The influence of ambient and den temperature on the
above-ground activity of woodchucks (Marmota monax) in
south-central Pennsylvania Mary Riley and Bradley Rehnberg
Department of Biological Sciences, York College of Pennsylvania
Seasonal timetable In order to survive the physiological costs of
hibernation and successfully reproduce upon emergence, woodchucks
must acquire the necessary fat stores during spring, summer, and
fall (Davis 1976). Grizzell (1955) noted above-ground activity to
be limited to only 12 h per day. Typically woodchuck activity
displays a bimodal distribution in the summer favoring morning and
evening periods, and a unimodal distribution in the spring and fall
favoring midday periods (Barash 1989). Social interactions are
early in the growing season as adults are both establishing their
territories and mating. Interactions diminish as the growing season
progresses (Bronson 1964; Maher 2006). Why dwell in a den? Dens are
constructed below the frost line in well- drained soils with a
preference for loam and sandy loam soils (Moss 1940). Woodchucks
tend to forage near their dens, as dens provide protection from
both predators and most environmental extremes (Reichman and Smith
1990). Although winter den temperatures are only slightly higher
than ambient, the progression of winter den temperatures to summer
den temperatures has been found to be very gradual. This allows
dens to remain much cooler during the summer and function as a
refuge from high ambient temperatures (Zervanos and Salsbury 2003).
Objective Assess above-ground activity of the woodchuck as
influenced by ambient and den temperature during their active
season. Hypothesis Hypothesis Woodchucks will not go above ground
during high ambient temperatures. Conclusion Den temperatures
remained relatively constant throughout the day and closely
followed den temperature data gathered by Zervanos and Salsbury
(2003). This reinforces the ability of the den to function as a
refuge during the warmer months. Overall activity greatly increased
in the fall evenings preceding hibernation. This was consistent
with previous literature that states woodchucks display a unimodal
distribution of activity in the fall as temperatures become cooler
(Barash 1989). The presence of midday activity even when ambient
temperatures exceeded 30C and overall lack of morning activity was
unexpected. External factors to ambient temperature including
location along the Codorus and consistent morning human traffic may
have also influenced the regulation of activity periods. Behavioral
Observations Observed from late June 2010 through mid-November 2010
for a total of 18 observations. An observation was comprised of
three 1-hour intervals in the morning, midday, and evening. The aim
was to quantify the number of woodchucks present and total time
spent above-ground. Data Analysis Summer is defined to include
measurements from June, July, and August (n=9) while fall includes
measurements from September, October, and November (n=9). The mean
change in temperature between dens and ambient was determined.
Statistically assessed mean number observed and mean time spent
above ground with a two-way ANOVA and Bonferroni Post test. Methods
Results Acknowledgements Stam Zervanos and June Brown for their
assistance throughout the development and execution of my thesis.
Bruce Smith and the Army Corp of Engineers for assisting in
locating the study area. Jen Butts for accompanying me during
summer evening observations. Michael Riley for a multitude of
reasons. Literature Cited Barash, D. P. 1989. Marmot sociology and
behavior. Stanford University Press, Standford CA. Davis, D. 1976.
Hibernation and circannual rhythms of food consumption in marmots
and ground squirrels. The Quarterly Review of Biology 51:477-514.
Grizzell, R.A. 1955. A study of the southern woodchuck, Marmota
monax monax. The American Midland Naturalist 53:257-292. Maher,
C.R. 2006. Social organization in woodchucks (Marmota monax) and
its relationship to growing season. Ethology 112:313- 324. Maher,
C.R. 2009. Genetic relatedness and space use in the behavorially
flexible species of marmot, the woodchuck (Marmota monax).
Behavioral Ecology and Sociobiology 63:857-868. Moss, A.E. 1940.
The woodchuck as a soil expert. The Journal of Wildlife Management
4: 441-443. Reichman, O.J. and Smith, S.C.1990. Burrows and
burrowing behavior by mammals. Current Mammalogy 2:197-244.
Zervanos, S.M. and Salsbury, C.M. 2003. Seasonal body temperature
fluctuations and energetic strategies in free-ranging eastern
woodchucks (Marmota monax). Journal of Mammalogy 84: 299-310.
Ambient and Den Temperature Measurements Manually inserted
thermister probes into den entrances. Data logger recorded ambient
and den temperatures for four roughly 2- week intervals. Figure 1.
Area where study was conducted. White circles indicate dens with
single entrances (n=21) and colored circles indicate dens with
multiple entrances (n=33). Only defined zones were included during
observations. Den temperature collections were taken from den
entrances within and outside of the zones.