Fauna Series No. 5

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

CHAPTER THREE:
DALL SHEEP (continued)

Classified Counts of the Mountain Sheep

Basic for our understanding of mountain sheep ecology is a knowledge of lamb and yearling numbers. Knowing the size of the lamb crop, we can then determine by later counts what time of the year losses among them occurred. The counts in the spring give the size of the new lamb crop, and also the number of yearlings which have survived the winter, a critical period in the sheep's life. The counts in late summer and fall show the lamb survival during the summer.

The lamb and yearling ratios are obtained by comparing the lamb and yearling counts with the ewe counts, rather than with the combined counts of ewes and rams. The basic relationship desired is the ratio of young to the ewes. Knowing the number of rams, the ratio can then be given in terms of the entire population if desired. The figures would be more precise if the 2-year-old ewes could be consistently segregated. Small errors in yearling ratios in the fall counts may be present because at this time some yearlings are with the rams and the proportion of rams counted may vary. During the winter a certain number of ewes would die which brings in a variable that tends to raise the yearling-ewe ratio. But the errors introduced by the roughness of the data in these particulars are apparently slight and unimportant. In comparing spring and fall counts, specific localities are not compared, for unless total counts are available for the locality one would expect enough variation in the sampling to make comparisons unprofitable.

Figure 46: A ewe with two lambs which appeared to be her twins. No other twins were identified as such. [Polychrome Pass, September 25, 1939.]

In counting the mountain sheep, they were generally classified as ewe, lamb, yearling, and ram. When the 2-year-old animals were tabulated the sexes were lumped. Young rams—those up to 3 or 4 years old—were tabulated separately on a few occasions. These partial tabulations of 2-year-olds and young rams are retained in Table 8 and the 2-year-olds also in Tables 9 and 10 (pp. 130 and 131), but it should be understood that this differentiation is far from complete.

When only a part of a band was classified the record was not placed in the tabulations. Since ewes with lambs tend to segregate when the lambs are young, special effort was frequently made to classify the whole ewe population in an area in order to include the bands with many lambs and those with few lambs and thus attain a representative figure. Classifications were begun in early spring before the lambs were born and were continued through the summer and early fall, except in 1941 when the work was terminated in early August.

Sheep from practically all parts of the range were classified during each of the three summers that counts were made. The opportune time to obtain classifications by locality is early June before the sheep have begun their summer migrations and late enough to include most of the lambs. In the fall the sheep return to the spring haunts, so the populations can again be classified by locality and the summer losses can be determined.

No dependence should be placed on the sex ratios shown in the classified counts, because rams and ewes segregate for most of the year, and unless one is certain that a complete count in an area has been made one does not know what proportion of the ewes or the rams in the area has been included.

The greater number of ewes in most of the tables is largely due to the special efforts made to classify the ewe bands. The data from the skull collections indicate that the sex ratio among the sheep is about 50—50. In the recent skull material there were 91 females and 93 males above the yearling age. In the old material there were 213 females and 274 males, but here the number of male skulls may be greater only because they are found more readily. Ewe skulls are more easily covered by gravel and hidden by vegetation. In the recent material, however, the skulls of ewes are found almost as easily as those of rams because hair remains are often present to direct one to the skull, and few skulls have as yet been covered by debris.

The lambing period may vary considerably in different years. In 1908 Sheldon (1930, p. 366) saw the first lamb of the season on May 25. Dixon (1938, p. 216) states that in 1926 he encountered the first lamb on May 5, and in 1932 on May 31. In 1939 I saw two lambs on May 14, and one was reported on May 11. Soon after May 14 many lambs appeared, and by June 1 most of them had been born. In 1940 the lambing was late. None was seen until June 4. By the middle of June the lambing was largely finished. In 1941 two lambs were seen in Nenana Canyon on May 8. Most of the 1941 lambs were born about the middle of May.

The majority of the lambs seem to be born within the first 2 weeks of the lambing period. During the third and fourth weeks there are additional arrivals, and sometimes births occur quite late in the season. On July 19, 1939, a lamb was seen which was about half the size of the others. It appeared to be about 2 weeks old which would place its birth early in July. Twins are a rarity. Only one pair was noted. Often a ewe is followed by two lambs (or more), but if a person watches long enough he generally will see a second ewe appear and claim one of the lambs.

Sheep classified totaled 4,985 in 1939, 1,157 in 1940, and 2,732 in 1941. There were many duplications since the total number of sheep in the park is not more than 1,500.

TABLE 8—Classified Spring Counts—1939¹

1 The best counts are included, following as nearly as possible the close of the lambing period. There is very little duplication, and figures give lamb crop and yearling survival for majority of mountain sheep in Mount McKinley National Park.

2 Arranged from east to west.

TABLE 9 —Classified Fall Counts—1939¹

¹Includes the most complete fail counts available for different localities. Lamb survival through the summer and proportion of yearlings are shown.
²Arranged from east to west.

TABLE 10.—Classified Spring Counts—1940¹

¹The sample is small but representative. The lamb crop is extremely small and the yearling survival low.
²Arranged from east to west.

TABLE 11.—Classified Fall Counts—1940¹

¹The small lamb crop revealed in spring count is again evident here.
²Arranged from east to west.

TABLE 12.—Classified Spring Counts 1941¹

¹Shows 1941 lamb crop and survival of 1940 lambs to yearling age. There is a minimum of duplication.
²Arranged from east to west.

TABLE 13—Classified Summer Counts—1941¹

¹Field work was terminated in early August, so these counts were made earlier than the late-season counts for 1930 and 1940.
²Arranged from east to west.

TABLE 14.—Comparison of lamb-ewe and yearling-ewe ratios for the years 1939, 1940, 1941

¹The "spring count" and "fall count" consist of the most complete counts in each locality over the range rather than a total of all counts during the spring or fall season. Thus duplication was avoided.
²"Total count" consists of all the classified counts made during time year.

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