Western terrestrial garter snake
|Western terrestrial garter snake|
|T. elegans terrestris|
(Baird & Girard, 1853)
6 sspp., see text
Most western terrestrial garter snakes have a yellow, light orange, or white dorsal stripe, accompanied by two stripes of the same color, one on each side. Some varieties have red or black spots between the dorsal stripe and the side stripes. It is an immensely variable species, and even the most experienced herpetologists have trouble when it comes to identification. They are medium-sized snakes, usually 46–104 cm (18–41 in).
Like many species of North American garter snake, the western terrestrial garter snake possesses a mildly venomous saliva. Specimens collected from Idaho and Washington produced venom with myonecrotic (muscle tissue-killing) effects when injected into the gastrocnemius muscles of mice. Several cases of mild human envenomation with local edema and other symptoms (but without any systemic symptoms) have occurred from the wandering garter snake subspecies, including in Colorado.
This species is the only garter snake species with a well-documented tendency to constrict prey, although the constriction is inefficient when compared with the constriction of many other snakes (such as the gopher snake), involving disorganized, loose, and sometimes unstable coils and a longer time required to kill prey. Snakes from Colorado populations of terrestrial garter snakes appear to be more efficient at killing their prey by constriction than those from Pacific Coast populations.
Thamnophis elegans is found in southwestern Canada and the western United States, as far east as western Nebraska and the Oklahoma Panhandle. An isolated population occurs in Baja California, Mexico.
Several subspecies or races have been identified, although the validity of some is debated.
- Thamnophis elegans arizonae Tanner and Lowe, 1989 – Arizona Garter Snake
- Thamnophis elegans elegans (Baird and Girard, 1853) – Mountain Garter Snake
- Thamnophis elegans hueyi Van Denburgh and Slevin, 1923 – San-Pedro-Martir Garter Snake
- Thamnophis elegans terrestris Fox, 1951 – Coast Garter Snake
- Thamnophis elegans vagrans (Baird and Girard, 1853) – Wandering Garter Snake
- Thamnophis elegans vascotanneri Tanner and Lowe, 1989 – Upper Basin Garter Snake
Thamnophis elegans occurs in a wide variety of habitats, including grasslands, woodlands, and coniferous forests, from sea level to found of up to 3,962 m (12,999 ft). It is primarily terrestrial, although populations in the Great Basin and Rocky Mountains are semiaquatic.
The diet of Thamnophis elegans relies heavily on the potential prey available in the environment, and thus varies due geographical differences. This makes the western terrestrial garter snake an excellent example of polymorphism. There are two main variants that are most prevalent: coastal and inland. Since T. elegans is found along the west coast of the United States, a snake native to a coastal habitat would be found near the coast of the Pacific Ocean. On the other hand, if the snake population was considered to be inland, it would be found near water sources such as streams, ponds, or lakes inland rather than being near the coast.
Coastal populations' food sources mainly include terrestrial prey such as slugs, salamanders, small mammals, and lizards. In contrast, inland populations indulge in a semi-aquatic diet containing frog and toad larvae, leeches, and fish. Thus, aquatic food sources are a staple in the inland snake populations’ diet.
Coastal snakes are less likely to respond to chemical cues received from fish, which can be inferred by less rapid tongue-flicking when around them. They are also less likely to attack and ingest the fish. This preference in diet is so strong that the snake will starve before eating non-preferred prey types. Further, this appears to be genetically determined seeing as variation in diet is maintained in newborn snakes from both populations. When hunting, the Western Garter Snake’s actions are chemically and visually mediated on land and in water. Despite the habitat where foraging takes place, both ecotypes utilize similar techniques. This consists of attacks that are both aerial and underwater. These include craning, cruising, and diving. However, coastal snakes are less likely to participate these activities.
These differences in diet and foraging behavior between coastal and inland snake populations suggest that the species has undergone microevolution. Due to dietary and foraging differences between both variants of T. elegans, it can be implied that coastal populations have filled a niche in the environment that allows them to no longer rely on fish as a major food source.
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