Ecological Studies of the Timber Wolf in Northeastern Minnesota Part 10

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 345 cm. fibrous ma.s.s 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 a.s.sume 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 a.s.sume 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 circ.u.mstances 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.

[Ill.u.s.tration: _Figure 12.--When internal organs were present in kills, they were examined in the field. (Photo courtesy of L. D. Mech.)_]

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 cla.s.s 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 cla.s.ses (Pimlott _et al._ 1969).

[Ill.u.s.tration: _Figure 13.--Relative rates of predation on deer of various ages, based on comparisons of the ages of wolf-killed deer with those of a theoretical population (dashed line) and those of the hunter-killed population. See figure 7._]

These figures a.s.sume 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 cla.s.s 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 cla.s.s 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; Ma.s.sachusetts, Shaw 1951).

Third, there is no reason to believe that in our area rifle hunting is especially selective for any particular age cla.s.ses. 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 a.s.sumption 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 cla.s.ses 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).

s.e.x Ratio

Statistical tests comparing a number of subsamples of both wolf-killed deer and hunter-killed deer showed a series of significantly different s.e.x 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 s.e.x 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 s.e.xable fawn wolf-kills were females and 44 percent of 63 s.e.xable adult wolf-kills were females.

If the s.e.x 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 s.e.x 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 s.e.xable 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 s.e.x 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 s.e.x 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 s.e.x ratio of the deer herd, because it would take much less to influence a population characteristic having two cla.s.ses (s.e.x) than one having 14 (age).

One additional difference in the s.e.x ratio was found between two other subsamples of the wolf-kill--that is, the wolf-kill before and after an unusually high snow acc.u.mulation, 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 acc.u.mulation, 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.