A Comparison of Abert squirrels, pine squirrels and fox squirrels with respect to life in the cold

by Justina Thorsen 2006 Winter Ecology – Spring 2006 Mountain Research Station – University of Colorado, Boulder

Topic under Consideration

 Abert, fox and pine squirrels have overlapping ranges in Boulder Mountain Parks (BMP). What are their different adaptations to the cold winters on the Colorado Front Range?

The Tree Squirrels



Sciurus

= derived from Greek terms “shadow” and “tail”  Thus, an animal who sits in the shadow of it’s tail (Gurnell 1987; MacClintock 1970) 

Tamias

= food caching  Bushy tails for: balance, communication and T body regulation  Tails often approach 1/3 of body length  Hindfeet with 5 digits, forefeet with 4 digits  May be furry in winter (Gurnell 1987)

Three Species Considered

Tamiasciurus hudsonicus Sciurus aberti Sciurus niger

Where Are They?

Distribution of the Abert squirrel

Where are They?

Distribution of the Fox Squirrel

Where are They?

Distribution of the pine squirrel

Nest Types

 Dreys: twig and leaf nests built in trees   Winter dreys are more elaborate than summer ones & must withstand inclement weather Winter dreys tend to be circular; summer dreys saucer shaped  Dens: holes or cavities in tree trunks  Holes: underground or in rock (Gurnell 1987)

Squirrel in it’s drey

Body Size and Bergman’s Rule

 In order of increasing size: pine, abert, fox  Pine squirrels are smallest N. American tree squirrel  Occupy the coldest, most northern habitats in N. America and the highest altitudes in Colorado  Fox squirrels are largest N. American tree squirrel  Occupy the lowest altitudes in Colorado

Coat Color

 Abert squirrels of Colorado tend to be melanistic  Farentinos (1971) estimated 56.7% melanism  Pine squirrels are ashy gray with white undersides  Fox squirrels are peanut colored with rusty undersides

Is Black Beneficial?

 Golightly and Ohmart (1978) found that:  Sun basking is common among Abert squirrels on winter afternoons  Sun basking resulted in increased T Body  Basking squirrels had higher T Body and nesting squirrels than inactive

Food Utilization

  Fox and pine squirrels are food generalists and opportunistic eaters Abert’s are food specialists and are ecologically dependent upon Ponderosa pines  The primary food for all squirrels is tree seeds and fruits  Secondary foods: berries, mushrooms and other plant matter

Selective herbivory of Abert squirrels

  Diet consists almost entirely of ponderosa pine tissues and other closely associated species (Snyder 1998, States and Wettstein 1998) Abert squirrels eat mycorrhizal fungi in the summer months and inner bark during winter months (States and Wettstein 1998)

Food Caching of Pine Squirrels

 Pine squirrels establish middens which they guard defensively  They cut and cache green cones in late summer to ripen and provide winter food sources (MacClintock 1970)  Middens often have underground networks of tunnels and nest cavities

Fall Gluttony of Fox Squirrels

 Fox squirrels prepare for the unpredictability and scarcity of food resources during winter by overeating in the fall (Steel and Koprowski 2001)  Fox squirrels may scatterhoard food

Levels and Patterns of Activity

 Inclement weather reduces activity, although active squirrels may be seen in severe weather  Wind and rain significantly reduce T body Ohmart 1978) (Golightly and  Fresh snow severely restricts movements (Golightly and Ohmart 1978)  Pine squirrels become subnivean and subterranean when T air is very low (Gurnell 1987)

Summary of Adaptations

 Aberts:  Behavioral – sun basking  Morphological – ear tufts, black coat  Physiological – utilization of inner bark  Pine Squirrels:  Behavioral – middens, territorial  Fox Squirrels:  Behavioral – fall gluttony, scatterhoarding  Morphological/Physiological - BAT

Conclusions

 The three species of tree squirrels each exhibit unique sets of adaptations to life in the cold.

 All three species experience similar environmental pressures where their ranges overlap (on the Front Range of BMP)  All three species exhibit a degree of inhibition from inclement weather (including: wind, rain, fresh snow, low T air )

Selected References and Literature Cited

Armstrong, D.M. 1987. Rocky Mountain Mammals, revised edition. Boulder, Colorado: Colorado Associated University Press. 223 pp Farentinos, R.C. 1971. Social dominance and mating activity in the tassel-eared squirrel (

Sciurus aberti ferrus

). Unpublished. PhD Thesis for the University of Colorado, Boulder. 73 pp.

Golightly, RT Jr. and RD Ohmart. 1978. Heterothermy in free ranging Abert’s squirrels (

Sciurus aberti

). Ecology, 59(5): 897-909.

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

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

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 Reinhold Company. 184 pp.

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. 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.

Snyder, M.A. 1998. Abert’s squirrels (

Sciurus aberti

) in Ponderosa pine (

Pinus ponderosa

) 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: 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 squirrel (

Sciurus aberti

). 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. p 185-194.

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