Fauna Series No. 6

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

LIFE HISTORY OF THE DEATH VALLEY BIGHORN

Food

Because of the extreme variability in the quantity, quality, and location of food supplies, food habits and feeding behavior are two of the most difficult pieces to fit into the jigsaw puzzle of the bighorn survival pattern. Almost the only generality that can be made about them is that they are effectively unpredictable.

Kinds

We cannot name any one plant as the mainstay of the bighorn diet. Nor can we say of any area that here they feed in the winter or that here they spend their summers.

The types of food can be described as including, to a locally limited degree, all of the four main groups of plants common to the desert bighorn ranges everywhere—grasses, annual nonwoody flowering plants, shrubs, and trees.

Owing to their shallow root system and the lack of humus in the soil, dependable supplies of grasses, can be found only in live groundwater areas, in high washes where runoffs are fairly frequent, and on rolling plateau land at elevations so situated as to make substantial snowfall a dependable feature of annual precipitation.

During the 8 years of this study there have been recorded but two "good flower years." That is to say, years when the casual visitor could be assured of finding enough flowering plants in any one place at any one time to call it a display. At best, these displays are spotty from the valley floor to the mountaintops because the rainfall necessary to their development is spotty, even in the wettest years.

Given enough water and sunlight, annuals on the valley floor and fans at the mouths of canyons may begin to mature in December, as at Badwater in 1954, and spread up the slopes with the advancing seasons through the summer, and if autumn rains are propitious there may be flowers blooming somewhere at some elevation every day of the year. This does not happen every year, by any means, but the fact that it happens some years is of vital importance to the bighorn, because even if the sheep do not find the plants in time to utilize the succulent inflorescence, the dried plants become a vital backlog of food which is no longer dependent on season, sunlight, or rainfall. In fact, much of this supply of food plants seems to become palatable to the bighorn only after it is completely dehydrated.

Present evidence indicates a preponderance of shrub utilization in the diet. The deeper, more extensive root systems of the woody plants and their general resistance to excessive heat, light, and aridity have made some acceptable shrubs available to the sheep at all elevations and at all seasons. The general scarcity of grasses throughout the entire region is, of course, good reason for the bighorn to lean heavily on browsing rather than grazing.

A very limited utilization of the mesquite represents the only tree browsing we have observed in Death Valley.

With very few exceptions, only those plants actually observed by us in the process of utilization are included in this report, because of the difficulty in so many cases of determining whether the browsing was done by sheep, packrats, jackrabbits, or chuckwallas.

On November 17, 1956, a band of nine sheep had been observed for several hours feeding at the base of a cliff 200 yards above the wash in shadow. The plants being used were carefully located by relative number, color, and position for future checking. The next day we found that the dried stems of Aster and the small, dark-green leafed Encelia, with some dried white bursage (Franseria dumosa) had been utilized. This was the first time we had a positive observation of utilization of the dried Franseria. But more unusual than that was a patch of green wetleaf (Boerhaavia annulata) that had been eaten well back to the ground! I sat down on a large boulder to add this second "first" to my notes when I saw that my large boulder was covered with chuckwalla (Sauromalus obesus) droppings, and bits of the wetleaf were scattered around among the small openings beneath. I added wetleaf to the chuckwalla's list, but not to the bighorn's.

The single circumstance of cropping of a plant is no basis for inclusion in a bighorn diet list. The close-cropped, hedged appearance of many Franseria plants in the vicinity of Corkscrew Spring had been puzzling for years until June 13, 1957, when a chuckwalla was observed to crawl up into a Franseria dumosa bush and, spreading all four legs, to move about on top of the plant in a sort of swimming motion. snipping off the newer shoots in a hedge-clipping effect that explained to us for the first time why so many Franserias look the way they do.

Rodents and rabbits are common deceivers in this branch of research. During the Cottonwood Mountains bighorn-burro study in the winter of 1955, the bunchgrass over the entire mountaintop on which we stood was "mowed" to the surface, not by burros (or bighorn) but by rabbits. There was no burro or sheep sign present, but close inspection revealed the ground to be literally overlaid with disintegrated rabbit pellets.

A typical exception to the rule, resulting in the inclusion of a plant that was not seen to be eaten, was recorded on June 5, 1957, in Monument Canyon (1-1/2 miles southwest of the road to Dante's View). On the way up the canyon I was aware of walking through an exceptionally tall and thick growth of Phacelia sp. (probably calthifolia). I noticed that now and then the tips of some had been utilized, and occasionally even faint track impressions of bighorn showed up in the coarse gravel between plants. The sun was in my eyes, however, and since I was looking for water I did not see what had really happened there until I was on my way back with the sun behind me. When I reached the Phacelia beds then, I saw very clearly that many of the flowering plants had been cropped almost to the ground, and that bighorn tracks were around all those cropped, and it was plain to see where the animals had moved from one plant to the other. On the other hand, where no faint tracks showed, no cropping had occurred. Tips broken off but not eaten still retained some color, and the general condition indicated a time lapse of about a week.

Reports from other observers of plants utilized by sheep are not included here for two reasons. Primarily, this list is based on what we have observed, not on what we have read or have been told. Secondly, these exclusions reduce the margin for error to a considerable degree, because mistakes in actual feeding observations are almost as common as those made by a misinterpretation of browsing evidence. Observers with us in the field have repeatedly pointed out incidents of apparent browsing on creosotebush, pigmy cedar, and rabbitbrush (Chrysothamnus), when close inspection revealed the presence of a forage plant growing up through the one seeming to be eaten.

Observed utilization of individual species of plants has been so varied as to preclude the effective use of a standard chart of plant distribution and utilization. This, of course, is but a further projection of the great climatic and altitudinal variation which characterizes the habitat as a whole, focusing on the instability of precipitation and the resultant unpredictability of seasonal plant development. While we have found no plant that we can indicate as the most important in the bighorn diet, or one that is more important to rams and one to ewes, etc., some are beginning to attain a general relative status, and as such they are listed. It must be emphasized that none of these plants was ever observed in full year-round utilization, but when they were utilized, it was shown in the following tabulation:

Forage plants observed to be used by bighorn

[Nomenclature follows Kelsey and Dayton (1942); where plant is not listed by them, nomenclature follows Jaeger (1950)]

¹ Probably more nearly in general year-round use than any other plant.

The difficulty involved in ascribing seasonal importance to any of the plants listed will become evident as the varying intensity and time of their use is described.

During the Badwater observations from December 18, 1954, to January 24, 1955, three plants made up 75 percent of the diet of the band of four ewes and two lambs: Desertholly (Atriplex hymenelytra), in full bloom from a warm rain in November, 35 percent; honeysweet (Tidestromia oblongifolia), having matured in October but still not completely dormant, 25 percent; and Parish euphorbia (Euphorbia parishii), 10 percent. The remaining 30 percent was composed of dried annuals of previous seasons, utilized systematically in alternating periods throughout the day, and new green shoots of annuals as they appeared.

When four of the band returned, February 12, they ignored the perennials of their former diet and concentrated on the lush annuals entirely.

We did not see honeysweet utilized again until one of the same sheep returned 3 years later to a fan 5 miles south of Badwater on December 3, 1957. This was the ewe named Droopy, and a young ram, and they divided their attention, when they ate at all, between desertholly and honeysweet.

Groundcherry (Physalis crassifolia) and Parish euphorbia, both utilized only slightly less than the first two in January 1955, have not reentered the list since.

Desertholly has maintained a relatively important place in subsequent observations with the exception of about 4 months during the winter of 1955—56, when two bands of bighorn ignored it and honeysweet during their entire stay in the Furnace Creek Wash area.

Bebbia juncea and Stephanomeria sp. had, in the meantime, assumed the importance holly and honeysweet had held at Badwater. The two plants in about equal ratio formed roughly 60 percent of the diet of a band of eight bighorn from December 28, 1955, to January 17, 1956. Both shrubs were utilized in all stages of development, from new green shoots to inflorescence and even to previous years' dried materials. Stephanomeria was, as a matter of fact, hunted out by scent and pawed free of soil during its early development while it still consisted only of white shoots 3 to 4 inches below the ground. The remaining 40 percent of food, represented more or less equal use of dried annuals, perennial inflorescence both fresh and dry, and Eriogonum inflatum which ranged from the dried woody stems of previous years down through the green leaves of this year's growth. All of these were pawed up and eaten with almost as much relish as Stephanomeria.

However, after 2 weeks Eriogonum inflatum was dropped from their diet for a week. Ephedra, desertholly, honeysweet, Franseria dumosa, and Grayia spinosa were available, but they were not utilized until just as the band was on its way out of the wash for the last time on January 18, when some of these were suddenly included again. That day the sheep made feeding history. They had ignored Ephedra in all our previous observations, but now, as though marking more changes than one in their pattern, they ate it, the entire band gathering about one bush and eating for 15 minutes. A short time later, we were surprised, to say the least, to find them wiggling their noses down among cottontop cactus (Echinocactus polycephalus) needles to lap up the "cotton" with evident relish. For the first time in over a week, they also stopped at Eriogonum patches and sent up great clouds of dust as they all pawed for the freshest shoots before moving on.

Dried Aster and Encelia tops came in for their usual share of use on this particular day, but the greatest surprise was the use of dried cottontop cactus thorns and of stingbush. The 6-month-old lamb known as Bad Boy chewed away on dried, blackened cottontop cactus thorns for a full 15 minutes. He was joined, though without much relish, from time to time by three or four others. We had seen evidence of possible use of stingbush before, but this was the first time we had actually observed it. They browsed, not on the succulent undergrowth, but on the dried prickly tips. This was not just nipping and passing on, but real feeding for several minutes. Dormant desertholly was included during the period of observation referred to, but Hymenoclea salsola and Franseria dumosa, high on many lists of bighorn forage plants, was still untouched. Some Grayia spinosa and Boerhaavia tops were browsed.

A single ewe and lamb followed the same pattern from February 2 to March 13, 1956, when they were joined by four more adults and another lamb. From March 13 to April 1 this band of seven fed almost exclusively on Bebbia and Stephanomeria. For 1 week they mixed the lush green Bebbia and Stephanomeria of the washes with the dormant vegetation of Paleomesa above the washes toward Pyramid Peak. This change was similar to the one made by the previous band just before they left the wash for the last time—dormant desertholly, Encelia, Aster, Eriogonum, and Grayia spinosa leading the list, as though the sheep were gradually breaking themselves into a change they knew was coming. They alternated their feeding in the wash with lengthening periods on the mesa, until one final feast on April 8; and then, with the forage in the wash the most luxuriant it had been all year, they left.

Two days later two of the ewes and a lamb returned and for 2 days ate nothing but the inflorescence of Hymenoclea and Franseria dumosa, with Bebbia and Stephanomeria suddenly secondary on the list. They had been gone for 3 days when the first ewe and her lamb returned alone. They were back on Stephanomeria and Bebbia again, with only desultory nipping now and then at Hymenoclea and Franseria.

This brief utilization of Hymenoclea and Franseria, it should be emphasized, was only of the inflorescence, lapping the buds and blossoms into the mouth with the tongue and then tossing the head to strip only the flowering parts from the plant. This is the only time we have seen this happen. Previous observations had shown no utilization of Hymenoclea whatever, and it stands today as the only record, although this is one of the most widely distributed plants in the wash community.

Franseria utilization had been previously recorded as varying from extremely casual and intermittent use to none at all. It is not only common in washes but is likely to be associated with the creosotebush saltbush (Larrea-Atriplex) communities throughout the Death Valley region. Heretofore, nothing but dried inflorescence had been taken. We were not to observe further utilization of Franseria until October 2, 1957, when a band of seven browsed for several hours vigorously on whole Franseria plants for the first and only time during this study.

Bebbia and Stephanomeria resumed their importance in sheep diet in October 1956 and retained it until the end of December 1956. Then with the exception of a few weeks of travel use in and out of Nevares Spring we observed no utilization of either plant from January 1, 1957, to August 1958. Furnace Creek Wash, Big Wash (a tributary to Furnace Creek Wash), and all the washes above Navel Spring were as lush with it as they had ever been in 8 years, but repeated and extensive surveys there disclosed no bighorn use and only an occasional sign of an animal or small band crossing the washes, apparently on their way from the back-country to Navel Spring for water.

So Bebbia and Stephanomeria have begun to emerge as extremely important to the bighorn in years of drought when practically the only areas that get enough moisture to produce green feed are the washes that form the stem and "handle" of the huge drainage fans near and beyond the base of such mountains as Pyramid Peak. These two plants are doubly important in that they are almost as acceptable to the bighorn in their dormant stage as in any other. In this stage they may rest unused but in perfect storage for several seasons, should general rains in the higher country provide enough forage to make regular use of the washes unnecessary.

From these observations it seems reasonable to suggest a correlation between the relatively dry months in the high country from November 1954 through December 1956 with the concurrent use of the washes, and the immediate and continued disuse of the washes with the resumption of relatively regular and heavy rainfall in the back country.

The importance of Eriogonums and Encelias should not be underestimated, since they are utilized at all stages of growth, have a wide altitudinal distribution, and are likely to be found in use almost as regularly, if not in such quantity, as Atriplex hymenelytra.

Most of this study has been conducted below the 4,000-foot level and has therefore not included much of the Ephedra range, but on those occasions when sheep did come in contact with it observations indicated a moderate to heavy use of it at higher elevations. The same observations were made with regard to Atriplex canescens, Grayia spinosa, Stipa speciosa, Hilaria jamesii, and Hilaria rigida.

The importance of some plants will be emphasized later in connection with water and the special part they play in bighorn watering behavior. In general, however, it should be pointed out here that the amount and kinds of food found near or at a water supply can be the determining factor in the survival of many animals every year.

During our 30 days of continuous observation at Nevares Spring in the summer of 1957, we found a great variation in the utilization of the available food supply. With our 12- and 16-power glasses from our general observation camp, we could check the use of any of the 40 plants listed below, which we believe comprise all plants of the Nevares community:

Plants of the Nevares Spring Area*

[*Species identified at Stanford University by Roxana S. Ferris, assistant curator of the Dudley Herbarium, and Dr. Wallace R. Ernst. **Utilized by bighorn during 30-day observation period]

Gramineae—Grass Family:
    Andropogon glomeratus (Walt.) B.S.P.—Bluestem**.
    Arundo donax L.—Giant reed.
    Distichlis spicata (L.) Greene—Saltgrass**.
    Imperata hookeri Rupr.—California satintail.
    Sporobolus airoides Torr.—Alkali sacaton, or hairgrass dropseed**.

Cyperaceae—Sedge Family:
    Fimbristylis thermalis Wats.—Hotspring fimbristylis.
    Schoenus nigricans L.—Black sedge.
    Scirpus olneyi Gray.—Olney bulrush.

Juncaeae—Rush Family:
    luncus cooperi Engelm.—Cooper rush.

Iridaceae—Iris Family:
    Sisyrinchium sp.—Blue-eyedgrass**.

Orchidaceae—Orchid Family:
    Epipactis gigantea Dougl.—Giant Helleborine.

Saururaceae—Lizard's-tail Family:
    Anemopsis californica (Nutt.) Hook.— Yerhamansa.

Polygonaceae—Buckwheat Family:
    Eriogonum inflatum Torr. and Frem.—Desert trumpet**.
    Eriogonum sp.—Eriogonum**.

Chenopodiaceae—Goosefoot Family:
    Atriplex hymenelytra (Torr.) Wats.—Desertholly**.
    Suaeda sp.—Seepweed.

Amaranthaceae—Amaranth Family:
    Tidestromia oblongifolia (Wats.) Sta.— Honeysweet**.

Mimosaceae—Mimosa Family:
    Prosopis juliflora var. torreyana (L.) Benson—Western honey mesquite**.
    Prosopis pubescens Benth.—Screwbean, or tornillo.

Zygophyllaceae—Caltrop Family.
    Larrea tridentata (D.C.) Coville—Creosotebush.

Euphorbiaceae—Spurge Family:
Euphorbia parishii Greene—Parish euphorbia**.

Tamaricaceae—Tamarisk Family:
    Tamarix aphylla (L) Karst (T. articulata Vahl.)—Athel.
    Tamarix pentandra Pall. (T. gallica L.)—Salt cedar.

Losaceae—Loasa Family:
    Eucnide urens Parry.—Desert rock nettle, or stingbush**.

Cactaceae—Cactus Family:
    Echinocactus polycephalus Engelm. and Bigelow—Cottontop cactus**.
    Opuntia basilaris Engelm.—Beavertail cactus.

Lythraceae—Loosestrife Family:
    Lythrum californicum Torr. and Gray—California loosestrife.

Labiatae—Mint Family:
    Salvia funerea M. E. Jones—Death Valley sage.

Compositae—Sunflower Family:
    Aster abatus Blake—Mohave, or desert, aster**.
    Baccharis sergiloides Gray—Squaw waterweed
    Bebbia juncea var. aspera Greene—Sweetbush, or rush hehhia**.
    Cirsium mohavense Jepson--Mohave thistle**.
    Encelia farinosa Gray—Incienso, or Encelia**.
    Franseria dumosa Gray—Burro-weed, or white bursage**.
    Gutierrezia microcephala (D.C.) Gray—Threadleaf.
    Peucephyllum schotti Gray—Pigmy cedar, desertfir, or sprucebush.
    Pluchea sericea (Nutt.) Coville—Arrowweed.
    Solidago confinis Gray—Goldenrod.
    Stephanomeria—Wirelettuce**.

Only 18 of the 40 species were utilized at all, and some of these may have been eaten accidently through their juxtaposition with other plants.

Of the grasses, alkali sacaton (Sporobolus airoides) was commonly and heavily used, with Andropogon a poor second and Distichlis spicata a poorer third. Utilization of Prosopis juliflora was very light as we had expected, but the surprise plant of the summer was the Mohave thistle (Cirsium mohavense). Both fresh and dry inflorescence of the thistle were devoured voraciously by the ewes, casually by the lambs, and not at all by the rams!

In a sense it can be said that use of the phreatophytes (deep-rooted plants adapted to live in moist situations) is seasonal in that they are utilized only when bighorn are coming in to water during hot and dry seasons.

The point that must be emphasized here is that the dry seasons are not limited to the seasons of the year, but may, and often do, last for several years at a time. So in a broader sense the word seasonal should probably be applied to the pluvial cycles which play such a dominant part in the survival patterns of the bighorn.

During our extended reconnaissance work preceding the 1955 census, we found no utilization of spring areas for several months prior to and including June. During this period when the burden was removed from the spring-area vegetation it should not, however, be presupposed that it had been shifted to any other one area or elevation. Much has been said about the bighorn leaving the springs for the high country when the rains come. The use of a broader term than high country is indicated by the 4-month observations below sea level in the Badwater area. Localized rainfall brought on the vegetation which brought the sheep down from Lemonade Spring to the fans in only that one area of the Black Mountains. At the same time, other bands were being observed at the expected higher elevations above the supply of ephemeral food plants at Badwater.

So it would appear difficult if not impossible to discuss bighorn food by season without discussing water and rainfall and their effect on vegetation and their use in relation to wet and dry periods that do not coincide with the calendar year.

For example, forage demands on spring areas was constant for 18 months from July 1955 until January 1957, when pressure on the spring vegetation was relieved by general rains until August. After that date came a 3-month period of use at the springs again; then rainfall once more released the sheep into the back-country and to the vegetation there which they had not been able to utilize until the arrival of a wet cycle or season. The distance from free water is the limiting factor in the availability of much of the best forage in the monument and is counteracted only by rainfall when it comes.

Much work remains to be done on the back-country phase of bighorn life history. We have observed bands at 4,000 and 5,000 feet elevation in upper Echo Canyon, where presumably the Nevares Spring bighorn browse when conditions allow them to remain away from their permanent water supply. With only a few hours observation obtainable, Hilaria sp., Stipa speciosa, and Ephedra sp. were the only plants we could definitely add to the bighorn food list from there.

On June 7, 1955, we observed a band of nine rams in the Cottonwoods at an elevation of 8,000 feet. They traveled and browsed so rapidly that no positive identification of plant utilization could be made. However, in the July census of the same year, Fred Jones reported from the same area the utilization of the following plants: Littleleaf mahogany (Cercocarpus intricatus), Sphaeralcea ambigua, winterfat (Eurotia lanata), low sagebrush (Artemesia arbuscula), Stipa speciosa, Ephedra, snowberry (Symphoricarpus longiflorus), and desert peach (Prunus fasciculata).

The relative importance of ephemeral plants as forage is not yet defined. The probability that some flower is blooming at some elevation in the monument every day of the year may be an important factor in bighorn distribution. That variety of diet is of great importance to them is indicated by their habit of incompletely using a plant or an abundant supply of plants. Abandonment of an area owing to exhaustion of ephemeral food supply has not been observed here. The Badwater band left the area while these annuals were at their peak. The last one to leave after feeding entirely on the blossoms of Atrichoseris platyphylla for 10 days left a lush field of the plants nodding shoulder high (to a sheep) on the fan, to become part of a "hayfield" for use in another year.

The big flower year of 1958 brought a band of 14 bighorn down from the Grapevine Mountains to an elevation of 1,600 feet at Death Valley Buttes on February 9, where they stuffed themselves for 8 days on Qenothera and Chaenactis with a seasoning of Encelia, Monoptilon, Mohavea, and Phacelia thrown in. Then they went back into the Grapevines while the flowering plants were at their optimum production. We were inclined to attribute this exodus to the fact that substantial stands of flowering annuals and perennials were appearing at higher elevations farther away from roads and human molestation.

At 6,000 feet elevation the same self-limiting utilization of ephemeral plants had apparently taken place on Funeral Peak when we surveyed the high trails there June 6, 1957.

The last group of plants to be considered has been labeled "supplementary food" because of the singular aspect of its utilization. Growing Echinocactus polycephalus was observed eaten but once, August 23, 1957. At that time, an entire plant, including many of the thorns, was devoured by a mature ram in half an hour. (See fig. 43.)

The lapping up of the "cotton" from among the thorns of cottontop cactus (Echinocactus) was engaged in casually by both ewes and lambs on two or three occasions, but the manner and extent of their use of the dried thorns, shells, and roots of long-dead Echinocactus plants illustrates the universal tendency of the bighorn to ingest dead wood and plant fiber.

On February 19, 20, and 26, 1956, a month-old lamb ate ordinary gravel in Furnace Creek Wash, and one mature ewe did likewise near Navel Spring on November 25, 1956. These are the only indication that supplementary minerals may be added to the diet in a separate form.

We have very few observations indicating an age class and sex differential in food utilization. Isolated incidents, of course, prove nothing, but can, in some instances, point toward possibilities. In February 1956, a 4-week-old lamb ate creosotebush (Larrea tridentata), Kleenex, and paper bags, which serves as a reminder that the young of all species go through an experimental stage of trying to eat everything they can get into their mouths. So to that extent, young bighorn probably ingest a number of items not relished by their elders. The most pregnant of the ewes at Furnace Creek Wash was the most avid devourer of the "milky" new shoots of Stephanomeria, and only ewes and lambs were observed eating thistles at Nevares Spring. However, the fact that only one ram was observed eating cottontop cactus is no reason to conclude that ewes and lambs do not.

In general, evidence points strongly toward lambs, both male and female, beginning to learn during the first week of life to eat everything their mothers do, and by the time they are 2 weeks old they are, to a limited degree, actually doing it.

As far as we have been able to determine, there are no areas of sheep range in Death Valley that are occupied separately by rams, or by bands of ewes during lambing. Bands of rams have been observed without ewes and lambs in certain areas, but limited data prohibit conclusions that ewes and lambs do not use the same areas at times and utilize the plants there.

Shortages

A general shortage of food critical enough to threaten seriously the survival of the species has not occurred here in the past 10 years, if ever. There have been several dry cycles severe enough to affect other species, but the bighorn status during those times can be inferred from the situations that had developed by the end of the particularly dry summer conditions which prevailed throughout the monument in 1956.

On June 7 of that year we found Hanaupah Canyon the driest we had ever seen it, with no food in the canyon washes or on the mountain walls above them. Nine burros, all bony, were subsisting with difficulty on a diet of watercress.

Three days later in Wildrose Canyon we found no flowers, no insects or birds, no lizards, rodents, cottontails, or jackrabbits. One dead coyote, reported to us 5 days earlier, had already been picked clean when we found it. As we approached the carcass, a live coyote in very poor condition got to its wobbly legs and made off. Miners throughout the area reported the least wildlife they had seen in many years. Yet on November 25 with the 2-year drought still unbroken, we observed 18 bighorn at Navel Spring in optimum condition. This was the largest band we had seen anywhere at that time.

Overgrazing has been caused in the past by domestic stock—cattle in the Grapevines and Cottonwoods and wild burros in the Panamints, Cottonwoods, and Black Mountains.

The cattle pressure has been alleviated in the Grapevines and Cottonwoods. Burros have been eliminated from the entire Amargosa Range and are being controlled in the Panamints and Cottonwoods. If present policies of the National Park Service continue in effect, there need be no further shortages induced by an exotic influence.

The only natural shortage of food to be expected, then, involves a shortage of water, and the two become a kind of inseparable bipartite hydrofloral factor of the greatest importance to bighorn survival. If food supplies become too attenuated between water supplies either in distance or time, bighorn survival is threatened.

Back-country forage in the case of Navel and Nevares Springs, for example, covers an area of extremely rough terrain from 8 to 12 airline miles, or a probable 12 to 20 sheep-travel miles, deep. Therefore, a bighorn in an extended dry cycle, if still dependent on its permanent water source, would have to make a 40-mile trip to utilize the farthest limits of its reserve source of food. This would eliminate the very young, the very old, and the sick.

In our 30-day observation at Nevares Spring we found that during the driest, hottest weather the ewes and lambs came to water every 3 to 5 days. (See Table 2.) It was not too uncommon for them to remain within observation range from 4 to 6 hours after drinking. And since bighorn apparently do not habitually travel during the night, it seems obvious that they were making no effort to reach the relatively lush forage of the back-country before they returned for water. It would seem doubtful if they could reach it in time to make the trip of any practical value during their longer periods away from water. Hence, the longer the hot and dry period continues, the more likely that a compound shortage of food and water will become acute.

TABLE 2.—Visits of sheep to Nevares Spring, August and September 1957

[E, ewe; L, lamb; R, ram]

Water unequivocally enters the picture as a compounding factor, because the lack of it near enough to the food supply effectively eliminates the supply itself. One rainstorm and the shortage immediately ceases to exist as long as pothole water lasts, and perhaps longer if precipitation is heavy enough to bring on new plant growth, whether annual or perennial. A zone of succulence is apparently a fair, if temporary, substitute for free water as far as the bighorn is concerned.

That the nature of feeding periods and the amounts eaten contribute substantially to the survival of the bighorn in Death Valley is indicated by the difficulty encountered when attempting to trace sheep by browsing sign. Habitually they tend to move so steadily and to eat so little of any one plant that browsing sign is often difficult to find even in areas where plants have been carefully noted during observed browsing periods. The animals' habit of nipping from a plant in passing rather than standing and devouring it to the ground, not only leaves a supply of food for their own future or for other animals but allows for a healthier plant recovery, ground cover, and finally, intact watersheds with a minimum of soil loss and a maximum of water retention.

Bighorn feeding behavior is as variable and unpredictable as bighorn food habits. The length of time and the time of day they may feed is influenced by so many overlapping factors that it becomes almost impossible to say what is dominating their browsing activity at any given time. A list would certainly include the following: The nature of food, the general condition of the animals when they find the food, the type of leadership present, the weather, the terrain, and the time and distance from water. Herd composition, age class, and sex, with especial reference to reproduction, may temporarily dominate both feeding and watering behavior.

The most compelling and pervasive influence is the quality and quantity of the food available. All age classes and sexes spend less time eating when food is rich and abundant. The six ewes, six lambs, and two rams in the flowerbeds on Death Valley Buttes for 8 days in February 1958 alternated average feeding periods of 90 minutes with average rest and play periods of 3 hours. The Furnace Creek Wash band, with the same age class and sex ratios represented but lacking the abundance of ephemeral food plants, reversed the ratio, alternating 3-hour feeding periods with 1-hour rest periods as a rough average.

The Badwater band, in early winter, with the ram ratio absent and also lacking an abundance of ephemerals, was perhaps the most regular in its feeding behavior of any band we have observed. Of no other band have we been able to be so definite. They browsed among the crags from the first streaks of dawn until sunrise; worked down to the fan from 8 to 9 a.m.; fed in the washes and on the fans until 10 or 10:30; then had their siesta for 30 to 45 minutes; and went back up the slope to browse at higher elevations until noon. Then they took another siesta for 1 to 1-1/2 hours. They repeated this pattern in the afternoon, with a siesta at 2 or 2:30; fed on back to the edge of rising elevations at dusk; and then climbed up to 100 to 500 feet by dark. Occasionally we heard them moving for an hour or 2 after dark.

The unusual regularity of the Badwater feed schedule appeared to be largely due to the personality of the old leader, sometimes called The Matriarch. The strength of her leadership was constantly emphasized by the persistent but futile efforts of another ewe to abrogate her regimen. (See "Herd Leadership.")

For 8 days the leader arose at dawn, led the band down and out on the fans, chose the feeding grounds and siesta sites, led the way to the potholes and pawed the first water, and chose the time and place for night bedding. They alternated an average of 50-minute siestas with 2-hour-and-l5-minute feeding periods.

When the leader returned on February 12 with only three of her band, she again took up a regular schedule, but ephemeral abundance was now reflected in longer resting and shorter feeding periods—70-minute siestas compared with 50 minutes in December, and feeding periods of 1 hour and 54 minutes compared to 2 hours and 15 minutes before the ephemerals matured.

The band of five which joined the ewe known as Old Mama and her lamb in Furnace Creek Wash, March 13, 1956, were gaunt, rough coated, and in every way indicated undernourishment in appearance and action. They buried their heads in the large Bebbia and Stephanomeria bushes and stayed for several minutes at one bush, then ran to the next. For the first few days they were reluctant to leave the wash for resting, and often in the morning would break into a run and bypass Old Mama on their way into the wash. After about a week their hunger seemed to abate to the extent that they more or less followed the patterns of the old leader.

Weather sometimes controls the length and time of feeding periods, especially for bands watering at lower elevations, where almost no feeding is done by some bands in the middle of hot and humid days.

During dry cycles when sheep may have to travel farther and farther from their water source they tend to line up in traveling through those areas where forage is exhausted. But as soon as they reach food supplies again they spread out and slow down enough to nip a plant as they pass it. This tends to produce longer and more mobile feeding periods. The same is true of the more barren and cliffy terrain, where the low-density ground cover necessitates longer browsing periods than is the case in washes and spring areas.

It is possible that one of the reasons for age class and sex segregation is to be found in differences in food habits and feeding behavior. During the rut, ewes with still nursing lambs are trying to grab as much green forage from the spring areas as they can get, with the added harassment by the rams. So for days at a time, getting food at all is on a catch-as-catch-can basis for both ewes, lambs, and rams.

On the other hand, the pregnant ewe apparently intensifies her search for succulents with the imminence of parturition, and at the same time begins to feed alone and farther away from the rest of the band.

During the spring the rams are likely to be found in mobile bands farther away from water than the ewes and lambs, suggesting a wider browsing range and a greater tolerance for dormant vegetation.

The tendency to browse in the early morning and late afternoon and evening is pronounced only in the hottest weather at lower elevations.

We have no definitive data on amounts eaten. We know that amounts vary with the quality as well as the quantity of food available, and this variation appears to be maintained by choice.

The average browsing time at Nevares Spring was 37 minutes. This comprised a series of the shortest browsing records we have, but no exhaustion of supply motivated abandonment of browsing.

Generally speaking, there appears to be among the bighorn very little aggressive competition for food. Rams in the rut fight for hours, then placidly feed side by side. Sometimes an older animal may oust a younger one from a particular choice plant, and lambs tend to get into the middle of plants like Bebbia or Stephanomeria, from which they may be removed by their elders. A maturing young ram occasionally will get rough and jostling with adult ewes, but he quickly withdraws if the ewe resists him at all.

The importance of herd leadership in food habits and feeding behavior is becoming increasingly evident. It begins in the first few days of the lamb's life when it is learning by emulation to do everything its mother does, even to the point of trying to help her chew her cud and straining to defecate when its mother does. Leadership in this sense may materialize from any quarter and does not always come from the accepted leader of the hand. The youngest in the band may be the boldest and often leads the way in minor enterprises.

On the other hand, when droughts become severe, bands tend to dissolve. As the 1957 summer advanced and the drought continued, we seldom saw more than three bighorn browsing together, usually a ewe, her lamb, and a ram temporarily en famille.

A Factor in Bighorn Distribution and Survival

As a factor in the distribution and survival of bighorn, food habits and feeding behavior emerge close to the top of the list, for we have here what amounts to a conservation program a la bighorn, with four main points: (1) By eating very little of any one plant at a time, the bighorn seldom destroy a plant by browsing. (2) By constant traveling while feeding, they allow plants to recover. (3) By wide dispersal in small bands they further minimize browsing damage. (4) By their ability to subsist for indefinite periods on completely dormant and even dead plant material, they can with immunity survive droughts resulting in the decimation of many other species of desert animals. (5) Their apparent ability to substitute green forage for free water allows them to reach distant food supplies that otherwise would be unavailable to them.

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