Pacific Coast Aquatic Garter Snake
Timmi Claveria (2001)

Thamnophis atratus

The Pacific Coast Aquatic Garter Snake, or Thamnophis atratus, is a part of the large group of garter snakes worldwide, all of which belong to the Columbridae Family and the Squamata Order. Garter snakes happen to be the most prevalent of the Colubrid snakes in North America. Thamnophis atratus used to be grouped with Thamnophis elegans, but in 1987 it was demonstrated that the two are indeed different species. It is now recognized that T. atratus has two subspecies: T. a. atratus and T. a. hydrophilus.

Many species of garter snakes look similar, since all are defined by sharply keeled dorsal scales and an undivided anal plate. However, the Pacific Coast Aquatic Garter Snake has several distinguishing features that can help in its identification. Pacific Coast Aquatic Garter Snakes are slender, medium sized snakes. Females are larger than males, but have shorter tails. Tail lengths range from 23.5-28.9% (of body length) in males and 22.1-27.4% in females. They usually do not grow to be more than 33 inches in length. They have a narrower head than other garters and a more pointed snout as well. T. atratus usually has 8 upper labial scales (the 7th being longer than the 6th), 10 lower labial scales, and 19 or 21 dorsal scale rows at mid-body. The internasal scales are longer than they are wide and are pointed anteriorly. The dorsal side of the snake is usually a pale gray to brown color. A narrow (compared to those in other garters) vertebral stripe is present, dull and indistinct in T. a. hydrophilus, bright yellow or orange in both subspecies, or absent (rarely in T. a. hydrophilus). The lateral stripes can either be bright and distinct (except in some T. a. atratus), dull and vague (T. a. hydrophilus), or lacking (some T. a. atratus and some T. a. hydrophilus. On each side of the vertebral stripe are 2 alternating rows of distinct black spots. The ventral side is lighter in color, almost pink, and is usually unmarked. The throat is a bright yellow in T. a. atratus, but not in a T. a. hydrophilus. In addition, round pupils characterize the eyes.

The Pacific Coast Aquatic Garter Snake can be found around permanent or semi-permanent bodies of water, usually shallow, rocky creeks, and swiftly flowing streams. They prefer areas with exposed boulders and riparian vegetation and can be found in a variety of habitats including coastal oak woodland and coastal chaparral. Pacific Coast Aquatic Garter Snakes range from SW Oregon to NW California, and south, about half way down the California coast. In SW Oregon they are found in lower elevations, but in California they have been sighted anywhere from sea level to an elevation of 6,240 feet. Specific Oregon counties in which they can be found include Coos Bay County, Douglas County, Curry County, Jackson County, and Josephine County.

In their habitats, Pacific Coast Aquatic Garter Snakes eat sculpin, minnows, goldfish, small trout, toads, salamanders, frogs, and amphibian larvae. Garter snakes rely on their tongue and vomeronasal organ to search for the odors of their prey. Very young snakes rely mainly on sit-and-wait tactics to obtaining food. They ambush their prey from rocks on the edge of shallow waters. Juveniles still use this method, as well as using the more active foraging style of the adult snakes. The adults forage in fast moving deeper (~19 inches) water. T. atratus have been observed foraging by anchoring their tail to submerged rocks and lunging at passing prey. Studies have shown variation in feeding frequency and prey type based on the snakes’ stage of life. Very young snakes ate mainly fish and tadpoles, and had the lowest frequency of full stomachs (18%). Juveniles also ate mainly fish and tadpoles, but had a higher frequency of full stomachs (37%). Adult diets had the most variation, consisting of larval and young salamanders, fish, and tadpoles, but had a frequency of full stomachs closer to that of the very young garter snakes (20%). As a meal, Pacific Coast Aquatic Garter Snakes are a tasty treat to other larger snakes, birds, and mammals.

Thamnophis atratus is an active diurnal snake. During the summer, the majority of their activity takes place during the morning and afternoon hours. During the cooler spring and fall seasons, or even at the higher elevations, the snakes limit their activity to the warmer afternoons. At night, the Pacific Coast Aquatic Garter Snake retreats to holes and crevices. In the winter, T. atratus have been known to migrate to inland montane localities to hibernate. However, on warmer winter days they can sometimes emerge from their holes to bask in the sun. In milder areas, instead of migrating, the snakes will hibernate in rotting logs or old mammal burrows.

Soon after emergence from hibernation, courtship and mating begin. This occurs in late March and April. The young snakes are born alive, in a secluded spot such as dense vegetation or loose bark of rotting logs. Clutch size ranges from 3-12, but average around 8. Births can occur from August to October.

Other behaviors characteristic of the Pacific Coast Aquatic Garter Snake include not being territorial, escaping from danger by diving into water and hiding beneath rocks or under exposed tree roots, and emitting a repulsive musk from the post anal glands when disturbed.

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