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 virginia.n.u.s_) 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 succ.u.mb 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 princ.i.p.al 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 acc.u.mulated snow, and was generally regarded as having one of the heaviest snowfalls and acc.u.mulations 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 acc.u.mulation was 39 inches, the highest acc.u.mulation 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).
[Ill.u.s.tration: _Figure 1.--Snow depth and penetrability by deer and wolves near Isabella, Minnesota, 1966-67._]
[Ill.u.s.tration: _Figure 2.--Snow depth and penetrability by deer and wolves near Isabella, Minnesota, 1967-68._]
[Ill.u.s.tration: _Figure 3.--Snow depth and penetrability by deer and wolves near Isabella, Minnesota, 1968-69._]
The snow penetrability in 1966-67 remained high throughout January, February, and March. During the following winter, penetrability fluctuated more, but even at its greatest, it was relatively unimportant to deer because the total snow depth was so low. During 1968-69, however, penetrability was a very important aspect of snow condition. It was so high during late January and early February, when snow acc.u.mulation 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 acc.u.mulation 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.
[Ill.u.s.tration: _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 sh.o.r.es 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 sh.o.r.elines 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.
[Ill.u.s.tration: _Figure 5.--Under unusually deep snow conditions, deer used lake sh.o.r.es heavily. (Photo courtesy of L. D. Mech.)_]
In considering wolf mobility in snow, two types of movement must be recognized: the trot used during general travel, and the bounding used while chasing prey. The trot is an easy gait of about 5 m.p.h. on firm footing (Mech 1970), and can be continued for hours at a time. During periods of deep snow and high penetrability, most wolf travel is on frozen waterways, roads, snowmobile trails, and animal trails, including the wolves" own pathways, which become well packed with frequent use (fig. 6, 7A, B). Such travel was observed during each of the three winters of this study.
[Ill.u.s.tration: _Figure 6.--Wolves travel single file in deep snow.
(Photo courtesy of L. D. Mech.)_]
[Ill.u.s.tration: _Figure 7.--(A) A single wolf must break his own trail through the snow. (Photo courtesy of L. D. Frenzel.) (B) Regular use by a pack keeps trails open. (Photo courtesy of L. D. Mech.)_]
The second type of wolf movement affected by snow is the leaping and bounding a.s.sociated 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.
[Ill.u.s.tration: _Figure 8.--Wolves run at a shallow angle, thus hindering them in deep snow. (Photo courtesy of D. H. Pimlott.)_]
During the winter of 1968-69, wolves also bogged down a great deal in snow when chasing deer. However, after January 1969 the snow was so deep that deer were floundering even more than wolves in many cases.
The fact that wolves could run in the trail broken by deer probably also gave the wolves an advantage under the conditions that severely restricted deer movements.
The above observations of snow conditions, deer movements, and wolf movements during the three winters of the study are in accord with observations made on the differences in the ability of the wolves to capture deer during the same period. Two indices support the conclusion that wolves had a much easier time catching deer during February and March 1969 than earlier in the winter and in the two previous winters: (1) the degree of utilization of wolf-killed deer, and (2) the kill rate of radiotagged wolves.
During the winters of 1966-67 and 1967-68, and in December and early January 1968-69, most wolf-killed deer found had been thoroughly eaten, and the bones--if present at all--were well chewed and scattered at each kill (fig. 9). All skin and flesh from the skull were eaten, and the mandible was usually separated from the skull. During late February and early March 1967, few fresh kills were even found, and wolves were returning several times to old kills that had been cleaned up many days before.
[Ill.u.s.tration: _Figure 9.--Usually the remains of a wolf kill are well chewed and scattered before the wolves abandon them. (Photo courtesy of L. D. Mech.)_]
[Ill.u.s.tration: _Figure 10.--During a period of especially deep snow, wolves abandoned many kills before pulling apart the skeletons. (Photo courtesy of L. D. Mech.)_]
However, in late January 1969 a substantial change began taking place.
The skeletons of most kills found were almost intact, the flesh having been eaten from around the bones (fig. 10). Appreciably more skin was usually left on the carca.s.s, 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 carca.s.ses. 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 carca.s.ses 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 carca.s.ses 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 carca.s.s (which presumably he killed) every 3 days, and he spent only 1 or 2 days at each. In two cases two new carca.s.ses 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 a.s.sociates 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 acc.u.mulation 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.