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Fauna Series No. 6








Life History





Fauna of the National Parks — No. 6
The Bighorn of Death Valley
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Death Valley National Monument comprises 2 million acres of extremely picturesque Great Basin desert terrain. Death Valley itself is approximately 190 miles long and from 5 to 20 miles wide. About 550 square miles of the valley is below sea level, reaching the lowest point in this hemisphere at —282 feet near Badwater.

The valley is surrounded by very rugged mountains which rise abruptly to elevations of 5,000 to 11,000 feet, with practically no foothills. These mountains, together with their great alluvial fans, compose the major habitat of the Death Valley bighorn.

Winters are mild, but midsummer temperatures of over 120° F. are not uncommon, and an alltime high for this hemisphere of 134° F. has been recorded. At sea level the extremely erratic rainfall varies between one-tenth of an inch and 4 inches per year, averaging 2 inches. In the mountains the annual total is higher, but it is not recorded. These extremes of climate and of terrain have profoundly influenced the ecology of the bighorn.

The present study was undertaken because very little has been published or known about the day-to-day life history and ecology of the desert bighorn anywhere. It was realized also that studies conducted under the critical environmental conditions of Death Valley might yield findings of a significant and possibly somewhat different nature from those obtained elsewhere.

Intensive fieldwork commenced December 18, 1954, when a month-long uninterrupted series of dawn-to-dusk observations of a clearly identifiable band of bighorn was undertaken. This type of intensive behavior watching set the overall pattern for the study. From December 18, 1954, to March 1, 1961, a total of 1,693 hours was devoted to such observations.

Although the study was conducted chiefly in the Black and Funeral Mountains, it includes a brief preliminary survey of the piñon belt of the Grapevine Mountains. All mountain ranges of the monument were visited many times during more than 6 years of fieldwork.

No single food item can be called a mainstay in the diet of Death Valley bighorn. Grasses, annual nonwoody flowering plants, shrubs, and trees—all are used at times; however the available food supply consists mainly of browse because the region's erratic precipitation does not consistently provide crops of grasses and annual nonwoody flowering plants. Forty-three plant species were observed to be eaten. Use of 12 of these was heavy, 12 moderate, and 19 light. Desertholly (Atriplex hymenelytra) received more nearly year-round use than any other plant. A general shortage of food critical enough to threaten seriously the survival of the species has not occurred in Death Valley in the past 10 years, if ever.

Available water supplies constitute a major limiting factor in bighorn numbers and distribution. It appears that Death Valley bighorn need water the year around, drinking every 3 to 5 days in hot and dry weather and every 10 to 14 days in cold weather, with lambs needing relatively more than adults. They can subsist for longer periods without water in an emergency. They seem to utilize permanent water sources and the surrounding forage only when necessary, withdrawing to outlying ranges whenever rainfall, ephemeral water supplies, and forage conditions permit.

As a corollary of this study, a list of more than 300 water sources was compiled from all available records, and to a large extent the list was subsequently checked in the field. Of these, 90 were mapped for the first time, and 43 were established as of prime importance to the bighorn.

A major danger to the future existence of the bighorn lies in a continued and accelerating usurpation of its ancestral water supplies by man.

Bighorn appear to prefer to remain in one home area if conditions allow them to, being born, living, and dying within a radius of 20 miles of their home water supply. However, unlike some other ungulates, they will move rather than starve. The exhaustion of non-permanent water supplies probably is the most frequent cause for moving.

The general daily activity of bighorn is divided between the effort of securing food and water and resting from that effort, with morning and evening play, especially on the part of the lambs. We have no record of nighttime activity, except in a few instances of their being frightened from their bedding grounds.

By nature, the bighorn would appear to be unwary and trusting, but it quickly acquires a wariness from experience which makes it prefer isolation from human activities. However, deliberate attempts on the part of humans to conduct themselves within limits that are clearly acceptable to bighorn sometimes can lead to a gradual but marked reversal from wariness. In such cases the individual personality of the leader of the band, and her previous experiences with humans, become major factors in determining the degree of herd tolerance to human presence.

Herd leadership possibly holds the key to the future of bighorn human relationships. If the leader (always a ewe) is unafraid of people, an entire band may become tame within days. This matriarchal leadership plays an important part in the overall life history of the naturally gregarious bighorn, controlling to a large extent group movements and reactions to the environment.

The climbing ability of the bighorn is as phenomenal as legend would have it and can scarcely be exaggerated within reason, but under ordinary circumstances the animals take the easiest route and rest often.

Fighting as an expression of serious hostility on the part of either sex, at any age, seems almost nonexistent.

Bighorn are probably no more nor less sanitary than other ruminants in a natural state, seldom defecating or urinating in their water supplies, but doing both in their vacated beds. Their droppings exhibit an almost infinite range of variation in size, often without correlation to size of animal. Appearances of age or recency likewise can be extremely deceptive. As a result of these variations and many others, the technique of bighorn dropping classification is useful in sign reading, but it demands more study than had been anticipated if it is ever to form an exact science.

Communication among bighorn is in general carried on by the same means as among other similar animals, employing the voice to a lesser degree than some and vision to a greater degree than most. Their sense of smell and hearing is probably average for ruminants.

The mating season is spread over a possible 9 months, reaching a peak in August and September. Since the gestation period is about 6 months, the peak of the lambing season is reached in January and February, with occasional lambs dropped throughout the year except during the peak of the mating season.

In general, the rams travel to meet the ewes during the rut and may visit several spring areas for this purpose.

The "fighting" between males seems to be less of a fight than a ritualistic contest. Serious injury is rare and occurs primarily as a result of some miscalculation or accident. We have seen nothing to indicate enmity as a basis for the bouts. Momentary contestants may feed, water, and travel together between clashes, and the presence of a ewe neither precipitates nor prevents a contest.

A third ram, usually a younger noncontestant, often makes off with the ewe, if there is one present, while the battle continues. Contestants are generally well matched older rams and are deferred to by the younger ones. If a younger, lighter ram challenges an older and heavier ram, he is usually ignored. All the prolonged contests we have seen have been between rams in their prime, with the longest, most grueling bouts taking place between those that are 9 to 12 years of age.

Mature rams appear to be indifferent to lambs, sharing no responsibility for their rearing but presenting no hazard to them.

The ewe during the mating season plays the role expected of the female. Her flight from the male often appears ritualistic in that she waits for him when he pauses in the chase and seems not seriously to seek to outrun him.

The ewe seeks isolation to bear her one lamb, shows solicitude for it during the first few days, but leaves it on its own almost from the beginning.

The lamb begins to learn immediately by emulation of its mother. It eats rough food within 2 weeks, spends most of its time with other lambs when available, learns self-protection through play, is weaned anywhere from a month in rare cases to 7 months in equally rare ones, with a probable average weaning age of 4 to 5 months.

The young ewe becomes essentially an adult at 18 months and can breed then. The ram matures more slowly. It leaves the ewe bands to join the off-season bachelor ram bands at the age of 3 years. About a year later it begins to seek out the ewes at the springs during the rut.

Aside from the cause of a prevalent cough among lambs, very little evidence of disease or parasites was found. Apparently, if a Death Valley bighorn survives its first year, it will have a life expectancy of at least 10 years. Determining the age of animals by counting annual horn growth rings is feasible but becomes increasingly difficult after the first 7 years, for new growth is represented by narrow rings instead of large segments after that age.

With the exception of the ram segregation during the off seasons, the low-density forage and population situation in Death Valley appears generally to preclude the forming of specific herds larger than family groups. The average band appears to be mainly a family group of three to five, with the numbers increasing on good ranges in good years and vice versa.

Except in areas withdrawn from bighorn use by water diversion and human encroachment, distribution of the animals appears to be general throughout most of the monument region wherever adequate water and natural forage exists. There is some indication of an exception to this in the piñon belt of the Grapevine Mountains, where deer are relatively numerous and appear to be unfavorably competitive. More ecological study is needed in all the timbered areas of the habitat.

The number of bighorn is estimated currently (1961) at between 600 and 900.

We believe that the best census method for this region is through the prolonged waterhole count as employed at Nevares Spring in 1957, augmented by an intensive application of the associative principle of sign reading and the system of field identification of individual animals described in the text and shown in the illustrations. We believe that, in Death Valley, traumatic interference with their normal activity by trapping and marking, the use of dart guns, etc., would preclude the type of observation by which the data of this report were gathered.

The specific cause of the annual 90 percent lamb mortality remains a major mystery, with malnutrition heading the list of suspects. Adult bighorn appear generally to die of old age.

We found no evidence of significant predation.

Man and what he has brought with him comprise the only significant threat to the bighorn survival.

The wild burro is part of this picture, but its threat to the bighorn survival is indirect, and present control methods are reducing the threat to a minimum. While all ecological wisdom urges the complete elimination of all exotic species from the biota, it cannot be argued that the burro is eliminating the bighorn, since healthy herds of both species are thriving together on some ranges and have done so for at least 25 years.

Only unchecked human encroachment appears actually to threaten the future status of the bighorn.


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

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