PREDATION ON ADULT CARIBOU
Despite the fact that there is much wolf predation on adult caribou one must be cautious and examine all accounts of such predation critically. A surprising number of reports, both written and spoken, do not bear scrutiny. One evening in 1939 some boys told us that they had just witnessed two wolves pull down a cow caribou. They gave a vivid account of the incident and told us where the hunt had occurred. After supper three of us set out to examine the carcass. We found it, but the animal had been dead at least a week. The hair slipped all over the back and even on the legs. The boys had probably seen one or two wolves at the carcass and had made a good story of it.
Another similar incident is also interesting. A man had come into Fairbanks from the direction of Circle with a report that wolves were killing great numbers of caribou, and described how a pack of wolves had killed a number of them. The story passed many lips. A man interested in wolves hunted up the observer to get particulars. The observer said he saw tracks of a band of wolves and one caribou they had killed. For the sake of accuracy, he was asked if one wolf could not make many tracks. The observer admitted that one wolf might make as many tracks as he had seen. He was asked how he knew the wolves had killed the caribou and if it wasn't possible that hunters had killed it since it was near the road. The observer did not know about that. In this instance, although the wolves may well have been killing many caribou, the observer had little evidence on which to base his assertions. It illustrates how an erroneous evaluation of a situation from the quantitative standpoint may be built up in the public mind.
Most of the predation on adult caribou seems to occur during the fall and winter months. During the calving period and throughout the summer the calves, when available, are preyed upon and apparently only a few adults are then killed. But sometimes there are stray adults and no calves and then probably adults are hunted. It seems that wolves run the adult caribou until they are exhausted, for in a short chase the grown animals can run away from the wolves. At times perhaps wolves may maneuver so as to bring a caribou down more quickly. Weak and lame animals would not be expected to run very far.
On October 14, 1940, tracks were seen which showed that at least four or five wolves had been chasing a large bull caribou which had galloped in a straight course through a patch of spruce. Along the trail there was some blood which may have come from bruised feet rather than from wounds inflicted by the wolves. The chase emerged from the spruces and went out on the gravel bar where tracking became difficult. But it was obvious that a large caribou had been chased by the wolves. A carcass was found later in the general area, but there was no way of knowing whether or not it was that of the animal that had been chased by wolves.
On March 19, 1941, on the rolling hills between Savage and Sanctuary Rivers I came upon the trails of four galloping caribou. Trails of two wolves showed that they were running after them. The chase had swung over to the north side of a broad basin, through a fringe of spruce, across to the south side, and up a rather steep ridge. Two caribou continued climbing the ridge to the top, but the other two, when near the top, cut down the ridge, galloping through a heavy growth of willows which filled the bottom of the basin. All the way the snow had been about 18 inches deep, but here it was even deeper, and the caribou broke through to their bellies. When the wolves came to the place where the four caribou had split, it may be significant that they gave chase to those which had failed to climb the ridge and had run down hill. After running down hill the two caribou floundered through the deep drifts among the willows in the hollow, then gained a slope on the other side largely free of snow. Here they followed some low ridges where the travel was easy and then started to climb the long slopes of the Outside Range of mountains. It was too late to follow the tracks far up the mountain so I never learned if the wolves overtook the two caribou. I had followed the chase for 3 or 4 miles.
On the morning of July 2,1941, I saw three wolves on the middle of East Fork bar. They were from the East Fork den and consisted of the gray female, the black-mantled male, and the light-faced male. They watched me for a few minutes, then the black-mantled male trotted slowly to the east slope. The light-faced male trotted a few yards to the carcass of a caribou but in a moment followed the other male. The female trotted up the bar toward the den. Four hours later I examined the carcass which proved to be that of a young cow. It was still warm, hence the wolves must have killed it that morning. There were several bloodshot tooth marks on the neck. The animal had not been hamstrung. At this time the caribou herds were far to the west so that the only caribou available were stragglers.
Otto Geist of Fairbanks told me that he once saw a caribou run past him with tongue out, obviously weary. A little later a wolf appeared on the trail and it also seemed weary. The next morning Geist followed the trail down river to the end of the chase. The wolf had killed the caribou, had eaten his fill, and was sleeping on the river bank opposite the carcass.
It is well known that wolves kill adult caribou but it is difficult to learn what proportion of the caribou killed are below standard in strength. It is hard to know how "nip and tuck" the relationships are between the two species; how many healthy caribou chased by wolves escape, and how many succumb.
PREDATION ON CARIBOU CALVES
In the spring the wolves prey extensively on the calves. The first day or two after birth they cannot run fast enough to give the wolves a chase, but in a few days they can almost keep up with the cows and then they force the wolves to do their best. In no instance was the wolf seen stalking caribou. Such maneuvers are unnecessary, for the wolf has no difficulty in approaching within a few hundred yards of them. Generally the caribou seem not to be worried much by wolves unless chased. Bands were frequently noted watching the wolves when they could have been moving away to a more secure position.
The wolf's method of hunting calves seems to give an opportunity for the elimination of the weaker animals. Usually the wolf chases a band of cows containing several calves. The speed of the calves is only slightly less than that of the wolves, at least on level terrain, so they make the wolf do his best, and the chase continues long enough for a test of the calves. The weakest, the one with the least endurance, falters after a time and drops behind the others, and this is the one the wolf captures. In some instances I suppose a calf falls behind because it is younger than the others, but after these animals are a few weeks old the differences in time of birth probably are unimportant, and the one actually weaker than the others is the one that succumbs. Thus the wolf appears to be a factor in maintaining quality in the herds.
There may be more weak animals in populations than has been generally realized. In this connection some observations of the elk in Jackson Hole, Wyo., are pertinent. In the spring of the year there is much variation in the strength of the yearlings. If a large herd is started running, some of the yearlings may be seen to collapse, exhausted by their efforts. Most of these animals at this time are able to rise again after a rest, being weak only from the winter hardships. Others are apparently diseased for they die soon after tumbling. In the presence of a predator probably these weaker animals would be eliminated as the winter progressed.