FOOTNOTES:

SpecimennumberSexAgeCause ofdeathConditionYearsM-28M5½WolvesRight 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-29F5½WolvesFront foot: "A 3×4×5 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-37F7½WolvesHind 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-115M4½HunterRight front hoof: Broken at tip.M-196F4½WolvesLeft 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-227M9½WolvesLeft 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).

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.

[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.

Figure 11.—The jaws and legs of kills were inspected closely for abnormalities. (Photo courtesy of L. D. Frenzel.)Figure 11.—The jaws and legs of kills were inspected closely for abnormalities. (Photo courtesy of L. D. Frenzel.)

Figure 12.—When internal organs were present in kills, they were examined in the field. (Photo courtesy of L. D. Mech.)Figure 12.—When internal organs were present in kills, they were examined in the field. (Photo courtesy of L. D. Mech.)

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 Mechet 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

ConditionWolf-killsHunter-killsLevel ofsignificanceDeer insampleDeer withconditionDeer insampleDeer withconditionNumberNumberPercentNumberNumberPercentPercentDental abnormalities14285.625951.9[34]90Jaw necrosis, lumps,or fractures[33]14264.225910.4[34]95Pathology of lower limbs7556.712610.895

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.

[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.

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 4½-year-old animals to 15 percent for 5½-year-olds (table 4). Indeed, 48 percent of the wolf-kills were aged 5½ and over, which compares favorably with the Ontario figure of 58 percent for these age classes (Pimlottet al.1969).

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.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 figure7.

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 5½ years old or older, and the percent killed in each year class dropped off suddenly from 13 percent aged 4½ to 6 percent aged 5½. 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, Pimlottet al.1969; southern Minnesota, Ericksonet 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. Pimlottet 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 Pimlottet 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 membersof 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 4½ 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 Mechet al.(p. 51).

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 (Pimlottet 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 (Ericksonet 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 Mechet 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.

Figure 14.—Arthritis in right hind foot of specimen M-28. (Photo courtesy of University of Minnesota Veterinary Diagnostic Laboratory.)Figure 14.—Arthritis in right hind foot of specimen M-28. (Photo courtesy of University of Minnesota Veterinary Diagnostic Laboratory.)

Figure 15.—Infection and fibrous mass in a front foot of specimen M-29. (Photo courtesy of University of Minnesota Veterinary Diagnostic Laboratory.)Figure 15.—Infection and fibrous mass in a front foot of specimen M-29. (Photo courtesy of University of Minnesota Veterinary Diagnostic Laboratory.)

Figure 16.—Injury to left front foot of specimen M-196. (Photo courtesy of L. D. Mech).Figure 16.—Injury to left front foot of specimen M-196. (Photo courtesy of L. D. Mech).

Figure 17.—Healed fracture of left hind leg of specimen M-227. (Photo courtesy of University of Minnesota Veterinary Diagnostic Laboratory.)Figure 17.—Healed fracture of left hind leg of specimen M-227. (Photo courtesy of University of Minnesota Veterinary Diagnostic Laboratory.)

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, Mechet 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 (Ericksonet 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 5½ 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 Pimlottet 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.

[35]C. W. Severinghaus. Unpublished data.

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.

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.

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.

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 (Pimlottet 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 Mechet 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.

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 Mechet al.(p. 1). Close inspection of wolf-killed deer was made from the ground (Mech and Frenzel,p. 35).

Snow measurements for each winter are shown infigures 1through3. 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).

Figure 1.—Snow depth and penetrability by deer and wolves near Isabella, Minnesota, 1966-67.Figure 1.—Snow depth and penetrability by deer and wolves near Isabella, Minnesota, 1966-67.

Figure 2.—Snow depth and penetrability by deer and wolves near Isabella, Minnesota, 1967-68.Figure 2.—Snow depth and penetrability by deer and wolves near Isabella, Minnesota, 1967-68.

Figure 3.—Snow depth and penetrability by deer and wolves near Isabella, Minnesota, 1968-69.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 totalsnow 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.

Figure 4.—During the winter of 1968-69, the snow was unusually deep in the study area. (Photo courtesy of L. D. Frenzel.)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.

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