2008 SOUTHEASTERN NATURALIST 7(2):359–366
Use of Trees by the Texas Ratsnake (Elaphe obsoleta)
in Eastern Texas
Josh B. Pierce1,*, Robert R. Fleet2, Lance McBrayer3,
and D. Craig Rudolph1
Abstract - We present information on the use of trees by Elaphe obsoleta (Texas
Ratsnake) in a mesic pine-hardwood forest in eastern Texas. Using radiotelemetry,
seven snakes (3 females, 4 males) were relocated a total of 363 times from April 2004
to May 2005, resulting in 201 unique locations. Snakes selected trees containing
cavities and used hardwoods and snags for a combined 95% of arboreal locations.
Texas Ratsnake arboreal activity peaked during July and August, well after the peak
of avian breeding activity, suggesting arboreal activity involves factors other than
Snakes within the eastern ratsnake complex (Elaphe obsoleta Say
[Texas Ratsnake], E. alleghaniensis Holbrook [Eastern Ratsnake], and E.
spiloides Duméril, Bibron & Duméril [Gray Ratsnake; following the taxonomy
of Burbrink 2001) are well known for their climbing abilities (Durner
and Gates 1993, Jackson 1976, Mullin et al. 2000, Stickel et al. 1980);
however, time spent in trees varies temporally and/or geographically (Blouin-
Demers and Weatherhead 2001, Durner and Gates 1993, Fitch and
Shirer 1971, Mullin et al. 2000). Possible explanations for arboreal behavior
in snakes include foraging (Beaupre and Roberts 2001), ecdysis (Stickel
et al. 1980), escape from predators (Rudolph et al. 2004), oviposition
(Brothers 1994, Clark and Pendleton 1995), thermoregulation (Shine et al.
2005), mating (Bullock 1981), and winter dormancy (Stickel et al. 1980).
However, the most frequently documented behavior associated with tree
use within North American ratsnakes is predation on nesting birds (Aldrich
and Endicott 1984; Blem 1979; Fendley 1980; Fitch 1963; Gress and Weins
1983; Hensley and Smith 1986; Jackson 1970, 1978; Mullin and Cooper
2002; Mullin et al. 2000; Neal et al. 1993; Stickel et al. 1980; Withgott and
Amlaner 1996). The peak of avian nesting has been shown to overlap with
the exploitation of arboreal prey in Texas Ratsnakes in Kansas (Fitch 1963)
and Gray Ratsnakes in Ontario (Weatherhead et al. 2003). During avian
1 Wildlife Habitat and Silviculture Laboratory (maintained in cooperation with the
Stephen F. Austin State University Arthur Temple College of Forestry and Agriculture),
Southern Research Station, USDA Forest Service, 506 Hayter Street,
Nacogdoches, TX 75965. 2Department of Mathematics and Statistics, Stephen F.
Austin State University, Nacogdoches, TX 75962. 3Department of Biology, Georgia
Southern University, PO Box 8042, Statesboro, GA 30460. *Corresponding author
360 Southeastern Naturalist Vol.7, No. 2
nesting, eggs and juvenile birds are especially vulnerable to consumption
by snakes. Therefore, ratsnakes might benefit energetically if arboreal activity
coincided with avian nesting. Neal et al. (1993) demonstrated that
ratsnakes were more active on Picoides borealis Vieillot (Red-cockaded
Woodpecker) nest trees during the nesting season. However, other factors
such as ecdysis, escape from predators, thermoregulation, mating, and winter
dormancy may also play important roles in ratsnake climbing, causing
climbing behavior to be the same throughout the active season. Thus, the
objective of our study was to describe the arboreal microhabitat use of Texas
Ratsnakes in eastern Texas, paying particular attention to their arboreal
activity during avian nesting.
Our study was conducted on the Stephen F. Austin Experimental Forest
(SFAEF) and adjacent private property located approximately 13 km
southwest of Nacogdoches, TX. The SFAEF is part of the Angelina-Sabine
National Forest and is administered by the USDA Forest Service’s Southern
Research Station (Wildlife Habitat and Silviculture Laboratory, Nacogdoches,
TX). The SFAEF consists of 1036 ha of forest, with bottomland
hardwood forest comprising approximately two thirds and upland pine and
mesic forests making up the remainder.
The dominant overstory species of the bottomland hardwood forest on
the SFAEF are Quercus lyrata Walt. (overcup oak), Fraxinus pennsylvanica
Marsh. (green ash), Q. phellos Linnaeus (willow oak), and Liquidambar
styracifl ua Linnaeus (sweetgum). Mesic sites are characterized by overstory
trees consisting of Pinus taeda Linnaeus (loblolly pine) and Q. falcata Michx.
(southern red oak), with Q. stellata Wangenh. (post oak), Cornus sp.
(dogwood), Q. marilandica Muenchh. (blackjack oak), Carya sp. (hickory),
Sassafras albidum (Nutt.) (Sassafras) Nees, and sweetgum generally composing
the midstory (Johnson 1971). The upland pine forest consists mostly
of P. echinata P. Mill. (shortleaf pine) and loblolly pine, with oak, hickory,
and sweetgum being common (Chambless 1971). The SFAEF has been subjected
to limited timber harvesting in recent decades, and canopy trees of
most forest habitat types are 70+ years old (Conner et al. 2003).
Snakes were captured with drift fence and funnel-trap arrays (Burgdorf
et al. 2005, Fitch 1951) from 29 March to 20 June 2004. Eleven Texas
Ratsnakes were equipped with radiotransmitters, but two of the snakes’
transmitters were found unattached to the snakes 4 and 6 weeks after release,
therefore too few data were obtained for any analyses. Of the remaining nine
snakes, seven were used in all data analyses, and two were used in only the
arboreal habitat characterization due to their deaths from unknown causes.
2008 J.B. Pierce, R.R. Fleet, L. McBrayer, and D.C. Rudolph 361
Captured individuals were returned to the laboratory where they were
weighed to the nearest gram and measured (total length and snout–vent
length [SVL]); sex was determined by probing for hemipenes (Schaefer
1934). Each snake was marked by subcutaneous injection of a passive integrated
transponder (PIT tag). Transmitters (60 x 11 x 5 mm; ≈6.7 g) were
implanted subcutaneously following the techniques of Weatherhead and
Anderka (1984). Transmitters weighed less than 2% of snake body masses. After
surgery, snakes were kept in the laboratory and monitored for at least five
days, then were released at the point of capture.
Snakes were tracked at various times throughout the day and were relocated
at intervals of 2 to 7 days. Relocations were made from 16 April 2004
to 5 May 2005. Relocation site coordinates were obtained using a global
positioning system (GPS; Garmin™ eTrex) unit. At each snake location, we
recorded air temperature (using a mercury thermometer 1.5 m above ground
in a shaded location near the snake), macrohabitat type (upland pine, mesic
forest, bottomland hardwood), stand basal area (using a one-factor metric
prism), percent canopy closure (using an ocular tube 11.5 cm long by 5.0 cm
in diameter), and snake activity (i.e., motionless, basking, traveling). Snakes
were considered arboreal when found ≥2 m above the ground in a tree ≥3
cm diameter at breast height (dbh) (Dueser and Shugart 1978). When snakes
were found in trees, the height of the snake, tree species, dbh, vine presence,
and cavity presence were recorded.
To assess potential infl uence of arboreal nesting birds on snake microhabitat
use, seasons were divided into the general avian nesting season and
the peak of avian nesting. The typical nesting season for arboreal nesting
birds inhabiting eastern Texas is from March to July, with April and May
having the greatest temporal concentration of nesting activities (Hamel
1992). Although colder temperatures did not prevent or eliminate snake
movement, climbing activity was reduced. Since we wanted to determine
when the snakes use trees most often during the months that are warm
enough for them to climb, November, December, January, and February
were excluded from monthly arboreal analysis.
To determine whether arboreal locations used by snakes were different
than what was available, habitat characteristics of trees used by snakes and
randomly selected trees were compared. One random tree was chosen for
each arboreal snake relocation by walking 10 to 200 paces (determined by a
random number generator) in a randomly chosen direction from each snake
relocation site (Blouin-Demers and Weatherhead 2001). The tree nearest to
each random location was selected and tree species, dbh, stand basal area,
and presence of cavities and vines were recorded and compared to these
same characteristics of trees used by snakes. Stand basal area and dbh were
compared across used and random locations using paired t-tests. Chi square
tests were used to test if snakes occupied trees containing vines and cavities
more than expected, and to determine if snakes chose certain tree types
362 Southeastern Naturalist Vol.7, No. 2
(hardwoods, pines, or snags [any dead tree which was either hollow or contained
a cavity]) over available tree types. Relocations in trees where snakes
were observed more than once were only included once in the analysis of
arboreal microhabitat use (Blouin-Demers and Weatherhead 2001). Thus,
only the characteristics of unique arboreal microhabitats were compared to
characteristics of random trees. All statistical analyses were performed at
an alpha level of 0.05 using SAS® software, Version 9 (SAS Institute 2003).
Proportional data were arcsine–transformed to achieve normality.
Use of trees
Snake locations were difficult to determine precisely when snakes
were positioned high in trees. However, the specific tree could often be
determined with a specific cavity or branch identified as the snake location.
Snakes (n = 7; 4 males and 3 females) were found in trees (≥2.0 m above
ground) during 96 of 363 (26.5%) observations. All three females used trees
more often than any male; however, a low sample size precluded statistical
comparisons. Male (18 of 38 relocations; 47.4%) and female (15 of 19 relocations;
78.9%) snakes climbed most frequently during July (Fig. 1). Four of
seven individuals climbed most frequently during July; only one snake was
found in a tree less than 60% of relocations during July (28.6%).
During the avian nesting season (March–July), snakes used trees proportional
to other active months (August–October; χ2 = 0.322, df = 1, P =
0.571). Similarly, tree use did not differ between the peak of avian nesting
(April–May) and non-peak (June–October, March) months (χ2 = 2.700,
df = 1, P = 0.100).
Arboreal habitat characterization
Only 40 of the 105 (n = 9 snakes) arboreal relocations were unique. The
dbh of trees used by snakes (mean = 18.5 cm) was significantly larger than
Figure 1. Percentage of relocations in trees by month for Elaphe obsoleta (Texas
Ratsnakes) from April 2004 to May 2005 in eastern Texas. The avian nesting season
is from March to July, with a peak in nesting during April and May.
2008 J.B. Pierce, R.R. Fleet, L. McBrayer, and D.C. Rudolph 363
that of random trees (mean = 11.4 cm; t = -4.39, df = 39, P < 0.001). However,
stand basal area did not differ between used (mean = 25.5m2/ha) and
random (mean = 27.4m2/ha) locations (t = -1.46, df = 39, P = 0.154). The
presence of vines did not differ between used (27.5%) and random (35%)
trees (χ2 = 0.524, df = 1, P = 0.469). Cavities, however, were found in 77.5%
of the 40 used trees, but in none of the random trees. Snakes used tree types
significantly different than those available (χ2 = 13.867, df = 2, P = 0.001).
Hardwoods (30 of 40 unique arboreal locations) and snags (6 of 40 unique
arboreal locations) were used more often than expected, whereas pines were
used less often than expected (4 of 40 unique arboreal locations).
Ratsnakes are known to prey on birds (Aldrich and Endicott 1984; Blem
1979: Fendley 1980; Fitch 1963; Gress and Weins 1983: Hensley and Smith
1986; Jackson 1970, 1978; Mullin and Cooper 2002; Mullin et al. 2000;
Stickel et al. 1980; Withgott and Amlaner 1996) and small mammals (Fitch
1963, Stickel et al. 1980). Although prey items were not recorded for our
population, our snakes did not climb trees most often during the peak of avian
nesting, which seems to support the idea that ratsnake climbing behavior
is not associated, at least exclusively, with predation on birds (Weatherhead
et al. 2003).
At the SFAEF in eastern Texas, arboreally nesting, roosting, or foraging
mammalian prey of suitable size for the Texas Ratsnake include Glaucomys
volans Linnaeus (southern fl ying squirrel), Sciurus niger Linnaeus (eastern
fox squirrel), Sciurus carolinensis Gmelin (eastern gray squirrel), Peromyscus
gossypinus LeConte (cotton mouse), Ochrotomys nuttalli Harlan (golden
mouse), Neotoma fl oridana Ord (eastern woodrat) and microchiropterans
(Schmidly 2004). Texas Ratsnakes are known to prey on fl ying squirrels
(Dennis 1971; D.C. Rudolph, US Forest Service, Nacogdoches, TX, pers.
comm.), and fl ying squirrels are abundant in the SFAEF (Conner et al. 1995).
Flying squirrels have two nesting seasons, one from March to April, and a
second during August (Schmidly 2004), giving ratsnakes potential arboreal
prey throughout their activity season.
On 92 of 105 arboreal relocations (87.6%), snakes were located in trees
containing cavities. On three occasions, shed skins were observed in tree
branches below sites where snakes were previously located. Snakes may
have been using trees as pre-molt basking locations, as has been documented
for Eastern Ratsnakes (Stickel et al. 1980). In eastern Texas, snakes preferentially
climbed trees containing cavities. Cavities within trees may provide
snakes a refuge from predators and the elements, and/or access to mammalian
prey. In addition to their strong vomeronasal sense (Halpern 1992),
snakes use visual cues to locate potential arboreal prey (Eichholz and Koenig
1992, Mullin and Cooper 2002). The presence of a cavity may be a cue used
by snakes to climb trees for further investigation (Neal et al. 1993). Thirtyone
of 40 unique arboreal locations at the SFAEF were associated with trees
364 Southeastern Naturalist Vol.7, No. 2
that contained cavities, while the remaining 9 trees appeared to be without
cavities. Hardwoods were used more often than expected, while pines were
used significantly less than expected. The use of hardwood trees in excess
of their availability may be linked to the use of cavities. In the southeastern
US, in the absence of Red-cockaded Woodpeckers, living pines do not typically
contain cavities (Conner et al. 2004), while mature hardwoods often
have cavities (Holloway et al. 2007). Snakes used trees that were larger than
those chosen at random, perhaps indicating that trees containing cavities are
usually mature trees.
In conclusion, Texas Ratsnakes in the SFAEF preferentially climbed
large hardwoods containing cavities. Texas Ratsnakes may use trees for
access to prey, for basking sites, and/or as predator avoidance sites (Werler
and Dixon 2000). The peak of snake arboreal activity did not coincide with
the peak of avian nesting, suggesting that avian prey availability is not the
primary purpose for climbing.
This study was partially supported by Stephen F. Austin State University. We are
grateful to S. Williams, R.Allen, K. Kowalczyk, S. Fleet, R. Conner, and B. Burt for
their assistance with this research. R. Thill, C. K. Adams, M. Kwiatkowski, D. Saenz,
and two anonymous reviewers provided valuable comments on earlier drafts of the
manuscript. We would also like to thank Mr. F. Molandes for allowing us to track
snakes on his property. Transmitters were constructed by P. Blackburn (Stephen F.
Austin State University College of Math and Science). The use of trade, equipment,
or firm names in this publication is for reader information only and does not imply
endorsement by the US Department of Agriculture of any product or service.
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