A Comparison of Abert A Comparison of Abert squirrels, pine squirrels and squirrels, pine squirrels and fox squirrelsfox squirrels with respect to with respect to

life in the coldlife in the cold

by by Justina ThorsenJustina Thorsen

20062006

Winter Ecology – Spring 2006Mountain Research Station – University of Colorado,

Boulder

Topic under ConsiderationTopic under Consideration Abert, fox and pine squirrels have overlapping Abert, fox and pine squirrels have overlapping

ranges in Boulder Mountain Parks (BMP). ranges in Boulder Mountain Parks (BMP). What are their different adaptations to the cold What are their different adaptations to the cold winters on the Colorado Front Range?winters on the Colorado Front Range?

The Tree SquirrelsThe Tree Squirrels

SciurusSciurus = derived from Greek terms “shadow” = derived from Greek terms “shadow” and “tail” and “tail”

Thus, an animal who sits in the shadow of it’s tail (Gurnell Thus, an animal who sits in the shadow of it’s tail (Gurnell 1987; MacClintock 1970)1987; MacClintock 1970)

TamiasTamias = food caching = food caching Bushy tails for: balance, communication and Bushy tails for: balance, communication and

TTbodybody regulation regulation Tails often approach 1/3 of body lengthTails often approach 1/3 of body length

Hindfeet with 5 digits, forefeet with 4 digitsHindfeet with 5 digits, forefeet with 4 digits May be furry in winter (Gurnell 1987)May be furry in winter (Gurnell 1987)

Three Species ConsideredThree Species Considered

Sciurus aberti

Tamiasciurus hudsonicus

Sciurus niger

Where Are They?Where Are They?

Distribution of the Abert squirrel

Where are They?Where are They?

Distribution of the Fox Squirrel

Where are They?Where are They?

Distribution of the pine squirrel

Nest TypesNest Types Dreys: twig and leaf nests built in treesDreys: twig and leaf nests built in trees

Winter dreys are more elaborate than summer ones & must Winter dreys are more elaborate than summer ones & must withstand inclement weatherwithstand inclement weather

Winter dreys tend to be circular; summer dreys saucer shapedWinter dreys tend to be circular; summer dreys saucer shaped

Dens: holes or cavities in tree trunksDens: holes or cavities in tree trunks Holes: underground or in rockHoles: underground or in rock

(Gurnell 1987)(Gurnell 1987)

Squirrel in it’s dreySquirrel in it’s drey

Body Size and Bergman’s RuleBody Size and Bergman’s Rule In order of increasing size: pine, abert, foxIn order of increasing size: pine, abert, fox

Pine squirrels are smallest N. American tree Pine squirrels are smallest N. American tree squirrelsquirrel

Occupy the coldest, most northern habitats in N. America and the highest Occupy the coldest, most northern habitats in N. America and the highest altitudes in Coloradoaltitudes in Colorado

Fox squirrels are largest N. American tree squirrelFox squirrels are largest N. American tree squirrel Occupy the lowest altitudes in ColoradoOccupy the lowest altitudes in Colorado

Coat ColorCoat Color Abert squirrels of Colorado tend to be Abert squirrels of Colorado tend to be

melanisticmelanistic Farentinos (1971) estimated 56.7% melanismFarentinos (1971) estimated 56.7% melanism

Pine squirrels are ashy gray with white Pine squirrels are ashy gray with white undersidesundersides

Fox squirrels are peanut colored with rusty Fox squirrels are peanut colored with rusty undersidesundersides

Is Black Beneficial?Is Black Beneficial? Golightly and Ohmart (1978) found that:Golightly and Ohmart (1978) found that:

Sun basking is common among Abert squirrels on Sun basking is common among Abert squirrels on winter afternoonswinter afternoons

Sun basking resulted in increased TSun basking resulted in increased TBodyBody

Basking squirrels had higher TBasking squirrels had higher TBodyBody than inactive than inactive and nesting squirrelsand nesting squirrels

Food UtilizationFood Utilization Fox and pine squirrels are food generalists and Fox and pine squirrels are food generalists and

opportunistic eatersopportunistic eaters Abert’s are food specialists and are Abert’s are food specialists and are

ecologically dependent upon Ponderosa pinesecologically dependent upon Ponderosa pines The primary food for all squirrels is tree seeds The primary food for all squirrels is tree seeds

and fruitsand fruits Secondary foods: berries, mushrooms and Secondary foods: berries, mushrooms and

other plant matterother plant matter

Selective herbivory of Abert Selective herbivory of Abert squirrelssquirrels

Diet consists almost Diet consists almost entirely of ponderosa entirely of ponderosa pine tissues and other pine tissues and other closely associated closely associated species species (Snyder 1998, States and (Snyder 1998, States and Wettstein 1998)Wettstein 1998)

Abert squirrels eat Abert squirrels eat mycorrhizal fungi in the mycorrhizal fungi in the summer months and summer months and inner bark during winter inner bark during winter months months (States and Wettstein (States and Wettstein 1998)1998)

Food Caching of Pine SquirrelsFood Caching of Pine Squirrels Pine squirrels establish middens which they Pine squirrels establish middens which they

guard defensivelyguard defensively They cut and cache green cones in late They cut and cache green cones in late

summer to ripen and provide winter food summer to ripen and provide winter food sources sources (MacClintock 1970)(MacClintock 1970)

Middens often have underground networks of Middens often have underground networks of tunnels and nest cavitiestunnels and nest cavities

Fall Gluttony of Fox SquirrelsFall Gluttony of Fox Squirrels Fox squirrels prepare for the unpredictability Fox squirrels prepare for the unpredictability

and scarcity of food resources during winter and scarcity of food resources during winter by overeating in the fall by overeating in the fall (Steel and Koprowski 2001)(Steel and Koprowski 2001)

Fox squirrels may scatterhoard foodFox squirrels may scatterhoard food

Levels and Patterns of ActivityLevels and Patterns of Activity

Inclement weather reduces activity, although Inclement weather reduces activity, although active squirrels may be seen in severe weatheractive squirrels may be seen in severe weather

Wind and rain significantly reduce TWind and rain significantly reduce Tbodybody (Golightly and (Golightly and Ohmart 1978)Ohmart 1978)

Fresh snow severely restricts movements (Golightly and Fresh snow severely restricts movements (Golightly and Ohmart 1978)Ohmart 1978)

Pine squirrels become subnivean and subterranean when Pine squirrels become subnivean and subterranean when TTairair is very low (Gurnell 1987) is very low (Gurnell 1987)

Summary of AdaptationsSummary of Adaptations Aberts:Aberts:

Behavioral – sun baskingBehavioral – sun basking Morphological – ear tufts, black coatMorphological – ear tufts, black coat Physiological – utilization of inner barkPhysiological – utilization of inner bark

Pine Squirrels:Pine Squirrels: Behavioral – middens, territorialBehavioral – middens, territorial

Fox Squirrels:Fox Squirrels: Behavioral – fall gluttony, scatterhoardingBehavioral – fall gluttony, scatterhoarding Morphological/Physiological - BATMorphological/Physiological - BAT

ConclusionsConclusions The three species of tree squirrels each exhibit The three species of tree squirrels each exhibit

unique sets of adaptations to life in the cold.unique sets of adaptations to life in the cold. All three species experience similar All three species experience similar

environmental pressures where their ranges environmental pressures where their ranges overlap overlap (on the Front Range of BMP)(on the Front Range of BMP)

All three species exhibit a degree of inhibition All three species exhibit a degree of inhibition from inclement weather from inclement weather (including: wind, rain, fresh snow, low (including: wind, rain, fresh snow, low TTairair))

Selected References and Literature Selected References and Literature CitedCited

Armstrong, D.M. 1987. Rocky Mountain Mammals, revised edition. Boulder, Armstrong, D.M. 1987. Rocky Mountain Mammals, revised edition. Boulder, Colorado: Colorado Associated University Press. 223 pp Colorado: Colorado Associated University Press. 223 pp

Farentinos, R.C. 1971. Social dominance and mating activity in the tassel-eared squirrel Farentinos, R.C. 1971. Social dominance and mating activity in the tassel-eared squirrel ((Sciurus aberti ferrusSciurus aberti ferrus). Unpublished. PhD Thesis for the University of Colorado, Boulder. 73 ). Unpublished. PhD Thesis for the University of Colorado, Boulder. 73 pp.pp.

Golightly, RT Jr. and RD Ohmart. 1978. Heterothermy in free ranging Abert’s squirrels (Golightly, RT Jr. and RD Ohmart. 1978. Heterothermy in free ranging Abert’s squirrels (Sciurus Sciurus abertiaberti). Ecology, 59(5): 897-909.). Ecology, 59(5): 897-909.

Gurnell, J. 1987. The Natural History of Squirrels. New York, New York: Facts on File Gurnell, J. 1987. The Natural History of Squirrels. New York, New York: Facts on File Publications. 201 pp.Publications. 201 pp.

Hamilton, WJ III and F Heppner. 1967. Radiant solar energy and the function of black Hamilton, WJ III and F Heppner. 1967. Radiant solar energy and the function of black homeotherm pigmentation: an hypothesis. Science, 155: 196-197.homeotherm pigmentation: an hypothesis. Science, 155: 196-197.

Keith, JO. 1965. The Abert squirrel and its dependence on Ponderosa pine. Ecology, 46: 150-163.Keith, JO. 1965. The Abert squirrel and its dependence on Ponderosa pine. Ecology, 46: 150-163.

MacClintock, D. 1970. Squirrels of North America. New York, New York: Van Nostrand MacClintock, D. 1970. Squirrels of North America. New York, New York: Van Nostrand Reinhold Company. 184 pp.Reinhold Company. 184 pp.

Larson, MM and GH Schubert. 1970. Cone crops of ponderosa pine in central Arizona including Larson, MM and GH Schubert. 1970. Cone crops of ponderosa pine in central Arizona including the influence of Abert squirrels. USDA Forest Service Research Paper RM-58, Rocky Mtn. the influence of Abert squirrels. USDA Forest Service Research Paper RM-58, Rocky Mtn. Forest and Range Experiment Station, Ft. Collins Colorado. Pp. 15.Forest and Range Experiment Station, Ft. Collins Colorado. Pp. 15.

Marchand, PJ. 1987. Life in the Cold. Hanover: University Press of New England. 304 pp.Marchand, PJ. 1987. Life in the Cold. Hanover: University Press of New England. 304 pp.

Snyder, M.A. 1998. Abert’s squirrels (Snyder, M.A. 1998. Abert’s squirrels (Sciurus abertiSciurus aberti) in Ponderosa pine () in Ponderosa pine (Pinus Pinus ponderosaponderosa) forests: directional selection, diversifying selection. In: Steel J.F. and D.A. Zegers ) forests: directional selection, diversifying selection. In: Steel J.F. and D.A. Zegers (eds). Ecology and Evolutionary Biology of Tree Squirrels. 6th Special Publication. VA: (eds). Ecology and Evolutionary Biology of Tree Squirrels. 6th Special Publication. VA: Virginia Museum of Natural History. p 195-201.Virginia Museum of Natural History. p 195-201.

States J.S. and P.J. Wettstein. 1998. Food habits and evolutionary relationships of the tassel-eared States J.S. and P.J. Wettstein. 1998. Food habits and evolutionary relationships of the tassel-eared squirrel (squirrel (Sciurus abertiSciurus aberti). In: Steel J.F. and D.A. Zegers (eds). Ecology and Evolutionary ). In: Steel J.F. and D.A. Zegers (eds). Ecology and Evolutionary Biology of Tree Squirrels. 6th Special Publication. VA: Virginia Museum of Natural History. Biology of Tree Squirrels. 6th Special Publication. VA: Virginia Museum of Natural History. p 185-194.p 185-194.

Steel, MA and JL Koprowski. 2001. North American Tree Squirrels. Washington: Smithsonian Steel, MA and JL Koprowski. 2001. North American Tree Squirrels. Washington: Smithsonian Institute Press. 201pp.Institute Press. 201pp.