Fauna Series No. 4

Fauna of the National Parks — No. 4 Ecology of the Coyote in the Yellowstone

CHAPTER VIII:
BIGHORN IN RELATION TO COYOTES

LAMB SURVIVAL

If coyotes were preying on bighorn, lambs would be the main victims, so a particular effort was made to determine lamb survival. There are, of course, many factors other than the coyote which might reduce the lamb crop so survival of lambs is not necessarily a criterion of coyote predation.

Knowledge of the lamb survival is, however, one of the first steps in learning what factors are important in maintaining the population. This phase of the study deserves much more time than I have been able to give it, but I feel that sufficient information was gathered to indicate some of the probable relationships between coyotes and bighorn.

It should be recognized that there are some difficulties in making a lamb census in winter, and in classifying the yearlings and older animals. Frequently I have found yearlings mistaken for lambs, and yearlings might at times be mistakenly classified as older animals. Lambs vary greatly in size. I have seen a male lamb in winter that was actually as large as a female yearling traveling with the same ewe. I do not wish to over emphasize the difficulties, for after some experience most of the animals can be rather readily classified, even though an animal will often be found which requires careful scrutiny and comparison to be properly identified. The general tendency is to identify yearlings as lambs.

Although I have quite a number of figures on lamb survival, I have omitted discussing several aspects of this question, especially survival of yearlings, feeling that further data are needed.

Season of 1936.—Few observations were made of the 1936 lambs and those were not made until the following spring. On May 16, three ewes, three lambs, and three young rams were seen at Junction Butte. Ranger Condon reported 32 ewes and 28 lambs in the Junction Butte region during the winter of 1936—37. This seems to be a high lamb ratio and possibly a few yearlings were classified as lambs, but as Ranger Condon is an excellent observer I am inclined to believe in the accuracy of his observation. Careful checking in the spring by Condon showed the loss of only one of the 27 lambs and this one was reported killed by coyotes. The condition of the animal at the time it was eaten was not known. Coyotes were plentiful on this bighorn range and were suffering from a shortage of food, yet they apparently preyed on only one lamb during the entire winter and this one may have been sickly.

Figure 41— Bighorn feeding on a range that was closely grazed, mainly by elk.
Loose discarded seed stems of needle grass were the chief food items.
Mount Everts, March 6, 1939.

Season of 1937.—During the summer, three counts, the last one quite complete, were made of the bighorn on Mount Washburn. These animals apparently winter on Mount Everts.

In November some representative counts of the sheep on Mount Everts were made and additional counts were made later in the winter, as follows:

¹ Includes yearlings.

Although the wintering bands on Mount Everts are composed of those summering on several ranges, including Mount Washburn, still the winter lamb count is only about half the summer count of lambs on Washburn alone. The ewe count is larger, as would be expected.

The various counts strongly suggest that there was an appreciable loss of lambs occurring sometime between late summer and the month of November. If predators were responsible for the loss, one would expect losses during the summer when the lambs are most helpless, and also during the winter months, when, as will be shown, there was no noticeable loss. It may be that the lambs that are seen coughing considerably in summer succumb in the late summer and fall. In September 1938 a sick lamb was found on the Gros Ventre Range by a hunter. This incident is in accord with conclusions that may be drawn from the observations made in Yellowstone. Marsh (1938) reports lambs dying at the National Bison Range from acute pneumonia at the age of 2 or 3 months. Another possibility is that the added exertion entailed by migration eliminates the weaker animals during that period, thus reducing the weaker animals over a short period rather than over a long one. It might be suggested that the bighorn in their migration are at times more vulnerable to coyote attack through being away from protecting cliffs. Vulnerability to predation while on cliffs is very low; nevertheless, even on the summer and winter ranges, bighorn are often in contact with coyotes when away from crags. They apparently are able to protect themselves from coyote attacks if necessary when not among cliffs.

The fawns which were on the winter range on January 18 apparently suffered little or no loss during the remainder of the season. On that date 40 ewes, 6 yearlings, and 13 lambs were counted. A few days before the count of 28 ewes, 5 yearlings, and 9 lambs was made on March 26; 4 ewes with 2 lambs were noted at Bear Creek, and 8 ewes with 2 lambs were seen on the north slope of Rattlesnake Butte. It is almost certain that these bighorn had spread out from Mount Everts and therefore in late March there were still about 13 lambs in the Mount Everts population, the same number as in January.

At Junction Butte and vicinity, the following counts were made in the winter of 1937—38:

Apparently the bighorn distribution in 1937—38 varied from that of the preceding winter. This is not surprising in view of the great difference in snow conditions. The counts, I feel, are incomplete for this area but the figures, as far as they go, indicate the winter survival of the two lambs seen on the range on November 22.

During the winter of 1937—38 a total of 193 bighorn was counted and, of about 146 classified into age and sex groups, only 19 were definitely recognized as lambs. There was some possibility of duplication in individuals counted, but it is considered small.

Season of 1938.—During the summer, counts of the bighorn on Mount Washburn were made, two of which were rather complete.

¹ Includes yearlings.

It is evident that the lamb survival during the summer was excellent for the lamb counts made on July 13 and September 2 are the same.

I saw several coyotes near the bighorn on Mount Washburn. The fire guard saw coyotes near them all summer and expressed concern over the safety of the lambs with coyotes so common on the mountain. Of 21 coyote droppings found on the bighorn range on Mount Washburn, none contained bighorn remains.

In the summer of 1938 not a single young ram, older than a yearling, was seen on Mount Washburn, while the preceding summer as many as six were found. It is possible that the young rams were off by themselves, although one would expect them to be found hear the ewes. The young rams seem especially susceptible to scabies. One was known to have died from this affliction, and others were affected the preceding winter. It is likely that some of the young rams in the Washburn population had succumbed to the disease.

During the winter of 1938—39 the following counts were made of bighorn wintering on Mount Everts and on Junction Butte in the vicinity of Tower Falls.

The number of lambs that appeared on the winter ranges in 1938 was about twice the number that came down in 1937. The yearling count was smaller in November 1938 than it was the preceding year. This correlated with the poor lamb crop in 1937 and the apparently high survival in 1936.

On Mount Everts the November lamb count was 23 and the late February count was 17, indicating a possible loss of 6 lambs. It is possible that a few lambs were overlooked for during this period several ewes with their lambs had been seen apart by themselves. They thus may have wandered away from the area where the count was made. A winter survival of 17 of the 23 lambs is satisfactory, and seems especially high after one has examined the poor range utilized. Although all but one lamb seemed to be in fair condition in early March, it is likely that the weaker ones had succumbed. The emaciated lamb seen may also have failed to survive the remaining part of the winter.

The lamb count at Tower Falls was very incomplete. The two lambs were seen with lone ewes off by themselves a half mile and a mile away from the other bighorn, so that it appeared some of the ewes with lambs were living alone. A more extensive count may have revealed other lambs. The figures are too few to compare with the earlier count, which was also small.

If coyotes were an important enemy of bighorn on the winter range one would not expect the high survival of lambs found on Mount Everts during the winters of 1937—38 and 1938—39, for coyotes during both years were abundant in that area. As the bighorn on Everts were frequently found feeding over a quarter of a mile from cliffs, they would have been vulnerable to attack if their safety depended entirely upon cliffs. Although the bighorn often retreat to cliffs when coyotes, man, or any other source of danger is discovered, they sometimes simply bunch up in the face of such danger. The latter action under ordinary circumstances probably is sufficiently protective to make them safe from coyote attack. It is particularly significant that there were no lamb losses so far as known on Mount Washburn during the summers of 1937 and 1938 even though coyotes were common on this summer range and were frequently seen near the bighorn. It is important to consider that much of the time these animals were feeding on open slopes away from any cliffs.

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