Population Of Desert Bighorn Sheep Research Articles

Comparing contemporary models to traditional indices to estimate abundance of desert bighorn sheep

AbstractAerial surveys for large ungulates produce count data that often underrepresent the number of animals. Errors in count data can lead to erroneous estimates of abundance if they are not addressed. Our objective was to address imperfect detection probability by developing a framework that produces realistic and defensible estimates of bighorn sheep (Ovis canadensis) abundance. We applied our framework to a population of desert bighorn sheep (O. c. nelsoni) in the Great Basin, Nevada, USA. We captured and marked 24 desert bighorn sheep with global positioning system (GPS)‐collars and then conducted helicopter surveys naïve to the locations of collared animals. We developed a Bayesian integrated data model to leverage information from telemetry data, helicopter survey counts, and habitat characteristics to estimate abundance while accounting for availability and perception probability (i.e., detection given availability). Distance to ridgeline, terrain ruggedness, tree cover, and slope influenced perception probability of sheep given they were viewable from the helicopter. There was also annual variation in perception probability (2018: median = 0.64, credible interval [CrI] = 0.37–0.87; 2019: median = 0.81, CrI = 0.49–0.97). The abundance estimates from the integrated data model decreased from 2018 (594; 95% CrI = 537–656) to 2019 (487; 95% CrI = 436–551). In addition, accounting for availability and imperfect perception resulted in greater estimates of abundance compared to traditional directed search methods, which were 340 for 2018 and 320 for 2019. Our modeling framework can be used to generate more defensible population estimates of bighorn sheep and other large mammals that have been surveyed in a similar manner.

Local differences in maximum temperature determine water use among desert bighorn sheep populations

AbstractThe availability of surface water is assumed by many wildlife managers to be important to desert bighorn sheep (Ovis canadensis nelsoni). Yet scientific evidence suggests bighorn sheep populations can sometimes fulfill their water requirements from forage alone. To investigate how water use changes with environmental conditions, we analyzed the surface water use and movement patterns of 4 desert bighorn populations across a range of meteorological conditions in the Mojave Desert in California, USA. We compared ground‐based weather data to the daily use of surface water sources for 65 known individuals, and examined the movement patterns of 18 of these individuals with global positioning system collars in relation to visitation to surface water sources. Maximum daily temperature most closely predicted the percent of bighorn visiting water daily, with visitation increasing approximately 30% between 30°C and 40°C. Individual animals switched from a tortuous foraging movement to directed movement when accessing surface water. Day length, maximum daily temperature, minimum daily temperature, solar radiation, and the interaction between day length and minimum temperature explained the distance desert bighorn were located from the nearest water source. Our results indicate that local meteorological conditions are important determinants of desert bighorn surface water use, and that managers should monitor water sources when the maximum daily temperature exceeds 35°C. Individual animals meet their water requirements with surface water under hot and dry conditions, likely because it is temporally or energetically efficient given their other resource needs.

Seasonal prevalence of gastrointestinal parasites in desert bighorn sheep (Ovis canadensis) in northern Mexico.

This study aimed to describe the shedding pattern of gastrointestinal parasite eggs by a wild population of desert bighorn sheep (DBS; Ovis canadensis) in northern Mexico. Seventy-five fresh faecal samples were collected from the ground in each season within an hour after being expelled by these animals. The generalized linear mixed model showed that eggs per gram of faeces were highest in winter (577 ± 399) and lowest in summer (260 ± 198). Generalized linear models revealed that Strongyloides spp. predominated during most seasons with a peak in summer (85% of faecal samples analysed) and the absence of this helminth in winter. Nematodirus spp. was another helminth present in three seasons, with the presence of this nematode in 35% of the faecal samples in spring and 0% in summer. Other parasites in DBS faeces included Bunostomun spp., Trichostrongylus spp., Cooperia spp., Mecistocirrus digitatus, Haemonchus contortus, Chabertia ovina and Eimeria ovinoidalis. There were differences among seasons in the percentage of these helminths and coccidia in faecal samples for all these parasites. It was concluded that helminths egg output in DBS in a semi-arid rangeland is lowest in summer and spring and highest in autumn and winter. Furthermore, it was shown that DBS in the study site do not suffer from severe parasite burden. Therefore, this nematode parasite burden is compatible with the conservation and well-being of this particular population.

Accurate population estimation of Caprinae using camera traps and distance sampling

With most of the world’s Caprinae taxa threatened with extinction, the IUCN appeals to the development of simple and affordable sampling methods that will produce credible abundance and distribution data for helping conserve these species inhabiting remote areas. Traditional sampling approaches, like aerial sampling or mark-capture-recapture, can generate bias by failing to meet sampling assumptions, or by incurring too much cost and logistical burden for most projects to address them. Therefore, we met the IUCN’s challenge by testing a sampling technique that leverages imagery from camera traps with conventional distance sampling, validating its operability in mountainous topography by comparing results to known abundances. Our project occurred within a captive facility housing a wild population of desert bighorn sheep (Ovis canadensis) in the Chihuahuan desert of New Mexico, which is censused yearly. True abundance was always within our 90% confidence bounds, and the mean abundance estimates were within 4.9 individuals (average) of the census values. By demonstrating the veracity of this straightforward and inexpensive sampling method, we provide confidence in its operability, urging its use to fill conservation voids for Caprinae and other data-deficient species inhabiting rugged or heavily vegetated terrain.

Desert bighorn sheep responses to human activity in south-eastern Utah

Abstract ContextHuman activity in wildlands can influence wildlife populations by decreasing or degrading habitat, decreasing survival, or through displacement. For bighorn sheep (Ovis canadensis), some human activities are detrimental and may result in physiological stress, altered activity budgets, or habitat abandonment. From 1979 to 2000, human recreation increased over 300% in areas occupied by desert bighorn sheep (O. c. nelsoni) in south-eastern Utah. Concurrently, the population of desert bighorn sheep occupying the Potash Bighorn Sheep Management Unit of south-eastern Utah was in steep decline. AimsWe investigated behavioural responses of desert bighorn sheep to different levels of human activity in this area from 2002 to 2003. MethodsWe divided the study population into two areas on the basis of the level of human use, i.e. low and high. We observed desert bighorn sheep at seven sites within these areas, including three with a high level of human use and four areas of a low level of human use. Key resultsDesert bighorn sheep spent significantly less time grazing and more time vigilant in areas of a high level of human use than in areas of a low level of human use. ConclusionsHuman recreational use of desert bighorn sheep habitat has the potential to negatively affect foraging by desert bighorn sheep. ImplicationsWe raise a cautionary flag because recreational use in bighorn sheep habitat near Moab, Utah, continues to increase and bighorn numbers continue to decline. When managers consider proposals to increase areas or trails for human recreation, they should incorporate ways to minimise impacts and reduce stressors to bighorn sheep.

Genetic assessment of a bighorn sheep population expansion in the Silver Bell Mountains, Arizona.

BackgroundThe isolated population of desert bighorn sheep in the Silver Bell Mountains of southern Arizona underwent an unprecedented expansion in merely four years. We hypothesized that immigration from neighboring bighorn sheep populations could have caused the increase in numbers as detected by Arizona Game and Fish Department annual aerial counts.MethodsWe applied a multilocus genetic approach using mitochondrial DNA and nuclear microsatellite markers for genetic analyses to find evidence of immigration. We sampled the Silver Bell Mountains bighorn sheep before (2003) and during (2015) the population expansion, and a small number of available samples from the Gila Mountains (southwestern Arizona) and the Morenci Mine (Rocky Mountain bighorn) in an attempt to identify the source of putative immigrants and, more importantly, to serve as comparisons for genetic diversity metrics.ResultsWe did not find evidence of substantial gene flow into the Silver Bell Mountains population. We did not detect any new mitochondrial haplotypes in the 2015 bighorn sheep samples. The microsatellite analyses detected only one new allele, in one individual from the 2015 population that was not detected in the 2003 samples. Overall, the genetic diversity of the Silver Bell Mountains population was lower than that seen in either the Gila population or the Morenci Mine population.DiscussionEven though the results of this study did not help elucidate the precise reason for the recent population expansion, continued monitoring and genetic sampling could provide more clarity on the genetic demographics of this population.

Detection of bacterial-reactive natural IgM antibodies in desert bighorn sheep populations.

Ecoimmunology is a burgeoning field of ecology which studies immune responses in wildlife by utilizing general immune assays such as the detection of natural antibody. Unlike adaptive antibodies, natural antibodies are important in innate immune responses and often recognized conserved epitopes present in pathogens. Here, we describe a procedure for measuring natural antibodies reactive to bacterial antigens that may be applicable to a variety of organisms. IgM from desert bighorn sheep plasma samples was tested for reactivity to outer membrane proteins from Vibrio coralliilyticus, a marine bacterium to which sheep would have not been exposed. Immunoblotting demonstrated bighorn sheep IgM could bind to a variety of bacterial cell envelope proteins while ELISA analysis allowed for rapid determination of natural antibody levels in hundreds of individual animals. Natural antibody levels were correlated with the ability of plasma to kill laboratory strains of E. coli bacteria. Finally, we demonstrate that natural antibody levels varied in two distinct populations of desert bighorn sheep. These data demonstrate a novel and specific measure of natural antibody function and show that this varies in ecologically relevant ways.

Estimation of regional sheep abundance based on group sizes

ABSTRACTWe used sightability models with aerial counts to estimate regional abundance of desert bighorn sheep (Ovis canadensis) in southwestern Arizona during 1992–2012. Because of incomplete spatial and temporal coverage of surveys, we integrated sightability estimates using Bayesian state‐space models. Although some game management units (GMUs) exhibited negative average growth rates during this period, temporal variability in growth was large. The combination of temporal variability and statistical uncertainty in the sightability estimates led to the selection of a regional model having zero mean growth from initial abundance. Hence, the model indicated a relatively stable population of desert bighorn sheep in southern Arizona, beginning with approximately 2,900 bighorn sheep in 1992 to the current number of approximately 2,300 sheep (2012). Our method provides statistically valid estimates of overall regional abundance not otherwise possible given the incomplete spatial coverage of surveys. Additionally, results have implications for other studies, common in wildlife management, where managers seek to integrate information across surveys with partial time‐series. © 2014 The Wildlife Society.

Local extinction and unintentional rewilding of bighorn sheep (Ovis canadensis) on a desert island.

Bighorn sheep (Ovis canadensis) were not known to live on Tiburón Island, the largest island in the Gulf of California and Mexico, prior to the surprisingly successful introduction of 20 individuals as a conservation measure in 1975. Today, a stable island population of ∼500 sheep supports limited big game hunting and restocking of depleted areas on the Mexican mainland. We discovered fossil dung morphologically similar to that of bighorn sheep in a dung mat deposit from Mojet Cave, in the mountains of Tiburón Island. To determine the origin of this cave deposit we compared pellet shape to fecal pellets of other large mammals, and extracted DNA to sequence mitochondrial DNA fragments at the 12S ribosomal RNA and control regions. The fossil dung was 14C-dated to 1476–1632 calendar years before present and was confirmed as bighorn sheep by morphological and ancient DNA (aDNA) analysis. 12S sequences closely or exactly matched known bighorn sheep sequences; control region sequences exactly matched a haplotype described in desert bighorn sheep populations in southwest Arizona and southern California and showed subtle differentiation from the extant Tiburón population. Native desert bighorn sheep previously colonized this land-bridge island, most likely during the Pleistocene, when lower sea levels connected Tiburón to the mainland. They were extirpated sometime in the last ∼1500 years, probably due to inherent dynamics of isolated populations, prolonged drought, and (or) human overkill. The reintroduced population is vulnerable to similar extinction risks. The discovery presented here refutes conventional wisdom that bighorn sheep are not native to Tiburón Island, and establishes its recent introduction as an example of unintentional rewilding, defined here as the introduction of a species without knowledge that it was once native and has since gone locally extinct.

Conservation genetics and the persistence and translocation of small populations: bighorn sheep populations as examples

Understanding and evaluating the factors that influence the persistence of small populations and establishment of new populations are basic goals of conservation biology. Genetic effects due to genetic drift and inbreeding can have important impacts on the success of new populations. Many bighorn sheep populations in western North America have had low numbers and many have gone extinct. Here, the possible effects of genetic drift and inbreeding are evaluated in three populations of desert bighorn sheep initiated in the 1970s from translocations. One of these has no molecular genetic data but has substantial demographic data (Aravaipa Canyon), one has both extensive demographic data and some molecular genetic data (Red Rock), and one has limited demographic data and some molecular genetic data (Tiburon Island). Overall, either from theoretical pedigree analysis and population genetic estimates from demographic history (Aravaipa, Tiburon) or from molecular data (Red Rock, Tiburon), it appears that the levels of genetic drift and inbreeding are substantial in all of these populations. This impact was larger when higher variance in male reproductive success was assumed. In other words, it appears that genetic factors are and will be important in the establishment and persistence of these populations. These examples in long-term monitored bighorn sheep populations are relevant to many endangered species in similar situations where demographic data are available but there is little or no historical molecular genetic data.

Evaluation of Methods Used to Estimate Size of a Population of Desert Bighorn Sheep (Ovis canadensis mexicana) in New Mexico

Reliable estimates of size of populations are critical for successful management of translocated desert bighorn sheep (Ovis canadensis mexicana). As costs decrease and quality improves, remote cameras are increasingly used as a non-invasive tool to monitor populations of wildlife, but their efficacy has yet to be evaluated in a diversity of species and habitats. Here we investigate whether remote cameras, in combination with a simple mark-resight model, produce estimates of size of populations of desert bighorn sheep comparable to those derived from surveys conducted on the ground and using helicopters in the Fra Cristobal Range of south-central New Mexico. We determined that estimates of size of populations derived from remote cameras were comparable to those produced from direct observations obtained by surveys from helicopters and on the ground, that ca. 25 sequential samples of photographs produced comparable estimates of size of populations, but that photographs of single desert bighorn sheep dramatically lowered estimates of size of populations, and that placement of remote cameras on wildlife guzzlers in July produced the greatest number of photographs in the shortest time. Our results suggest that use of remote cameras may be an accurate, low-cost, and non-invasive means of estimating size of populations of desert bighorn sheep in New Mexico. Further research is warranted, ideally in tandem with ongoing surveys on the ground and using helicopters, in other management areas.

Using Genetic Tools to Track Desert Bighorn Sheep Colonizations

ABSTRACT Understanding colonization is vital for managing fragmented populations. We employed mitochondrial DNA haplotypes and 14 microsatellite (nuclear DNA) markers to infer the origins of newly established populations of desert bighorn sheep (Ovis canadensis nelsoni) and to assess loss of genetic diversity during natural colonizations. We used haplotype distribution, F‐statistics, Bayesian population clustering, and assignment tests to infer source populations for 3 recent colonies and identified a previously undetected colonization from multiple source populations. Allelic richness declined in 3 of 4 colonies in comparison to the primary source populations, but not as much as has been reported for translocated populations. Heterozygosity declined in only one colony. We also demonstrated that both native and translocated desert bighorn sheep have naturally recolonized empty habitats and suggest that colonization may partially offset population extinction in the region as long as connectivity is maintained. Genetic techniques and mitochondrial DNA haplotypes we described will allow managers to determine the origins of future colonizations by bighorn sheep in California, USA, and prioritize protection of linkages between known sources and colonies.

Surface Mining and Ecology of Desert Bighorn Sheep

Understanding how mining operations influence use of habitat and movements by bighorn sheep (Ovis canadensis) is critical to long-term management of populations that inhabit areas in and around mining operations. We studied responses of a population of desert bighorn sheep to a surface-mining operation in the Silver Bell Mountains, Pima County, Arizona. We incorporated two study periods with different levels of activity at the mine: closure (1993–1995) and operation (2003–2005). We captured and radiocollared ≥22 bighorn sheep in each period and monitored size of home ranges, size of core areas, and use of the mine. Home ranges and core areas of adult males during the breeding season were larger during closure than during operation. During the non-breeding season, there was evidence that home ranges of adult males were larger during closure compared to when the mine was in operation. Core areas of adult males were similar in size during the non-breeding season of both periods. During the breeding season, adult males used the mine more while the mine was in operation than during closure. During the non-breeding season, however, adult males did not use the mine more during either period. Home ranges and core areas of females were not different in size during both seasons and periods of closure and operation of the mine. During both seasons, females used the mine more during operation than during closure. Overall, the population of desert bighorn sheep used an active mine more than it did when the mine was closed.

Predicting population survival under future climate change: density dependence, drought and extraction in an insular bighorn sheep

1. Our understanding of the interplay between density dependence, climatic perturbations, and conservation practices on the dynamics of small populations is still limited. This can result in uninformed strategies that put endangered populations at risk. Moreover, the data available for a large number of populations in such circumstances are sparse and mined with missing data. Under the current climate change scenarios, it is essential to develop appropriate inferential methods that can make use of such data sets. 2. We studied a population of desert bighorn sheep introduced to Tiburon Island, Mexico in 1975 and subjected to irregular extractions for the last 10 years. The unique attributes of this population are absence of predation and disease, thereby permitting us to explore the combined effect of density dependence, environmental variability and extraction in a 'controlled setting.' Using a combination of nonlinear discrete models with long-term field data, we constructed three basic Bayesian state space models with increasing density dependence (DD), and the same three models with the addition of summer drought effects. 3. We subsequently used Monte Carlo simulations to evaluate the combined effect of drought, DD, and increasing extractions on the probability of population survival under two climate change scenarios (based on the Intergovernmental Panel on Climate Change predictions): (i) increase in drought variability; and (ii) increase in mean drought severity. 4. The population grew from 16 individuals introduced in 1975 to close to 700 by 1993. Our results show that the population's growth was dominated by DD, with drought having a secondary but still relevant effect on its dynamics. 5. Our predictions suggest that under climate change scenario (i), extraction dominates the fate of the population, while for scenario (ii), an increase in mean drought affects the population's probability of survival in an equivalent magnitude as extractions. Thus, for the long-term survival of the population, our results stress that a more variable environment is less threatening than one in which the mean conditions become harsher. Current climate change scenarios and their underlying uncertainty make studies such as this one crucial for understanding the dynamics of ungulate populations and their conservation.

Water Developments and Desert Bighorn Sheep: Implications for Conservation

In recent times the construction and maintenance of wildlife water developments for the stated purpose of conserving desert bighorn sheep (Ovis canadensis) populations has been controversial, especially within sensitive lands of the southwestern United States. A major portion of the controversy is whether wildlife water developments provide a benefit to populations of desert bighorn sheep and whether the associated incursion into the naturalness of an area is justified. Desert bighorn sheep are a valued natural resource and they exist today in small, isolated populations threatened with a variety of human-related impacts (e.g., disease, development, climate change, habitat fragmentation, water diversion). In this article I summarize the available published literature related to desert bighorn sheep and wildlife water developments and review the effects and consequences of water developments in desert bighorn sheep conservation. Based on my review, I contend that recent criticism of water developments has failed to adequately consider anthropogenic factors that can influence wildlife populations and their habitats. My review found that desert bighorn sheep benefit from water developments and that the role of active management of wildlife habitat, including the development of free-standing water for bighorn sheep where this component is unavailable, is justified as a means of mitigating negative anthropogenic influences in an otherwise natural setting.

Elevation and connectivity define genetic refugia for mountain sheep as climate warms

Global warming is predicted to affect the evolutionary potential of natural populations. We assessed genetic diversity of 25 populations of desert bighorn sheep (Ovis canadensis nelsoni) in southeastern California, where temperatures have increased and precipitation has decreased during the 20th century. Populations in low-elevation habitats had lower genetic diversity, presumably reflecting more fluctuations in population sizes and founder effects. Higher-elevation habitats acted as reservoirs of genetic diversity. However, genetic diversity was also affected by population connectivity, which has been disrupted by human development. Restoring population connectivity may be necessary to buffer the effects of climate change on this desert-adapted ungulate.

Head to Head

Turner et al. (2004) developed a habitat selection model for a population of desert bighorn sheep (Ovis canadensis) in the Peninsular Ranges of southern California that is listed as a threatened and endangered population by the state of California and the federal government, respectively. We are concerned that the recent publication of an article by Turner et al. (2004) could be detrimental to the management and recovery of bighorn sheep in the Peninsular Ranges because it lends credibility to a flawed analysis of bighorn sheep habitatuse patterns. The model attempts to extrapolate conclusions from a limited subset of bighorn sheep data that is not representative of the study area and was not gathered in a manner conducive to the analysis methods used by the authors. The authors classified habitat pixels as “active” or “inactive” based on the presence-absence of bighorn sheep observations without considering monitoring intensity. Turner et al. (2004) also failed to consider the implications of basing their model almost entirely on a bighorn sheep subpopulation known to have atypical habitat selection patterns. This subpopulation in the northwestern Santa Rosa Mountains frequently used food and water sources within hillside urban areas. Because the Turner et al. (2004) model was developed using data primarily from this atypical subpopulation, the model has low external validity and is unlikely to accurately predict habitat selection by other bighorn sheep subpopulations in the Peninsular Ranges. Furthermore, with the NW sub-population used in model development now excluded from urban areas, the Turner et al. (2004) model is unlikely to accurately predict habitat selection patterns of even this sub-population. We suggest the Turner et al. (2004) model is at best only applicable to this subpopulation between the years 1994–1998.

Precipitation, density, and population dynamics of desert bighorn sheep on San Andres National Wildlife Refuge, New Mexico

Understanding the determinants of population size and performance for desert bighorn sheep (Ovis canadensis mexicana) is critical to develop effective recovery and manage- ment strategies. In arid environments, plant communities and consequently herbivore populations are strongly dependent upon precipitation, which is highly variable seasonal- ly and annually. We conducted a retrospective exploratory analysis of desert bighorn sheep population dynamics on San Andres National Wildlife Refuge (SANWR), New Mexico, 1941-1976, by modeling sheep population size as a function of previous popu- lation sizes and precipitation. Population size and trend of desert bighorn were best and well described (R2 = 0.89) by a model that included only total annual precipitation as a covariate. Models incorporating density-dependence, delayed density-dependence, and combinations of density and precipitation were less informative than the model contain- ing precipitation alone (AAICc=8.5-22.5). Lamb:female ratios were positively related to precipitation (current year: F134=7.09, P=0.012; previous year: F, 33=3.37, P=0.075)but were unrelated to population size (current year: F1 34=0.04, P=0.843; previous year: F| 33 =0.14, P=0.715). Instantaneous population rate of increase (r) was related to population size (F| 33 = 5.55; P=O.O25). Precipitation limited populations of desert bighorn sheep on SANWR primarily in a density-independent manner by affecting production or survival of lambs, likely through influences on forage quantity and quality. Habitat evaluations and recovery plans for desert bighorn sheep need to consider fundamental influences on desert bighorn populations such as precipitation and food, rather than focus solely on proximate issues such as security cover, predation, and disease. Moreover, the concept of carrying capacity for desert bighorn sheep may need re-evaluation in respect to highly variable (CV =35.6%) localized precipitation patterns. On SANWR carrying capacity for desert bighorn sheep was zero when total annual precipitation was <28.2 cm.

Habitat use by desert bighorn sheep during lambing

Habitat used for predator escape may be a factor limiting restoration of desert bighorn sheep (Ovis canadensis mexicana) in small mountain ranges in New Mexico. Female bighorn sheep seek isolation in discrete areas for parturition. Although parturition sites are used only for <3 days they play an important role in neonate survival. We compared habitat characteristics at pre-parturition (n = 21), parturition (n = 38), random (n = 38), and post-parturition sites (n = 21). At each site we calculated distance to steep terrain, elevation, ruggedness, slope, and visibility. Parturition and post-parturition sites were higher in elevation and more rugged than sites used during pre-parturition. Post-parturition sites were closer to terrain with 100% slope than the pre-parturition or parturition sites. Post-parturition sites had higher visibility and steeper slopes than the pre-parturition sites. Parturition sites were steeper, higher in elevation, more rugged, and had lower visibility than the random sites. Fidelity to parturition sites was observed on 1 of 38 occasions. Because lambing sites are used for short periods they are poorly understood and often ignored by land managers. Additional research is recommended in other desert bighorn sheep populations to understand better and predict habitat use during the lambing period.

VIGILANCE, PREDATION RISK, AND THE ALLEE EFFECT IN DESERT BIGHORN SHEEP

Knowledge of how predation risk affects population survivorship is important for understanding predator–prey relationships and designing effective conservation strategies. The Allee effect (inverse density dependence) can be generated when antipredator strategies become inefficient in small groups of prey, thus making the population more susceptible to catastrophic population collapse and extinction. Many populations of desert bighorn sheep (Ovis canadensis) are declining, and predation is, in many cases, a major mortality factor. We studied desert bighorns at the Red Rock Wildlife Area, New Mexico, USA, to assess predation risk in different group types (ram, ewe, mixed) and age–sex classes. Multiple regression analysis revealed that predation risk (as estimated by individual vigilance) increased with smaller group size and greater inter-individual distance for all bighorns, with groups of <5 individuals subject to the greatest risk. Although rugged terrain is thought to provide refuge from predators, habitat ruggedness did not influence vigilance. The biggest males in ram groups experienced the greatest predation risk in that they were in the smallest groups, were most likely to be solitary, and were spaced farther apart from conspecifics. Although big rams spent twice as much time vigilant as other age–sex classes, collective alertness was lowest for ram groups. The conclusion that big rams were most at risk from predation was partially supported by the recent predation history of the population and previous studies in which mountain lion (Felis concolor) kills were biased toward rams. We discuss the management implications of these results for small populations subject to Allee effects, including reintroduction and/or translocation practices and selective removal of problem predators. We suggest that the use of multivariate techniques to simultaneously explore the influence of multiple factors and the use of vigilance as a correlate of predation risk would be useful management tools for assessing seasonal and class-specific vulnerability to predation.