Squirrels and relatives III

Tree squirrels

Class Mammalia

Order Rodentia

Suborder Sciurognathi

Family Sciuridae

Subfamily Sciurinae

Thumbnail description
Small to medium sized rodents, hind limbs disproportionately long; most species have a prominent tail; fur color extremely variable

Size
Head and body length 2.95–18 in (75–460 mm), tail almost as long again; 0.53 oz–6 lb (15–3,000 g)

Number of genera, species
21 genera; 117 species

Habitat
Forest, woodland, scrub, urban areas, parks, and gardens

Conservation status
Endangered: 1 species; Vulnerable: 14 species; Lower Risk/Near Threatened: 20 species; eight subspecies are considered threatened

Distribution
Found on all continents except Antarctica and Australia

Evolution and systematics

Tree squirrels are fascinating rodents whose arboreal roots are thought to go back to the Eocene (54–38 million years ago [mya]). The first members of the squirrel family which were "tree squirrel like" in their dental morphology appear in the fossil record during the Oligocene (37–25mya). These include specimens from France, Sciurus dubius, dated to the early to middle Oligocene and members of the genus Protosciurus, which occurred in North America from the early Oligocene to the early Miocene (24–5 mya). The Miocene fossil record also contains a large number of genera of flying and ground squirrels from Europe, Asia, and North America as well as genera of tree squirrels that are still extant today.

The Sciuridae squirrels are divided into ground and tree squirrels in the subfamily Sciurinae and flying squirrels in the subfamily Pteromyinae. The number of recognized genera and species varies among authors. Including arboreal giant and pygmy squirrels, there are 21 described genera with approximately 117 species: Tree squirrels (Callosciurus, Funambulus, Funisciurus, Glyphotes, Heliosciurus, Microsciurus, Paraxerus, Prosciurillus, Sciurus, Sundasciurus, Syntheosciurus, and Tamiasciurus), giant squirrels (Epixerus, Protoxerus, Ratufa, Reithrosciurus, and Rubrisciurus), pygmy squirrels (Exilisciurus, Myosciurus, Nannosciurus, and Sciurillus).

Some genera such as Sciurus or Callosciurus also contain a significant number of described subspecies. However, the precise status of some of these is in doubt and as new morphological and genetic data become available the number of recognized species may change.

Physical characteristics

The arboreal squirrels range in their size from the incredible pygmy squirrels of tropical forests weighing fractions of an ounce (a few tens of grams) to the giant squirrels of India several pounds (kilograms) in weight. Males and females are similar and there is no sexual dimorphism in either color or size. Their bones are relatively light and their skeleton shows adaptations for climbing. Their hind legs are disproportionately long. They have long toes with curved claws. Their tails, almost as long as their bodies, are used for balance, signaling, and thermoregulation. Their tails gave the genus Sciurus its name, stemming from the ancient Greek with skia meaning shade or shadow and oura meaning tail. They have different sets of sensory hairs, vibrissae, on the head, feet, forelegs, on the underside of their body, and at the base of the tail. These help them orient themselves and move through their three dimensional environment.

Distribution

Tree squirrels are found throughout the world. They are present on all continents with the exception of Australia and Antarctica. A gray squirrel (Sciurus carolinensis) population introduced to Australia went extinct in the 1970s. They are absent

from the polar regions, Madagascar, parts of southern South America, north Africa, and Australasia.

Habitat

Tree squirrels occur in forests, woodlands, gardens, urban areas, and agricultural landscapes. Space and habitat use is extremely

flexible and linked to the spatial and temporal distribution of resources. Social organization encompasses individual territoriality and overlapping home ranges with territorial female core areas in habitats where food is sparse and patchily distributed. In contrast, male space use appears to be affected by access to females during the breeding period. There are three known strategies: males adapt their use of space to overlap with as many females as possible; make

excursions out of their normal home range to visit estrus females; or track female movements. In areas where several species co-occur such as in African rainforest habitats, Emmons in 1980 found that two of nine species were restricted to specific habitats, foraging by the different species occurred at different heights on and above the ground in the trees, and there was a temporal partitioning with differences in the timing of the active period. In addition, the species differed in body size with resulting differences in their diet.

Behavior

Tree squirrels do not hibernate and do not form colonies. Most species are solitary, but some African species such as Heliosciurus rufobrachium or Funisciurus anerythrus are known to move in pairs or small groups. Tree squirrels use olfactory, vocal, and visual signals to communicate. Chemical signals are important in rodent communication and tree squirrels have a number of scent glands in the facial area around the mouth and cheeks. "Cheek-rubbing" is sometimes linked to biting of the substrate, and urine is used to mark the base of specific trees and the underside of branches. Anal dragging is also reported for some species in which anal glands and perhaps fecal materiel are used to produce a signal. Little is known about the different functions of marking in relation to orientation or inter-individual communication or even the potential for disease transmission.

Vocalizations of squirrels can be linked to several behavioral contexts. These include contact seeking or distress of young, alarm calls, calls associated with mating, and aggressive sounds. In contrast to forest species, estrus females in savanna species such as Paraxerus cepapi or Funisciurus congicus use vocal calls rather than olfactory clues to attract males. Some rainforest species have been reported to lower the frequency and to increase the length of calls as an adaptation to the dense vegetation around them.

Squirrels also make use of their fluffy tails, and tail waving and shivering is sometimes observed when males approach

females during mating. Tail flicking as well as foot stamping, linked to aggressive postures such as piloerection (hairerection), are seen during agonistic encounters.

Feeding ecology and diet

Fruit, leaves, and bark are important in the diet of tropical giant squirrels, whereas nuts as well as conifer and deciduous tree seeds are a significant component of North American and European squirrels. A large variety of other food items are also consumed including fungi, berries, buds, shoots, tree sap, and insects. Anecdotal evidence also indicates that some species may opportunistically prey on bird eggs in nests. Squirrels are also known to satisfy their calcium demands by gnawing bones.

Squirrels, like many birds, hide food items to consume them at a later time. Two main strategies have been observed: scatter hoarding in which one or several items are cached at different locations throughout the squirrels territory or home range and larder hoarding in which a large amount of food is stored in one or a small number of sites. Caching is thought to be an adaptive strategy and food stores are important in periods of adverse weather conditions and low natural food availability. Its intensity varies with habitat type and the behavior has been linked to increased survival and reproductive success. The "stealing" of European red squirrel, Sciurus vulgaris, caches by the larger introduced gray squirrel, S. carolinensis, has been suggested as a factor contributing to the decline of the native red squirrel in Italy by Wauters and others in 2002.

Reproductive biology

Tree squirrels have a polygamous mating system in which either sex can have more than one mate. Most knowledge comes from studies in North America and Europe where mating behavior among the different species of tree squirrel is very similar. Females are in estrus for less than one day and males compete for access to the females. During "mating chases" in which a female is often followed by several males, she will try and avoid pursuing males and mate in sheltered locations with reduced risk of attack and injury. Generally, there is a hierarchy among the males pursuing the female with the dominant males closest to the female. Approaches of males towards the female are cautious with withdrawal movements, vocalizations, and flicking actions of the tail to check whether the female will accept the male.

The onset of estrus is not synchronous in females within a population and depends on body condition. Males do not contribute to parental care. Litter size and reproductive success

are linked to habitat quality and food availability with some females succeeding in raising more than one litter a year.

Squirrels nest in tree cavities (dens), nests (dreys), or in some cases holes in the ground. Dreys are elaborate circular structures made up of a coarse outer layer of leaves or needles and sticks and a woven inner layer of grass, moss, bark, feathers and other similar materials. Communal nesting, particularly during periods of adverse weather occurs in a number of species.

Conservation status

The IUCN lists 14 species (Epixerus ebii, E. wilsoni, Funambulus tristriatus, Funisciurus isabella, Paraxerus alexandri, Prosciurillus weberi, Sciurus anomalus, S. arizonensis, S. richmondi, S. sanborni, Sundasciurus brookei, S. moellendorffi, S. steerii, and Syntheosciurus brochus) in the Lower Risk category, 14 species as Vulnerable (Callosciurus pygerythrus, C. quinquestriatus, Funisciurus carruthersi, Myosciurus pumilio, Paraxerus cooperi, P. palliatus, P. vexillarius, P. vincenti, Prosciurillus abstrusus, Ratufa indica, R. macroura, Sundasciurus jentinki, S. rabori, and S. samarensis), and one species as Endangered (Sundasciurus juvencus). The European red squirrel, Sciurus vulgaris, is considered endangered in parts of its range, and the United States Endangered Species Act list two subspecies as endangered. This includes the Mt. Graham red squirrel (Tamiasciurus hudsonicus grahamensis) of Arizona and the Delmarva fox squirrel (Sciurus niger cinereus) of the Delmarva Peninsula to southeastern Pennsylvania.

The main causes for species declines are human induced habitat loss, degradation, and fragmentation as well as persecution and hunting. However, there is insufficient information on the behavior, ecology, population dynamics, and threats to many Asian, South American, and African species and a need for basic ecological studies to allow adequate conservation assessments.

Significance to humans

Squirrels have been hunted for sport, their fur, and their meat. Some species are considered a pest or nuisance due to damage to crops or trees or pose a threat to native species due to introductions outside their natural range. However, many people take pleasure in their presence in parks and gardens and enjoy watching their ingenious ability to overcome obstacles in search of food. Squirrels are also notably present in the folklore of all countries where they are found, included in children's tales, sayings, and weatherlore: "Squirrels gathering nuts in a flurry, will cause snow to gather in a hurry."

Species accounts

List of Species

Red squirrel

Sciurus vulgaris

taxonomy

Sciurus vulgaris Linnaeus, 1758, Uppsala, Sweden.

other common names

French: Ecureuil roux; German: Eichhörnchen; Spanish: Ardilla roja.

physical characteristics

Head and body length 7–9.5 in (180–240 mm), tail 5.5–7.6 in (140–195 mm); weight 7.8–15.4 oz (220–435 g); sexes similar, large variation in coat color across range. Upper fur varies from red-brown or chestnut to very dark in appearance. Fur of some subspecies in winter, such as the Yenisei squirrel (S. v. jenissejensis), Siberia, is an intense bluish gray with dark gray mottling. Ear tufts grow in autumn and are prominent during the winter but are absent during the summer.

distribution

From Ireland and Britain in the west, across the Palaearctic to Hokkaido in Japan.

habitat

Deciduous, mixed, and coniferous woodlands, also occurs in suburban areas, gardens, and parks.

behavior

The species is not territorial but occupies home ranges which overlap with other conspecifics. Core areas of females within the home range are thought to be exclusive in some habitats. Habitat use flexible in large conifer forests with frequent home range shifts by individuals tracking seed crops in different conifer species. Red squirrels are diurnal and active all year.

feeding ecology and diet

Main diet is tree seeds and fungi. In poor seed years red squirrels will also feed on buds, shoots, and berries

reproductive biology

Promiscuous mating system, males and females do not form pair bonds. Breeding starts December–January and continues until early October when the last litters are weaned. There are two breeding peaks within a year; spring (February–April) and summer (May–August) litters respectively. Breeding may be delayed or missed in years with poor food supplies.

conservation status

Dramatic range and population declines in Britain, Ireland, and parts of Italy due to competition by the introduced North American gray squirrel. Habitat fragmentation and disease outbreaks such as the parapox virus in England have also contributed to population declines. The species is considered endangered in the United Kingdom.

significance to humans

Traditionally hunted for its fur. The species now is a cherished sight in parks and gardens. Significant animal in folklore, in

Nordic myths the squirrel Ratatosk was the carrier of messages between the eagle at the top of Yggdrasil the ash tree spanning the worlds to the serpent Niddhogg below.

Gray squirrel

Sciurus carolinensis

taxonomy

Sciurus carolinensis Gmelin, 1788, "Carolina." United States.

other common names

English: Eastern gray squirrel, migratory squirrel; French: Ecureil gris; German: Grauhörnchen; Spanish: Ardilla gris.

physical characteristics

Head and body length 9.4–11.2 in (240–285 mm), tail 7.5–9.5 in (190–240 mm); weight 10.6–26 oz (300–720 g); sexes similar. Grizzled black to pale gray coat color mixed with reddish brown on feet, hips, and head, underpart white to pale gray.

distribution

Eastern United States, with introductions to Montana, Oregon, Washington, and California, also introduced to Quebec, New Brunswick, British Colombia, Manitoba, and Vancouver Island in Canada and Britain, Ireland, and South Africa.

habitat

Mature continuous forest and woodland. Highest observed densities in stands of oak, walnut, and hickory, very low densities in conifer dominated plantations colonized by gray squirrels in Britain.

behavior

Diurnal activity pattern, active all year. Gray squirrels considered solitary and both sexes disperse. However, female-biased philopatry and the formation of female kin groups observed in high quality parkland in Kansas.

feeding ecology and diet

Gray squirrels feed predominantly on nuts as well as tree seeds of both deciduous and coniferous species. Other food items include, flowers, buds, fungi, and fruit. Gray squirrels hoard seeds such as acorns. The squirrels may react to a chemical cue in the shell of the acorns and caching decisions were found to be based on perishability with longer lived items being cached rather than consumed. In addition, gray squirrels increase the longevity of their food stores by preventing germination through embryo excision. This is particularly the case for white oak (Quercus alba) acorns.

reproductive biology

Polygamous. Females can have young within their first year of age. There are two breeding peaks, December to February and May to June (breeding in South Africa begins in October). Males are attracted to estrus females from considerable distances away and start following the females, sometimes several days before onset of estrus. Gestation period is approximately 44 days, followed by approximately 70 days of lactation. Litter size ranges from 1–7 with an average of about 2.3.

conservation status

Common; not threatened

significance to humans

Hunted for sport and food. Adored by many in city parks and gardens, although gardeners dislike them for digging up bulbs and competing with birds at feeders. Considered a pest in Britain due to causing significant damage to deciduous and

coniferous trees in woodlands through bark stripping and due to competition with red squirrels.

Eastern fox squirrel

Sciurus niger

taxonomy

Sciurus niger Linnaeus, 1758, South Carolina, United States.

other common names

English: Cat squirrel, stump-eared squirrel.

physical characteristics

Head and body length 10–14.5 in (260–370 mm), tail 7–13 in (180–330 mm); weight 17.5–49.5 oz (500–1,400 g). Coat color variable.

distribution

Eastern and central United states reaching southern Canada in the north. Introduced to Ontario in Canada and New Mexico, Texas, California, Oregon, Colorado, Idaho, and North Dakota within the United States.

habitat

Forests, woodlands, urban areas, and agricultural landscapes.

behavior

Diurnal activity pattern, period of greatest activity in winter is in the early morning. Social system characterized by dominance hierarchy. Main factors influencing dominance status of individuals are their sex and age. Fox squirrels will forage in open fields near woodland. The behavior of caching seeds in grassland thought to be important in seed dispersal and habitat succession.

feeding ecology and diet

Tree seeds and nuts as well as buds and flowers of trees.

reproductive biology

Polygamous. Reproductively active from mid-December onwards. Similar to gray squirrels, there are two breeding peaks within a year but not all females produce two litters. Mean litter size ranges from 2–3.5 young.

conservation status

Delmarva Peninsula fox squirrel, S. n. cinereus, considered endangered. Logging and changes to habitat composition have been suggested as causes for decline. The Big Cypress fox squirrel, S. n. avicennia, in Florida is locally regarded as threatened.

significance to humans

Hunted for food and sport.

Abert squirrel

Sciurus aberti

taxonomy

Sciurus aberti Woodhouse, 1853, Arizona, United States.

other common names

English: Tassel-eared squirrel; Spanish: Ardilla de Abert.

physical characteristics

Head and body length 10–12.5 in (260–320 mm), tail (190–260 mm); weight 16–29 oz (450–820 g). Peppered gray upper fur, underside white. White conspicuous eye ring. Tail is gray fringed in white. Ear tufts or tassels can reach up to 1.5 in (40 mm) in February and March, very reduced or absent during the summer. Some populations in New Mexico and Colorado have a high incidence of melanism. Kaibab race characterized by dark gray to black belly and white tail.

distribution

Northern Mexico, Arizona, New Mexico, Utah, Colorado, and Wyoming.

habitat

Ponderosa and yellow pine forests, mixed conifer forests.

behavior

Diurnal activity pattern. Home range size reported to vary seasonally linked to the availability of resources, with male home ranges larger than female ranges.

feeding ecology and diet

Ponderosa pine seeds, bark, buds, and flowers. Other food items include insects, acorns, mistletoe berries, and fungi. Fungi are an important food source in the summer and Abert squirrels may be important spore dispersal agents.

reproductive biology

Polygamous. Gestation approximately 40 days, mean litter size 3.4 young.

conservation status

There have been suggestions that existing logging and forest management practices may impact negatively on some populations.

significance to humans

None known.

Arizona gray squirrel

Sciurus arizonensis

subfamily

Sciurinae

taxonomy

Sciurus arizonensis Coues, 1867, Arizona, United States.

other common names

None known.

physical characteristics

Head and body length 9.6–12.6 in (244–321 mm), tail 7.5–11 in (200–285 mm); weight 18–31 oz (520–870 g). Upper body grizzled silver gray, in winter with brownish yellow dorsal stripe. Underside white, tail is black fringed with white; with orange to rusty brown hairs ventrally. White eye ring.

distribution

Arizona, New Mexico, and Sonora, Mexico.

habitat

Riparian deciduous forest, mixed broadleaf forests at elevations of 3,600–9,200 ft (1,100–2,800 m).

behavior

Diurnal activity pattern. Nests are located in a variety of species including sycamores, walnut trees and Apache pines, most found in oak trees. Recorded vocalizations include chucking and barking alarm calls.

feeding ecology and diet

Diet includes corns, walnuts, conifer seeds, berries, tree flowers, and alder buds.

reproductive biology

Polygamous. Breeding activity linked to flower emergence and most females in estrus in April and early May. Observed litter size ranges from 2 to 4.

conservation status

Introduction of Abert squirrels into the Catalina Mountains may adversely affect existing populations. Arizona gray squirrels have been listed as a Category 2 species by the Fish and Wildlife Service. Considered endangered in Mexico due to severe habitat loss through agricultural development and logging.

significance to humans

Visits picnic sites in the mountains. Focus of scientific study.

Peters's squirrel

Sciurus oculatus

taxonomy

Sciurus oculatus Peters, 1863, Mexico.

other common names

English: Black-backed or spectacled squirrel.

physical characteristics

Head and body length 9.5–11 in (250–280 mm), tail (250–270 mm); weight 14.5–19.5 oz (410–550 g). Upper fur grizzled gray or containing a band of black. Tail is black with suffusion of white. Underparts white or mixed with pale yellow or ochre.

distribution

Mexican plateaus from Morelos to San Luis Potosi.

habitat

Oak, pine, and fir forest between 4,900–11,810 (1,500–3,600) m elevation.

behavior

Animals are capable of moving swiftly through the trees. Little known about their space use and social organization.

feeding ecology and diet

Known to feed on acorns, almonds, and wild figs.

reproductive biology

Little known about breeding behavior, females observed to be in breeding condition in July in Veracruz.

conservation status

Agricultural development caused significant loss of forest habitat and the species is considered fragile.

significance to humans

Occasionally hunted for food in some areas.

Variegated squirrel

Sciurus variegatoides

taxonomy

Sciurus variegatoides Ogilby, 1839, El Salvador.

other common names

Spanish: Ardilla chiza.

physical characteristics

Head and body length 8.7–10.6 in (240–270 mm), tail 9.4–12.5 in (240–320 mm), 15.5–32 oz (440–900 g). Variable in color, shiny coat of black to grizzled yellowish gray, underside white, feet whitish speckled with black.

distribution

From central Panama, Costa Rica, Nicaragua, Honduras, El Salvador, Guatemala north to the southern Chiapas in Mexico.

habitat

Tropical evergreen forests, nut palm forests, as well as deciduous tropical forests.

behavior

Arboreal squirrel with diurnal activity pattern. Nest located on stems near the main trunk. Territories may range over 5.9 acres (2.4 ha).

feeding ecology and diet

Fruits, nuts, seeds of hard shelled fruits, and fungi. Main components of an observed diet included the seeds of guacimo, mango, and hogplum.

reproductive biology

Polygamous. Breeding period thought to be in April or May. Litter size variable, approximately 4–6 young.

conservation status

Some populations in Mexico considered fragile.

significance to humans

Occasionally hunted.

North American red squirrel

Tamiasciurus hudsonicus

taxonomy

Tamiasciurus hudsonicus (Erxleben 1777), Hudson Bay, Canada.

other common names

English: Pine squirrel, chickaree; French: Ecureuil de l'Hudson; Spanish: Ardilla de Douglas.

physical characteristics

Head and body length 7–9 in, (178–230), tail 3.5–6.3 in (90–160 mm); 6.4–8.8 oz (180–250 g). Fur reddish brown or olive gray, underside white or faint yellowish. Tail with yellowish to rusty tips with a black band. White eye ring.

distribution

Boreal forests of Canada and the northern United States from Alaska in the west to Quebec and Main in the east. Fragmented populations in the Apalachian mountains and as far southeast as North Carolina, introduced to Newfoundland. Southward in the west, the species occurs in the Cascade and Rocky mountains and on Vancouver island. There are also isolated populations in subalpine "sky-island" habitats in New Mexico and Arizona.

habitat

Coniferous forests as well as coniferous-deciduous stands. Other habitats include aspen, red pine and Norway spruce. Red squirrels are territorial and a single individual of either sex occupies and defends a territory.

behavior

Diurnal activity pattern. They use vocal calls during courtship and to defend their territory. Four vocalizations and an alarm

call were connected with territorial behavior in a study of the species in the Cascade Mountains, British Colombia.

feeding ecology and diet

Main diet are conifer and deciduous tree seeds and fungi. Other items include tree buds, flowers and sap, bark and insects. The species larder hoards cones in a central cache site (midden) that typically mark the center of the territory. Scatterhoarding of cones has also been observed. Whitebark pine cones are an important food of grizzly bears in Yellowstone and they are know to excavate red squirrel middens.

reproductive biology

Polygamous. Timing of breeding varies throughout the range. Courtship is thought to begin when bare patches of ground appear at the end of winter. During the breeding period males leave their territories more often and females allow males to enter their territories. Females come into estrus for only one day. Litter sizes vary between 3–6 young.

conservation status

Isolated populations such as the Mt. Graham red squirrel subspecies in Arizona are protected and considered endangered.

significance to humans

Hunted or trapped for fur in some areas.

Pallas's squirrel

Callosciurus erythraeus

taxonomy

Callosciurus erythraeus (Pallas, 1779), Assam, India.

other common names

English: Red-bellied squirrel; French: Ecureuil à ventre rouge; Spanish: Ardilla de Pallas.

physical characteristics

Head and body length 7.8–10.2 in (200–260 mm), tail 6–7.6 in (160–195 mm); 11–16 oz (310–460 g). Very variable in color, Upper fur olive brown agouti, underside reddish (varying from maroon to creamy buff).

distribution

From Bhutan and Assam in the east through Myanman to southern China, south to Indochina, Thailand, and Malaya. Also occurs on Taiwan, introduced to Japan and Cap d'Antibes in France.

habitat

Broadleaf evergreen forest, dipterocarp forest, bamboo-rich forest, tropical rainforest. Overlapping home range system of space use, less overlap among females than males.

behavior

Diurnal activity pattern. Observed mobbing snakes in Japan to defend young. Distinct recorded vocalizations for aerial and terrestrial predators leading conspecifics to adopt different escape strategies. Alarm calls also used following mating. This is thought to be a possible strategy by males to delay second matings by the female and increase the probability of paternity.

feeding ecology and diet

Fruit, seeds, nuts, and insects.

reproductive biology

Promiscuous. Reproductively active throughout the year, peaks March to August. Average litter size 1.4 young.

conservation status

Common; not threatened.

significance to humans

Introduced population in France causes damage to trees, may compete with native red squirrel. Considered a pest on oil palm plantations.

Resources

Books

Brown, D. Arizona's Tree Squirrels. Phoenix: Arizona Game and Fish Department, 1984.

Corbet, G. B., and J. E. Hill. A World List of Mammalian Species. 3rd ed. London: British Museum of Natural History, 1991.

Gurnell, J. The Natural History of Squirrels. London: Christopher Helm, 1987.

Gurnell, J., and P. W. W. Lurz, eds. The Conservation of Red Squirrels, Sciurus vulgaris. London: People's Trust for Endangered Species, 1997.

Steele, M. A., and J. Koprowski. North American Tree Squirrels. Washington, DC: Smithonian Institution Press, 2001.

Steele, M. A., J. F. Merritt, and D. A. Zegers, eds. Ecology and Evolutionary Biology Of Tree Squirrels. Martinsville, VA: Virginia Museum of Natural History Special Publication Number 6, 1998.

Wilson, D. E., and D. M. Reeder, eds. Mammal Species of the World: A Taxonomic and Geographic Reference. 2nd ed. Washington, DC: Smithonian Institution Press, 1993.

Periodicals

Best, T. L. "Sciurus oculatus." Mammalian Species 498 (1995).

Best, T. L., and S. Riedel. "Sciurus arizonensis." Mammalian Species 496 (1995).

Black, C. C. "Holarctic evolution and dispersal of squirrels (Rodentia: Sciuridae)." Evolutionary Biology 6 (1972): 305–322.

Emmons, L. H. "Sound communication among African rainforest squirrels." Zeitschrift für Tierpsychologie 47 (1978): 1–47.

——. "Ecology and resource partitioning among nine species of African rainforest squirrels." Ecological Monographs 50 (1980): 31–54.

Hadj-Chikh, L. Z., M. A. Steele, and P. D. Smallwood. "Caching decisions by grey squirrels: a test of handling time and perishability hypotheses." Animal Behaviour 52 (1996): 941–948.

Koprowski, J. "Sciurus niger." Mammalian Species 479 (1994).

——. "Sciurus carolinensis." Mammalian Species 480 (1994).

——. "Natal philopatrie, communal nesting, and kinship in fox squirrels and gray squirrels." Journal of Mammalogy 77 (1996): 1006–1016.

Moore, J. C. "Relationships among the living squirrels of the Sciurinae." Bulletin of the American Museum of Natural History 118 (1959): 153–206.

Steele, M. A. "Tamiasciurus hudsonicus." Mammalian Species 586 (1998).

Steele, M. A., G. Turner, P. D. Smallwood, J. O. Wolff, and J. Radillo. "Cache management by small mammals: Experimental evidence for the significance of acorn-embryo excision." Journal of Mammalogy 82 (2001): 35–42.

Wauters, L. A., G. Tosi, and J. Gurnell. "Interspecific competition in tree squirrels: Do introduced grey squirrels (Sciurus carolinensis) deplete tree seeds hoarded by red squirrels (S. vulgaris)?" Behavioural Ecology and Sociobiology 51 (2002): 360–367.

Other

IUCN Red List of Threatened Species. [February 12, 2003]. <http://www.redlist.org/>

United States Fish and Wildlife Service Endangered Species Programme. [February 12, 2003]. <http://endangered.fws.gov/>

Peter W. W. Lurz, PhD