Dall sheep
The Dall sheep (Ovis dalli), a subspecies of thinhorn sheep, is a wild sheep species native to the alpine mountain ranges of Alaska and northwestern Canada, including the Yukon and parts of British Columbia and the Northwest Territories.[1][2] Distinguished by its white or creamy wool coat and lack of facial markings unlike darker sheep species, it inhabits steep, rocky terrains at elevations often exceeding 2,000 meters, where it forages on grasses, sedges, and lichens.[1][3] Adult rams feature massive, keratin-based horns that curl backward and flare outward, potentially reaching spreads of 33 inches after 7-8 years of growth, serving for territorial defense and mating rivalries during the fall rut.[1][4] Ewes bear shorter, more slender horns, and both sexes form herds for predator avoidance, with rams often solitary outside breeding season.[5] Classified as Least Concern by the IUCN due to stable populations in largely undisturbed habitats, the species faces localized pressures from hunting and habitat fragmentation, though regulated trophy hunting targets mature rams to sustain herds.[6][7]Taxonomy and genetics
Classification and nomenclature
The Dall sheep (Ovis dalli) is a species within the genus Ovis, recognized under binomial nomenclature as described by Edward William Nelson in 1884.[8] Its taxonomic classification places it in the kingdom Animalia, phylum Chordata, class Mammalia, order Artiodactyla, family Bovidae, subfamily Caprinae, and genus Ovis.[5] The species is distinguished from related North American sheep like the bighorn (Ovis canadensis) by its thinner horns and predominantly white pelage in the nominate subspecies.[9] The specific epithet dalli honors William Healey Dall (1845–1927), an American naturalist and explorer who contributed to Alaskan fauna studies, though he did not describe the species himself.[10] Early descriptions sometimes subsumed it under Ovis montana dalli, reflecting historical lumping with Rocky Mountain sheep taxa, but modern taxonomy elevates it to full species status based on morphological and genetic distinctions.[11] Common names include Dall's sheep (reflecting the eponymous honorific) and thinhorn sheep, the latter emphasizing the slender, flared horns compared to thicker bighorn counterparts.[5] Indigenous names vary, such as Imnaiq in coastal Inupiaq dialects of Alaska.[12]Subspecies distinctions
The species Ovis dalli, known as thinhorn sheep, includes two principal subspecies distinguished by pelage coloration, horn morphology shared with the species level, and largely allopatric distributions. The nominate subspecies O. d. dalli (Dall's sheep) features a predominantly white coat in adults, with lambs showing light gray pelage that lightens over time, and occupies alpine habitats across Alaska and northern Yukon Territory.[5] In contrast, O. d. stonei (Stone's sheep) exhibits a darker pelage ranging from slate gray to nearly black, often with blackish markings on the legs, face, and back, and a white rump patch in some individuals, primarily inhabiting mountainous regions in southern Yukon Territory and northern British Columbia.[5][13] These morphological distinctions, particularly in coat color, serve as the traditional basis for subspecies classification, though intermediate forms occur in zones of contact, such as parts of Yukon where intergradation produces variably colored individuals historically termed Fannin sheep.[14] Horns in both subspecies are slender, sharply curved, and flared at the bases—thinner than those of bighorn sheep (Ovis canadensis)—with males averaging 80–100 cm in length and females smaller.[15] Genetic analyses, including microsatellite and mitochondrial DNA studies, reveal population structuring that aligns with these subspecies boundaries, indicating moderate genetic differentiation despite ongoing gene flow in overlap areas.[16] Such evidence supports the validity of the subspecies designations, though some clinal variation suggests color differences may partly reflect adaptive responses to local environments rather than strict genetic isolation.[17]| Subspecies | Adult Coat Color | Primary Geographic Range |
|---|---|---|
| O. d. dalli | White to off-white | Alaska; northern Yukon and Northwest Territories |
| O. d. stonei | Slate gray to black, with markings | Southern Yukon; northern British Columbia |
Genetic diversity and population structure
Thinhorn sheep (Ovis dalli), encompassing Dall's sheep (O. d. dalli) and Stone's sheep (O. d. stonei), exhibit high levels of genetic diversity within populations, with expected heterozygosity (HE) averaging 0.722 across 919 individuals genotyped at 12 microsatellite loci from 24 sampled areas in Alaska and Yukon.[16] This diversity reflects the species' occupation of much of its historical range without major bottlenecks, though localized isolation contributes to variability.[17] Population structure shows moderate genetic differentiation among regions (FST = 0.160), driven by isolation-by-distance patterns (r = 0.75, P < 0.001) and barriers like mountain ranges that restrict dispersal, delineating at least eight subpopulations.[16] Subsequent analyses using 153 single-nucleotide polymorphisms (SNPs) on approximately 2,000 harvested rams identified genetic clusters largely aligning with subspecies color morphs, but with boundaries diverging from traditional maps due to historical glacial refugia and post-glacial expansion.[18] Admixed zones, including Fannin sheep populations, display hybrid genotypes with predominant Dall's ancestry from secondary contact between subspecies.[19] For Stone's sheep, genotyping of ~2,800 individuals at 123 DNA markers revealed finer-scale differentiation, indicating 1–2 interconnected populations with gene flow across borders but smaller effective sizes than inferred from visual surveys, as admixed individuals were often misclassified as Dall's, reducing estimates by ~20%.[20][21] These patterns underscore limited philopatry and topography's role in maintaining structure, informing management to preserve connectivity amid habitat fragmentation.[16]Physical description
Morphology and measurements
Dall sheep (Ovis dalli) possess a stocky build suited to alpine environments, featuring a compact torso, short legs relative to body size, and cloven hooves with prominent dewclaws that enhance grip on steep, rocky terrain.[5] The pelage comprises a dense woolly undercoat overlaid with longer, coarser guard hairs, offering thermal insulation and some camouflage against snowy backdrops.[22] In the nominate subspecies O. d. dalli, the coat is uniformly white or cream-colored, accented by black facial markings around the muzzle, eyes, and nostrils; the subspecies O. d. stonei exhibits darker gray to black pelage with white underparts and leg interiors, though body proportions remain similar across forms.[5][23] Sexual dimorphism is pronounced, with rams substantially larger than ewes. Adult rams typically measure 130–180 cm in head-body length, stand 91–102 cm at the shoulder, and weigh 72–82 kg, though exceptional individuals may exceed 90 kg.[24][25][22] Ewes are smaller, with head-body lengths of 132–162 cm, comparable shoulder heights but masses of 45–50 kg on average.[5][22] Lambs at birth weigh approximately 4–5 kg and grow rapidly, reaching 20–25 kg by weaning at 3–4 months.[26] Tail length is short, measuring about 10 cm in adults of both sexes.[24] Seasonal body mass fluctuations occur, with losses up to 16% in winter due to nutritional stress.[22]
Horn characteristics and sexual dimorphism
Dall sheep horns consist of a bony core covered by a keratin sheath and grow continuously from the base throughout the animal's life, with growth accelerating in spring and summer and halting in winter, forming visible annuli that serve as age indicators similar to tree rings.[15] Horns are typically amber or tan in color.[27] Male Dall sheep, or rams, possess massive horns that flare outward before curling backward in a tight spiral, often achieving a full curl after seven to eight years of growth.[28] Mature ram horns commonly measure around 33 inches in length for full-curl specimens, though exceptional individuals exceed 40 inches.[29] These horns develop rapidly after the first year, distinguishing rams from ewes by age three.[22] Female Dall sheep, or ewes, exhibit slender, gently backswept horns that curve only slightly and remain shorter and less robust than those of rams.[15] Ewe horns typically do not exceed 15 inches in length and lack the pronounced flaring and tight curling seen in males.[30] Sexual dimorphism in Dall sheep horns is pronounced, with rams' larger, more elaborate structures adapted for intrasexual combat, while ewes' smaller horns provide minimal secondary sexual traits.[30] This dimorphism emerges post-maturity, as juvenile rams initially resemble ewes in horn morphology before divergent growth patterns manifest.[22] Both sexes retain horns permanently, unlike antlers in deer, underscoring the species' caprine heritage.[15]Distribution and habitat
Geographic range
The Dall sheep (Ovis dalli), also known as thinhorn sheep, occupies alpine and subalpine habitats across northwestern North America. Its distribution is confined to rugged mountain ranges in Alaska and adjacent Canadian territories, where populations are estimated at approximately 100,000 individuals as of recent surveys.[31][5] In Alaska, Dall sheep inhabit extensive areas of the Brooks Range, Alaska Range, Chugach Mountains, and Kenai Peninsula, marking the southern limit of their range in the state.[1][2] These populations extend northward into the Yukon Territory, covering much of its mountainous terrain.[5] Further east, sheep occur in the Mackenzie Mountains of the western Northwest Territories.[2][32] The southeastern extent reaches the extreme northwestern corner of British Columbia, particularly in the Stikine Plateau and Cassiar Mountains, though densities are lower compared to core Alaskan and Yukon areas.[5][11] No viable populations exist outside this contiguous range, with historical records confirming absence in regions south of the Kenai Peninsula or east of the Mackenzie Mountains.[33][34]Preferred habitats and adaptations
Dall sheep inhabit rugged alpine terrains in the mountain ranges of Alaska and northwestern Canada, preferring relatively dry, open habitats consisting of alpine ridges, meadows, and steep slopes featuring cliffs and crags that serve as escape terrain from predators.[15] These environments provide high visibility for detecting threats and access to forage while minimizing cover for ambush predators.[35] Sheep generally avoid forested or shrubby areas, venturing into them only briefly during extreme weather, as such habitats limit mobility and increase predation risk.[15] Elevational preferences range from sea level to over 1,800 meters, but Dall sheep most commonly occupy sites between 300 and 1,500 meters, shifting to higher altitudes in summer for cooler conditions and peak forage availability, and lower elevations in winter for reduced snow depth and milder temperatures.[22] Habitat selection emphasizes steep slopes (often exceeding 30 degrees) and high terrain ruggedness near precipitous cliffs, which facilitate rapid evasion of carnivores like wolves and bears.[35] In winter and spring, they favor south-facing aspects, shallower snow cover, and areas with higher vegetation greenness to balance security, thermal moderation, and energetic efficiency in locomotion, selecting slopes under 45 degrees to conserve energy during movement.[35] Adaptations enabling exploitation of these precipitous habitats include a lightweight build relative to other sheep species, enhancing agility on sheer rock faces, and hooves with cleft structures and grippy pads that provide superior traction on slick, uneven surfaces.[5] Their dense underwool and guard hairs offer insulation against subzero temperatures and high winds, while behavioral strategies such as grouping on exposed ridges amplify vigilance against predators.[36] These traits, combined with precise habitat choice, allow Dall sheep to persist in environments with limited vegetative cover and severe climatic extremes, where forage quality fluctuates seasonally but security remains paramount.[35]Behavior and natural history
Social structure and group dynamics
Dall sheep (Ovis dalli) display a social structure marked by pronounced sexual segregation outside the breeding season, with adult rams forming all-male bachelor groups and ewes associating primarily with lambs and yearlings in nursery groups.[37] This segregation persists year-round except during the rut, from mid-November to mid-December, when rams join ewe groups to compete for mates.[37] Young rams typically remain with maternal groups until about two years of age, after which developing horns prompt their transition to bachelor bands.[38] Bachelor groups, often comprising up to 15 mature rams, establish linear dominance hierarchies through agonistic interactions including head-on horn clashes, bluff charges, leg kicks, and mounting displays, which intensify prior to the rut to determine access to ewes.[37] [39] Older, larger rams generally lead these groups, influencing movement and resource access.[40] Ewe groups exhibit matrilineal bonds and cooperative behaviors, such as nursery formations where one or more ewes provide vigilance over multiple lambs while others forage, enhancing collective predator detection in alpine environments.[37] [5] Group sizes vary by season, habitat quality, and composition, with mean sizes reported as approximately 3.4 individuals for ram groups and 4.3 for ewe groups in late summer observations, though larger aggregations of 50 or more can form in optimal foraging areas.[41] Sexual segregation aligns with differences in risk tolerance and foraging needs, as ewes select steeper, rockier terrains for lamb protection against predators like wolves and grizzly bears, while rams occupy broader ranges prioritizing forage availability.[42] [37] Larger group sizes correlate with increased travel distances for resources and amplified anti-predator benefits through shared vigilance.[37] Post-rut, rams and ewes resume segregation, with juveniles practicing dominance behaviors through play-fighting to prepare for adult social roles.[37] This fission-fusion dynamic supports survival in rugged, predator-rich habitats by balancing foraging efficiency, mate competition, and threat avoidance.[37]Foraging and diet
Dall sheep (Ovis dalli) are herbivores that primarily graze on graminoids, including grasses and sedges, which constitute approximately 70% of their diet, supplemented by forbs, lichens, mosses, and shrub leaves or stems.[43][22] Diets vary by region and season, with summer foraging featuring a broad selection of available alpine plants due to high abundance, while winter rations are more limited to wind-exposed dry grasses, sedge stems, lichens, and mosses.[26][15] In the Brooks Range of Alaska, female Dall sheep exhibited seasonal dietary shifts, relying mainly on grasses in early winter, transitioning to sedges in February, and returning to grasses in early spring, with shrubs forming a consistently minor component.[44] Such selectivity reflects adaptations to nutritional availability in harsh alpine environments, where forage quality influences habitat use and energy balance.[35] Dall sheep regularly visit natural mineral licks, especially in spring and summer, to ingest soil rich in sodium, magnesium, and other elements deficient in vascular plants, often traveling many miles despite predation risks, which facilitates inter-band mixing and gene flow.[26][45] High potassium in emerging spring forage exacerbates sodium imbalances, driving lick use to maintain electrolyte homeostasis.[45] Foraging typically occurs on open slopes and meadows accessible via agile climbing, prioritizing security and forage patches over dense cover.[22]Reproduction and life history
Dall sheep exhibit a distinct breeding season, or rut, occurring from November to early December, during which mature rams compete aggressively through horn-clashing displays to establish dominance and access to ewes.[1] Ewes typically breed annually, with pregnancy rates varying by population health and environmental conditions; in monitored Alaska populations, rates have ranged from 44% to 76% in recent studies.[46] Gestation lasts approximately 173 days, or 5.5 to 6 months.[5] Lambing occurs primarily in late May to early June, with peak births in mid-May in central Alaska populations; ewes isolate themselves in steep, rugged cliffs to minimize predation risks during this vulnerable period.[1][47] Litters consist of a single lamb in nearly all cases, though twins are occasionally documented at rates below 5%.[5] Neonatal lambs weigh around 4-5 kg at birth and exhibit rapid growth, reaching weaning at 3-5 months when they consume solid forage alongside milk.[23] Sexual maturity is attained by ewes around 2-3 years of age, with many first breeding in their third year, while rams mature similarly but rarely secure mates until 4-7 years due to dominance hierarchies favoring older males.[48][49] Lamb survival to one year averages 15-30% in predator-influenced areas, impacted by factors such as weather and forage availability, with ewes forming nursery groups post-lambing to enhance collective vigilance.[50] In the wild, Dall sheep lifespan rarely exceeds 12 years, though exceptional individuals reach 16 for rams and 19 for ewes; mortality accelerates after age 8 due to dental wear and predation.[1][5]Ecology and interactions
Predation and mortality factors
Primary predators of Dall sheep (Ovis dalli) include gray wolves (Canis lupus), grizzly bears (Ursus arctos horribilis), coyotes (Canis latrans), and golden eagles (Aquila chrysaetos), with impacts varying by region and age class.[51][52] Predation disproportionately affects lambs, where neonatal mortality rates can reach 30-50% in the first few months, primarily from coyotes and eagles in coastal and central Alaskan ranges.[53][52] In interior populations with denser wolf and bear presence, such as the Richardson Mountains, these larger carnivores account for most documented kills across age classes, contributing to population declines observed since the 1990s.[51][54] Coyote predation on lambs predominates in studies from the Alaska Range and Chugach Mountains, comprising up to 43% of radiocollared lamb deaths, often within the first two weeks post-birth when ewes isolate for lambing.[53][55] Golden eagles target vulnerable neonates on open slopes, documented as a secondary but significant cause, with kills peaking in May-June.[53][52] Wolves primarily prey on ewes and rams in rugged terrain, exerting selective pressure that favors individuals with superior escape abilities, while bears opportunistically take lambs and weakened adults during foraging overlaps.[56][51] Adult survival exceeds 85-90% annually in low-predation areas, but drops in wolf-dominated habitats like Denali, where packs historically limited populations by targeting solitary rams or separated ewes.[57][56] Indirect predation effects arise from cyclic prey like snowshoe hares, which boost coyote numbers during peaks, elevating lamb mortality; hare lows correspondingly reduce coyote survival and reproduction, improving sheep lamb recruitment.[58] Non-predatory mortality factors include avalanches, falls from cliffs, starvation during harsh winters, and late snowmelt, which delays forage access and increases exposure for gestating ewes and yearlings—potentially raising overall die-offs by 20-30% with one-month delays.[59][60] These stressors compound predation risks, as weakened sheep become more vulnerable, though direct predation remains the leading verifiable cause of death in monitored cohorts.[61][55]Disease and parasites
Dall's sheep (Ovis dalli) host a variety of gastrointestinal helminths, with nine species identified in Yukon populations, including the abomasal nematode Marshallagia marshalli exhibiting the highest prevalence (up to 100% in some samples) and infection intensity.[62] [63] These parasites generally impose low burdens and do not appear to significantly impair host fitness, though seasonal variations in abundance correlate with forage availability and weather conditions.[62] Protostrongylid nematodes, such as the meningeal lungworm Parelaphostrongylus odocoilei and the pulmonary lungworm Protostrongylus stilesi, infect Dall's sheep across their range, with P. stilesi prevalent in Northwest Territories populations.[64] [43] P. odocoilei, transmitted via gastropod intermediates like slugs and snails, can migrate to the central nervous system and lungs, contributing to verminous pneumonia and respiratory distress, particularly in first discovered in Alaskan Dall's sheep in 1999.[64] [65] These parasites are endemic and associated with sporadic bacterial co-infections leading to pneumonia in both captive and wild thinhorn sheep (O. dalli), though mortality is typically low in free-ranging populations.[66] [67] Viral diseases include contagious ecthyma (orf), caused by the orf virus, which manifests as proliferative skin lesions around the mouth and feet, most severely affecting lambs and occurring throughout Alaska where Dall's sheep inhabit.[68] [69] Secondary bacterial infections often exacerbate these lesions, but the condition is self-limiting in adults and does not typically cause population-level impacts.[69] Bacterial pneumonia agents, notably Mycoplasma ovipneumoniae, have been detected in captive Dall's sheep during epizootics, such as one in the late 1980s resulting in high morbidity from respiratory stress and secondary infections.[70] In wild Alaskan populations, however, M. ovipneumoniae occurs in healthy individuals without observed illness or die-offs, distinguishing Dall's sheep from more susceptible bighorn sheep (Ovis canadensis).[71] Endemic bacterial pathogens contribute to verminous pneumonia alongside protostrongylids, but overall, Dall's sheep exhibit robust health with parasites and diseases rarely driving significant mortality.[66] [43]Competition with other species
Dall sheep (Ovis dalli) inhabit steep, rocky alpine terrains in Alaska and Yukon that limit overlap with most other ungulates, resulting in minimal documented interspecific competition for forage or space. Primary potential competitors include mountain goats (Oreamnos americanus), which co-occur in select coastal ranges such as the Kenai Peninsula, where both species graze on overlapping resources like grasses (Festuca spp.), sedges (Carex spp.), and lichens during summer.[72] However, mountain goats preferentially select even steeper cliffs and escape terrain, facilitating niche partitioning that reduces direct resource conflict.[73] Lower-elevation herbivores like moose (Alces alces) and caribou (Rangifer tarandus) show negligible competitive interaction with Dall sheep, as their foraging occurs primarily in valleys and tundra below the sheep's preferred elevations above 1,500 meters.[35] Empirical studies attribute Dall sheep population dynamics more to predation, weather extremes, and intrinsic forage limitations than to interspecific competition, with no evidence of density-dependent effects from co-occurring species driving declines.[51] In the Northern Richardson Mountains, potential resource overlap with other ungulates has been noted but not quantified as a limiting factor.[35]Population dynamics
Historical abundance and declines
Dall sheep populations in Alaska were estimated at around 75,000 individuals during the 1960s through the 1980s, reflecting relative stability in suitable alpine habitats prior to significant modern declines.[74] By the mid-1980s, surveys indicated approximately 73,000 sheep statewide.[75] Earlier historical abundance remains poorly quantified due to limited pre-20th-century data, though indigenous subsistence hunting, documented since at least the 16th century, suggests sustained presence without evidence of overexploitation leading to range-wide crashes.[1] Statewide estimates declined 21% from about 56,740 sheep in 1990 to 45,010 in 2010, with more pronounced drops in specific regions thereafter.[76] In the Baird Mountains of Noatak National Preserve, adult sheep numbers fell 50% from 800 in 1989 to under 400 by 1991.[77] Further surveys in Arctic parks showed a 65% reduction in Noatak from 2011 to 2014, with estimates dropping to 784 adults (95% CI: 583-1,080).[78] Across Alaska, recent declines range from 50-70% in various ranges since the 1990s, though populations remained stable or at historical highs through the early 1990s before downturns accelerated.[79][80] These declines are primarily linked to environmental factors rather than harvest pressure, as annual hunting takes (typically under 1,000 sheep) represent a small fraction of populations and have not correlated with observed drops.[75] Severe winters, including deep snow persistence, rain-on-snow events, and late springs, reduce forage availability and increase mortality, particularly among lambs and weaker individuals.[81][82] Such stochastic weather variability, to which alpine-adapted Dall sheep are highly sensitive, drives natural fluctuations, with recent patterns suggesting amplified impacts from climatic shifts.[78][83] Predation by wolves and other carnivores contributes to mortality but has not reversed declines despite localized control efforts.[51] Disease outbreaks, such as Mycoplasma ovipneumoniae, pose emerging risks but are not primary historical drivers.[84]Current status and trends (including 2024-2025 data)
Dall sheep (Ovis dalli) maintain a global conservation status of Least Concern, with range-wide populations exhibiting general stability amid incomplete monitoring data. However, regional trends reveal significant variability, including sharp declines in specific locales attributed to severe weather, low lamb recruitment, and elevated adult mortality. In Alaska, thinhorn sheep populations, encompassing Dall's sheep, have declined 40-70% since 2010 peaks across both managed and unmanaged areas, exacerbated by extreme winters and poor pregnancy rates ranging from 18-96% in monitored units like the Talkeetnas.[85][86] Protected areas document pronounced decreases. Yukon-Charley Rivers National Preserve surveys recorded just 70 sheep in core units during July 2023, a 78% drop from historical averages, leading to a closure of sheep hunting south of the Yukon River and within the preserve for the 2024-2025 seasons to support potential recovery. In Gates of the Arctic National Park and Preserve's Southeast unit, adult sheep estimates plummeted 61% from 1,969 (95% Bayesian credible interval: 1,805-2,189) in 2015 to 766 (583-1,067) in 2022, following earlier crashes linked to harsh conditions.[87][88] Harvest metrics reflect these pressures. Alaska reported 369 Dall rams taken in 2024, well below the 1962-2024 average of 916, amid broader concerns over sub-legal rams and declining hunter success. In Yukon, thinhorn sheep number approximately 22,500 as of 2024, with 234 rams harvested via 1,781 licenses; yet surveys in Game Management Zone 5 showed 16-63% declines in select sheep management units from 2022-2023, tied to late snowmelt and deep snowpack.[86][85] The 2025 range-wide assessment underscores persistent weather-driven instability in Alaska, including 19-34% adult mortality rates in recent years, while Yukon's populations remain relatively stable overall but vulnerable to climate-induced events. Ongoing monitoring emphasizes predation diversion and habitat considerations to mitigate further losses.[86]Influencing environmental factors
Dall sheep populations are primarily shaped by alpine climate variability, including winter snow regimes and summer vegetation productivity, which directly impact foraging access, energy balance, and survival rates. Deep snow accumulation limits winter range distribution, as sheep prefer wind-swept slopes that expose forage, with excessive snowfall correlating to reduced habitat suitability across Alaska and Yukon territories.[89] Freeze-thaw events in winter create icy crusts that impede movement and predator evasion, lowering adult survival; frequencies of such events negatively affect Dall sheep in northern ranges, where populations exhibit heightened sensitivity compared to southern conspecifics.[90] Summer forage quality, measured by normalized difference vegetation index (NDVI), influences lamb recruitment, with higher productivity enhancing nutritional intake and preconditioning sheep for subsequent winter stresses; lamb survival improves following productive summers, particularly when paired with milder winters lacking excessive freeze-thaw cycles.[91] Warmer winter temperatures can boost survival rates under low freeze-thaw conditions but may exacerbate insect harassment and heat stress in northern latitudes, interacting with broader climatic shifts.[92] Ongoing climate alterations, including reduced snow persistence and shrub encroachment into alpine tundra, threaten habitat integrity by diminishing open foraging areas and altering plant phenology, potentially reducing dietary diversity and increasing competition for thermal refugia during extreme weather.[93] [94] These factors compound in peripheral populations, such as those in Wrangell-St. Elias and Yukon-Charley Rivers, where rugged topography provides security but amplifies vulnerability to precipitation-driven avalanches and thermal extremes.[35] Empirical models indicate that variable snow depths during lambing seasons prompt habitat shifts toward lower-elevation, less snowy slopes, balancing forage needs against predation risks.[95]