Fauna Series No. 6

Fauna of the National Parks — No. 6 The Bighorn of Death Valley

LIFE HISTORY OF THE DEATH VALLEY BIGHORN

Structure

Size

This report will add little of its own to the present data pertaining to the size of the desert bighorn. Details of measurements of desert bighorn are contained in the statistics from the Desert Game Range (Aldous et al, 1958) and from Arizona (Russo, 1956). We have no reason to believe that the Death Valley bighorn are significantly larger than those measured and weighed in Arizona or smaller than those in Nevada.

One lamb at the Desert Game Range (Deming, 1955) measured 17-1/2 inches at the shoulder when it was 32 hours old.

We have no other measurements of lambs, but a relative size is indicated by the fact that a male lamb can walk under its mother's belly when it is born, may weigh as much as she does in 1 year and twice as much as she does in 4 years.

The seven rams and five ewes from Arizona weighed considerably more and measured longer than those from Nevada, but measured less at the front shoulder. These morphological differences (table 10) also are present to some degree within the Death Valley population.

TABLE 10—Measurements of bighorn from Nevada and Arizona

The Badwater sheep were relatively stocky, heavy bodied, and short necked compared with the rangy "ewe-necked," high-shouldered animals at Nevares; and those at Navel Spring ran somewhere between the two, the first band being stocky, the second rangy and slim, but not ewe necked. Some of these differences may be noticed in the photographs of the sheep, at the end of this report.

Horns

Horns appear during the third month, with some indication that they develop sooner and faster on males than on females. Close inspection usually discloses a thickening at the base of the male's horn in observable contrast to the consistent flatness of the female's horn; the female buttons normally appear in almost parallel extrusion, while an outward divergence is noticeable in the male horns from their inception. Young horns tend toward a gray-blue color during the first year, turning yellowish in maturity.

By the end of the first year, the casual observer is likely to confuse the yearling ram with adult ewes, but the experienced fieldworker will readily observe the thickness and spread of the rams' horns as well as the fact that the male curl is already in evidence. (See fig. 32.)

As the bighorn matures, no two sets of horns develop alike, varying with the individual animal as definitely as do fingerprints in man.

The variation in rams' horns takes place on a larger scale than in ewes' and is therefore more observable in the field.

To the field observer the "horn-prints" serve two main functions—as age indicators and as identification marks.

Something of the scope of the variation in Death Valley ram horn development is indicated by the measurements of six skull and horn sets collected during this survey as shown in table 11.

TABLE 11.—Measurements of Death Valley ram horns, from skulls

Ewe and lamb skulls and horn sets are seldom found in the Death Valley region, possibly because they are more easily consumed or carried away by carnivores or scavengers. Smaller skulls and leg bones have been carried out of our camp by ravens, coyotes, and kit foxes.

The one ewe skull collected is above average but relatively typical of the Nevares type as shown here:

The length of annual ring growth decreases with age, until the rings of later years may become quite indistinct wrinkles around the base of the horns of a 10- or 12-year-old ram. Photographs included in this report clearly show this decreasing growth rate with age.

Irregular annual variation in ring width suggests a correlation with wet and dry cycles and concomitant dietary fluctuations, or in extreme cases, sickness. (See fig. 31.)

Aging by annual ring count cannot be positive since the extent of variation in ring growth is not known. The method is more reliable with rams than with ewes, because the rings are likely to be much more clearly defined in the rams' horns. Figures 68, 69, 70, 71, 72, 73, 74, 75, 76 present fairly accurately the development of rings in a ram of known age. Figures 63, 64, 65, 66, 67 illustrate the less well defined ring development often occurring in ewes. The unique set of "horn-prints" of Droopy fig. (6), on the other hand, would appear to indicate an age of 7 or 8 years with reasonable certainty. An illustration of relative horn development of both sexes at 6 and 18 months of age is shown in figs. 32 and 33.

Brooming of tips is almost universal in the Death Valley rams. Jones (1950) reports a complete lack of it in the Sierra bighorn. This suggests the existence in Death Valley of climatic or dietary factors not prevalent in the Sierra Nevada that render the horns susceptible to brooming under abrasion or impact. Jones saw no evidence of horns being used to dig for food, nor have we.

The manner of resting occasionally indulged in, with a horn point resting on the ground, as described previously, could contribute to brooming if the horntips were already dessicated by excessive sun and aridity.

Our evidence, however, indicates "fighting" as the principal direct cause of brooming. (See "Fighting with Other Males.") The type of miscalculation illustrated in figure 59 could end in the loss of a horntip.

It should be noted that the captive ram at the Desert Game Range, with no access to fighting rams, has no brooming. (See figs. 68—76.)

Track Measurements

Track measurements played a lesser part in our field study than had been anticipated, since positive classification of tracks by age class and sex has proven, except in extreme cases, to be of doubtful authority. Owing to the overlap in hoof size of the smaller rams and larger ewes, it is often impossible to know which has left the track. (See fig. 42.) The largest sharp track we have measured was 4 inches long. Allowing for spread of the matrix receiving the imprint, a male front hoof of approximately 3-3/4 inches was indicated. The average ram track measures about 3-1/2 inches by 2-1/2 inches for the front hoof (depending on the nature of the mold). Ewe tracks generally average about the same width as rams but one-half inch shorter. The hind tracks work out about the same for both sexes, being approximately 2-1/2 inches long and 2 inches wide. The smallest lamb track we have measured was 1-1/4 inches long.

The weather, the nature of the soil matrix or mold of the deposit, the relative weight of the animal at the time of deposition, and the nature of the gait employed while traveling are all factors to be considered in calculating the possible variations in the track measurements of one animal, and they will be considered further under "Sign Reading."

Pelages

Pelages of Death Valley bighorn present a picture that is confused by the extreme variations of altitude, climate, foraging conditions and uncertainties as to the dates of seasonal shedding and pelage renewal. Note the smooth, even texture of the Badwater band throughout the winter at low, warm elevations and on lush forage. (See figs. 1 and 7.) Figure 19 illustrates the even, but typically long and dull pelage of sheep on good forage but at higher (2,000 to 4,000 feet) elevations. Figure 30 shows the heavy, rough, bleaching coat characteristic of the band of five which came into Furnace Creek Wash on March 13, 1956. The heavy pelage and excessive hunger suggested the recent abandonment of a high, depleted range. These sheep remained blond after shedding in contrast to the dark brownish gray of the original band of eight from the mesa.

Present evidence points toward a darker pelage during the summer than winter, although the interseasonal variations described above complicate classification.

Death Valley lambs are generally a bluish gray at birth, with occasional tints of brown and indistinct markings. (See figs. 34, 35, 36.) The rump patch gradually becomes more distinct but remains yellowish until the lamb is about 6 weeks of age, when the patch becomes still more distinct and a clearer white (fig. 61), with a distinct rump patch, and white leg-lining, the dark body color matching the mother's.

There appear to be some regional or family characteristics of coloration. The Badwater band of six evidenced a rich chocolate-brown color of adults and immatures alike, with no noticeable variation between even the 5-month-old lamb and its 4-year-old mother.

On March 2, 1955, this notation was made of a band of six in Echo Canyon:

We were immediately struck by the differences in shape and color of this band as compared with the Badwater band. Longer of body, leg, and neck to a noticeable degree. Paler in color without exception. Spring shedding had hardly begun, whereas it was nearly completed with the Badwater band.

A question arose: Why were all the Badwater sheep short necked, short legged, and thick bodied, and dark desert varnish in color, while these in Echo Canyon were all long necked, slim bodied, long legged, and uniformly gray in color?

The Natural Bridge band were rangy in build and light in color, but all ran, not gray or brown. Their faces and feet were lighter, too, almost white, and one resembled a palomino.

Are these bands formed, at least in part, on a family basis?

The Willow Creek herd within our observations have been uniformly dark with blackish tints and distinct white markings. (See fig. 47.)

On September 19 and 20, 1955, we photographed a band of seven at Indian Pass that ranged in color from dark mahogany to pale cream. A band of 18 Paleomesa sheep were so uniformly brown in color that identification beyond horn-print distance was impossible.

That entire bands in the same area may differ in color from each other was demonstrated in the Death Valley Buttes by the 1958 band of 14 cream-colored sheep and the 1961 band of 27, ranging in color from very dark gray to light tan.

The 28 at Quartz Spring, 1960, were all gray with 2 of the darkest we have seen anywhere.

Pelage is somewhat of an indicator of relative physical condition to the extent that systemic or glandular malfunction, regardless of the causative agent, is usually reflected in a rough, lacklustre coat. (See fig. 39.)

Many of the 1956 Nevares population had a mottled appearance, apparently caused by incomplete molt. We saw practically no sign of this in any of the mature ewes or rams in the succeeding years through 1960, but some of the younger rams retained "shawls" throughout the summer.

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Last Modified: Thurs, May 16 2002 10:00:00 pm PDT
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