Desert ungulates must contend with high solar radiation, high ambient temperatures, a lack of water and cover, unpredictable food resources, and the challenges these factors present for thermoregulation and water balance. To deal with the conflicting challenges of maintaining body temperature within acceptable limits and minimizing water loss, desert ungulates use a variety of physiological, morphological, and behavioral mechanisms. The mechanisms involved in thermoregulation and water balance have been studied in many domestic and wild African and Middle Eastern ungulates; studies involving ungulates inhabiting North American deserts are limited in comparison. Our objectives were to review available scientific literature on thermoregulation, water balance, and the effects of dehydration in desert ungulates. We discuss the physiological, morphological, and behavioral mechanisms used by ungulates to maintain temperature and water balance in arid environments, and the implications for research and management of desert ungulates in western North America.

The United States Fish and Wildlife Service's (USFWS) designation of critical habitat for the endangered Nelson's bighorn sheep (Ovis canadensis nelsoni) in the Peninsular Ranges of southern California has been controversial because of an absence of a quantitative, repeatable scientific approach to the designation of critical habitat. We used 12,411 locations of Nelson's bighorn sheep collected from 1984–1998 to evaluate habitat use within 398 km2 of the USFWS‐designated critical habitat in the northern Santa Rosa Mountains, Riverside County, California. We developed a multiple logistic regression model to evaluate and predict the probability of bighorn use versus non‐use of native landscapes. Habitat predictor variables included elevation, slope, ruggedness, slope aspect, proximity to water, and distance from minimum expanses of escape habitat. We used Earth Resources Data Analysis System Geographic Information System (ERDAS‐GIS) software to view, retrieve, and format predictor values for input to the Statistical Analysis Systems (SAS) software. To adequately account for habitat landscape diversity, we carried out an unsupervised classification at the outset of data inquiry using a maximum‐likelihood clustering scheme implemented in ERDAS. We used the strata resulting from the unsupervised classification in a stratified random sampling scheme to minimize data loads required for model development. Based on 5 predictor variables, the habitat model correctly classified >96% of observed bighorn sheep locations. Proximity to perennial water was the best predictor variable. Ninety‐seven percent of the observations were within 3 km of perennial water. Exercising the model over the northern Santa Rosa Mountain study area provided probabilities of bighorn use at a 30 times 30‐m2 pixel level. Within the 398 km2 of USFWS‐designated critical habitat, only 34% had a graded probability of bighorn use to non‐use ranging from ≥1:1 to 6,044:1. The remaining 66% of the study area had odds of having bighorn use <1:1 or it was more likely not to be used by bighorn sheep. The USFWS designation of critical habitat included areas (45 km2) of importance (2.5 to ≥40 observations per km2 per year) to Nelson's bighorn sheep and large landscapes (353 km2) that do not appear to be used (<1 observation per km2 per year).

Temporal geographic variation in lambing seasons was statistically assessed for 22 populations, including 5 ecological races, of North American bighorn sheep (Ovis canadensis ssp.) from the Canadian National Parks (52° N) to western Texas (30° N). Throughout their distribution, bighorn lambing occurs coincident with the period of vegetative development when the environmental regime ameliorates neonate survival. Analyses generally demonstrate later and shorter lambing seasons in higher latitudinal populations (P < 0.001). Inception of lambing occurs later with colder temperatures, increased snowfall, at higher latitudes and elevations, and with later and shorter growing seasons [Formula: see text]. Additionally, a significant (P < 0.001) divergence in the reproductive "strategy" (median onset and duration of lambing) exists between bighorn herds of the Sierra Nevada Mountains, California (37° N) and the Desert National Wildlife Range, Nevada (36° N) as a result of two distinct, but adjacent environmental regimes. Lambing in northern populations is cued to a brief, relatively predictable period of plant growth. Southern bighorn protract lambing such that some recruitment coincides with relatively unpredictable plant growth, triggered by erratic rains.

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Ungulates adapted to desert areas are able to survive extreme temperatures and limited water availability. This ability is largely due to behavioral, morphological, and physiological adaptations that allow these animals to avoid or tolerate extreme environmental conditions. The physiological adaptations possessed by ungulates for thermoregulation and maintenance of water balance have been the subject of numerous studies involving a wide range of species. In this report we review the behavioral, morphological, and physiological mechanisms used by ungulates and other desert mammals to maintain water and temperature balance in arid environments. We also review some of the more commonly used methods for studying the physiological mechanisms involved in water balance and thermoregulation, and the influence of dehydration on these mechanisms.

TURNER, J. C. 1984. Diurnal periodicity of plasma cortisol and corticosterone in desert bighorn sheep demonstrated by radioimmunoassay . Can. J. Zool. 62: [2659][2660][2661][2662][2663][2664][2665]. A sensitive and specific radioimmunoassay (RIA) for cortisol and corticosterone was validated for desert bighorn sheep (Ovis c-trnodensis c.remnobare.s) plasma. Statistical and physiological validation of assay accuracy, precision, sensitivity, and specificity demonstrated the RIA to be reliable. Coefficients for intra-and inter-assay variation were 1.20 and 2.35% for cortisol and 3.50 and 3.97% for corticosterone, respectively. Sensitivity was 3.3 and 4.9 pg for cortisol and corticosterone, respectively. A distinct diurnal rhythm in plasma cortisol and corticosterone synchronous with photoperiod was found in free-ranging and captive bighorn sheep. A significant difference ( P < 0.01) existed between the mean 24-h plasma cortisol concentration for captive (33.04 + 5.97 ng/mL) and free-ranging (49.20 + 8.17 ng/mL) bighorn. Corticosterone levels were not significantly distinct between captive and free-ranging bighorn. The mean scotophase cortisol concentration in free-ranging class 111 rams (60.97 + 3.21 ng/mL) was significantly greater ( P < 0.05) than the mean scotophase cortisol concentration (54.03 t 5.97 ng/mL) of the free. ranging group. Absence of group interactions in solitary rams may be sufficiently stressful so as to entrain increased plasma cortisol concentrations. TURNER, J . C. 1984. Diurnal periodicity of plasma cortisol and corticosterone in desert bighorn sheep demonstrated by radioimmunoassay . Can. J . Zool . 62: 2659 -2665. Des techniques radioimmunologiques sensibles et specifiques qui servent 2 I'analyse du cortisol et de la corticosterone plasmatiques ont i t @ essayees chez le mouflon bighorn (Ovis c-trntrdensis c-rrm~~ohtrrrs). L'exactitude, la precision, la sensibilite et la spkcificite de ces techniques ont ete approuvees statistiquement et physiologiquement. Les coefficients de variation intraet inter-analyses ont et@ ivalu@s a 1.20 et 2,35% dans le cas du cortisol et h 3.50 et 3.97% dans le cas de la corticostirone. La sensibilite est de 3.3 pg dans le cas du cortisol et de 4,9 pg dans le cas de la corticostirone. Le cortisol et la corticosterone plasmatiques suivent un rhythme diurne particulier associe h la photoperiode chez les mouflons libres et les moutlons gardes en captiviti. II existe une difference significative ( P < 0.01) entre les concentrations plasmatiques moyennes (evaluees sur 24 h) du cortisol des mouflons en captivite (33.04 t 5,97 ng/mL) et des moutlons libres (49.20 t 8.17 ng/mL). Les concentrations de corticosterone ne different pas significativement chez les deux groupes de moutlons. La concentration moyenne de cortisol de scotophase est significativement plus ilevie ( P < 0,051 chez les brebis libres de classe 111 (69,97 f 3.21 ng/mL) que chez les autres animaux libres. L'absence des interactions de groupe entraine suffisamment de stress chez les brebis solitaires pour...