Fish and Wildlife Service (2009) is a good source of current information on conservation status as well as natural history.
The genus Sciurus includes some 28 species (Thorington and Hoffmann 2005), ranging in forested areas throughout most of North America and Eurasia (including Japan) and southward in the Western Hemisphere to Brazil and Bolivia. About half of the species in the genus occur in Mexico and Central America. Taxonomy of the subfamily Sciurinae was reviewed by J. Moore (1959). Colorado is home to 2 quite distinctive species of Sciurus. Both species are managed as small game mammals.
Sciurus aberti ABERT'S SQUIRREL
Description
Also called "tassel-eared squirrel," Abert's squirrel, in its rich winter pelage, has to rank as one of Colorado's most beautiful mammals. A large tree squirrel, the species is polymorphic in coat color along the Front Range (Nash and Ramey 1970). The gray phase has a white belly and dark lateral stripes; other color phases are solid: jet black, dark brown, or pale brown. Proportional prevalence of various color phases varies with location, although all three color phases may occur in a single litter. The predominant color along the Front Range is black. Coat color polymorphism and geographic variation have been studied, and the former was found to be stable in populations since the turn of the century (C. Ramey and Nash 1976a, 1976b; Hoffmeister and Diersing 1978; Hancock and Nash 1982). The ears have conspicuous tufts of hair, most noticeable in fresh winter pelage. In southwestern Colorado, almost all Abert's squirrels are salt-
Digitized by Goog le and-pepper gray, often marked with a broad dorsal patch of rust-red hairs. Measurements are: total length 450-580 mm; length oftail200-300 mm; length ofhindfoot 65-75 mm; length of ear 35-45 mm; weight 550-750 g. The skull resembles that of the fox squirrel but lacks the typical reddish tinge common in the latter species. The ventral anterior border of the orbit is opposite the first molar, and the third upper premolar is large and well developed.
PHOTOGRAPH 8-16. Abert's squirrel (Sciurus aberti). Photograph by J. Perley Fitzgerald.
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Abert's squirrels are common in ponderosa pine forests in southern and central Colorado, and their distribution and pattern of color variation have generated interest (C. Ramey and Nash 1971). During the Late Pleistocene, Abert's squirrels apparently occurred only as far north as central Arizona and New Mexico, so their occurrence in Colorado is a result of post-Pleistocene dispersal (R. Davis and Brown 1989). Lamb et al. (1997) demonstrated the phytogeographic continuity of Abert's squirrels from Colorado's Eastern Slope with populations to the south in New Mexico. Extensions of the known range have occurred in recent years in southeastern (Mellott and Choate 1984) and western (R. Davis and Bissell 1989) Colorado. According to D. Armstrong (1972) and E. Hall (1981 ), Sciurus aberti mimus occurs in southwestern Colorado; S. a. ferreus occurs in the Front Range, Sangre de Cristos, and Wet
o1git1zed by Goog le Mountains. However, Hoffmeister and Diersing ( 1978) treated S. a. mimus as a junior synonym of S. a. aberti, as substantiated by genetic studies (Wettstein et al. 1995; Lamb et al. 1997). In the early twentieth century, the species was not common in Colorado but populations increased after closure of the hunting season (Wade 1935). At the present time, Abert's squirrel is classified as a small game mammal by the Colorado Division of Wildlife.
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MAP 8-32. Distribution of Abert's squirrel (Sciurus aberti) in North America.
D1git1zed by Google MAP 8-33. Distribution of Abert's squirrel (Sciurus aberti) in Colorado.
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Abert's squirrels are dependent on ponderosa pine for both nesting sites and food, and thus are restricted to open montane forests (J. Keith 1965; D. Patton 1977; M. Snyder 1992), although R. Forbes (1997) reported foraging on acorns beneath snow in the Sandia Mountains of New Mexico. Dodd et al. (2003) emphasized the importance for S. aberti in Arizona of larger trees and patches of trees with interlocking canopies. The squirrels feed on inner bark, seeds, twigs, buds, and young cones of ponderosa pines. Only about 10 percent of the weight of twigs cut is actually consumed. Abert's squirrels in northeastern Arizona were
o1git1zed by Goog le found to decrease the potential cone crop of individual trees by 20 percent. By direct predation on developing seeds, the squirrels reduced cone crop by an additional 25 percent over 2 years (Allred et al. 1994). Havlick (1984) studied Abert's squirrels in sympatry with fox squirrels (S. niger) and pine squirrels (Tamiasciurus hudsonicus) in the Boulder Mountain Parks. Fox squirrels mostly were observed in moist riparian systems, pine squirrels utilized mixed coniferous forests, and Abert's squirrel was observed only in ponderosa pine woodlands. Because fox squirrels are a relative newcomer to the foothills of the Colorado Front Range, long-term quantitative studies of populations and their interactions would be invaluable. Abert's squirrels appear to select trees with low levels of certain monoterpenes and can reduce ponderosa pine cone production by up to 74 percent, depending on numbers of squirrels and the rate of cone production (Capretta et al. 1980). During winter, the squirrels feed almost exclusively on phloem (inner bark) of particular individual trees. This high-volume, low quality diet is associated with a distinctively large gastrointestinal tract (Murphy and Linhart 1999). Target trees represent fewer than 10 percent of the trees in stands populated by Abert's squirrels along the Front Range, and they differ chemically and physiologically from non-target trees (M. Snyder 1992). The squirrels are probably important agents of natural selection in southwestern ponderosa pine woodlands, but not the only agents. M. Snyder (1998) noted that each of the other 3 major specialized consumers on Pinus ponderosa-North American porcupine (Erethizon dorsatum), western pine beetle (Dendroctonus ponderosae ), and dwarf mistletoe (Arceuthobium vaginatum)-is biased toward trees with a different chemical signature, so the trees are caught in a complex system of diversifying selection. Abert's squirrels also eat fungi, both aboveground (epigeous) and belowground (hypogeous) forms. Fungal spores in the squirrel's feces may help spread mutualistic mycorrhizal associates of ponderosa pine (States and Wettstein 1998). Carrion also is eaten, and bones and antlers are often gnawed for their mineral content. Food caches are not established.
D1git1zed by Google Abert's squirrels are diurnal and active year-round. Winter foraging focuses on terminal twigs in the tree crown. They gnaw through the base of a twig, strip the needle clusters, remove the outer bark, eat the inner bark, and drop the remaining part of the twig. These feeding sites are recognizable by the severed needle clusters and bare, discarded twigs at the base of trees (M. Snyder 1992). States et al. (1988) detailed seasonal patterns offoods and foraging by Abert's squirrels in northeastern Arizona. At night and when inactive during the day, they retreat to tree nests. Occasionally animals of different ages and sexes share the same nest, especially in winter and spring. Nests are typically in ponderosa pine, with most nest trees having a diameter of more than 0.3 m. Nests are either constructed from pine twigs up to 60 em in length and 2 em in diameter or "excavated" in the mass of twigs that proliferates from dwarf mistletoe infestations. Nest chambers are lined with grass and other soft materials. Nests are 5-18 m aboveground. The nest is frequently located on the south side of the tree and near the trunk; such areas would receive early sun and be somewhat protected from wind. Abert's squirrels appear to do best in ponderosa pine stands with close to 60 percent canopy closure and over 220 stems per hectare (Farentinos 1972c; D. Patton 1977, 1984). Nest trees also differed in chemical composition from non-nesting trees (M. Snyder and Linhart 1994). In Arizona, Abert's squirrels nested preferentially in larger trees and nests tended to be on the south or east side of the bole. Where sympatric in the Pinaleiio Mountains of Arizona, pine squirrels and Abert's squirrels used different kinds of nests. Pine squirrels mostly used tree holes (especially in quaking aspen), whereas Abert's squirrels built nests of branches and twigs in conifers, especially in Douglas-fir. Although not territorial, Abert's squirrels are mostly solitary except during the breeding season. Home ranges in Colorado ranged from about 5 to 20 ha, depending on season and sex of the animal. Mean home range size for both sexes is about 8 ha. Males typically have larger home ranges, especially when breeding, although adult females may cover large distances in summer (Farentinos 1979).
D1git1zed by Google Abert's squirrels show a variety of visual displays and have several vocalizations. Displays include tail-flaring, tail-flicking, and foot-stomping, all of which are associated with varying degrees of alarm. Calls include alarm barks, tooth chattering, screams, and clucks. Changes in position of the tail and ears and general body posture are used during agonistic displays in the mating season, communicating degrees of dominance or subordinance (Farentinos 1974). As in other tree squirrels, courtship is marked by frantic chases, with males following estrous females up and down trees and over the forest floor. Such chases may continue for several hours and result in multiple copulations by the same or different males. Dominant males mate first, but in prolonged chases subordinate males may also have a chance to mate. Aggressive behaviors are typical during these chases, with the female attacking her male admirers or making vocalizations and threat displays. Males also engage in male-male aggressive behaviors during the chase (Rice 1957; Farentinos 1972b, 1980). Most mating occurs in April or May in Colorado, with only 1litter per year. (Winter breeding has been reported from Arizona, but its generality is not known; Pogany et al. 1998.) Gestation takes about 46 days. Litter size averages about 3 (range, 2 to 4; Farentinos 1972b). Young squirrels are naked at birth. The eyes are open and the pelage is well developed by about 6 weeks of age. Young animals may leave the nest at about 9 weeks of age but still rely on the female until about 2.5 months old. Population dynamics of the species are poorly known. Sex ratios strongly favor males, 1.4:1.0 (J. Keith 1965; Farentinos 1972b ). Population estimates range from 12 to 30 animals per square kilometer in the Black Forest ofEl Paso County, and from 82 to 114 animals per square kilometer near Boulder (Farentinos 1972a). Spring populations are lowest. Adults made up 53-65 percent of populations near Boulder. Annual mortality ranged from 22 to 65 percent in Arizona, with the speculation that severe snow conditions can lead to high mortality (R. Stephenson and Brown 1980). Drennan et al. (1998) successfully used track stations to index the abundance of Abert's squirrels, chipmunks, and ground squirrels in northeastern Arizona.
D1git1zed by Google Information on predation is sparse. Hawks (especially northern goshawks) may be the most capable predators (Beier and Drennan 1997; Drennan and Beier 2003), as most terrestrial carnivores are not very successful at catching this arboreal species. Martens generally occur at higher elevations than do these squirrels. Food availability is probably important, as ponderosa pine seed crops are variable. A review of the biology of Abert's squirrel in comparison with other western tree squirrels was provided by W. Smith et al. (2003). Abert's squirrels are listed as small game mammals in Colorado (Table 4-1), with an open season from mid-November to mid-January, but data on harvest (Table 4-3) are unavailable. Nash and Seaman (1977) and J. Keith (2003) reviewed the literature on the species.
Sciurus niger FOX SQUIRREL
Description
The fox squirrel is a large tree squirrel, gray-brown above and shading to reddish brown laterally. The belly is rufous, yellowish brown, or whitish. The tail is bushy, grayish red dorsally and red below. Measurements are: total length 450-600 mm; length of tail 170-300 mm; length ofhindfoot 50-75 mm; length of ear 19-3 3 mm; weight 400-1, 100 g. The skull is very similar to that of Abert's squirrel although it often has a reddish tinge. The skull of a fox squirrel will fluoresce red under ultraviolet light, a character unique to this mammal and caused by normal erythropoietic porphyria (Flyger and Levin 1977).
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