Deducting rest-activity patterns in free-ranging fox squirrels (Sciurus niger) using low-cost temperature data loggers
Fig. 1: One of the collared fox squirrels in the
life trap
Most
data on rest-activity cycles come from direct observations of focal
animals or radio telemetry. Both methods are discontinuous,
labor-intensive, and require the researcher’s presence on site possibly
leading to alterations of the activity patterns studied (observer
effect). The purpose of the study was therefore to test if low-cost
thermo-sensitive data loggers (iButtons) attached to a neck collar can
detect the rest-activity patterns of free-ranging fox squirrels (Sciurus
niger L.) throughout the year.
Fox
squirrels were tagged with collars containing iButton data loggers
DS1922L-F5 encased in dental acrylic resin attached to zero-degree
cable ties shrink-wrapped for comfort. Data loggers were programmed to
record skin temperature every 5, 10 or 30 minutes at a thermal accuracy
of 0.0625 or 0.5 °C. This allowed for 14-171 days of continuous
temperature data. A total of 1347 days of data were collected during 4
winters and 3 spring/summer periods.
Temperature
logs of detached loggers confirmed the accuracy of the iButton factory
calibration by comparing recorded temperatures Tc to ambient
temperatures (Ta) measured by Siena Heights University’s nearby weather
station. Some detached collars remained in the animal's leaf nest for a
prolonged time before the nest disintegrated and the collars
fell to the ground. The time course of Tc provides data on nest use and
the insulation of the leaf nest and allows for a detective-story like
reconstructions of the time course of events between the detachment and
recovery of the collars (Figures 2 and 3).
Fig. 2: 18
days of temperature recordings of the first detached collar of fox
squirrel #147 in Winter 2012/13. The upper (red) solid line graph shows
collar temperature (Tc) while the lower (blue) dotted line shows
ambient temperature (Ta). Black bars on top of the graph indicate the
duration of night after civil twilight (LD). After detachment on
December 26, the squirrel was not in his leaf nest. Tc shows low
amplitude fluctuations that might be due to episodes of warming
sunshine and cooling shade (a). From December 27 on, the squirrel spend
the night inside the nest, first further away, then closer to the
collar (b). The clearly unimodal minima indicate only a single short
activity period when the squirrel was not in the nest. Between December
22 and January 5, snow was accumulating up to 5 inches and there was
continuous frost. On January 5, the leaf nest was partially destroyed
and the collar fell into the snow. This kept the sensor at an average
temperature of 0°C cutting the extreme cold and warm ends of Ta
fluctuations (c). On January 11, the snow completely melted due to an
abrupt increase in Ta. Now, the sensor follows Ta, however, due to
being surrounded by wet leaves, Tc was about 5°C lower than Ta measured
at the SHU football field (d).
Figure
3: Seven days of temperature recordings of the second and third collar
detached by fox squirrel #147 on January 6 (collar 2, upper dotted line
graph in green) and February 4, 2013 (collar 3, intermediate solid line
graph in red), respectively, while the squirrel was in its overnight
nest. The lower solid line in blue shows the ambient temperature (Ta).
Black bars on top of the graph indicate the duration of night after
civil twilight (LD). High collar temperatures (Tc) indicate times when
a squirrel was present in the nest; low temperatures indicate absence
of squirrels. Please note that the time course of temperature
recordings of both collars is almost perfectly aligned. The Squirrel
that is spending the night inside this nest seems to sleep closer to
collar 2 than to collar 3. Every night, Tc increases with the duration
of the presence of the animal in the nest. Differences in the Tc
fluctuations may be due to the exposure of the collar to the periphery
of the nest that changes insulation with weather conditions, as well as
different proximity to the sleeping squirrel.
Figure
4 shows the temperature patterns of three female fox squirrels (#104,
133, and 148) for the same week in late spring 2013. Please note that
the recording of #104 started on May 30, and the collar of #133 was
switched on June 4. Also note, that #148 did not return to her nest on
June 2 and returned to her nest only for a short night rest on June 4
and June 5. Black arrow indicate time when these collared squirrels
were re-trapped confirming the assumption of ONA during temperature
lows. Also note that only on the hottest day, a daily pattern was less
visible.
Fig.
4: Collar temperature patterns (Tc, red, upper line graph) of three fox
squirrel (#104, #133, and #148) in spring 2013 with the corresponding
ambient air temperature (Ta, blue, bottom line graph), and the duration
of night (LD, shown as black bars, top panel). Please note that
the recording of #104 started on May 30, and the collar of #133 was
switched on June 4. The darker color of the Tc-line graphs of #104 and
#133 is due to having more values (5 minute intervals as to 30 min
interval in #148). #148 did not return to her nest on June 2 and
returned to her nest only for a short night rest on June 4 and June 5.
Black arrow indicate time when these collared squirrels were
re-trapped. During this re-trapping, the collar of squirrel #133 was
exchanged. As the new device needed to adjust to the animal’s
temperature, data were cut out to not disturb the pattern. Fox
squirrels #148 was sampled at 30 minutes intervals, while #104 and 133
were sampled at 5 minute intervals.
Figure
5 shows the temperature patterns of the female fox squirrel #104 during
four different months. Note the individual “signature” in all patterns
of this squirrel breaking up most days into several activity bouts in
all months. Also note the absence of out-of-nest activity on February
13. The timing coincides with the birth season and could indicate
prolonged nest time due to birth and parental care. This would also
explain the only short outside time in the evening and during the
following night.
Fig.
5: Weekly body temperature patterns (Tb, shown in red, upper panel) of
four fox squirrel (#104, #111, #133, and #147) in spring 2012/2013 with
the corresponding ambient air temperature (Ta, shown in blue, bottom
panel), and the duration of night (LD, shown as black bars, top panel).
Collar
temperature patterns clearly indicated onsets and offsets of diurnal
activity and showed a substantial individual variability of
rest-activity profiles between animals tracked during the same seasons.
Low-cost thermo-sensitive data loggers were capable to detect key elements of the rest-activity cycles of free-ranging fox squirrel. The observed variability of rest-activity patterns disagrees with long-held notions of common seasonal patterns in local population and provides a new baseline to address pattern changes due to habitat alterations and global change. The high percentage of rest in the life of fox squirrels identifies this species as a good example of a slow life history strategy.
Scheme
Low-cost thermo-sensitive data loggers were capable to detect key elements of the rest-activity cycles of free-ranging fox squirrel. The observed variability of rest-activity patterns disagrees with long-held notions of common seasonal patterns in local population and provides a new baseline to address pattern changes due to habitat alterations and global change. The high percentage of rest in the life of fox squirrels identifies this species as a good example of a slow life history strategy.
Scheme