Stone's Sheep Research Articles

Stone’s sheep (Ovis dalli stonei) lambing and nursery habitat selection

Wildlife conservation often focuses on mitigating disturbance in critical habitats like reproductive ranges. We studied lambing and nursery habitat selection by Stone’s sheep ( Ovis dalli stonei (J.A. Allen, 1897)) in the Cassiar Mountains in British Columbia. We estimated the timing of parturition and delineated lambing and nursery habitats based on movement behaviours of collared females ( n = 18) and vaginal implant transmitters. We identified 23 lambing events in 2018 ( n = 4), 2019 ( n = 13), and 2020 ( n = 6). The median birth date was 17 May and ranged from 3 May to 14 June. Females remained in lambing habitats from 1.5 to 11.3 days with a median of 5.5 days. We examined habitat selection during the lambing and nursery periods at the home-range scale using resource selection functions, and at a finer scale using integrated step-selection analyses. Females selected southwest slopes in rugged terrain at mid-elevations, suggesting selection for warmer micro-climates and features that facilitate predator avoidance. Females avoided habitats near roads during the lambing period but showed selection for habitats near roads during the nursing period. We developed predictive maps of suitable lambing and nursery ranges to inform land planning to help reduce overlap of anthropogenic disturbance with potential reproductive habitats.

Diverse migration patterns and seasonal habitat use of Stone's sheep (Ovis dalli stonei).

We describe temporal and spatial patterns of seasonal space-use and migration by 16 GPS-collared Stone's sheep (Ovis dalli stonei) from nine bands in the Cassiar Mountains of northern British Columbia, Canada. Our objectives were to identify the timing of spring and fall migrations, characterize summer and winter ranges, map and describe migration routes and use of stopover sites, and document altitudinal change across seasons. Our last objective was to assess individual migration strategies based on patterns of geographic migration, altitudinal migration, or residency. Median start and end dates of the spring migration were 12 and 17 Jun (range: 20 May to 05 Aug), and of the fall migration were 30 Aug and 22 Sep (range: 21 Aug to 07 Jan). The median area of winter and summer ranges for geographic migrants were 630.8 ha and 2,829.0 ha, respectively, with a broad range from about 233.6 to 10,196.2 ha. Individuals showed high fidelity to winter ranges over the limited duration of the study. The winter and summer ranges of most individuals (n=15) were at moderate to high elevations with a median summer elevation of 1,709 m (1,563-1,827 m) and 1,673 m (1,478-1,751 m) that varied <150 m between ranges. Almost all collared females (n=14) exhibited changes in elevation use that coincide with abbreviated altitudinal migration. Specifically, these females descended to lower spring elevations from their winter range (Δ>150 m), and then gradually moved up to higher-elevation summer ranges (Δ>150 m). In the fall, they descended to lower elevations (Δ>100 m) before returning to their higher winter ranges. The median distance travelled along geographic migration routes was 16.3 km (range: 7.6-47.4 km). During the spring migration, most geographic migrants (n=8) used at least one stopover site (median = 1.5, range: 0-4), while almost all migrants (n=11) used stopover sites more frequently in the fall (median = 2.5, range: 0-6). Of the 13 migratory individuals that had at least one other collared individual in their band, most migrated at about the same time, occupied the same summer and winter ranges, used similar migration routes and stopover sites, and exhibited the same migration strategy. We found collared females exhibited four different migration strategies which mostly varied across bands. Migration strategies included long-distance geographic migrants (n=5), short-distance geographic migrants (n=5), vacillating migrants (n=2), and abbreviated altitudinal migrants (n=4). Different migratory strategies occurred within one band where one collared individual migrated and two did not. We conclude that female Stone's sheep in the Cassiar Mountains displayed a diverse assemblage of seasonal habitat use and migratory behaviors. By delineating seasonal ranges, migration routes and stopover sites, we identify potential areas of priority that can help inform land-use planning and preserve the native migrations of Stone's sheep in the region.

Ancient hybridization patterns between bighorn and thinhorn sheep.

Whole-genome sequencing has advanced the study of species evolution, including the detection of genealogical discordant events such as ancient hybridization and incomplete lineage sorting (ILS). The evolutionary history of bighorn (Ovis canadensis) and thinhorn (Ovis dalli) sheep present an ideal system to investigate evolutionary discordance due to their recent and rapid radiation and putative secondary contact between bighorn and thinhorn sheep subspecies, specifically the dark pelage Stone sheep (O.dalli stonei) and predominately white Dall sheep (O.dalli dalli), during the last ice age. Here, we used multiple genomes of bighorn and thinhorn sheep, together with snow (O.nivicola) and the domestic sheep (O.aries) as outgroups, to assess their phylogenomic history, potential introgression patterns and their adaptive consequences. Among the Pachyceriforms (snow, bighorn and thinhorn sheep) a consistent monophyletic species tree was retrieved; however, many genealogical discordance patterns were observed. Alternative phylogenies frequently placed Stone and bighorn as sister clades. This relationship occurred more often and was less divergent than that between Dall and bighorn. We also observed many blocks containing introgression signal between Stone and bighorn genomes in which coat colour genes were present. Introgression signals observed between Dall and bighorn were more random and less frequent, and therefore probably due to ILS or intermediary secondary contact. These results strongly suggest that Stone sheep originated from a complex series of events, characterized by multiple, ancient periods of secondary contact with bighorn sheep.

Vegetation and prescribed fire: Implications for Stone's sheep and elk

ABSTRACTFire is the dominant single natural disturbance influencing northern mountain and boreal landscapes. We evaluated fire‐induced changes in forage resources for 2 focal ungulates, Stone's sheep (Ovis dalli stonei) and elk (Cervus canadensis), in northern British Columbia, Canada. We implemented 4 prescribed fires and monitored short‐term vegetation (quantity and quality) and ungulate (Stone's sheep and elk) responses. We took measurements prior to burning, the year of burning, and 1 year after burning in treatment areas and adjacent unburned control areas in winter and summer at 2 scales. At a fine scale, we used vegetation transects and pellet counts; at the landscape scale, we used Landsat imagery for vegetation and aerial survey flights for animals. Following prescribed fire, shrub cover declined and burned communities increased in herbaceous cover. Plant species diversity also declined but increased to almost that of unburned areas by 1 year after burning. Burning increased quality of forage most (2–7 percentage points more digestibility and 0.3–6.3 percentage points more crude protein than unburned areas) in the summer of the burn. In winter, forage biomass and available digestible dry matter increased to pre‐burn levels by 1 year after burning. Stone's sheep and elk always used burned areas more than unburned control areas in winter at both scales. Whereas elk used sites with higher forage quantity, Stone's sheep appeared to respond to forage quality at the fine scale. Ungulate grazing during the 2 years of this study did not alter forage quantity on burned or unburned sites, as determined from range exclosures. Based on our findings in this area, maximum benefits to elk would be achieved from large prescribed fires on south‐aspect slopes that result in the greatest amounts of forage biomass. Smaller prescribed fires at high burn intensities on west‐aspect slopes intermixed with rocky outcrops and talus scree (less frequented by elk) would most benefit Stone's sheep requirements for high‐quality forage and escape terrain. © 2018 The Wildlife Society.

Genome-wide set of SNPs reveals evidence for two glacial refugia and admixture from postglacial recolonization in an alpine ungulate.

Past glaciation events have played a major role in shaping the genetic diversity and distribution of wild sheep in North America. The advancement of glaciers can isolate populations in ice-free refugia, where they can survive until the recession of ice sheets. The major Beringian refugium is thought to have held thinhorn sheep (Ovis dalli) populations during times of glacial advance. While isolation in the major refugium can account for much of the genetic and morphological diversity seen in extant thinhorn sheep populations, mounting evidence suggests the persistence of populations in smaller minor refugia. We investigated the refugial origins of thinhorn sheep using ~10000 SNPs obtained via a cross-species application of the domestic sheep ovine HD BeadChip to genotype 52 thinhorn sheep and five bighorn sheep (O. canadensis) samples. Phylogenetic inference revealed a distinct lineage of thinhorn sheep inhabiting British Columbia, which is consistent with the survival of a group of thinhorn sheep in a minor refugium separate from the Beringian refugium. Isolation in separate glacial refugia probably mediated the evolution of the two thinhorn sheep subspecies, the white Dall's sheep (O. d. dalli), which persisted in Beringia, and the dark Stone's sheep (O. d. stonei), which utilized the minor refugium. We also found the first genetic evidence for admixture between sheep from different glacial refugia in south-central Yukon as a consequence of post glacial expansion and recolonization. These results show that glaciation events can have a major role in the evolution of species inhabiting previously glaciated habitats and the need to look beyond established refugia when examining the evolutionary history of such species.

Changes in horn size of Stone's sheep over four decades correlate with trophy hunting pressure.

Selective harvest may lead to rapid evolutionary change. For large herbivores, trophy hunting removes males with large horns. That artificial selection, operating in opposition to sexual selection, can lead to undesirable consequences for management and conservation. There have been no comparisons of long-term changes in trophy size under contrasting harvest pressures. We analyzed horn measurements of Stone's rams (Ovis dalli stonei) harvested over 37 years in two large regions of British Columbia, Canada, with marked differences in hunting pressure to identify when selective hunting may cause a long-term decrease in horn growth. Under strong selective harvest, horn growth early in life and the number of males harvested declined by 12% and 45%, respectively, over the study period. Horn shape also changed over time: horn length became shorter for a given base circumference, likely because horn base is not a direct target of hunter selection. In contrast, under relatively lower hunting pressure, there were no detectable temporal trends in early horn growth, number of males harvested, or horn length relative to base circumference. Trophy hunting is an important recreational activity and can generate substantial revenues for conservation. By providing a reproductive advantage to males with smaller horns and reducing the availability of desirable trophies, however, excessive harvest may have the undesirable long-term consequences of reducing both the harvest and the horn size of rams. These consequences can be avoided by limiting offtake.

Changes in horn size of Stone's sheep over four decades correlate with trophy hunting pressure

Changes in horn size of Stone's sheep over four decades correlate with trophy hunting pressure

Resource separation by mountain ungulates on a landscape modified by fire

AbstractFire restructures plant communities and can be an important modifier of ecosystems. Increases in forage quantity and quality in burned areas attract large ungulates, and may result in changes to animal distributions. In mountainous northern British Columbia where prescribed fire is used to enhance ungulate range, there is concern that expanding elk (Cervus elaphus) populations will move in response to burning into the traditional ranges of another grazing species, Stone's sheep (Ovis dalli stonei), and have adverse effects on them. We compared patterns of resource selection and use by both species on a landscape with 138 prescribed burns varying from 0 to 31 years of age. Seasonal range sizes of global positioning system (GPS)‐collared individuals were smallest in winter and late winter and largest in summer for both female Stone's sheep and elk. Both species selected for south aspects and against conifer stands in all seasons. Stone's sheep selected for burned areas in fall, winter, and late winter and selected to be close to a burn in every season except summer. Elk selected for burned areas in every season, with the highest selection for burn‐shrub areas. Stone's sheep typically used younger burns, whereas elk were less specific and often used older burns. The highest potential for seasonal overlap in the use of burned areas occurs during winter and late winter, but Stone's sheep and elk at current population levels overlap minimally because they partition their use of the landscape through elevation and topography. Stone's sheep always selected and used steeper rugged terrain than elk, and were at higher elevations, often in rocky areas. Elk avoided alpine and rocky areas, and were at lower elevations than Stone's sheep, on flatter less rugged terrain. We recommend continued monitoring of post‐fire effects on resource selection and use by these sympatric grazers, particularly relative to the distribution of expanding elk populations, which could enable competitive interactions and change predator‐prey dynamics. © 2015 The Wildlife Society.

Seasonal habitat use and selection by grizzly bears in Northern British Columbia

AbstractWe defined patterns of habitat use and selection by female grizzly bears (Ursus arctos) in the Besa‐Prophet watershed of northern British Columbia. We fitted 13 adult females with Geographic Positioning System (GPS) radio‐collars and monitored them between 2001 and 2004. We examined patterns of habitat selection by grizzly bears relative to topographical attributes and 3 potential surrogates of food availability: land‐cover class, vegetation biomass or quality (as measured by the Normalized Difference Vegetation Index), and selection value for prey species themselves (moose [Alces alces], elk [Cervus elaphus], woodland caribou [Rangifer tarandus], Stone's sheep [Ovis dalli stonei]). Although vegetation biomass and quality, and selection values for prey were important in seasonal selection by some individual bears, land‐cover class, elevation, aspect, and vegetation diversity most influenced patterns of habitat selection across grizzly bears, which rely on availability of plant foods and encounters with ungulate prey. Grizzly bears as a group avoided conifer stands and areas of low vegetation diversity, and selected for burned land‐cover classes and high vegetation diversity across seasons. They also selected mid elevations from what was available within seasonal ranges. Quantifying relative use of different attributes helped place selection patterns within the context of the landscape. Grizzly bears used higher elevations (1,595 ± 31 m SE) in spring and lower elevations (1,436 ± 27 m) in fall; the range of average elevations used among individuals was highest (500 m) during the summer. During all seasons, grizzly bears most frequented aspects with high solar gain. Use was distributed across 10 land‐cover classes and depended on season. Management and conservation actions must maintain a diverse habitat matrix distributed across a large elevational gradient to ensure persistence of grizzly bears as levels of human access increase in the northern Rocky Mountains. © 2011 The Wildlife Society.

Habitat selection by a focal predator (Canis lupus) in a multiprey ecosystem of the northern Rockies

Abstract Large predators respond to land cover and physiography that maximize the likelihood of encountering prey. Using locations from global positioning system–collared wolves (Canis lupus), we examined whether land cover, vegetation productivity or change, or habitat-selection value for ungulate prey species themselves most influenced patterns of selection by wolves in a large, intact multiprey system of northern British Columbia. Selection models based on land cover, in combination with topographical features, consistently outperformed models based on indexes of vegetation quantity and quality (using normalized difference vegetation index) or on selection value to prey species (moose [Alces americanus], elk [Cervus elaphus], woodland caribou [Rangifer tarandus], and Stone's sheep [Ovis dalli stonei]). Wolves generally selected for shrub communities and high diversity of cover across seasons and avoided conifer stands and non-vegetated areas and west aspects year-round. Seasonal selection strategies we...

Using stable isotopes to define diets of wolves in northern British Columbia, Canada

Wolves (Canis lupus), as both opportunistic and specialist predators, can limit and regulate ungulate dynamics. As part of understanding predator–prey interactions in the largely undisturbed system of the Besa-Prophet area in northern British Columbia, we used stable isotopes of carbon and nitrogen to infer seasonal diets of 5 wolf packs. We selected the hair, tissue, or blood sample of each prey species that could best index within-season diet composition. Seasonal isotopic differences for a given sample type were as much as 0.28% d 13 C and 0.97% d 15 N. The large biomass species of moose (Alces americanus) and elk (Cervus elaphus) dominated the diets of wolves, but caribou (Rangifer tarandus) and Stone’s sheep (Ovis dalli stonei) also were locally or seasonally important to some packs. Mean isotopic determinations of summer food habits were correlated positively (P , 0.001) with proportions of prey by species determined from scat samples. This general agreement lends support for the tissue to diet discrimination values used in the Bayesian modeling and indicates that the longer-term dietary estimates from stable isotopes were reflective of shorter-term recent ingestion. Although moose have been assumed to be the most important prey item for wolves throughout the year in northern British Columbia, our results indicate that dietary dynamics of wolves in the Besa-Prophet area are more complex than previously reported.

Bilateral Perirenal Hemorrhage in two Stone's Sheep (Ovis Dalli Stonei): A Possible Manifestation of Malignant Catarrhal Fever

Malignant catarrhal fever (MCF)-like disease was diagnosed at postmortem in 2 Stone's sheep (Ovis dalli stonei). On gross examination, the predominant abnormality in both sheep was severe perirenal hemorrhage and multiple renal infarcts. Microscopically, there was severe, multisystemic lymphocytic arteritis. Both sheep were positive for Ovine herpesvirus 2 (OvHV-2) on polymerase chain reaction, and partial sequencing of the viral DNA polymerase genes from the 2 sheep revealed >99% homology, with 96% similarity to the reference GenBank OvHV-2 viral sequence. Based on the histological lesions, polymerase chain reaction results, and viral DNA polymerase gene sequencing, a diagnosis of OvHV-2-mediated MCF was made. Massive perirenal hemorrhage has not been described previously as a manifestation of MCF.

Coat darkness is associated with social dominance and mating behaviour in a mountain sheep hybrid lineage

Natural hybridization can produce novel traits when morphologically different populations hybridize, and can introduce variation in traits that become associated with sexual selection. Evidence from breeding experiments and genetic markers indicate that the great variation in coat darkness and the unique coat patterns found in Stone's sheep Ovis dalli stonei , populations, have resulted from an ancient hybridization event between thinhorn sheep, O. dalli, and bighorn sheep, O. canadensis . Behavioural evidence gathered in 2003 and 2004 in Yukon Territory, Canada, showed that higher dominance rank was correlated with increasing darkness in rams, and comparatively darker rams were seen more often tending oestrous ewes. The relationship between ram darkness and dominance and use of the tending mating tactic was still significant after age or horn size were controlled. In sum, our study demonstrates an interesting example of the potential for hybridization to generate diversity. In the case of mountain sheep an ancestral hybridization has resulted in a subspecies with unique coat colour characteristics that are associated with social dominance and mating behaviour.

Use of Natural Licks by Four Species of Ungulates in Northern British Columbia

Use of licks influences the movements and distribution of ungulates. We recorded attendance patterns, duration of visits to licks, and time spent licking by 4 ungulate species at wet and dry licks to examine possible influences on the timing of use of licks in northern British Columbia, Canada. Within-species licking intensity (based on regressions between time spent licking per visit and duration of visits to licks) was higher for elk (Cervus elaphus) than for moose (Alces alces) at wet licks, and higher for mountain goats (Oreamnos americanus) than for Stone’s sheep (Ovis dalli stonei) at dry licks. Lick response variables (duration of visits to licks, time spent licking per visit, and proportion of time spent licking) did not vary significantly among early, mid-, and late summer seasons, but there were species-specific differences in the timing of highest attendance. High use of wet licks by both female and male elk in late May followed vegetation greening at low elevations. Average attendance by female elk was highest in late June, coinciding with high lactation demands. Attendance by moose at wet licks was highest in mid-July, potentially coinciding with other aspects of forage phenology such as increased plant defensive compounds. Attendance by Stone’s sheep and mountain goats at dry licks was high in early July, following forage change at high elevations and again in early August, potentially related to the tradeoff between lactation demands and predation risk. Across species, consumption of lick soils allows ungulates to improve rumen function and nutrient absorption during the transition to spring and summer forage and to supplement elemental intake by females during the nutritional stress associated with lactation.

PROTOSTRONGYLID PARASITES AND PNEUMONIA IN CAPTIVE AND WILD THINHORN SHEEP (OVIS DALLI)

We describe health significance of protostrongylid parasites (Parelaphostrongylus odocoilei and Protostrongylus stilesi) and other respiratory pathogens in more than 50 naturally infected Dall's sheep (Ovis dalli dalli) from the Mackenzie Mountains, Northwest Territories (1998-2002) as well as in three Stone's sheep (O. d. stonei) experimentally infected with P. odocoilei (2000-2002). Histological lesions in the brain and distribution of P. odocoilei in the muscles of experimentally and naturally infected sheep were consistent with a previously hypothesized "central nervous system to muscle" pattern of migration for P. odocoilei. Dimensions of granulomas associated with eggs of P. odocoilei and density of protostrongylid eggs and larvae in the cranial lung correlated with intensity of larvae in feces, and all varied with season of collection. Prevalence of P. stilesi based on the presence of larvae in feces underestimated true prevalence (based on examination of lungs) in wild Dall's sheep collected in summer and fall. Similarly, counts of both types of protostrongylid larvae in feces were unreliable indicators of parasitic infection in wild Dall's sheep with concomitant bacterial pneumonia associated with Arcanobacterium pyogenes, Pasteurella sp., and Mannheimia sp. Diffuse, interstitial pneumonia due to P. odocoilei led to fatal pulmonary hemorrhage and edema after exertion in one experimentally infected Stone's sheep and one naturally infected Dall's sheep. Bacterial and verminous pneumonia associated with pathogens endemic in wild Dall's sheep in the Mackenzie Mountains caused sporadic mortalities. There was no evidence of respiratory viruses or bacterial strains associated with domestic ruminants, from which this population of wild sheep has been historically isolated.

Habitat selection by female Stone's sheep in relation to vegetation, topography, and risk of predation

Stone's sheep (Ovis dalli stonei) are susceptible to anthropogenic disturbances, but few data have been available to help minimize impacts. We used the movements of individuals to define use and availability, and resource-selection functions (RSF) and the information-theoretic approach to examine seasonal habitat selection and interannual variation in selection of attributes by groups of Stone's sheep. Movement rates of Stone's sheep followed consistent yearly patterns, with highest rates occurring in summer and fall. Models that contained vegetation, topography, and risk of predation best explained resource selection. Topographic features ranked better, however, than components of vegetation or risk of predation from grizzly bears (Ursus arctos) and wolves (Canis lupus) at explaining habitat selection. Selection strategies were variable among groups of Stone's sheep and between years within groups even though consistencies in selection for steep slopes, ridge-like topography, south aspects, and upper elevations were common. This research provides the first comprehensive analysis of habitat selection by Stone's sheep, which show strong fidelity to seasonal ranges but also exhibit behavioural plasticity in selection of attributes within those ranges.

CHEMICAL COMPOSITION OF LICK SOILS: FUNCTIONS OF SOIL INGESTION BY FOUR UNGULATE SPECIES

Perspectives on the importance of natural salt licks to ungulates have been broadened beyond the role of providing sodium. This study examined the chemical compositions of wet and dry licks in north-central British Columbia, and defined the benefits of licks to elk (Cervus elaphus), moose (Alces alces), Stone’s sheep (Ovis dalli stonei), and mountain goats (Oreamnos americanus). We analyzed soils for buffering compounds (carbonates) and estimated available elements from extractions with solutions at a low, digestive-tract pH. Even though the 2 types of licks are visually distinct, with different concentrations of chemical components, they serve similar functions as concentrated sources of sodium, carbonates, magnesium, and sulfate. Sodium and sulfate concentrations were typically higher at both wet and dry licks than control sites. Carbonates and magnesium were elevated in soils from dry licks and magnesium also was high in inflow waters to wet licks. We estimated elemental intake by ungulates from the composition of forage samples. Forages used by all 4 ungulate species were too low in sodium to meet requirements. Spring and summer forages contained high potassium levels. Licks in our study, therefore, provide ungulates with supplemental sources of sodium that are particularly beneficial to offset increasing demands during lactation and with carbonates to help stabilize rumen pH after forage changes in spring. Supplemental sources of magnesium may be actively sought by ungulates when high levels of dietary potassium affect absorption. To assess the importance of clay in soils ingested at licks, we determined the clay mineral types at licks and compared the buffering capacity of clay-filled fecal material collected at licks with fecal material collected away from licks. Further studies are needed to define the roles of clay in improving forage palatability and digestibility.

Behaviour, habitat associations, and intrasexual differences of female Stone’s sheep

Stone’s sheep ( Ovis dalli stonei Allen, 1897) in northern British Columbia segregate sexually during most of the year, and intrasexually between maternal and nonmaternal females during spring and early summer. Our objective was to quantify intrasexual habitat use of female Stone’s sheep relative to maternal status using measures of behaviour and habitat use. We reviewed three hypotheses of intersexual segregation (predation-risk, forage-selection, and activity-budget hypotheses) to determine if they also explained intrasexual segregation of female Stone’s sheep. Female Stone’s sheep spent the majority of their active time foraging. Nursery groups spent shorter durations of time active, more time active in solid rock escape features, and less time active in shrub habitat than nonmaternal groups. The best predictive model describing intrasexual differences incorporated distance to nearest escape feature and size of nearest escape feature. Regardless of maternal status, female Stone’s sheep used low-elevation plant communities early in the growing season that were characterized by more shrub species and increased cover. As the growing season progressed, they tracked a phenological stage, moving up in elevation and associating with communities that contained increasing amounts of moss and lichen cover. The patterns in behaviour, habitat use, and vegetation associations between intrasexual groups of Stone’s sheep were best characterized by the predation-risk hypothesis.

Phylogenetic Analysis of Snow Sheep (Ovis nivicola) and Closely Related Taxa

Based on mitochondrial cytochrome b gene sequence analysis, the history of true sheep (Ovis) began approximately 3.12 million years ago (MYA). The evolution of Ovis resulted in three generally accepted genetic groups: Argaliforms, Moufloniforms, and Pachyceriforms. The Pachyceriforms of the subgenus Pachyceros comprise the thin-horn sheep Ovis nivicola (snow sheep), Ovis dalli (Dall and Stone sheep), and Ovis canadensis (Rocky Mountain and desert bighorn). North America wild sheep (O. canadensis and O. dalli) evolved separately from Eurasian wild sheep and diverged from each other about 1.41 MYA. Ancestral stock that gave rise to snow sheep, Moufloniforms, and Argaliforms occurred 2.3 MYA, which then gave rise to two different extant lines of snow sheep that diverged from each other about 1.96 MYA. The more recent nivicola line is genetically closer to the North American wild sheep and may represent a close association during the refugium when Alaska and Siberia were connected by the Bering land bridge. The earlier period of evolution of the Pachyceriforms suggests they may have first evolved in Eurasia, the oldest ancestor then giving rise to North American wild sheep, and that a canadensis-like ancestor most likely gave rise to nivicola. Cytogenetic analysis further validates that the standard diploid number for modern nivicola is 52.

DEVELOPMENT AND PATHOGENESIS OF PARELAPHOSTRONGYLUS ODOCOILEI (NEMATODA: PROTOSTRONGYLIDAE) IN EXPERIMENTALLY INFECTED THINHORN SHEEP (OVIS DALLI)

Recently, the protostrongylid nematode Parelaphostrongylus odocoilei has been reported in a new host species, thinhorn sheep (Ovis dalli). For the first time, we completed the life cycle of P. odocoilei in three Stone's sheep (O. dalli stonei) and two thinhorn hybrids (O. dalli stonei x O. dalli dalli), each infected with 200 third-stage larvae from slugs (Deroceras laeve). The prepatent period ranged from 68 days to 74 days, and shedding of first-stage larvae (L1) peaked at >10,000 L1 per gram of feces between 90 and 110 days postinfection. A total of 75, 27, and 14 adult P. odocoilei were recovered from skeletal muscles of three Stone's sheep. Starting in the prepatent period, all infected sheep lost weight and developed peripheral eosinophilia. At 2 wk before patency, two thinhorn hybrids developed neurologic signs (hind end ataxia, loss of conscious proprioception, and hyperesthesia) that resolved at patency. Eosinophilic pleocytosis and antibody to Parelaphostrongylus spp. were detected in the cerebrospinal fluid of the affected sheep, suggesting that the migration route of the "muscleworm" P. odocoilei may involve the central nervous system. Twenty days after treatment with ivermectin, neurologic signs recurred and larval shedding ceased in one infected thinhorn hybrid, whereas multiple treatments transiently suppressed but did not eliminate larval shedding in the other. During patency, two Stone's sheep with numerous eggs and larvae of P. odocoilei in the lungs died of respiratory failure following anesthesia or exertion. Parelaphostrongylus odocoilei has widespread geographic distribution, high prevalence, the possibility of causing neurologic and respiratory disease, resistance to treatment, and may constitute a significant emerging disease risk for thinhorn sheep.