Fauna Series No. 5

Fauna of the National Parks — No. 5 The Wolves of Mount McKinley

CHAPTER THREE:
DALL SHEEP (continued)

Do the Wolves Prey Mainly Upon the Weak Sheep? (continued)

DISCUSSION OF SKULL DATA

The wolf preys mainly on the weak sheep.—For an analysis of the data, the skulls have been placed in two groups according to the years in which the sheep died. One group is composed of skulls from animals which died between 1937 and 1941. The skulls in this group, at the time I found them, showed that the sheep had died in the last year or two. They had died during winters which were not severe, perhaps a little more favorable to them than the average winter. A crust on the snow in 1940 compensated somewhat for the otherwise favorable nature of these winters. This recent material portrays the current picture and is thought to present more accurate information of sheep mortality in the presence of wolves than does the old material.

The second group of skulls includes those from sheep which died previous to about 1937. These skulls are segregated because the winter conditions when the animals died are not so well known. A few of these skulls may date back to the days before the wolf became abundant, and an unknown number of them, no doubt, date back to the severe winters of 1929 and 1932 when apparently in many cases the strong and the weak alike succumbed in the deep snows. Skulls of sheep which died in 1929 and 1932 would probably show a lower proportion of old age and diseased animals and a greater number of healthy animals in their prime years. More of these old skulls are incomplete, several being represented by single mandibles. The incomplete skulls would be less likely to show presence of disease.

The data for each group are presented in Tables 4 to 7 (pp. 122 and 123). In Tables 4 and 6 the number of sheep in yearly age classes is given, and in Tables 5 and 7 the yearly age classes are combined into the 2- to 8-year and old-age classes. The lamb and yearling data are the same in both sets of tables. Besides being segregated according to ages. the skulls are segregated according to sex and the presence or absence of severely diseased skull bones.

TABLE 4—Skulls of 221 sheep which died between about 1937 and 1941, showing number of diseased and nondiseased animals in annual age classes. Sexes of lambs and yearlings are combined.

¹Exact age not known because of absence of horns, but teeth showed that the animals were 9 years of age or older.
²Lambs and yearlings.
³Adults.

TABLE 5.—Skulls of 221 sheep which died between about 1937 and 1941, segregated into four age groups. Skulls showing serious diseased conditions are segregated from the normal ones.

¹Lambs and yearlings.
²Adults.

TABLE 6.—Skulls of 608 sheep which died before about 1937, showing number of diseased and nondiseased animals in annual age classes. Sexes of lambs and yearlings are combined since usually they are not known.

¹Exact age not known because of absence of horns, but teeth showed that the animals were 9 years of age or older.
²Lambs and yearlings.
³Adults.

TABLE 7.—The skulls of 608 sheep which died previous to about 1937 are here segregated into four age groups. Skulls showing serious diseased conditions are segregated from the normal ones.

¹Lambs and yearlings.
²Adults.

In the recent material there are 221 skulls. Of these, 153, or 69 percent, are in the old-age class; 21, or 9 percent, are in the 2- to 8-year class but are severely diseased ; 10, or 4 percent, are in the 2-to 8-year class and show no sign of disease; 7, or 3 percent, are from yearlings showing signs of disease; 22, or 10 percent, are yearlings with no evidence of disease; and 8, or 3 percent, are lambs.

In the old material there are 608 skulls. Of these, 348, or 57 percent, are in the old-age group; 64, or 10 percent, are in the 2- to 8-year class but are severely diseased; 75, or 12 percent, are in the 2- to 8-year class, without evidence of disease; 3, or .5 percent, are from yearlings and show signs of disease; 85, or 14 percent, are from yearlings with no indication of disease; 33, or 5 percent, are lambs.

These figures are remarkable because they show that most of the sheep dying belonged to the weak classes. In the absence of predation we would expect the mortality to be distributed among the weak, namely the old, diseased, and the young, but in the presence of a strong predator like the wolf, known to be preying extensively on the sheep, it is interesting that so few animals in their prime are represented. In the recent material 211 skulls, or 95 percent, were from the weak classes in the population and only 10 skulls, or 5 percent, were from sheep in their prime which were healthy. In the old material 533, or 88 percent, of the skulls were from weak animals and only 12 percent from sheep in their prime which were healthy so far as known. The figures for both groups are roughly similar, considering the fact that in many cases disease or weakness would not be shown in bony remains.

It is significant that the incidence of disease is much higher in the skulls of the 2- to 8-year class than in the old-age class. In the recent material, 68 percent of the skulls in the 2- to 8-year class show evidence of serious disease, as compared with only 20 percent in the old-age class. In the old material 46 percent of the skulls in the 2- to 8-year class show a serious diseased condition, while diseased animals comprised only 25 percent in the old-age group. From these figures it becomes apparent that disease is an important factor in the predation among sheep in the 2- to 8-year group and not so important in the old-age group. In the old-age group the effects of age in weakening the animal are, as we would expect, more important than disease.

It is of interest to note that in the old-age group the youngest animals (those 9 years old) are the part of the group showing the highest incidence of disease. This is logical, for the older the animals become the more their weakness would be due to old age rather than disease. In the old material the incidence of disease is significantly higher among the ewes than among the rams. Possibly more of the ram skulls date back to the hard winters of 1929 and 1932 when conditions were so severe that disease was not an important factor. This is suggested as an explanation, since ram skulls would be expected to last longer than ewe skulls. In the recent material there is no appreciable sexual difference in disease incidence.

It is not known how many of the dead sheep the wolves killed but it is certain that they killed many of them. The figures on mortality are so uniform in showing that the weak are the ones dying that for our purpose we do not need to know how many of the sheep the wolf killed. Because of the high percentage of vulnerable sheep (95 percent in recent material) among the dead, whatever predation was done by the wolf would necessarily mainly affect the weak sheep. Perhaps we should expect the wolf predation to operate in this way, for prey-predator relationships between large animals, where numbers are limited, must be rather finely adjusted from the standpoint of species preservation. The wolf and the mountain sheep have existed together under conditions largely as at present for a long time, so that an adjustment between them, whereby both can survive, should be expected. If the wolf were powerful enough to capture the sheep indiscriminately it would long ago have exterminated them, for the law of diminishing returns in the case of mountain sheep hunting would not begin to function very early in the process of decrease. The adjustment seems to be such that, in mountain habitat having a considerable degree of ruggedness and extent, the wolf catches a few of the strong animals but preys mainly on the weak members of the population. This conclusion is supported by the skull data, by field observations of animals killed, and by the hunting methods whereby successive bands of sheep are chased by the wolf as he courses over the hills.

Some related big game-predator findings.—Many persons have taken for granted that the large predators prey upon the weak. It seemed to them that such a relationship was necessary and logical. Others have been skeptical of any such relationship, feeling that the predators preyed indiscriminately on all classes. Actual data have been scarce because quantitative information bearing on this type of problem is difficult to accumulate. I have not reviewed the literature thoroughly, but two reports of studies bearing on the subject have come to my attention. Some observations which I made on the coyote and mule deer in Yellowstone National Park are pertinent. Because these three studies are closely related to the wolf-sheep problem, the conclusions will be given briefly.

Some such relationship as exists among the wolves and Dall sheep also appeared to exist between the mule deer and the coyote in Yellowstone National Park (Murie, 1941). On the better ranges, where the fawns were strong, there was little predation, but on a poor range, where the fawns weakened as the winter progressed, they were preyed upon by coyotes. The prey-predator adjustment seemed to be one in which, as a rule, the prey was not taken until somewhat weakened. Here the predation affected mainly the weak and tended to quicken the adjustment of the deer population to the condition of the range.

Sigurd F. Olson (1938, p. 335) in a paper on the wolves of northern Minnesota, stated: "Long investigation indicates that the great majority of the killings are of old, diseased, or crippled animals. Such purely salvage killings are assuredly not detrimental to either deer or moose, for without the constant elimination of the unfit the breeding stock would suffer."

In an interesting study of the cougar, Hibben (1937), reported on 11 deer which had been killed by cougars. He wrote as follows concerning these deer: "The fact that, of the 11 deer, all showed abnormal or subnormal characteristics is almost too complete to be mere coincidence, and yet coincidence it may be. It is certainly logical that not every deer which a lion kills is subnormal. It is claimed by many hunters that the lion kills at will and takes the best as he chooses. The evidence does not seem to support this theory. Often in the course of this survey lions have been followed which were hunting. These were obviously trying to catch any deer they could. One lion made at least three attempts before he secured a meal. When he did get one, it was * * * the one with the abscessed neck. * * * All the evidence seems to show that the lion catches what he can and that may very well be the less ablebodied prey." Here again is a large predator whose ability to catch its prey seems to be closely adjusted to the ability of the prey to escape.

EFFECT OF WOLF PREDATION ON THE SHEEP AS A SPECIES

It is with a definite hesitancy that I venture to discuss the effects of the wolf on the sheep as a species because of the limitation of knowledge in this general field. The ways in which Nature operates to bring about or retain transformations are still rather mysterious. There is much that we do not know about fundamental premises, such as the causes of variations, which may alter our evaluation of other phases of the problem such as the elimination of the weak. The mathematics involve figures which we do not have, and a time element which we cannot evaluate. A slight trend which may to us seem insignificant could perhaps be of major importance if given a long enough period in which to operate. But because the elimination of the weak at once suggests a racial benefit, it seems apropos to consider briefly predation on the different categories of weak sheep as it may have a bearing on natural selection.

The predation on the old-age group would seem to have little bearing on evolution since the old animals are the successful ones, and their characteristics have already been lodged in many offspring. But some sheep may become infirm at an earlier age than others and be the first of their age group to be eliminated by the wolf. The stronger would continue breeding another year or two and impart their characteristics to a few additional offspring. So even in this group predation may result in a greater number of offspring from the stronger sheep. Characters such as tooth durability related to longevity would be favorably affected. The influence of the wolf might be less than appears, however, for some of the early infirm sheep taken by the wolves probably would have been ineffective breeders. But at least a tendency for selection of favorable characters may be conceded.

Predation on sheep weak from disease may operate in more than one way. Disease itself may be a selective agency, eliminating the weak; or it may affect the strong and weak alike. Actinomycosis is perhaps generally acquired by both the weak and the strong, depending somewhat upon the character of the food and accidental presence of abrasions in the mouth. If the wolf should prey on a sheep which has recovered from this disease but has become weakened because of resulting malformed dentition, the wolf may be eliminating an animal with inherent resistance and thus neutralize the selective influence of the disease. If a sheep acquired the disease because of faulty tooth succession and recovered, then the wolf, in destroying it, would be a selective agent. Thus the wolf's predation on the diseased sheep might operate to neutralize selection due to disease or at times might be eradicating some unfit animals. On the whole, it seems that predation on the diseased animals is not an important factor in natural selection or survival.

Because losses are so heavy among sheep in their first year, it is among these animals that selection would have to operate to be most effective. But unfortunately it is in this group that the information regarding natural selection is least complete, because the condition of the yearlings at the time of death is usually not known. The most important information not obtainable from an examination of the skulls is the general physical vigor and strength of the yearlings that are taken by wolves. No doubt the crippled yearlings and those diseased are eliminated early. In the fall most of the lambs are probably strong enough to avoid the wolf under favorable circumstances.

As winter progresses all the sheep become thinner and some of the growing yearlings succumb, regardless of predation. Other yearlings weaken but would probably recover in the absence of the wolf. It is in the predation on these yearlings that the wolf would have an important bearing on selection.

The percentages involved in these theoretical calculations are not known. Some day we may find a situation which will give us this information on yearlings. The escape ability of the yearlings is so high that we would expect eventually to find the wolves largely eliminating the less able individuals in this group.

It is difficult to arrive at a definite conclusion. Many of the deaths among the sheep have no bearing on selection so far as wolf activities are concerned; others seem to be significant in this respect. What effect predation on the weak has on a species is hard for any of us to say in the present state of our knowledge. It may be much greater than many believe; it may be important along with other forces in Nature. If the sheep has reached a point, at least for the time being, in equilibrium with its environment, then perhaps the wolf is important in maintaining the type.

We have been discussing the elimination of the weak and the part this activity may play in the maintenance or improvement of the species. There is another angle that could be considered. As an evolutionary force the wolf may function most effectively by causing the sheep to dwell in a rocky habitat.

I have tried in this brief review to point out that much of the wolf predation, although affecting the weak sheep, may have a limited selective value, but that there is a tendency toward some selection which may be of great importance to the sheep as a species.

Continued >>>

Last Modified: Thurs, Dec 20 2001 10:00:00 pm PDT
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