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]).
Figure 11.—On frozen lakes, wolves often seem to have the advantage over deer, such as in this case where the wolf (center) has just killed a deer and is trying to discourage a raven from joining him in the feed. (Photo courtesy of L. D. Frenzel.)