOTES:

[36] _M. H. Stenlund. Personal correspondence to L. D. Mech, Oct. 10, 1969._

DISCUSSION AND CONCLUSIONS

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."

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.

SUMMARY

During the winters of 1966-67, 1967-68, and 1968-69, the interactions of wolves (_Canis lupus_) and white-tailed deer (_Odocoileus virginianus_) were observed in northeastern Minnesota from aircraft. Snow depth and supporting ability were also measured during these winters, and the ability of wolves to capture deer was compared for a period of usual snow conditions versus a period of extreme snow conditions.

It was found that during February and March 1969, when snow remained from 2.5 to 3.9 feet deep and failed to support running deer, wolves were able to capture deer more easily. This was evidenced by kills that were left partly or completely uneaten, and by a higher rate of predation by radiotagged wolves and their associates.

Although both wolves and deer floundered in the extremely deep snow, the relatively lighter weight-load-on-track of wolves evidently gave them a greater advantage than under the usual snow conditions, when wolves were observed floundering more than deer. This factor, plus a decline in the health and vigor of some segments of the deer population and a tendency for deer to congregate on frozen lakes, where wolves have an advantage, help explain the increased vulnerability of deer to wolf predation during the winters of deep snow.

ACKNOWLEDGMENTS

This study was supported by Macalester College, the Minnesota Department of Conservation, the USDA Forest Service, the U.S. Bureau of Sport Fisheries and Wildlife, and the New York Zoological Society. Pilots John Winship, Pat Magie, Jack Burgess, and Don Murray flew the observation planes during radiotracking. Miss Elizabeth Dayton, Mr. Wallace C. Dayton, and the Quetico-Superior Foundation, all of Minneapolis, financed Mech during the writing of this report.

Thanks are also due L. J. Verme, J. P. Kelsall, and J. M. Peek for their helpful reviews.

LITERATURE CITED

Foromozov, A. N. 1946. The snow cover as an environment factor and its importance in the life of mammals and birds. (Moskovskoe obshchestvo ispytatelei priroda) Materialy k poznaniyu fauny i flory SSSR, Otdel. Zool. n. 5 (XX). (Translation from Russian published by Boreal Institute, Univ. Alberta, Edmonton, Alberta.)

Kelsall, J. P. 1968. The caribou. Can. Wildl. Serv. Monog. 3, 340 p.

Kelsall, J. P. 1969. Structural adaptations of moose and deer for snow. J. Mammal. 50: 302-310.

Mech, L. D. 1966. Hunting behavior of wolves in Minnesota. J. Mammal. 47: 347-348.

Mech, L. D. 1970. The wolf: the ecology and behavior of an endangered species. 389 p. New York: Natural History Press, Doubleday.

Nasimovich, A. A. 1955. The role of the regime of snow cover in the life of ungulates in the U.S.S.R. Moskva, Akademiya Nauk SSSR. 403 p.

Pimlott, D. H., Shannon, J. A., and Kolenosky, G. B. 1969. The ecology of the timber wolf in Algonquin Provincial Park. Out. Dep. Lands and Forests Res. Rep. (Wildl.) 87, 92 p.

Rutter, R. J., and Pimlott, D. H. 1968. The world of the wolf. 202 p. Philadelphia and New York: J. B. Lippincott Co.

Stenlund, M. H. 1955. A field study of the timber wolf (_Canis lupus_) on the Superior National Forest, Minnesota. Minn. Conserv. Dep. Tech. Bull. 4, 55 p.

Verme, L. J. 1968. An index of winter severity for northern deer. J. Wildl. Manage. 32: 566-574.

THE POSSIBLE OCCURRENCE OF THE GREAT PLAINS WOLF IN NORTHEASTERN MINNESOTA

L. David Mech and L. D. Frenzel, Jr.

The timber wolf (_Canis lupus_) of northeastern Minnesota occupies an area within the range given by Goldman (1944) for the eastern timber wolf (_C. l. lycaon_ Schreber). However, this area is within 150 miles of the eastern edge of the former range of the Great Plains wolf (_C. l. nubilus_ Say), and there is some question as to whether the Minnesota wolf is really an intergrade between these two subspecies. Writing of _nubilus_, Goldman (1944, p. 444) stated: "Specimens from eastern Minnesota and Michigan seem more properly referable to _lycaon_, but relationship to _nubilus_ is shown in somewhat intermediate characters."