Western Terrestrial Garter: Thamnophis elegans

Dustin Fowkes

 

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 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).

References:

Kozloff, Eugene (1995) Plants and Animals of the Pacific Northwest, University of Washington Press, pg. 218-219.

Brodie, Edmund., Nussbaum, Ronald., and Storm, Robert (1982) Amphibians and Reptiles of the Pacific Northwest, University of Idaho Press, pg. 294-297.

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.

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.

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.

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.