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?

THE PROBLEM

In any predation problem involving big game it becomes important to learn if the predation is indiscriminate or if it affects primarily certain classes of vulnerable animals such as those past their prime, those weakened by disease, or the young. The possibility is generally recognized that through predation the weak and diseased are eliminated, so that in the long run what seems so harmful may be beneficial to the species. Perhaps the evolution of the mountain sheep has progressed to a point whore it is in equilibrium with its environment but still requires environmental stresses such as the wolf to maintain this equilibrium.

To learn what kind of predation is taking place and what effect it may be having on the population we must have information on the type of animals killed. There are difficulties in collecting quantitative data of this kind. First, it is hard to find a sufficient number of remains to give significant results. Furthermore, in the majority of cases, it cannot be determined whether or not the animal was killed by the predator. There are some criteria that can be used, but usually the evidence is missing. Bloody pieces of hide are a strong indication of wolf predation, but this evidence in time disappears. The location of remains may be significant. If they are found beneath a bluff the chances are that wolves maneuvered the animal down to the bottom, but one cannot be certain of the cause of death in these cases. After knowing the immediate cause of death one must ascertain the condition of the animal when it died.

The number of deaths which may be assigned definitely to the predator are usually a small proportion of the total. Such being the case, one must often broaden the scope of the study from determining the type of animal being killed by predators to the type of animal dying from all causes. If the results are highly uniform, then significant conclusions on predation can still be secured. For instance, if it is found that most of the animals were weak at the time of death, then it may be concluded that whatever predation there was affected the weak part of the population.

Fortunately not all evidence is destroyed when the animal is eaten. The age of the animal can be closely determined from the horns and, in the absence of horns, the age group can be approximated from the teeth. The condition of the animal in a great many cases can also be determined from the condition of the teeth and skull bones.

SEARCH FOR SHEEP SKULLS

Although I began to find an occasional sheep skull and other remains after arriving in the field, it. did not seem possible that enough skulls could be gathered for any quantitative treatment of them. But as I continued my excursions among the sheep hills I began to learn where carcasses were most likely to be found. Small tributary streams having narrow bars bounded by steep cliffs were the most likely localities. Skulls were found in all sorts of situations, but the best places were along these narrow streams, especially beneath the steep rocky slopes. About this time I acquired the services of two boys from a Civilian Conservation Corps camp. With their help I made a methodical search along all the likely streams and draws. Of course those streams on the best winter ranges yielded the greatest returns. In going up the narrow streams and draws one of us would walk in the center, and the other two would follow the sides of the stream bed. In places where numerous small draws cut up a ridge we sometimes would each follow a draw. There was competition to find the most skulls and the "best" ones, which were those showing the most severe diseased conditions. In time we exhausted all the more likely places and discontinued the organized search. But whenever it did not interfere with my main objective I usually followed a course over the hills where chances were best for finding skulls. Thus I continued to pick up a few of them in all sorts of locations.

When a wolf or bear (or both, as often happens) has finished with a sheep carcass there usually is not much left. Sometimes there may be only a few patches of hair or some broken pieces of long bones. But usually the entire skull or parts of it are present in the vicinity. The skull may be considerably damaged, so that the vital parts, the tooth rows and horns, are broken apart and scattered. Wolves often eat away the entire face and most of the brain case of even an old ram, and, if the wolves fail to chew up the skull, a grizzly may come along and do so. When the skull, or parts of it, were not found at the location of the largest bunch of hair or hide, or at the stomach contents, hunting proceeded farther and farther away, and we were often repaid for our perseverance. Knolls above the kill, where it was likely a wolf or fox would go to chew on a bone, were examined, often with success. I was always anxious to find at least one horn for age determination and, if possible, all the tooth rows, because sometimes two or three tooth rows might look normal and the fourth show a severe condition of necrosis. Hence it was desirable to have as many of the tooth rows as possible. In many cases only a single mandible could be found. Although the bones last a long time, the horns soften up when they remain on the ground where it is damp and disintegrate relatively soon. Many large ram horns found lying in the grass were so soft where they were in contact with the ground that their surfaces could be scraped away with one's finger.

The mandibles and upper tooth rows of the ram skulls, and sometimes the entire skull and horns, wore collected. Generally the skulls of the ewes were saved, since their horns and horn cores are not large and can be more easily carried.

Figure 38: These two horns show the variation in size in Dall sheep rams. The small horn is from a 7-year-old animal; the large horn is from a 4-year-old animal.

A total of 829 sheep skulls were gathered. All but about 100 of them were picked up the first year in the field.

Some of the skulls were recent in origin; others were of animals which had died several years ago. Those designated as recent were, for the most part, those of animals that had died since 1938, though some probably dated back to 1937. The accumulation of old skulls would be considerably greater if there were not so much movement of gravel in the stream beds during the spring freshets and summer rainstorms. Skulls lying in the loose gravel bars are usually carried away or completely covered with gravel. Fortunately, those on the grassy benches or the sides of the stream beds remain findable for a much longer period of time.

DETERMINATION OF AGE

The Dall sheep is an exceptionally suitable species for a study of predation because the age of the animals can be established rather accurately from an examination of the horns, and, in the absence of the horns, from the teeth, although the teeth, except for the first 2 years, are less satisfactory as an index of age.

Each year the horn makes its main growth during the summer at a time when much nutritive food is available. During the winter there is scarcely any growth in the horn, probably due to the poorer quality of feed available. This season leaves its mark upon the horn which in the early years may be a slight swelling, and in later years a groove, more accentuated than other wrinkles on the horn. Thus the annual growths in the horn made in summer are divided by annual growth rings formed in winter.

The horns of a captive ram which I examined did not follow the development sequence found in the wild, probably because there was not such a marked seasonal change in its food. The ram was 5-1/2 years old, yet had eight definite growth rings.

Figure 39: Two ewes, one showing diverging type of horns, the other, the more usual type, in which the horns diverge very little. [Cathedral Mountain, July 12, 1939.]

The base of the horn increases in circumference with age and the length of the annual growth increases the first 2 or 3 years, after which it tends to become shorter each year. The annual horn growths of a 12-year-old ram, measured on the outside of the curve, were as follows: First year, 1/2 inch; second year, 8-7/8 inches; third year, 6-3/4 inches; fourth year, 4-1/4; inches; fifth year, 3 inches; sixth year, 3-1/8 inches; seventh year, 2-1/2; inches; eighth year, 2-7/8 inches; ninth year, 2-1/8 inches; tenth year, 1-3/8 inches; eleventh year, 1 inch; twelfth year, 7/8 inch. The circumference of this horn was 9-3/4 inches at end of fourth year; 11 inches at end of fifth year; 11-1/2; inches at end of seventh year; and 12-5/8 inches at end of twelfth year.

The growth rings in ewes fall very close together in an old animal so that in the case of some horns it is difficult to get an accurate age determination. However, it is felt that the age can be rather accurately established in most cases.

Figure 40: Horns of a 6-year-old ewe. The upper one retains on the tip the entire first year's growth. Only an indication of the growth ring in the lower horn can be seen near the tip. The horns of many ewes and some rams have tips as in the lower specimen; this fact must be considered in determining age.

In determining age one must carefully examine the tip of a horn to learn how much of the first year's growth remains. The first year's growth is often worn almost completely away so that the annual growth ring at the end of the first year merges with the tip of the horn. Figure 40 shows the horns of a 6-year-old ewe in which the first year's growth is present in the upper horn and worn down to the annual ring in the lower horn. When the first year's growth of horn is worn away there is generally some indication of the annual growth ring at the tip. A clue to the amount of horn missing can be gained by examining the pattern of the annual growths present below the tip. Usually there is not much difficulty, however, in interpreting the tips of the horns.

When the horns are not found the age can be approximated by an examination of the teeth. The yearlings and 2-year-olds and probably the 3-year-olds can be definitely identified from the tooth formula.

A male lamb which died September 28 had horns an inch long. The premolars were fully erupted, and the first molars were just erupting. The lower molars were a trifle more advanced than the upper molars. There was scarcely any noticeable wear on the incisors.

In a yearling the first molars were fully functioning and showed some wear. There was no sign of the second and third molars.

In a 2-year-old female which died on June 8 the first and second molars were fully erupted, but the third molar was not. All the deciduous premolars except the third upper premolar on one side were shed. The first and second permanent incisors were functioning. The third and fourth deciduous incisors were still present.

Figure 41: The lower mandible is that of a 4-year-old ram and the upper one that of a 10-year-old ram. The bone has been chipped away to expose the molars. The last two molars of the 4-year-old animal retain practically their full length, while those of the 10-year-old animal are almost worn away. As the surface wears down the tooth pushes upward until, after 10 or 11 years, little of it remains.

In animals older than 2 or 3 years the approximate age can be determined by breaking away part of the jawbone and measuring the length of the molar teeth. In a young animal the roots of the molars reach to the base of the jawbone (Figure) 41. As the surface of the tooth wears away the tooth pushes upward, becoming shorter with age. The third molar in a 5-year-old measured 53 mm. in length; a 10-year-old, 17 mm.; and in a 13-year-old ram, 10 mm. There is considerable individual variation in the length of the molars so that one cannot allocate a skull to annual age classes without the possibility of some error. For instance, an 8-year-old sheep may have as long molars as a 7-year-old animal, or there may be an overlapping of 2 years. However, for deciding whether the animal should be placed in the old-age group or in the next younger group this method of age determination is very useful.

DETERMINATION OF SEX

The sex can be easily determined from the horns. The rams have large curved horns and heavy horn cores; the ewes have small, slightly curved horns and small horn cores. In the absence of horns, the sexes of sheep older than about 2 years can be determined by examining the mandibles. In the ewes, these bones, just anterior to the first premolars, are noticeably more slender than those of the rams.

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