Term paper
Western Terrestrial Garter: Thamnophis elegans
Garter
snakes are very common throughout the Pacific Northwest and highly abundant.
The members of the Thamnophis genus can be found just about everywhere
in the Pacific Northwest and are commonly seen in backyard gardens, especially
those with rocks (Kozloff 1995) . They are typically no longer than 60cm in
length and coloration can be highly variable, even among snakes of the same
subspecies (Kozloff). These highly successful snakes are of the Natriccinae
subfamily and unlike most snakes, they give birth to fully developed live
young. They have an interesting variation in life histories among some species.
The time it takes for a juvenile to reach reproductive maturation can differ by
three years depending on the environment. (Bronikowski and Arnold 1999) Thamnophis snakes have a highly
sensitive tongue which is chalk full of chemoreceptors. They are constantly
sticking their tongue out “tasting” the air for the scent of would be prey.
(Arnold 1991) Of the twenty two known species of Thamnophis, four of them reside in the northwest.(Brodie et al 1982)
The
four species present are the Pacific Coast Aquatic Garter Snake Thamnophis atratus, the Northwestern
Garter Snake Thamnophis ordinoides,
the Common Garter Snake Thamnophis
sirtalis, and the Western Terrestrial Garter Snake Thamnophis elegans. The last mentioned, Thamnophis elegans, will be the focus of this paper. Despite the
name Western Terrestrial Garter
snake, some subspecies of elegans are aquatic. Here in Oregon they can be
found just about everywhere in except for the highlands of the Cascade mountain
range (see figure 1). Elegans are
easily distinguished from the sirtalis species
because elegans have 8 upper labial
scales whereas the Common Garter snake has only 7, plus the sixth and seventh
upper labial scales are typically enlarged in terrestrial (not the aquatic) elegans subspecies due to larger than
normal salivary glands (Brown 1995).
There
are four subspecies of Thamnophis elegans
in the Pacific Northwest, the Klamath Garter snake Thamnophis elegans biscutatus, the Mountain Garter snake Thamnophis elegans elegans, the Coast
Garter snake Thamnophis elegans
terrestris, and the Wandering Garter snake Thamnophis elegans vagrans. (Brodie et al 1995) See figure 2 at the
end of the paper for pictures of the different elegans subspecies. Remember that there may be highly variable
coloration between individuals, even between those of the same subspecies!
The following
information in the next four
paragraphs was taken from Amphibians and Reptiles of the Pacific Northwest (see
bibliography).The Klamath Garter snake (T.
e. biscutatus) is by far the largest of the elegans subspecies with mature snakes being over 90 cm. T. e. biscutatus are found in or around
water and consequently they prey on mainly aquatic organisms. Mostly fish,
leeches, lampreys, frogs, tadpoles, and toads. This subspecies has a relatively
small range compared to other elegans and
resides mostly in the southern Klamath area. These snakes are typically brown
or black with a yellow or sometimes brownish stripe running down their back.
The Mountain
Garter snake (T. e. elegans) may be
found in southwestern Oregon, the Klamath-Siskiyou Mountains, and the Cascade
Mountains. T. e. elegans are usually
associated with water or a damp meadow however their diet consists of strictly
terrestrial organisms such as slugs, toads, frogs, lizards, and mice. Adult elegans elegans may grow to be over 60
cm. This subspecies is black or dark brown with light colored flecks. Their
stripe is usually a vibrant yellow or orange, very broad and sharp edged. Some
individuals may also have yellow or crème colored lateral stripes.
The Coast Garter
snake (T. e. terrestris) is rarely
found any further north than the coastal Klamath Mountains of Curry. Mature
adults may reach up to 60 cm and are reddish brown or olive in color. They have
a broad, straight edged yellow stripe on top of their backs and lateral olivish
or yellow stripes with red flecks. The red flecks are sometimes also present on
the snake’s belly. Like the Mountain Garter snake, terrestris subspecies eat terrestrial organisms such as slugs,
mice, lizards, and salamanders. A unique feature of this subspecies is that
some individuals have seven upper labial scales rather than eight (eight upper
labial scales is sometimes used to distinguish sirtalis and ordinoides from
elegans species which obviously won’t
work for some T. e. terrestris).
The Wandering
Garter snake (T. e. vagrans) can be
found all over Oregon, especially eastern parts, and in British Columbia,
Washington, and even Idaho (in Idaho vagrans
have been found at elevations more than 2,400m ! ). They are very versatile in
their feeding habits and have been known to eat fish, tadpoles, slugs, snails,
earthworms, mice, lizards, frogs, birds, and salamanders. In some parts of
British Columbia they have even been reported eating marine and seashore
invertebrates! This subspecies is typically dark in color with a thin yellow or
brown uneven stripe. Some individuals have no dorsal stripe or it is broken up
by flecks of black.
As you can see, Thamnophis elegans subspecies are highly
variable and figure 2 in the back of this paper will give you some idea of just
how different various subspecies look from one another. The rest of this paper
will focus on some behaviors of T.
elegans. Because most literature published today on Garter snakes focuses
on thermoregulation of active snakes, I have decided to give information on
retreat behavior. This is important because the Pacific Northwest being a
temperate zone, most ectotherms spend most of their time tucked away in their
retreats.
A study done by
R. Huey, C. Peterson, and S. Arnold recorded retreat site selection of over
2,000 Thamnophis elegans over a
fifteen year period at Pikes Point near Eagle Lake, California (Northern
California). The following information in the next four paragraphs is from this study (see bibliography). The
vegetation at this location is similar to that of Eastern Oregon and is
dominated by sagebrush and pine forests. The dominant rocks in the area are
basalt with sizes ranging from pebbles to car sized boulders. Daily changes in temperature
averaged 47 degrees during the active season. In their study, snakes were
surgically outfitted with temperature sensitive radiotransmitters so the
scientists could determine the snake’s location as well as obtain body
temperature data.
The scientists
found that elegans species of Garter
snakes typically use rocks that are greater than 15cm thick but less than 40 cm
as retreat sites. They emerge from their sites during midmorning to bask on
sunlit ground, then spend only a short part of their day foraging. They also
found that some snakes (especially gravid, shedding, or food digesting) do not
come out of their retreat rocks at all. Many do little more than peak their
head or tail out. Even on clear days! Even more remarkable, these snakes that
do not emerge for days at a time are able to maintain a relatively constant
body temperature. One gravid female was recorded as maintaining a body temp
range of 25.2 – 30.9 degrees celsius for a fourteen hour period and for a
twenty four hour period her body temp did not go lower than 23.6 degrees
celsius!
How do they do
it? Apparently this study found that temperatures underneath the rocks showed a
strong daily pattern which the snakes are able to take advantage of. Depending
on the size of the rock, thickness, and position of the rock to sunlight,
temperatures were lowest from 6:00 am to 1:00 pm. The highest temperatures were
recorded 2:00 pm and 11:00 pm. The temperatures at different positions
underneath the rock also varied. Temperatures underneath the edge of rocks
varied much more than temperatures at the center. In short, these elegans had a wide thermal gradient
available to them even when staying under a single rock.
Going back to the
fact that the snakes preferred rocks approximately 20cm thick, the researchers
found that daily thermal cycles under rocks were primarily dictated by rock
thickness and not by mass or shape of the rock. It seemed apparent to the
researchers that rocks about 20 cm thick provided the right environment for the
snakes to avoid extreme temperatures. However the authors might be missing a
piece to the puzzle. Maybe the heat produced by digestion, shedding, are developing
young plays a role in the snakes ability to maintain favorable body temps
without emerging. A good experiment to do next would be to compare digesting
and non-digesting body temps of the same individual in a lab where a controlled
environment is easy to obtain. It is remarkable that organisms which are
ectotherms can pull off such feats in such an environment. In my mind it would
be hard to maintain a body temperature range of 24 –39 degrees in an environment
that flucuates 47.5 degrees on a daily basis (at least without a jacket or
sleeping bag).
1)
Kozloff, Eugene (1995) Plants
and Animals of the Pacific Northwest, University of Washington Press, pg.
218-219.
2)
Brodie, Edmund., Nussbaum, Ronald., and Storm, Robert (1982) Amphibians and Reptiles of the Pacific
Northwest, University of Idaho Press, pg. 294-297.
3) Huey, Raymond.,
Peterson, Charles., and Arnold, Stevan (1989)Hot Rocks and Not-So-Hot Rocks: Retreat Selection by Garter Snakes and
its Thermal Consequences, Ecology
vol 70. No 4, pg. 931-944.
4) Brown, Herbert.,
Bury, Bruce., Darda, David., Diller, Lowell., Peterson, Charles., and Storm,
Robert (1995) Reptiles of Washington and
Oregon, Seattle Audubon Society, pg. 138-144.
5) Arnold, Stevan
(1992) Behavioral Variation in
Population. VI. Prey Responses by Two Species o f Garter Snakes in Three
Regions Of Sympatry, Animal Beahvior,
vol 44, pg. 705-719.
6) Bronikowski, Anne
and Arnold, Stevan (1999) The Evolutionary
Ecology of Life History Variation in the Garter Snake Thamnophis Elegans,
Ecology, vol 80(7), pg. 2314-2325.