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Abert's squirrel

Abert's squirrel (Sciurus aberti), commonly known as the tassel-eared squirrel, is a medium-sized arboreal rodent endemic to the ponderosa pine forests of the southwestern United States and north-central Mexico, distinguished by its prominent ear tufts and reliance on coniferous trees for survival.^[1]^[2] Adults typically measure 450–580 mm in total length and weigh 540–971 g, with a gray coat in most subspecies, though some exhibit black, brown, or reddish variations, and there is no notable sexual dimorphism.^[1] These squirrels are diurnal and solitary, inhabiting cool, dry mixed-conifer forests at elevations of 1,830–2,590 m, where they maintain home ranges of 8–90 ha depending on season and food availability.^[1]^[2] The species' distribution is closely tied to ponderosa pine (Pinus ponderosa) stands across Colorado, Utah, Arizona, New Mexico, and parts of Wyoming in the U.S., extending southward into Chihuahua and Durango in Mexico, with nine subspecies often isolated by geographic barriers.^[1]^[2] Habitat preferences favor mature forests with large-diameter trees (12–41 inches dbh) for nesting, typically placing dreys 16–90 ft high in the upper crowns of pines or associated species like Gambel oak or Douglas-fir.^[2] Their diet is specialized, consisting mainly of ponderosa pine seeds, buds, inner bark, and pollen cones, supplemented seasonally by fungi, acorns, and occasionally carrion; individuals may consume up to 45 twigs per day in winter to access cambium.^[1]^[2] Population densities vary widely from 2–122 squirrels per square kilometer, fluctuating with cone crop abundance and influenced by forest management practices.^[2]^[1] Reproduction occurs primarily from February to June, with a 43–46 day gestation period yielding litters of 1–5 young (average 3.5), and females provide sole parental care until juveniles become independent at about 10 weeks.^[1]^[2] Sexual maturity is reached around 327 days, and while lifespan in the wild is not well documented, captives have lived up to 7 years.^[1] Behaviorally, Abert's squirrels are non-territorial but may share winter nests in pairs, and they exhibit promiscuous mating; ear tufts, which are prominent year-round but shed briefly in late summer, likely aid in thermoregulation and communication.^[1]^[2] Conservationally, Abert's squirrel populations are considered stable overall (IUCN Red List: Least Concern as of 2016) but vulnerable due to habitat fragmentation from logging, fire suppression, and urbanization, with isolated subspecies at higher risk of local extirpation.^[1]^[3] It holds no federal endangered status in the U.S. but is classified as a Priority III species in Wyoming, emphasizing the need for preserving large, contiguous ponderosa pine stands to support its specialized ecology.^[2]

Taxonomy and Etymology

Etymology

The specific epithet aberti and the common name "Abert's squirrel" honor Colonel John James Abert (1780–1863), chief of the U.S. Army Corps of Topographical Engineers, under whose auspices the describing expedition was undertaken. Woodhouse named it "in honor of Col. J. J. Abert, Chief of the Corps of Topographical Engineers" to acknowledge his contributions to scientific exploration of the American Southwest.^[4] Abert's son, James William Abert (1820–1897), was a lieutenant, naturalist, and explorer who participated in several expeditions, including surveys of New Mexico and the Colorado River region, where he documented flora, fauna, and geography, contributing illustrations and reports that aided later explorations.^[2] The species was formally described as Sciurus aberti by physician and naturalist Samuel Washington Woodhouse in 1853, based on specimens collected during the 1851 Sitgreaves Expedition from Zuñi Pueblo, New Mexico, to the Colorado River.^[2] The genus Sciurus, shared with other tree squirrels, derives from Ancient Greek skiá ("shadow") and ourá ("tail"), alluding to the animal's characteristic bushy tail.^[2]

Taxonomy and Subspecies

Abert's squirrel, scientifically classified as Sciurus aberti, belongs to the order Rodentia within the family Sciuridae, subfamily Sciurinae, and tribe Sciurini.^[5] The species is placed in the genus Sciurus and subgenus Otosciurus, reflecting its close relation to other tree squirrels adapted to coniferous forests.^[1] First described by Samuel Washington Woodhouse in 1853 based on specimens from the San Francisco Mountains in northern Arizona, the taxonomy has evolved, with early classifications sometimes separating certain populations as distinct species.^[2] Nine subspecies of Sciurus aberti are currently recognized, each associated with isolated ponderosa pine (Pinus ponderosa) habitats across the southwestern United States and northern Mexico, where geographic barriers like the Grand Canyon limit gene flow and promote differentiation.^[2] These subspecies exhibit subtle variations in ear tuft size, pelage coloration, and body proportions, but all share the characteristic prominent ear tufts that give the species its common name, tassel-eared squirrel.^[1] Notably, the Kaibab squirrel (S. a. kaibabensis), once classified as a full species (S. kaibabensis), is now considered a subspecies due to genetic and morphological similarities, though it remains isolated on the Kaibab Plateau north of the Grand Canyon.^[2] The following table summarizes the recognized subspecies, their nominal descriptors, and primary geographic ranges:

Subspecies	Descriptor (Author, Year)	Primary Range
S. a. aberti	Woodhouse, 1853	Northern Arizona
S. a. barberi	Allen, 1895	Northwestern Chihuahua, Mexico
S. a. chuscensis	Goldman, 1937	Arizona-New Mexico border region
S. a. durangi	Thomas, 1903	Durango, Mexico
S. a. ferreus	True, 1891	Rocky Mountains, central Colorado; also southern Utah and northern Arizona
S. a. kaibabensis	Merriam, 1903	Kaibab Plateau, northern Arizona
S. a. mimus	Merriam, 1903	New Mexico-Colorado border
S. a. navajo	Durrant & Kelson, 1951	Southeastern Utah
S. a. phaeurus	Allen, 1893	Southern Chihuahua and Durango, Mexico

These subspecies are allopatric, with no overlapping distributions, underscoring the role of fragmented forest habitats in their evolution.^[2] Conservation efforts often focus on these isolated populations, as habitat connectivity could influence future taxonomic revisions.^[1]

Physical Characteristics

Size and Morphology

Abert's squirrels (Sciurus aberti) are medium- to large-sized tree squirrels, with total body lengths ranging from 450 to 580 mm, including a bushy tail that measures approximately 190 to 250 mm.^[1] Head-body lengths average 260 to 270 mm, while adult weights typically fall between 540 and 750 g, though individuals can reach up to 971 g, particularly in autumn when food resources are abundant.^[6]^[7] There is no pronounced sexual dimorphism in size, though males may be slightly smaller than females on average.^[1]^[7] Morphologically, Abert's squirrels exhibit adaptations suited to an arboreal lifestyle in coniferous forests. They possess long, strong hind legs and elongated rear paws that facilitate climbing, leaping between branches, and navigating the canopy of ponderosa pines.^[8] Their most distinctive feature is the prominent tufts of fur on the ears, often called "tassels," which can exceed 25 mm in length and are most pronounced during winter, potentially aiding in thermoregulation or display; these tufts reduce in size or disappear in summer.^[1] The tail is notably bushy and serves multiple functions, including balance during movement and signaling in social interactions.^[8] Overall body form is robust and streamlined for tree-dwelling, with a rounded head, sharp claws for gripping bark, and vibrissae (whiskers) around the muzzle for sensory navigation in dense foliage.^[1]

Coloration and Variation

Abert's squirrels exhibit a distinctive pelage characterized by a predominantly gray dorsal coloration, often with a subtle agouti pattern, and white or buff underparts. The most prominent feature is the prominent tufts of elongated hairs on the ears, which are particularly noticeable during winter and are shed in summer. The tail is broad and bushy, typically gray above with white-tipped hairs creating a frosted appearance, and white below. Many individuals display a narrow reddish-brown or rusty stripe along the midline of the back, which is more pronounced in northern populations and fades or disappears in southern ranges such as eastern Arizona and New Mexico.^[9]^[10] Geographic variation in coloration is evident across the nine recognized subspecies, with seven displaying the typical gray morph and two showing darker brown or black forms. For instance, the Kaibab squirrel (S. a. kaibabensis), endemic to the Kaibab Plateau north of the Grand Canyon, has a uniform dark gray to blackish pelage overall, lacking the white ventral contrast seen in the nominate form. Coat color generally shows continuous variation in the intensity of brown pigmentation, rather than discrete categories, influenced by local environmental factors and genetics.^[10]^[11] Color polymorphism is notable, particularly in northern Colorado populations of the subspecies S. a. ferreus, where a melanistic (all-black) morph occurs frequently, sometimes comprising the majority of individuals in certain areas. This melanism is genetically controlled by a recessive mutation (T370C) in the agouti signaling protein (ASIP) gene, distinct from mutations in other squirrel species and representing convergent evolution for dark pigmentation. The black morph is absent in southern populations, such as those in Arizona, New Mexico, and Mexico, highlighting regional genetic differentiation. Seasonal pelage changes are minimal, though ear tufts elongate in winter for thermoregulation.^[10]^[12]^[13]

Distribution and Habitat

Geographic Distribution

Abert's squirrel (Sciurus aberti) is endemic to the southwestern United States and northern Mexico, primarily inhabiting the Colorado Plateau and the southern Rocky Mountains. Its core range spans Colorado, Utah, Arizona, and New Mexico, with extensions into southeastern Wyoming where suitable ponderosa pine habitat occurs, and southward into the Sierra Madre Occidental of Chihuahua and Durango, Mexico.^[2]^[12]^[14] The species' distribution is closely tied to ponderosa pine (Pinus ponderosa) forests, typically occurring at elevations between 1,800 and 3,000 meters (5,900–9,800 feet), though it can range from 1,676 to 2,590 meters (5,500–8,500 feet) in Arizona. In Colorado, populations are found in the southern and central regions, including the Spanish Peaks area where the range extends eastward by approximately 70 kilometers beyond previously documented limits. Utah hosts isolated populations in the southeastern part, while Arizona's distribution covers central and northern mountainous areas, including the Kaibab Plateau and the Mogollon Rim. New Mexico's populations occupy the Sacramento, San Mateo, and other southern mountain ranges.^[2]^[15]^[2] Introduced populations have expanded the squirrel's range in Arizona, with successful establishments in the Graham Mountains (Pinaleno Mountains) and Santa Catalina Mountains since the 1940s, where they persist in mixed-conifer forests despite limited ponderosa pine. In Mexico, the distribution is restricted to higher-elevation pine forests in the northern states, reflecting post-Pleistocene dispersal patterns that shaped the modern range. No significant historical range contractions have been documented, though populations fluctuate with habitat availability and cone production cycles.^[2]^[16]^[14]

Habitat Preferences

Abert's squirrels primarily inhabit mature ponderosa pine (Pinus ponderosa) forests in the southwestern United States, favoring cool, dry interior environments at elevations typically ranging from 1,676 to 2,590 meters (5,500 to 8,500 feet) in Arizona.^[2] These forests often include associated vegetation such as Gambel oak (Quercus gambelii), pinyon pine (Pinus edulis), junipers (Juniperus spp.), quaking aspen (Populus tremuloides), and Douglas-fir (Pseudotsuga menziesii), which provide supplementary food and cover.^[2] Ideal habitat consists of uneven-aged stands featuring small, even-aged tree groups that form interlocking canopies, enabling efficient travel and predator avoidance between clusters.^[17] Structural preferences emphasize moderate tree densities of 375 to 625 trees per hectare, with a basal area of 34.4 to 45.3 square meters per hectare and average tree diameters of 28 to 33 centimeters, including some larger individuals over 51 centimeters for cone production.^[2] Squirrels select sites with dense canopy closure for nesting and food storage, often utilizing witches' brooms caused by dwarf mistletoe or tree cavities 9 to 15 meters above ground in trees around 40 centimeters in diameter.^[17] Understory components, such as small clusters of young trees and shrubs, support foraging for hypogeous fungi and buds while maintaining open park-like conditions that align with the species' arboreal lifestyle.^[16] Habitat selection is influenced by landscape mosaics, where patches of high-quality, untreated forest comprising at least 35% of the area sustain populations by preserving microclimates for seed production and fungal communities.^[16] In managed forests, squirrels avoid heavily thinned areas that reduce canopy interconnectivity or exceed 50% stem removal, as these disrupt access to resources and increase vulnerability.^[16] Seasonal variations may lead to shifts toward mixed-conifer stands during winter for added thermal cover, though ponderosa pine remains the core habitat throughout the year.^[2]

Associated Plant Communities

Abert's squirrels (Sciurus aberti) are primarily associated with ponderosa pine (Pinus ponderosa) forests, which form their core habitat across much of their range in the southwestern United States and northern Mexico.^[2] These communities are characterized by cool, dry conditions at montane elevations, typically between 1,830 and 2,590 meters (6,000–8,500 feet), where ponderosa pine dominates as the overstory with densities of 168–250 trees per acre and basal areas of 150–200 square feet per acre.^[2]^[1] The understory often includes grasses and forbs adapted to semi-arid environments, supporting the squirrel's foraging needs for cones, seeds, and fungi.^[2] In addition to pure ponderosa pine stands, Abert's squirrels inhabit mixed-conifer woodlands and savannas, where ponderosa pine co-occurs with species such as Gambel oak (Quercus gambelii), pinyon pine (Pinus edulis), junipers (Juniperus spp.), quaking aspen (Populus tremuloides), and Douglas-fir (Pseudotsuga menziesii).^[2] These associations fall within broader ecological systems like the Rocky Mountain Ponderosa Pine Woodland and Savanna, Colorado Plateau Pinyon-Juniper Woodland and Shrubland, and Rocky Mountain Gambel Oak-Mixed Montane Shrubland, often at elevations of 1,740–2,900 meters (5,700–9,500 feet) in regions such as the Front Range and San Juan Mountains of Colorado.^[18] Gambel oak provides acorns as a supplemental food source, while pinyon-juniper understories contribute seeds, enhancing habitat suitability in transitional zones.^[2]^[19] Introduced populations, such as those in the Pinaleño Mountains of Arizona, have expanded into mixed-conifer and spruce-fir forests, utilizing species like white fir (Abies concolor) and Engelmann spruce (Picea engelmannii) for cover and nesting when ponderosa pine is scarce.^[20] This adaptability highlights the squirrel's reliance on coniferous overstory for survival, though native populations remain most abundant in undisturbed ponderosa pine-dominated communities.^[20]^[1]

Ecology and Behavior

Abert's squirrels (Sciurus aberti) are strictly diurnal, remaining active from shortly before sunrise until shortly before sunset throughout the year, with no evidence of hibernation or torpor.^[21] Their activity patterns feature peaks in the early morning and late afternoon, particularly in Arizona populations where they are most vigorous during the first four hours after dawn and the last two hours before dusk; a midday lull often occurs, especially in summer, during which individuals may return to nests, bask in the sun, or seek shade for cooling.^[22] This diurnal rhythm persists even in inclement weather, supporting consistent foraging and other behaviors tied to their ponderosa pine habitats.^[21] Outside the breeding season, Abert's squirrels exhibit a largely asocial and solitary social structure, with adults being non-gregarious and spending most daylight hours foraging independently; they do not defend exclusive territories, allowing significant overlap in home ranges among individuals.^[23] However, they demonstrate tolerance for spatial proximity, occasionally sharing nests or resting sites without aggression.^[24] Communal nesting occurs primarily in winter, where multiple adults—including breeding females with males or other females—share dreys for social thermoregulation during cold periods, often with consistent pairing among the same individuals in mixed-sex groups; this behavior is less frequent during breeding to avoid unwanted mating or resource competition.^[25] During the April–May breeding season, social interactions intensify through mating chases involving oestrous females and groups of males, where a temporary dominance hierarchy emerges among males based on aggressive displays and home-range familiarity.^[23] The most dominant male in each chase typically copulates first with the female, and male aggression levels correlate positively with copulation success, though hierarchies are unstable and non-linear across bouts.^[23] Females mate with multiple males but do not receive paternal care post-copulation, reinforcing the species' predominantly solitary lifestyle beyond reproductive periods.^[23]

Reproduction

Abert's squirrels exhibit seasonal breeding, with mating occurring primarily from late February to early June, though activity can extend into other months depending on local conditions.^[1] During this period, females enter estrus for a single day, attracting groups of males that engage in prolonged chases through the trees, often lasting hours and involving aggressive interactions among males and between sexes.^[23] Mating is promiscuous, with the dominant male typically copulating first, followed by subordinates; a vaginal plug forms after insemination to prevent further mating by rivals. Males provide minimal parental care beyond guarding females post-mating, while females handle all rearing responsibilities.^[1] Sexual maturity is reached at approximately 327 days of age.^[1] Gestation lasts 40–46 days, with estimates varying by study; for instance, one observation reported 40 days, while another documented 46 days.^[21]^[23] Females typically produce one litter per year, born in late spring or early summer (June–July), though rare second litters may occur in favorable years.^[21] Litter sizes range from 1 to 5 young, averaging 3–3.5; newborns weigh about 12 g, measure 60 mm in length, and are born altricial—hairless, blind, and helpless.^[21]^[15] Young remain in the nest for 7–10 weeks, during which females provision them with food and protection; weaning occurs around 70–76 days when offspring reach about 355 g.^[1] Juveniles begin foraging independently by 10 weeks and achieve adult size by 15–16 weeks, dispersing from the natal area shortly thereafter.^[8] This reproductive strategy aligns with the species' dependence on seasonal cone production in ponderosa pine forests, ensuring offspring development coincides with peak food availability.^[21]

Nesting and Shelter

Abert's squirrels construct nests, known as dreys, primarily in the crowns of ponderosa pine (Pinus ponderosa) trees, which provide essential shelter year-round, particularly for nightly rest and protection from predators. These nests are typically built by females using small twigs (0.5 inches or less in diameter and 6–24 inches long) gathered from pine branches, forming a bulky structure lined with softer materials such as grass, moss, shredded bark, or occasionally fabric. Nest placement occurs in the upper third of the tree crown, at heights ranging from 16 to 90 feet above the ground, often on the southern or southeastern side of the tree for optimal sunlight exposure and stability. In optimal habitats, nests are situated in mature ponderosa pines with diameters at breast height (DBH) of 12–41 inches, favoring trees that are part of dense stands with interlocking canopies to enhance concealment and thermal regulation.^[2] Nest-site selection is influenced by both structural and chemical characteristics of the host trees. Squirrels preferentially choose larger, taller ponderosa pines with multiple access routes via branches, as these features maximize accessibility while minimizing exposure to wind and predators. Chemically, the phloem of nest trees exhibits lower concentrations of copper, iron, and silicon but higher levels of sodium and nonstructural carbohydrates compared to non-nest trees, suggesting that squirrels detect nutritionally beneficial trees through taste or olfaction. The xylem oleoresin in nest trees also contains a lower percentage of α-pinene, potentially reducing respiratory irritation. Discriminant function analysis of phloem chemistry accurately classifies nest versus control trees at 90.6%, underscoring the role of these cues in selection. In winter, squirrels may share nests in pairs (an adult female and subadult), while summer shelters can occasionally include cavities in Gambel oak (Quercus gambelii) or branches of cottonwood (Populus spp.), though ponderosa pine remains dominant.^[2]^[26] Nests are not territorial but communal, with multiple individuals, including non-related squirrels, sharing the same drey across seasons; a single squirrel may occupy several nests within its home range of 20–90 hectares. This flexibility supports year-round use, with dreys serving dual purposes as resting sites and breeding platforms during the spring mating period. In managed or restored forests, habitat alterations like canopy thinning can reduce nest availability by disrupting interlocking crowns, potentially increasing vulnerability. Introduced populations, such as in the Pinaleno Mountains of Arizona, adapt by using alternative conifers like Douglas-fir (Pseudotsuga menziesii) and corkbark fir (Abies lasiocarpa var. arizonica), selecting for even larger trees (DBH ~60 cm) with steep slopes and abundant access points, though native preferences center on ponderosa pine ecosystems.^[1]^[27]

Diet and Foraging

Abert's squirrels (Sciurus aberti) are primarily folivorous and mycophagous, with their diet centered on ponderosa pine (Pinus ponderosa) tissues, including seeds extracted from cones, inner bark of twigs, apical and staminate buds, and pollen.^[2] They consume up to 75 cones per day during peak seed availability from late spring to autumn, preferring fresh, green cones and selectively feeding on those with higher nutritional value based on the tree's chemical profile.^[2] Supplementary foods include hypogeous fungi (such as species in genera like Boletus and Rhizopogon), which provide higher protein and energy, as well as dwarf mistletoe (Arceuthobium spp.), tree sap, and occasionally pinyon pine (Pinus edulis or P. cembroides) nuts when available.^[1]^[28] Seasonal shifts in diet reflect the availability and quality of ponderosa pine resources, with inner bark comprising over 80% of intake during winter and spring when seeds and buds are scarce, supporting survival on this high-fiber, low-nutrient food for up to six months annually.^[28] In summer and autumn, foraging emphasizes seeds, fungi, and reproductive structures like pollen cones, with fungi becoming prominent during monsoon periods due to increased underground fruiting.^[28] Rare opportunistic items, such as carrion, bones, insects, or acorns from Gambel oak (Quercus gambelii), supplement the diet but constitute less than 5% of overall consumption.^[2] Water needs are met primarily through food moisture, though squirrels may drink from puddles or ponds during dry seasons.^[2] Foraging occurs year-round during daylight hours, with individuals actively clipping twigs, excavating fungi from soil near pine roots, or scaling trees to access cones and buds, often covering territories of 10-20 hectares while feeding on over 50% of available trees in high-quality habitat.^[29] They do not cache food extensively, relying instead on constant foraging, and exhibit solitary behavior except during brief mating or parental care periods.^[1] Morphological adaptations, including a gastrointestinal tract with 61% greater surface area and an enlarged cecum (125% larger than in generalist congeners), enable efficient fermentation of fibrous phloem and lignin-rich materials, facilitating nutrient extraction from their specialized diet.^[28] By dispersing fungal spores through scat, Abert's squirrels contribute to mycorrhizal networks that enhance ponderosa pine health and soil moisture retention.^[29]

Predators and Threats to Individuals

Abert's squirrels face predation primarily from avian and mammalian carnivores adapted to forested environments. The northern goshawk (Accipiter gentilis) is a key predator, exhibiting a strong dependence on the species as prey, with documented attacks and captures in ponderosa pine habitats.^[24] Other raptors, such as Cooper's hawks and red-tailed hawks, occasionally prey on adults and juveniles when opportunities arise.^[1] Mammalian predators include cougars (Puma concolor), bobcats (Lynx rufus), and coyotes (Canis latrans), which target squirrels on the ground or at low tree levels during foraging or travel.^[1] To mitigate predation risks, Abert's squirrels employ behavioral adaptations rather than physical defenses, as they lack specialized anti-predator morphology. They preferentially use areas with interlocking tree canopies for safe movement between pines, reducing exposure to aerial attacks.^[1] Individuals often lie flat on branches after sunning to blend with foliage and minimize visibility, and they avoid foraging on windy days when predator approach sounds may be masked.^[1] Alarm calls, including foot-stomping and vocalizations, alert conspecifics to threats from both predators and human disturbances.^[15] Beyond predation, direct threats to individual Abert's squirrels include vehicle collisions, particularly in areas near roads traversing ponderosa pine forests. Anecdotal observations indicate that these squirrels may not perceive approaching vehicles as threats, leading to higher mortality rates compared to more urban-adapted squirrel species.^[30] Human recreation, such as hiking and off-road activities, can disrupt foraging and increase stress, potentially elevating vulnerability to predators through altered behavior.^[31] Forest management practices like thinning or clear-cutting may fragment canopies, exposing individuals to greater predation risk during transit.^[32]

Conservation

Status and Population Trends

The Abert's squirrel (Sciurus aberti) is classified as Least Concern on the IUCN Red List, indicating that it does not qualify for a more threatened category and faces no major global extinction risk. NatureServe assesses it as globally secure (G5), reflecting a stable overall status with populations considered apparently secure across much of its range.^[33] In the United States, it has no special federal conservation status under the Endangered Species Act, though it is designated as a Priority III species in Wyoming, warranting consideration in resource management plans but not requiring intensive conservation efforts.^[2] Some subspecies in Mexico are considered threatened due to habitat fragmentation, but this does not alter the species-level assessment.^[16] Population trends for the Abert's squirrel are generally stable, with no evidence of widespread decline or significant increase across its core range in the southwestern United States and northern Mexico.^[1] Abundance fluctuates cyclically in response to the availability of ponderosa pine (Pinus ponderosa) cones, its primary food source, leading to periodic booms and busts rather than long-term directional changes.^[2] For instance, populations on the Kaibab Plateau in Arizona vary markedly year-to-year with cone crop productivity, but recover during favorable conditions without apparent cumulative loss.^[2] In central Arizona, densities have been estimated at a minimum of one squirrel per hectare in suitable habitat, supporting harvest levels of 50,000 to 100,000 individuals annually (primarily tree squirrels including Abert's), which suggests robust numbers capable of sustaining hunting pressure.^[2]^[34] Isolated populations, such as the Kaibab subspecies (S. a. kaibabensis) on the North Rim of the Grand Canyon, remain viable but vulnerable to localized habitat alterations like timber harvesting or fire, though monitoring indicates persistence without recent declines.^[1] Overall population size is not precisely quantified globally, but estimates in managed ponderosa pine forests range from 0.05 to 1.03 squirrels per hectare, with higher densities (up to 317 per square mile in fall) observed during peak resource years.^[2]^[35] In introduced areas, such as parts of the Sky Islands in Arizona and New Mexico, populations have expanded naturally since the 1940s, sometimes at the expense of native species like the Mount Graham red squirrel (Tamiasciurus hudsonicus grahamensis), but without impacting the species' overall stability.^[16] Conservation efforts emphasize habitat maintenance through selective logging to preserve old-growth ponderosa pines, ensuring continued population resilience.^[2]

Conservation Challenges

Abert's squirrel (Sciurus aberti) is classified as Least Concern on the IUCN Red List due to its relatively wide distribution across ponderosa pine forests in the southwestern United States and northern Mexico, with stable populations overall. However, certain subspecies, such as the Kaibab squirrel (S. a. kaibabensis), face heightened vulnerability, rated as T3 (Vulnerable) by NatureServe owing to their restricted range on the Kaibab Plateau.^[36] These isolated populations increase susceptibility to localized threats, complicating broad-scale conservation efforts.^[1] The primary conservation challenge stems from habitat alteration in ponderosa pine ecosystems, where historical fire regimes maintained open, park-like forests ideal for Abert's squirrels. Fire suppression over the past century has led to denser, multi-layered canopies with encroaching conifers, reducing forage availability and suitable nesting sites.^[37] Timber harvesting exacerbates this by fragmenting habitats and disrupting feeding patterns, particularly when clear-cutting removes key ponderosa pines without regard for population connectivity.^[33] In response, forest restoration treatments like selective thinning aim to mimic natural conditions and lower wildfire risk, but improper implementation can temporarily displace squirrels or degrade nest tree availability.^[32] Catastrophic wildfires and insect outbreaks, such as mountain pine beetle infestations, pose acute threats by destroying mature pine stands that provide cones and shelter. For instance, high-severity fires can eliminate large swaths of habitat, while beetle epidemics weaken trees, indirectly affecting squirrel nutrition and reproduction.^[38] In peripheral ranges like southeastern Wyoming, where populations are critically imperiled (S1), additional pressures from wind energy development and overgrazing further limit expansion.^[38] Conservation is further challenged by the species' occasional status as an invasive competitor in mixed forests, such as on Mount Graham in Arizona, where removal efforts to protect the endangered Mount Graham red squirrel (Tamiasciurus hudsonicus grahamensis) may impact local Abert's populations.^[39] Effective management requires protecting clusters of mature feed and nest trees, limiting harvests to small selective blocks under 20 hectares, and monitoring subspecies-specific trends to prevent isolation from becoming a barrier to gene flow.^[33] Ongoing surveys and habitat modeling are essential to balance restoration with these needs, ensuring resilience against climate-driven changes in fire frequency.^[38]

Management and Introduced Populations

Abert's squirrels (Sciurus aberti) have been intentionally introduced to several isolated mountain ranges in Arizona and New Mexico during the mid-20th century, primarily by the Arizona Game and Fish Department to enhance hunting opportunities as a game species.^[40]^[41] In 1941, individuals were translocated from northern Arizona populations to the Santa Catalina Mountains near Tucson, where they successfully established a stable population and subsequently dispersed to the adjacent Rincon Mountains.^[40]^[24] Similar introductions occurred in the Pinaleño Mountains (Mount Graham) in the 1940s, as well as to other "Sky Island" montane forests across southern Arizona and New Mexico, leading to self-sustaining populations with densities comparable to native ranges.^[42]^[16] These introduced groups demonstrate high establishment success, often persisting from as few as 15–45 founders, though they remain geographically isolated from core native populations in the southwestern United States and Mexico.^[38]^[1] Management efforts for Abert's squirrels focus on balancing habitat preservation with control measures in areas of introduction, particularly where they pose competitive or ecological risks to native species. In the Pinaleño Mountains, ongoing programs employ integrated pest management techniques, including live-trapping, translocation, and selective removal via shooting, to suppress Abert's squirrel numbers and support recovery of the endangered Mount Graham red squirrel (Tamiasciurus hudsonicus grahamensis), with which they compete for resources like ponderosa pine cones.^[39]^[43] Post-2017 Frye Fire monitoring documented a rebound in Mount Graham red squirrel abundance from 33 individuals in 2017 to 109 by 2021 and further to 233 by 2024, prompting adaptive adjustments and continued control of Abert's squirrels (with 743 individuals removed from FY2019–2022 and efforts ongoing as of 2025).^[43]^[44] Additionally, Abert's squirrels may transmit parasites to native squirrels, further justifying targeted reductions in overlap zones.^[45] Habitat management emphasizes ponderosa pine forest restoration through mechanical thinning and prescribed burns to mitigate wildfire risks while maintaining suitable foraging and nesting structures, as excessive clear-cutting fragments populations and reduces winter survival.^[32]^[1] Overall, introduced populations are considered stable and unmanaged in most non-conflict areas, but conservation strategies prioritize monitoring isolation and connectivity to prevent genetic bottlenecks.^[1]^[38]

References

[1]
Sciurus aberti (Abert's squirrel) - Animal Diversity Web
Abert's squirrels (Sciurus aberti) are concentrated in the mountainous regions of the southwestern United States and in north central Mexico.
[2]
Sciurus aberti - Forest Service - USDA
The Abert's squirrel is confined to the Colorado Plateau and the southern Rocky Mountains of Colorado, Utah, Arizona, and New Mexico.
[3]
https://www.iucnredlist.org/species/20010/22246472
[4]
[PDF] PATTERNS OF BODY AND TAIL LENGTH AND BODY MASS IN ...
Aug 25, 2008 · The data on body mass, head–body length, and tail length ... Sciurus aberti. 262.3. 221.7. 618.6. 274.0. 208.0. 593.8. 269.5. 242.3. 601.5.
[5]
https://itis.gov/servlet/SingleRpt/SingleRpt?search_topic=TSN&search_value=180173
[6]
Abert's Squirrel - Sciurus aberti - NatureWorks - New Hampshire PBS
Abert's squirrel is also know as the tassel-eared squirrel. It has long tufts or tassels of fur on its ears. Its fur is gray on the sides ...
[7]
Abert's Squirrel - Bandelier National Monument (U.S. National Park ...
Identified by their dark gray backs with a red-brown patch, white bellies, and ... tufted ears. Abert's squirrels feed on the cones, buds, and twigs of ...
[8]
https://nhpbs.org/natureworks/abertssquirrel.htm
[9]
National Registry of Natural Landmarks - NPS History
KAIBAB SQUIRREL AREA The Kaibab Squirrel Area, located within portions of the Kaibab ... These undisturbed stands illustrate the zonation of red, black, and white ...<|control11|><|separator|>
[10]
The Genetic Basis of Melanism in Abert's Squirrel (Sciurus aberti)
Feb 17, 2024 · This study examines Abert's Squirrel (Sciurus aberti), which has a melanistic morph whose genetic basis has not yet been established.
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Coat Color Polymorphism of Abert's Squirrel, Sciurus aberti, in ... - jstor
Coat color variation was studied in museum specimens and in nat- ural populations of Abert's squirrel (Sciurus aberti) in Colorado. The phanero- morphism ...
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"Role of post-Pleistocene dispersal in determining the modern ...
Abert's squirrel is a forest-dwelling mammal, dependent upon ponderosa pine, that now ranges from southern Wyoming to northern Mexico.
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[PDF] Abert's Squirrel (Sciurus aberti)
Jun 15, 1977 · (1975) indicated that these squirrels are common in mixed conifer forests in many New. Mexican canyons. Many populations are relatively isolated ...
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[PDF] Management and Conservation of Tree Squirrels - Forest Service
Abert's squirrels were introduced to Sky Islands in Arizona and New Mexico in the 1940s (Davis and Brown 1988) and may have contributed to decline in native ...Missing: distribution | Show results with:distribution
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[PDF] Impact of ponderosa pine thinning on Abert's squirrel (Scuirus aberti ...
Abert's squirrel habitat preferences have been described rather extensively and are summarized in Keith (2003). Ideal Abert's squirrel habitat is fairly ...
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[PDF] Guide to the Ecological Systems of Colorado
distinguished by a high cover of vegetation between scattered rocks, and plant community ... (Erethizon dorsatum), Abert's squirrel (Sciurus aberti), chipmunks ( ...
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https://assets.bouldercounty.gov/wp-content/uploads/2017/03/research-report-2008Worden.pdf
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Use of Mixed-Conifer and Spruce-Fir Forests by an Introduced ... - jstor
... Use of Mixed-Conifer and Spruce-Fir Forests by an Introduced Population of Abert's Squirrels (Sciurus aberti), The Southwestern Naturalist, Vol. 48, No. 2 ...
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The Abert Squirrel and Its Dependence on Ponderosa Pine
Squirrels are strictly diurnal and in 1954 were active and fed each day even in inclement weather. Abert squirrels build nests in the branches of ponderosa pine ...
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Utah Natural Heritage Program
Insufficient relevant content. The provided URL (https://fieldguide.wildlife.utah.gov/?Species=Sciurus%20aberti) only contains a species database title, "Wiki Options Login," and no specific information on Abert's squirrel reproduction, including breeding season, gestation, litter size, or development.
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Social dominance and mating activity in the tassel-eared squirrel ...
The behaviour of a wild population of tassel-eared squirrels was investigated from September 1969 to January 1971. Squirrels were live captured and marked ...
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Sciurus aberti (Abert's squirrel) | CABI Compendium
Distribution. Native to Mexico and parts of the USA including Colorado, Utah and Wyoming, Abert's squirrels appear to be continuing to expand their range ...
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Communal Nesting in Asocial Abert's Squirrels: the Role of Social ...
Feb 5, 2007 · Each female tree squirrel is in estrus for <1 d and mates with multiple males in a mating chase (Farentinos 1972a; Thompson 1977; Gurnell 1987).
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Nest-Site Selection by Abert's Squirrel: Chemical Characteristics of ...
Nest-site selection by Abert's squirrels was associated with specific chemical features of the phloem and xylem of nest trees. The phloem of nest trees had ...Missing: habits shelter<|control11|><|separator|>
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None
### Summary of Key Findings on Nest-Site Selection by Abert’s Squirrels (Edelman & Koprowski, 2005)
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Comparative Morphology of the Gastrointestinal Tract in the Feeding ...
Aug 6, 2025 · The foraging behaviors of Abert's squirrels are constrained by seasonally changing food sources that vary in nutritional quality, chiefly ...<|control11|><|separator|>
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Abert's Squirrel - Pajarito Environmental Education Center
Abert's squirrels were named after John James Abert, an American naturalist who influenced the in mapping of the American West in the 1800s. Abert's Squirrels ...
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Abert's Squirrels - National Park Service
Mar 31, 2012 · Also, Abert's squirrels have very large home ranges and only occur in mature ponderosa pine (Pinus ponderosa) stands, so increases in human ...
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[PDF] human-effects-aberts-squirrel-and-dusky-grouse.pdf - Boulder County
Mar 28, 2017 · We conducted a pilot study of the potential effects of recreation on Abert's squirrel (Sciurus aberti) and dusky grouse (Dendragapus obscurus).
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[PDF] Habitat use by Abert's squirrels (Sciurus aberti) in managed forests
The Abert's squirrel (Sciurus aberti) is dependent on ponderosa pine (Pinus ponderosa) for food and cover (Keith, 1965; Farentinos, 1972; Dodd et al., 2003).
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Sciurus aberti | NatureServe Explorer
Sciurus aberti Woodhouse, 1853 (TSN 180173). Taxonomic Comments: Under a proposed taxonomic arrangement for Sciurini (de Abreu-Jr et al. 2020a, de Abreu-Jr ...
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[PDF] Abert's squirrel - NFSL Digital Collections
Since 1990 the number of hunters has generally averaged between 12,000 and 18,000 a year with the average annual harvest being between 50,000 and 100,000 tree ...
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Tassel-Eared Squirrel Population, Habitat Condition, and Dietary ...
Aug 6, 2025 · Second, Abert's squirrels consumed more mushrooms throughout the year (Dodd et al., 2003; States & Wettstein, 1998), but also more phloem ...
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Sciurus aberti kaibabensis - NatureServe Explorer
Kaibab Squirrel - Natureserve Global Rank: T3. ... Conservation Status. NatureServe Status. Global Status: G5T3. Global Status (Rounded):. T3. Global Status Last ...
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Habitat use by Abert's squirrels ( Sciurus aberti ) in managed forests
... Abert's squirrel (Sciurus aberti) is dependent on. ponderosa pine (Pinus ponderosa) for food and cover. (Keith, 1965; Farentinos, 1972; Dodd et al., 2003).
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[PDF] Abert's Squirrel - Sciurus aberti - b State Species Abstract a
Six subspecies of Abert's Squirrel are recognized. Of those, only one occurs in Wyoming – S. a. ferreus 1. Description: Abert's Squirrel is a large-bodied ...
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[PDF] Abert's Squirrel Management in Support of Mount - usda aphis
Nov 17, 2021 · Currently, threats to the red squirrel include habitat degradation and loss through high-severity wildfire, fire suppression activities, insect ...
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Squirrels and Chipmunks of Saguaro National Park
Oct 3, 2025 · In 1941, the Arizona Game and Fish Department successfully introduced Abert's squirrel as a game species from northern Arizona into the Santa ...
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[PDF] Final Report - Western National Parks Association
Abert's squirrels were found in each of the high elevation ponderosa pine or mixed conifer forest sites.
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Mount Graham Red Squirrel | U.S. Fish & Wildlife Service
Additionally, Abert's squirrels were introduced to the Pinaleño Mountains in the 1940's as a game species. This species is now more abundant than red squirrels.
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[PDF] Abert's Squirrel Management in Support of Endangered Mount ...
Dec 20, 2022 · The research projects are being conducted to study the morpho- metric patterns in Abert's squirrels with the goal of documenting the ...
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TWS2020: Parasites may reduce endangered squirrels' survival
Dec 7, 2020 · Invasive Abert's squirrels may be passing harmful parasites to endangered native Mount Graham's squirrels in Arizona.Missing: challenges | Show results with:challenges