Movement Patterns of Prairie Rattlesnakes ( Crotalus v. viridis ) across a Mountainous Landscape in a Designated Wilderness Area

Abstract

Many snake species make lengthy linear migrations between overwintering sites and summer foraging or mating habitats. Although mountainous topography may restrict migratory movements, most previous studies on migratory snake populations have occurred in areas with low to moderate topographic relief. The objectives of this study were to describe the movement patterns of Prairie Rattlesnakes (Crotalus v. viridis) in a mountainous landscape, compare those patterns to those of migratory snake populations from areas with lower topographic relief, and test for variation in movement patterns between sexes and among years. We used radiotelemetry to monitor the movements of 21 male and 6 nonpregnant female Prairie Rattlesnakes in the Frank Church Wilderness in central Idaho during the summers of 2006-2008. Mean total distance moved during the entire activity season in 2008 was 4.46 km (range 1.38-7.67); mean maximum distance moved from the hibernaculum was 1.46 km (range 0.69-2.71). Although the movement distances reported here are intermediate to those reported for other migratory snake populations, they are similar to some distances reported from areas with low to moderate topographic relief. This suggests that rattlesnakes are capable of making considerable movements in a mountainous landscape, although factors such as prey availability could also contribute to differences in reported movement distances. Rattlesnakes displayed moderate fidelity to summer activity areas but had similar mean bearings during outbound migration across multiple years. We hypothesize that linear migrations reported from rattlesnakes in many populations actually represent the most-direct movement to annual foraging areas rather than true searching movements.

Full text

Journal of Herpetology, Vol. 49, No. 3, 377–387, 2015
Copyright 2015 Society for the Study of Amphibians and Reptiles
Movement Patterns of Prairie Rattlesnakes (Crotalus v. viridis) across a Mountainous
Landscape in a Designated Wilderness Area
JAVAN M. BAUDER,
1,2,3
HOLLY AKENSON,
4
AND CHARLES R. PETERSON
1
1
Department of Biological Sciences, Idaho State University, Pocatello, Idaho, USA
4
Wallowa Resources, 401 NE 1st St., Suite A, Enterprise, Oregon, USA
ABSTRACT.—Many snake species make lengthy linear migrations between overwintering sites and summer foraging or mating habitats.
Although mountainous topography may restrict migratory movements, most previous studies on migratory snake populations have occurred in
areas with low to moderate topographic relief. The objectives of this study were to describe the movement patterns of Prairie Rattlesnakes
(Crotalus v. viridis) in a mountainous landscape, compare those patterns to those of migratory snake populations from areas with lower
topographic relief, and test for variation in movement patterns between sexes and among years. We used radiotelemetry to monitor the
movements of 21 male and 6 nonpregnant female Prairie Rattlesnakes in the Frank Church Wilderness in central Idaho during the summers of
2006–2008. Mean total distance moved during the entire activity season in 2008 was 4.46 km (range 1.38–7.67); mean maximum distance moved
from the hibernaculum was 1.46 km (range 0.69–2.71). Although the movement distances reported here are intermediate to those reported for
other migratory snake populations, they are similar to some distances reported from areas with low to moderate topographic relief. This
suggests that rattlesnakes are capable of making considerable movements in a mountainous landscape, although factors such as prey
availability could also contribute to differences in reported movement distances. Rattlesnakes displayed moderate fidelity to summer activity
areas but had similar mean bearings during outbound migration across multiple years. We hypothesize that linear migrations reported from
rattlesnakes in many populations actually represent the most-direct movement to annual foraging areas rather than true searching movements.
Landscape features exhibit a multitude of influences on
animal movements. For example, the availability of key
resources such as food, water, mates, and refugia may vary
spatially across the landscape and their spatial arrangement
may influence animal movement patterns (McIntyre and Weins,
1999; Klaassen et al., 2006; Filipa-Loureiro et al., 2007).
Landscape features may also impose costs on animal move-
ments, thereby confronting individuals with tradeoffs between
the benefit of acquiring key resources and the costs associated
with movements (Werner and Anholt, 1993). Costs imposed by
landscape features may include risks of predation (Fortin et al.
2005), physiological stress (Daltry et al., 1998; Bartelt et al.,
2010), and increased energy expenditures (Johnson et al., 2006).
Such costs may be particularly important for migrating species
as a result of extended movements between spatially separate
habitats. Species whose migrations are restricted to fixed
migration routes may also be more susceptible to natural and
anthropogenic disturbances along those routes (Berger, 2004).
Understanding the movement patterns of migratory species and
the factors which may restrict those movements can help better
understand how those movement patterns may be affected by
both natural and anthropogenic disturbances.
Many populations of snakes from multiple taxa undertake
seasonal migrations between overwintering sites and summer
activity areas, particularly at northern latitudes (Gregory et al.,
1987; Larsen, 1987; Jorgensen et al., 2008; Smith et al., 2009;
Williams et al., 2012; Gardiner et al., 2013). In contrast,
conspecifics or closely related species at more-southerly
latitudes often show reduced movement distances or nonmi-
gratory movement patterns (Reed and Douglas, 2002; Rodri-
guez-Robles, 2003; Brown et al., 2008; Dugan et al., 2008; Klug et
al., 2011). Migrations in snakes may be caused by limited
numbers of suitable hibernacula, spatial separation between
hibernacula and foraging habitats, and attempts to minimize
conspecific competition near communal hibernacula (Hirth et
al., 1969; Gregory, 1984; Larsen, 1987; Jorgenson et al., 2008).
Migrations are often lengthy (e.g., over several kilometers) and
very linear (King and Duvall, 1990; Jorgenson et al. 2008;
Wastell and Mackessy, 2011; Martino et al. 2012). The high
directionality of these movements is hypothesized to be an
efficient search pattern in response to spatially unpredictable
prey or mates (King and Duvall, 1990; Duvall and Schuett,
1997). Although resource availability is known to influence
movement patterns in migratory snake species, relatively little
attention has been focused on the role of landscape character-
istics, such as habitat types, in influencing those movements. In
particular, the effects of topography have received relatively
little attention, perhaps because many studies on migratory
snake populations occurred in areas with low to moderate
topographic relief or did not comment on the effects of
topography (e.g., Brown et al., 2009; Gardiner et al., 2013).
Where resource availability in a given area is relatively stable
across time, it is often advantageous for an individual to exhibit
fidelity to those areas if the benefits of reusing that resource
exceed the costs of searching for new resources (Switzer, 1993).
Site fidelity has been demonstrated in diverse taxa at a range of
spatial and temporal scales spanning temporary refugia (Webb
and Shine, 1997; Beck and Jennings, 2003) to seasonal and
annual home ranges (Andersen, 1991; Janmaat et al., 2009;
Scouler et al., 2011; Trierweiler et al., 2013). Snakes in cold-
temperate climates should show strong fidelity to hibernacula
because most hibernacula are persistent over time, and the
consequences to fitness of surviving the winter are high. Many
studies of snakes in such environments do show strong fidelity
to specific hibernaculum (e.g., Blouin-Demers et al., 2007; Parker
and Anderson, 2007; Clark et al., 2008; Smith et al., 2009).
However, less attention has focused on fidelity to migration
routes and summer activity areas. If resources such as prey and
mates are fairly stable in space and time, we would expect
individuals to return to the same summer activity areas over
multiple years. In contrast, varying resource availability or
predictability (or both) may cause individuals to show low
fidelity to activity areas over time (Edwards et al., 2009). Most
2
Present address: Department of Environmental Conservation,
University of Massachusetts, Amherst, Massachusetts, USA
3
Corresponding author. E-mail: javanvonherp@gmail.com
DOI: 10.1670/13-153

studies monitoring snake movements over multiple activity
seasons have found that snakes do indeed use the same general
summer activity areas (Kapfer et al., 2008; Pattishall and
Cundall, 2008; Smith et al., 2009). However, few, if any, studies
have examined fidelity to migration routes over time in snakes.
An accurate description of ecological patterns and processes
is first necessary to understand the factors influencing those
patterns and processes. Therefore, the objective of this study
was to describe the movements of Prairie Rattlesnakes (Crotalus
v. viridis) in a mountainous landscape. To address this objective,
we asked three specific questions: 1) How do the movement
patterns of Prairie Rattlesnakes in a mountainous landscape
compare to those of populations from areas with lower
topographic relief; 2) do males and nonpregnant females
display differences in movement patterns that may be caused
by male mate-searching; and 3) do rattlesnakes display high
fidelity to summer activity areas? Additionally, we tested for an
effect of body size on several movement statistics because
movement patterns have varied with body size in multiple
snake taxa (Blouin-Demers et al., 2007; Jorgensen et al., 2008;
Hyslop et al., 2014).
MATERIALS AND METHODS
Study Area.—We conducted this study in the lower Big Creek
drainage of the Frank Church—River of No Return Wilderness in
central Idaho, USA (458050N, 1148510W, Fig. 1). Our field work
was based out of the University of Idaho’s Taylor Wilderness
Research Station (TWRS, 1,200 m). The topography of the lower
Big Creek drainage is characterized by steep valleys and high
ridges (1,100–2,780 m in about 4.8 km). Southerly aspects support
several species of xeric shrubs and grasses including mountain
mahogany (Cercocarpus ledifolius), big sagebrush (Artemisia
tridentata), Idaho fescue (Festuca idahoensis), and bluebunch
wheatgrass (Pseudoroegneria spicata). Cooler, northerly aspects
support Douglas fir (Pseudotsuga menziesii) and mallow ninebark
(Physocarpus malvaceus). Riparian vegetation includes black
cottonwood (Populus trichocarpa), Rocky Mountain maple (Acer
glabrum), hawthorn (Crataegus douglasii), serviceberry (Amelanch-
ier alnifolia), alder (Alnus spp.), chokecherry (Prunus virginiana),
raspberry (Rubus idaeus), thimbleberry (Rubus parviflorus), rose
(Rosa spp.), and other shrub species. Exotic cheatgrass (Bromus
tectorum) is also present throughout lower Big Creek. Exposed
rocky outcrops and bare talus slopes are widespread along the
valley sides. Large fires burned much of the Big Creek drainage
in August 2000, including most of the forested habitat near the
TWRS, and the effects of the fire are still clearly seen. A second
fire burned a wide area north of the TWRS during July 2006.
The overwintering locations of the vast majority of rattle-
snakes observed during this study occurred in three clusters
(0.05–1.96 ha) within 1.5 km of each other and within 800 m of
the TWRS. We refer to these clusters as hibernaculum
complexes. Within a complex, the number of rattlesnakes
captured at a given crevice or opening during spring egress
ranged from one to three and the distances between neighbor-
ing crevices or openings within a cluster was approximately 5–
75 m. All complexes observed during this study were along the
side of the Big Creek Valley or its tributary valleys. Two
complexes were north of Big Creek and the third was south of
Big Creek. The southern complex, approximately 1.96 ha,
consisted of scattered rock outcrops and small talus patches at
1,302–1,468 m elevation with a mean aspect of 2148. The first
northern complex consisted of two disjunct talus patches; a 0.58-
ha subcomplex at 1,245–1,311 m with a mean aspect of 968and a
0.04-ha subcomplex at 1,317–1,321 m with a mean aspect of
1418. The second northern complex consisted of several small
rock outcrops and was approximately 0.05 ha at 1,248–1,262 m
with a mean aspect of 1038. One rattlesnake overwintered
solitarily north of Big Creek in the south-facing junction of a
rock outcrop complex and talus slide at 1,264 m and was
approximately 1.2 km from the nearest hibernaculum complex.
Rattlesnake Radiotelemetry.—We surgically implanted radio
transmitters into 29 male and nonpregnant female rattlesnakes
during this 3-yr study. Rattlesnakes were captured near the
hibernacula 28 April–1 May 2006, 2 May and 8–10 May 2007, and
16 April–23 May 2008 and brought back to the TWRS. In 2007,
we were unable to capture a sufficient number of rattlesnakes
around the hibernacula in the spring, so we implanted
transmitters into five rattlesnakes that were opportunistically
encountered around the TWRS between 17 May and 18 June.
Rattlesnakes were anesthetized using Sevoflurane as an inhalant
following the procedures described in Reinert (1992). Transmit-
ters were implanted using the technique described in Reinert and
Cundall (1982). We used 3.8-g PD-2, 9-, 11-, and 13.5-g SI-2T, and
5-g SB-2T transmitters (Holohill Systems Ltd., Carp, Ontario,
FIG. 1. Map of the study area showing the Frank Church—River of
No Return Wilderness, the Taylor Wilderness Research Station, and the
three Prairie Rattlesnake (Crotalus v. viridis) hibernaculum complexes
used in this study.
378 J. M. BAUDER ET AL.

Canada). Transmitters were 5% of the rattlesnake’s body mass
at time of surgery. Each rattlesnake was held for 8–36 h before
being released at their respective capture sites and all rattlesnakes
were alert and responsive before release. Telemetered rattle-
snakes were monitored 11 May–6 August 2006, 17 May–11
August 2007, and 16 April–28 September 2008. We located each
rattlesnake using a three-element Yagi antenna (Wildlife Materi-
als International Inc., Murphysboro, Illinois, USA) and a Telonics
TR-2 receiver (Telonics Inc., Mesa, Arizona, USA) approximately
once every 2–4 d and recorded its position (Universal Transverse
Mercator, UTM) using a handheld GPS unit (Garmin GPSmap
76CS, Garmin International Inc., Olathe, Kansas, USA). GPS
accuracy ranged from 2–13 m (mean approximately 5 m). We
attempted to capture each rattlesnake to determine its mass at the
end of the 2006 field season and once per month in 2007 and
2008.
Movement Patterns.—To describe individual rattlesnake move-
ment patterns, we entered the UTM coordinates for all telemetry
locations that were separated by at least 1 d, including the
overwintering location, into ArcGIS 9.2 (ESRI, Inc., Redlands,
California, USA). We then calculated several movement statistics
that would allow us to describe these patterns. We measured the
straight line distance between each location for each rattlesnake
using the Animal Movement Extension (Hooge and Eichenlaub,
1997) in ArcView GIS 3.2 (ESRI, Inc.). We measured the
maximum straight line distance (i.e., displacement) a rattlesnake
moved from its spring capture point. These distances were
converted into topographic distances using 10-m digital elevation
models (DEM) in ArcGIS 9.2. Topographic distances were used in
all subsequent analyses. We measured the bearing of each
movement segment using Hawth’s Analysis Tools (Beyer, 2004)
in ArcGIS 9.2. These were converted to a turn angle (08to 1808)
representing the departure from the previous bearing, which
served as a measure of directionality. We also calculated an
inverse meandering ratio (Williamson and Gray, 1975) for each
rattlesnake as a second measure of directionality by dividing the
maximum distance moved from the hibernaculum by the total
distance moved and subtracting that value from one so that high
values represent high meandering. Movement rate was calculat-
ed for each movement segment as meters moved per 24 h.
Movements between consecutive radiotelemetry observations
that were separated by more than 7 d were excluded for rate and
absolute turning angle calculations to ensure that the most-
accurate data were used for these calculations. Movements less
than 5 m in length were excluded to facilitate comparisons with
previous studies (Jorgensen et al., 2008). Only snakes captured at
their hibernacula and monitored for an entire field season were
included in subsequent analyses to avoid biasing the results
towards snakes that had already begun moving away from their
hibernaculum.
Estimating Summer Activity Areas.—We estimated summer
activity areas (i.e., active season home ranges) using two
approaches. We calculated 100% minimum convex polygons
(MCP) and 95% fixed kernels (FK) for each rattlesnake in Home
Range Tools (Rodgers et al., 2005) and ArcGIS 9.2. To determine
the appropriate smoothing parameter (h), we decreased the
reference bandwidth (h
ref
) incrementally by 0.1 until we found
the smallest contiguous polygon with no lacuna that included all
telemetry observations (Berger and Gese, 2007). We also
calculated a 50% FK for each snake to represent areas of
concentrated activity. For each rattlesnake, many UTM coordi-
nates were identical, which reflected multiple telemetry obser-
vations of that rattlesnake at identical locations. Because these
duplicate UTM coordinates caused computational problems with
our kernel estimation, we altered these duplicate coordinates by
1–2 m to generate useful kernel estimates while retaining the
information provided by observing our telemetered rattlesnakes
in the same location on multiple occasions. Because of the
extensive topographic relief in our study area, planimetric
activity areas underestimated the size of the activity area
(Greenberg and McClintock, 2008). We therefore converted our
activity area polygons (MCP and 95% FK) and 50% FK into three-
dimensional TIN (triangular integrated networks) using a 10-m
DEM and calculated the topographic area of each TIN (Green-
berg and McClintock, 2008). We buffered each summer activity
area polygon by 5 m to allow the TIN to cover the full extent of
the polygon. Unless otherwise noted, all statistical analyses were
conducted in SAS 9.1 (SAS Institute, Carey, North Carolina,
USA). All means are reported 6one standard error.
Statistical Analysis.—We then conducted a series of analyses to
test for an effect of overwintering site, sex, initial body mass,
change in body mass, and year on several movement statistics
(i.e., total distance, maximum distance, mean rate, mean turning
angle).Weusedanunequalvariancet-test to test for differences
between rattlesnakes overwintering on either side of Big Creek
for each year separately. We use linear mixed-effects models with
the lme function (Pinheiro et al., 2014) in R 3.0.2 (R Core Team,
2013) to test for an effect of sex, initial body mass, and year on
total distance moved, maximum distance moved, rate, turn
angle, and meandering ratio. We initially included a sex by mass
interaction, but if this proved statistically nonsignificant (P<
0.05), we removed the interaction term from the model and made
inferences using only main effects. Maximum distance moved
and mean rate were log
10
transformed to meet assumptions of
normality. Data from all 3 yr were included and the number of
telemetry locations (e.g., telemetry fixes) for each individual in
each year was included to correct for differences in sampling
intensity among years. Individual was included as a random
effect to control for repeated measures from the same individual.
We also used linear mixed-effects models with the lme function to
test for an association between the percent change in body mass,
sex, and each movement statistic using the 2008 data, testing for
interactive effects as described earlier. The meandering ratio did
not meet the assumptions of these parametric tests despite
transformations. We therefore tested for differences in meander-
ing ratio between males and nonpregnant females and between
rattlesnakes overwintering north and south of Big Creek using a
Mann-Whitney U-test. We used a Spearman’s Rank Correlation
to test for an association between initial body mass and percent
change in body mass to determine if larger rattlesnakes lost more
weight and an unequal variance t-test to test if percent change in
body mass differed between males and females.
Fidelity to Summer Activity Areas.—To assess the degree of
fidelity to summer activity areas, we calculated the percentage of
95% and 50% FK overlap between years for rattlesnakes that
were captured at their hibernacula and monitored over an entire
field season for multiple years (N=12). Percent overlap was
calculated following Jenkins (2007) by dividing the area of
overlap (i.e., the area used in both years, only overlapping) by the
total cumulative use area (i.e., the total area used in both years,
overlapping and nonoverlapping). In calculating the total
cumulative use area, the area of overlap was not counted twice.
For example, if a rattlesnake’s home range during year 1 was 100
ha, during year 2 was 200 ha, and the area of overlap was 50 ha,
the index of overlap would be calculated as Overlap =50/
[(200-50)+100]. To test for differences in fidelity among years,
PRAIRIE RATTLESNAKE MOVEMENT PATTERNS 379

we compared the percentage of 95% and 50% FK overlap among
all three combinations of years (2006/2007, 2007/2008, and 2006/
2008) using a repeated measures analysis of variance (ANOVA).
We tested whether rattlesnakes followed the same bearing during
outbound and inbound migration in 2008. Migratory movements
were identified using the procedures described in Bauder (2010)
but generally consisted of lengthy, rapid, and linear movements
away from or towards the hibernaculum during the spring and
fall, respectively, in contrast to the shorter, less-directional
movements associated with foraging. We used a nonparametric
Moore’s test for circular uniformity of paired data to test if the
mean bearing of inbound and outbound migrations were
different (Zar, 1996). The null hypothesis under this test was
that rattlesnakes followed the same mean bearing during
outbound and inbound migration. Because outbound and
inbound migrations typically occurred in opposite directions,
we added 1808to the mean inbound migration bearing to
calculate the angular difference between mean bearing for
inbound and outbound migration. We used Moore’s test for
circular uniformity of paired data to test whether rattlesnakes
followed the same mean bearing during outbound migration
during different years. We only used snakes for which we had
complete outbound migration data (N=12). Because this test can
only be conducted between two samples, we ran the test for each
pair-wise year combination using Bonferroni corrections.
RESULTS
Rattlesnake Radiotelemetry.—We monitored the movements of 12
male rattlesnakes in 2006, 12 male and 3 nonpregnant females in
2007, and 16 males and 6 nonpregnant females during 2008 for a
total of 29 rattlesnakes (Table 1). Five rattlesnakes were
telemetered during all 3 yr, one in 2006 and 2007, 6 in 2007
and 2008, and 2 in 2006 and 2008 for a total of 49 snake-years.
Seven of these snake-years were partial data sets due to battery
failure or late capture dates. Twenty-seven (21 males and 6
nonpregnant females) rattlesnakes provided data from 44 snake-
years that met the criteria for inclusion in some or all of the
analyses. Mean body mass of these 27 individuals at the time of
transmitter implantation was 323 g (614.5 g, range 172–487 g) for
males and 207 g (613.97 g, range 138–255 g) for females. Mean
snout–vent length was 75.7 cm (61.57 cm, range 68.3–93.8 cm)
for males and 76.3 cm (61.90 cm, range 70.3–81.7 cm) for females.
Movement Patterns and Home Range Size.—Rattlesnakes moved
a mean total distance of 4.04 km (60.24 km) and a mean
maximum distance of 1.32 km (60.11 km; Table 2). The
maximum distance moved from a hibernaculum was 2.93 km
by a male in 2006. However, these overall means are likely biased
because not all individuals were monitored throughout the
activity season or the entire study. Additionally, the spring
hibernacula searches in 2007 probably occurred after peak
emergence, further biasing the results of that year to rattlesnakes
that emerged later in the spring. Because the 2008 data were
collected during the entire activity season, and snakes were
captured throughout their spring emergence, these results are
probably the most-accurate representation of rattlesnake move-
mentsinthislandscape.During2008,telemeteredrattlesnakes(N
=22) moved a mean total distance of 4.46 km (60.37 km) and a
mean maximum distance of 1.46 km (60.15 km). During 2008,
mean rate was 42.76 m/24 h (63.82 m/24 h), mean turn angle
was 70.928(62.848), and mean meandering ratio was 0.66
(60.03). Mean 95% and 50% FK size was 109.21 ha (622.60 ha)
and 23.98 ha (64.58 ha), respectively, while mean MCP was 48.34
ha (67.00 ha). Estimates for 95% FK and MCP that incorporated
topographic relief were a mean of 16% (range 6–28%) and 16%
(range 8–23%), respectively, larger than those that did not. Total
distance moved was strongly correlated with the size of the 95%
FK (r
s
=0.7502, P<0.0001) and 50% FK (r
s
=0.7594, P<0.0001),
as was maximum distance moved (95% FK, r
s
=0.8961, P=<
0.0001; 50% FK, r
s
=0.8776, P=<0.0001). Total distance moved
and maximum distance moved were also highly correlated with
MCP (r
s
=0.7864, P<0.0001 and r
s
=0.9060, P<0.0001,
respectively). For this reason, we did not use home range size in
analyses testing for differences in movement patterns between
overwintering locations, sex, and among years. We found no
differences in any movement statistics in each year between
rattlesnakes overwintering on the north and south side of Big
Creek.
There were few differences in movement patterns between
males and nonpregnant females, and the interactive effect
between sex and body mass (either initial body mass or change
in body mass) was not significant in any analyses (P‡0.2661).
There was no significant effect of sex on total distance moved (t
TABLE 1. Size and radiotelemetry tracking dates of 27 Prairie Rattlesnakes (Crotalus v. viridis) in the lower Big Creek drainage of central Idaho 2006–
2008. Means and ranges are presented for mass, SVL, number of days monitored with radiotelemetry, and number of radiotelemetry locations (i.e.,
telemetry fixes per individual).
Year NMass (g) SVL (cm)
Number of days
monitored
Number of
locations
Start date of
telemetry
End date of
telemetry
2006 11 ?314 (210–436) 81.6 (72.5–92.2) 85 (83–87) 27 (26–30) 11 MAY 6 AUG
2007 9 ?and 2 /287 (185–421) 78.2 (68.3–86.0) 85 (83–87) 30 (29–31) 17 MAY 11 AUG
2008 16 ?and 6 /303 (138–487) 81.8 (71.8–93.8) 144 (110–164) 39 (28–44) 16 APR 28 SEP
Mean NA 302 80.7 115 34 NA NA
TABLE 2. Mean annual movement statistics from 27 Prairie Rattlesnakes (C. v. viridis) monitored with radiotelemetry in the lower Big Creek
drainage of central Idaho 2006–2008. FK =fixed kernel and MCP =100% minimum convex polygon.
Total distance (km) Max. distance (km) Rate (m/24 h) Meandering ratio Turn angle (8) 95% FK (ha) 50% FK (ha) MCP (ha)
2006 4.20 1.42 59.98 0.66 87.79 113.50 24.42 53.37
2007 3.03 0.95 42.66 0.68 82.98 54.45 12.44 29.43
2008 4.46 1.46 42.76 0.66 70.92 109.21 23.98 48.34
Mean 4.04 1.32 47.04 0.67 78.22 96.30 21.14 44.79
380 J. M. BAUDER ET AL.

=-0.21, df =25, P=0.8324), log
10
maximum distance moved (t
=-1.16, df =25, P=0.2569), or log
10
mean movement rate (t=
0.07, df =25, P=0.9472). There was a marginal effect of sex on
mean turning angle (t=1.86, df =25, P=0.0751), with males
having a higher mean turning angle (81.01862.528) than
females (65.63865.798). There was a significant difference in
meandering ratio between males and nonpregnant females in
2008 (0.71 60.02 vs. 0.54 60.05, Z=-2.66, P=0.0077),
indicating that males meandered more than nonpregnant
females. There was no difference in percent change in body
mass between males and females in 2008 (t=0.98, df =4.34, P
=0.3769).
Initial body mass had some effect on rattlesnake movement
patterns. There was a significant positive effect of initial body
mass on the total distance moved (t=2.59, df =13, P=0.0223;
Fig. 2A) and the log
10
mean movement rate (t=2.51, df =13, P
=0.0259; Fig. 2B). There was no effect of initial body mass on
maximum distance moved (t=1.07, df =13, P=0.3032) or
mean turning angle (t=0.57, df =13, P=0.5813). Initial body
mass was correlated with meandering ratio, but only in 2008 (r
s
=0.4348, P=0.0431), and the strength of this association was
low. There was no correlation between initial body mass and
percent change in body mass in 2008 (r
s
=-0.3385, P=0.1444).
There was no correlation between meandering ratio and percent
change in body mass in 2008 (r
s
=-0.1528, P=0.5201). None of
the movement statistics we measured had an effect on change in
body mass in 2008.
Fidelity to Summer Activity Areas.—There were few differences
in rattlesnake movement statistics among years. After correcting
for the number of telemetry observations, there was no
significant difference in total distance moved, maximum distance
moved, or mean turning angle among years (P‡0.1446). There
was a marginal effect of year on log
10
mean movement rate (F
2,13
=3.66, P=0.0547) with higher movement rates in 2006
compared to 2007 and 2008 (Table 2). There were no significant
differences in the mean bearings of outbound migration for
rattlesnakes monitored during multiple years (R0=0.37–0.68, P
>0.60). The mean difference in outbound migration bearings
between years ranged from 29.618(65.458) between 2007 and
2008 to 70.038(643.228) between 2006 and 2007. Mean percentage
overlap in 95% FK home ranges was 35% (63.83%) and 18%
(62.73%) for 50% FK home ranges (Fig. 3). The percentage of 95%
FK overlap was not significantly different among the three pair-
wise year combinations (F
2,8
=0.10, P=0.9094) nor was the
percentage overlap of 50% FK (F
2,8
=0.97, P=0.4204).
DISCUSSION
Seasonally migrating snake populations in north-temperate
climates may show wide variation in movement distances both
within and among taxa (Table 3). For example, Prairie
Rattlesnakes from three separate populations in southern
Canada exhibited more than a 6-fold difference in mean
maximum distance moved (Didiuk, 1999; Jorgensen et al.,
2008; Gardiner et al., 2013) and all moved greater distances than
the Prairie Rattlesnakes in this study (Table 3). Similarly,
Martino et al. (2012) and Gardiner et al. (2013) found significant
differences in home range size and daily movement rate, but not
maximum distance moved, among three species of snakes
within the same study area. The movement distances observed
in our study were intermediate to those reported for other
seasonally migrating snake populations from multiple taxa in
areas with low to moderate topographic relief (Table 3).
Most studies of snake movements do not discuss how snakes
moved in relation to topographic features, although Williams et
al. (2012) found that Great Basin Gophersnakes (Pituophis
catenifer deserticola) at one study population (elevation 435–635
m) moved primarily away from hibernacula on hills into the
adjacent valley. Rattlesnakes in our study area did not restrict
their movements to valley bottoms and often moved over and
along ridges (Fig. 4). We also observed multiple crossings of Big
Creek and its smaller tributaries. Our observed movement
distances were also similar to or greater than those reported for
rattlesnakes from areas with less topographic relief (Jenkins,
2007; Parker and Anderson, 2007; Shipley et al., 2013). This
suggests that topography may not be wholly responsible for the
intermediate movement distances observed in our study
compared to those of other seasonally migrating snake
populations. Variation in movement distances among popula-
tions may be caused by variability in resource availability,
particularly suitable hibernacula, prey, and mates, which may
force individuals to travel further to locate sufficient resources
(Duvall et al., 1990; Pearson et al., 2005; Beaupre, 2008; Gardiner
et al., 2013). Additionally, variation in population sizes of
communal denning species could lead to higher population
densities in foraging habitats and may force individuals to
move greater distances to locate sufficient resources (Jorgensen
et al., 2008). Low topographic relief may provide snakes with
FIG. 2. (A) Relationship between initial body mass (g) and total
distance moved (km), and (B) log
10
mean movement rate (m/day) for
male (N=21) and female (N=5) Prairie Rattlesnakes (Crotalus v. viridis)
in the lower Big Creek drainage of central Idaho 2006–2008. Some
individuals were monitored for multiple years, so each point on the
graph represents one snake-year (N=41).
PRAIRIE RATTLESNAKE MOVEMENT PATTERNS 381

increased flexibility to travel further to locate summer foraging
habitat. This may be particularly important for snakes in areas
where hibernacula are limited, such as the northern edges of
their ranges. Indeed, some of the longest movement distances
reported for snakes occur near the northern edge of their range
where cold winters may limit the availability of suitable
hibernacula (Jorgensen et al., 2008; Martino et al., 2012).
Although we did not conduct extensive hibernacula surveys
during our study, we suspect that suitable overwintering sites
are relatively abundant within our study area given the
abundance of rocky, south-facing habitats and limited observa-
tions of solitary overwintering. However, topography may still
have a restrictive effect on rattlesnake movements within our
study area. We never observed rattlesnakes >2,000-m elevation
and observed strong selection for low elevations and gentle
slopes within 3 km of the hibernacula (Bauder et al., In press).
However, at the scale of the home range rattlesnakes showed
much less selection for topographic features. Topography may
therefore have an absolute restrictive effect on rattlesnake
movements in lower Big Creek but primarily at broad spatial
scales. Rattlesnakes could have moved up to 5 km from the
hibernacula and still remain below 2,000 m by following
drainages, yet the maximum distance moved from a hibernac-
ulum was 2.93 km.
Overlap between overwintering sites and summer foraging
habitat may lead to shorter or nonmigratory movements (Reed
FIG. 3. Summer activity areas (95% fixed kernel utilization distribution [UD]) and outbound migration movements for one female (A) and three
male (B–D) Prairie Rattlesnakes (Crotalus v. viridis) in the lower Big Creek drainage of central Idaho monitored for multiple years during 2006–2008.
Each panel displays summer activity areas and outbound migration movements for 2 yr. Thick summer activity areas correspond with thick outbound
migration movements. The three hibernaculum complexes are denoted by white triangles and rivers–creeks with dark gray lines as shown in Figure 1.
382 J. M. BAUDER ET AL.

TABLE 3. Mean movement distances, rates, and home range sizes (minimum convex polygon =MCP home range [HR]; 95% fixed kernel utilization distribution =UD HR) from studies of
seasonally migrating snake populations from north temperate latitudes. Single elevation values were used when the study reported a single elevation value for the study site. Total distance and
maximum distance are reported as kilometers, rate as meters moved per day, and home range estimates as hectares. Studies with two values represent the reported values for males and nonpregnant
females, respectively. Values with NA were not directly reported in the original study.
Study
a
Species Location Elevation range Habitat NTotal dist. Max. dist. Rate MCP HR UD HR
This study C. v. viridis Central Idaho 1148–1898 Bunchgrass/Douglas fir 22 4.46 1.46 43 48 109
King and Duvall (1990) C. v. viridis Southern Wyoming 2120 Sagebrush-steppe 16 3.51
2.76 2.57 196
b
NA NA
7 2.03 133
d
Didiuk (1999) C. v. viridis Southeast Alberta 600–>850 Prairie 5 40.00
c
17.08
c
NA NA NA
Jorgensen et al. (2008) C. v. viridis Southeast Alberta NA Prairie 19 8.17 2.76 211
b
NA NA
Gardiner et al. (2013) C. v. viridis Southwest
Saskatchewan 760–950
d
Prairie 23 NA 2.81 92 109 14
Shipley et al. (2013) C. v. viridis East-central Colorado 1728–1783 Prairie 10
43.71
3.65 0.57 116
b
105
b
18 NA
0.44 18
Cobb (1994) Crotalus oreganus
lutosus Southeast Idaho 1470 Sagebrush-steppe NA 10.00 4.80 NA NA NA
Jenkins (2007) C. o. lutosus Southeast Idaho 1596–1697 Sagebrush-steppe 32 5.08 1.47 NA NA 23
Parker and Anderson
(2007) C. o. concolor Southwest Wyoming 1840–2125 Sagebrush-steppe 21 4 2.12 0.78 NA 118 301
1.96 0.68 64 196
Brown et al. (2009) C. o. oreganus Southern British
Columbia 285–>700
d
Bunchgrass/pondersoa
pine 14 5.84 1.08
e
142
b
25 NA
Reinert et al. (2011) Crotalus horridus Central Pennsylvania 200–550
d
Deciduous hardwood
forest 10-12 5.53 2.11 37 89 NA
Wastell and Mackessy
(2011) Sistrurus catenatus
edwardsii Southeast Colorado 1380–1470 Prairie/ 12 4.53 1.89
e
32 42 104
sandhill
Smith et al. (2009) Agkistrodon
contortrix Central Connecticut 80–220 Deciduous hardwood
forest 10 8 4.32 0.77 NA 17 NA
1.82 0.36 5
Gardiner et al. (2013) Coluber constrictor
flaviventris Southwest
Saskatchewan 760–950
d
Prairie 23 NA 2.46 65 159 14
Martino et al. (2011) Pituophis catenifer
sayi Southwest
Saskatchewan 760–950
d
Prairie 6 NA 1.71 52 87 NA
Williams et al. (2012) Pituophis catenifer
deserticola Southern British
Columbia 330–645
d
Bunchgrass/pondersoa
pine 39 NA 0.52 NA 10.5 NA
a
Maximum distance reported by King and Duvall (1990) was calculated by summing the length of daily movement steps during outbound migration. Values from Brown et al. (2009) were for resident snakes only. Sample size
range for Reinert et al. (2011) was sample size range over the 4-yr study and values were the mean annual values average across all 4 yr.
b
Represents distance per movement.
c
Values were calculated from raw data provided in the source document.
d
Elevation ranges (in meters) were estimated from Google Earth using the description of the study area provided in the original document.
e
Represents range length.
PRAIRIE RATTLESNAKE MOVEMENT PATTERNS 383

and Douglas, 2002; Dugan et al., 2008; Shipley et al., 2013).
Bauder et al. (In press) found that small mammal surface
activity (used as a proxy for prey availability) was similar across
all habitats, that rattlesnakes foraged in most habitats in
proportion to their availability, and that some rattlesnakes fed
at their hibernaculum. These observations suggest that suitable
overwintering sites and prey are sufficiently abundant within
lower Big Creek to negate the need for more-extensive seasonal
migrations. However, additional data on the distribution and
availability of overwintering sites and prey and their interaction
with population density are required to test this hypothesis.
Many studies on snake movements have found differences in
total or seasonal movement distances between sexes and have
attributed such differences to male mate-searching, females
searching for oviposition sites, or differing energetic require-
ments between sexes (Gregory et al., 1987; Blouin-Demers and
Weatherhead, 2002; Pearson et al., 2005; Cottone and Bauer,
2013). Male and nonpregnant female rattlesnakes in our study
showed no differences in movement distances, either total
distance or maximum distance moved, or movement rate,
although a relatively small female sample size may have
prevented us from detecting differences. Males in many species
of North American vipers show larger home range sizes
compared to females (Roth, 2005; Waldron et al., 2006; Smith
et al., 2009; Anderson, 2010; Glaudas and Rodriguez-Robles,
2011; Putnam et al., 2013), although differences in distance
FIG. 4. Movement pathways for 11 selected Prairie Rattlesnakes (Crotalus v. viridis) in the lower Big Creek drainage of central Idaho monitored
during 2006–2008. Pathways represent movements from a single activity season. Pathways not showing a return to a hibernaculum were collected in
2006 or 2007 when radio tracking ceased in August.
384 J. M. BAUDER ET AL.

moved or home range size may be absent or marginal in some
populations (King and Duvall, 1990; Reed and Douglas, 2002;
Parker and Anderson, 2007). Larger home range sizes are
generally attributed to male mate-searching, which is a common
mating system among North American vipers (Duvall et al.,
1992). Although we did not observe sex-specific differences in
distance moved we did observe differences in the pattern of
movement. Several males undertook distinct mate-searching
movements which were generally long, rapid movements made
in July and August that tended to have low directionality
(Bauder, 2010). This likely explains the higher turn angles and
meandering ratios we observed in males.
The mate-searching movements observed in our study
contrast with those observed in Prairie Rattlesnakes in southern
Wyoming (King and Duvall, 1990; Duvall and Schuett, 1997). In
those populations, females continued to forage during the late
summer, but their spatial predictability was low because of the
spatial unpredictability of prey. Straight line mate-searching
movements allowed males to maximize their encounters with
females (Duvall et al., 1992; Duvall and Schuett, 1997). The
sharp contrast of our results and those of Duvall et al. suggests
that females within lower Big Creek were more-spatially
predictable, thereby allowing males to forgo linear mate-
searching movements. Even so, only about half of the males
in this study exhibited distinct mate-searching movements
during July and August. The male depicted in Figure 3C
undertook mate-searching movements in 2006 (indicated by the
bold activity area outline) but did not in 2008 (indicated by the
thin activity area outline). We are unsure why we did not
observe mate-searching movements in all males, and we are
unaware of how widespread this pattern is among other snake
populations exhibiting male mate-searching. Although larger
males may be more likely to exhibit courtship or mate-guarding
behaviors (Clark et al., 2014), we found no difference in initial
body mass between mate-searching and non–mate-searching
males (Bauder, 2010). Males exhibiting mate-searching gained
slightly less weight (1.7% vs. 6.9%) and spent less time foraging
(60 days vs. 81 days), on average, than did males where mate-
searching was unobserved, although these differences were not
significant (Bauder, 2010). This suggests that there may be some
opportunity cost to mate-searching which, given a restricted
activity season, may cause some males to forgo searching for
females at the expense of foraging. Alternatively, if receptive
females are relatively accessible within our study area, males
may have ample opportunities to encounter females while
foraging and therefore negate the need for additional searching.
Consistent with several other studies from multiple snake
taxa (Kapfer et al., 2008; Pattishall and Cundall, 2008; Smith et
al., 2009), rattlesnakes in our study generally showed fidelity to
summer activity areas although the degree of overlap in
summer activity areas was relatively modest (range 0.06–0.65).
In addition, telemetered rattlesnakes tended to follow the same
mean bearing during their outbound migrations over multiple
years. We observed comparatively less overlap in summer
activity areas, particularly in areas used for foraging, among
telemetered rattlesnakes within activity seasons (JMB, unpubl.
data) although fully addressing interindividual space use
overlap was beyond the scope of this study. Our rates of
activity area overlap were lower than those reported by Jenkins
(2007) for Great Basin Rattlesnakes (Crotalus oreganus lutosus)in
southeast Idaho (mean =0.63), although Jenkins (2007)
considered overlap of summer foraging areas whereas our
summer activity areas included inbound and outbound
migration. Additionally, portions of summer activity areas used
one year but not another were often used during mate-searching
(e.g., Fig. 3C). This results in less overlap in summer activity
areas between years if males do not conduct extensive mate-
searching movements every year, as suggested by our data.
Differing methodologies used to calculate activity area fidelity
in other studies also limits our ability to compare our results
more broadly. The tendency for rattlesnakes to use similar
migration bearings across time while showing modest activity
area overlap may suggest greater fidelity to migration routes
than to foraging areas. Fidelity to summer foraging areas should
be expected if prey resources remain fairly stable in space and
time. However, small mammal populations often fluctuate
widely over time (Gillespie et al., 2008; Boonstra and Krebs,
2012), and Beaupre (2008) found that Timber Rattlesnakes
(Crotalus horridus) foraged more and showed less reproductive
activities during years of low prey abundance. One might
therefore expect that snakes, such as rattlesnakes which prey on
species with dynamic life histories, would adjust their foraging
movements in response to shifts in prey abundance. However,
Jenkins (2007) found that Great Basin Rattlesnakes returned to
the same general foraging areas in subsequent years even in
years of low prey abundance. Movements during these years
were more directional, suggesting that rattlesnakes adjusted
their foraging movements within their summer activity areas to
increase their searching efficiency in response to perceived prey
availability (King and Duvall, 1990; Duvall and Schuett, 1997).
The patterns of fidelity to migration routes and summer
activity areas observed in this study may have some implica-
tions for the hypothesis that linear migration movements
represent efficient search patterns for widely distributed or
spatially unpredictable prey resources. If snakes do migrate to a
familiar summer activity area year after year along a familiar
route, then these movements are probably not true searching
movements. Rather, linear migratory movements may represent
the quickest and most direct route to a known foraging area. We
hypothesize that migratory movements are relatively fixed and
independent of perceived prey availability compared to
foraging movements made during the summer. This may
explain the linear migrations observed in many seasonally
migrating snakes (Larsen, 1987; King and Duvall, 1990; Wastell
and Mackessy, 2010). However, when a snake reaches its
summer foraging area, it may then adjust its movements in
response to perceived prey availability, perhaps continuing to
make linear movements if prey availability is low. Jenkins (2007)
found that Western Rattlesnakes in southeastern Idaho made
more linear movements during low prey years. Rattlesnakes still
returned to the same general foraging areas, suggesting that the
more-linear movements were in response to more-spatially
unpredictable prey resources. However, further research is
needed to evaluate if summer fidelity is observed in other
rattlesnake populations, particularly during fluctuations of prey
abundance, and to test the hypothesis that rattlesnakes modify
foraging movements, not migration routes, in response to prey
availability.
Acknowledgments.—We thank J. Akenson, R. Bauder and C.
Bauder, A. Brumble, T. Morrison, D. Hilliard, S. Cambrin, and
the students, staff, and researchers at the Taylor Wilderness
Research Station for assistance in the field. J. Kie, D. Delehanty,
E. Strand, and J. Rachlow provided valuable input during this
study. T. Peterson assisted with the statistical analysis. We
would like to thank Arnold Aviation for logistical support. This
PRAIRIE RATTLESNAKE MOVEMENT PATTERNS 385

research was supported by the DeVlieg Foundation, National
Geographic Society (Young Explorers Grant), the Wildlife
Conservation Society, the American Museum of Natural History
(Theodore Roosevelt Memorial Grant), Sigma Xi, and The
Explorers Club (Youth Activity Fund). The University of Idaho
College of Natural Resources and Department of Fish and
Wildlife Resources also supported this research through a
Berklund Undergraduate Research Scholarship and the Fund
for Excellence Award, respectively. The Orianne Society
supported JMB during the preparation of this manuscript. The
comments of G. Perry and two anonymous reviewers greatly
improved this manuscript. This research was approved by the
University of Idaho and Idaho State University IACUC (no.
2005-08 and no. 628REN1008, respectively) and the Idaho
Department of Fish and Game (permit no. 940706).
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Accepted: 27 July 2014.
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