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 (continued)

DISCUSSION

Variations in lamb crop. —The lamb crop in 1939 was about 49 percent; in 1940 about 16 percent; and in 1941 about 54 percent. The 1939 and 1941 lamb crops were probably slightly better than average, with relatively small losses occurring during lambing time. The analyses of wolf droppings indicated some predation on the 1939 lambs before the caribou moved into the region with their calves. Possibly this wolf predation was just sufficient in 1939 to lower the 1939 lamb crop below that of 1941. However, we are dealing with rather small differences, and the slight wolf predation on lambs probably had little effect on the ratio.

The 1940 lamb crop was abnormal in two respects; the season was very late and the number of lambs unusually small. In 1940 the first lamb was found on June 4. On this date the lambing had been practically completed in the 1939 and 1941 seasons. The lamb crop in 1940 was one-third or less than that of 1939 or 1941.

It is not easy to account for the small lamb crop. According to some of the trappers there is occasionally an exceptionally poor lambing year. It is possible that the small 1940 lamb crop is correlated with crusted snow conditions reported for January of that year. The ewes may have been affected in some manner, causing a lowering in the lamb numbers. Since the lambing was so late, possibly the early bred ewes were affected most by the snow conditions. It may be significant that in 1932, after the severe winter, the lambing was also late and the lamb crop very small. On the other hand, the winters preceding the high 1939 and 1941 lamb crops were favorable for the sheep. The crop, in part, may be dependent on winter conditions.

Ewe-lamb ratio normally about 50 percent.—A good average lamb-ewe ratio appears to be about 50 percent, although this is based on the classification of only three lamb crops, one of which was almost a complete failure. But a 50 percent lamb crop is about what one would expect in a good year judging from the reproduction rates of big game in general. The 2-year-old ewes are included in the ewe counts even though ewes probably do not produce lambs until they are 3 years old. However, their inclusion does not change the figures more than a few points, still leaving the ratio near 50 percent.

In 1941 L. J. Palmer (1941) classified a number of sheep in the Mount Hayes region of the Alaska Range east of Mount McKinley National Park. He did not segregate the yearlings so the figures are not strictly comparable. He counted 501 ewes and 214 lambs, and estimated that one-half of the 2-year-olds and yearlings survived. If one-half the yearlings survived, we would have about 100 to subtract from the ewe count, to make his ewe count comparable with mine. This would give us about 400 ewes. After making this adjustment, which still includes the 2-year-olds as in my figures, we get a lamb-ewe ratio of 53 percent. This percentage is surprisingly close to my "total" and "spring" counts the same year (Table 14). This close agreement between the figures for the Mount Hayes area, where wolf predation is probably less than at Mount McKinley National Park, and those in the park, strengthens the conclusion that a lamb-ewe ratio of about 50 percent is a good normal lamb crop.

Lamb and yearling ratios in different parts of range.—The lamb-ewe ratios tended to be uniform over the range during the 3 years that these ratios were determined.

In considering yearling survival, however, there does seem to be some real difference between localities. The yearling survival in the general East Fork area was below average, while at Savage and Sanctuary River Canyons and at Toklat River the yearling survival was consistently above average. In the areas where there is a slightly higher than average yearling ratio it appears that the country is rougher and of such a nature as to give greater protection to the yearlings from wolves. Sanctuary and Teklanika River Canyons with a mountain between them seem to be especially favorable localities for yearling survival. Although there apparently are local differences, the general average in different localities each year does not vary greatly.

Summer losses.—In comparing the spring and fall counts in each of the 3 years it is obvious that the losses of both lambs and yearlings during the summer were slight. There was relatively little wolf predation on lambs in 1939, and practically none in 1940 and 1941 so far as I could ascertain. The only sick animal noted was a young lamb observed in June by rangers.

Lamb survival first winter.—We do not know what the lamb-ewe ratio was in 1938, but judging from the number of yearlings present in 1939 it apparently was high. If it were somewhere near 50 percent, which seems likely, then more than half of the lambs survived the winter. Such a survival is probably sufficient to cause a definite upward trend in the population. The number of 2-year-olds in 1939 was also quite high, so that the sheep seemed to have done well for two successive years.

The excellent 1939 lamb crop suffered heavy casualties during the winter and a number of these casualties may have resulted from the crusted snow conditions prevailing for part of the winter or heavy wolf pressure due to scarcity of caribou in the sheep hills. The yearling-ewe ratio in 1940 varies from 15 to 19 percent (the lamb crop in 1939 was about 50 percent). About two-thirds of the lambs failed to come through the winter. Such a heavy loss no doubt means no increase, or a possible decrease in the population, for the yearling survivals would hardly compensate for the winter losses among the older animals.

As has been pointed out, the lamb crop in 1940 was only about a third the size of the crop in 1939 and 1941. Of this small crop, 20 percent or less survived the winter as yearlings, so that there were scarcely any yearlings in the population in the summer of 1941.

Thus for two successive years the survival of yearlings was so small that it is probable that the sheep population suffered a decline in numbers each year. So far as known, there was no disease among the yearlings except actinomycosis which generally affects only a nominal number of the animals. The cause of heavy losses appeared to be due to wolf predation.

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