Identifying Shasta County Garter Snakes

With the exception of the Pit River specific information, the identification data presented here is a summary of the data found at the Key to California Gartersnakes web page at CaliforniaHerps.com and information from Stebbins 2003, summarized for Shasta County Garter Snake identification. Pit River specific information is from Rossman and Stewart 1987.

Garter snakes can be highly variable within their range and therefore can be difficult to identify if you are not familiar with the typical look of a species within your area. For example, in parts of Mendocino County the Aquatic Garter Snake looks an aweful lot like our Terrestrial Garter Snakes. As such, finding a picture online or in a book that resembles a snake you found in the field may result in mis-identification. To properly identify a garter snake species, you really need to know the identification keys for the species that may be found in your area.

In Shasta County, the upper labials is probably the easiet way to distinguish the garter snakes. The Aquatic Garter Snake and Sierra Garter Snake are the most difficult to properly distinguish from each other.

The following sections should help you properly identify Garter snakes in Shasta County.

All images on this page are links to larger versions of the image.

The images currently labeled as Aquatic Garter Snake are actually a sister subspecies to our own. The images are from a Diablo Gartersnake. The specimen used is from Contra Costa County where it was found hunting California Red-legged Frog tadpoles. While the scale counts and shape are the same as what you would expect from our local subspecies, the coloration is different. The images will be replaced by local photos once I have some decent photos that demonstrate the proper identification keys.

Use Range Maps

The Terrestrial Garter Snake and Common Garter Snake can be found throughout Shasta County.

South of Lake Shasta, the Aquatic Garter Snake is found west of the Sacramento River and the Sierra Garter Snake is found east of the Sacramento River. They do not appear to have a contact zone south of Lake Shasta. They come close at the bottom of the county, but all evidence indicates those populations are in fact separated by the Sacramento River and do not seem to have any contact there either in Shasta County or in Tehama County to the south (Rossman and Stewart 1987 pg. 6,9).

Aquatic and Sierra Gartersnakes ©2014

General range map taken from CWHR, modified.
Museum records taken from following institutions: CAS Herps, CAS-SUR, MVZ Herps, LSUMZ Herps.

In the Pit River drainage north east of Lake Shasta, they have a contact zone and can be difficult to distinguish. Some apparent hybrids have been found, particularly in Rock Creek (Rossman and Stewart 1987 pg. 4). In the map to the left, localities where apparent hybrids have been found and/or where both species have been found should show up as red.

The contact zone should be considered to be from Potem Creek on the west to Lake Britton / Burney creek on the east. Potem Creek and west, you will not likely find a Sierra Garter Snake. Lake Britton / Burney creek and east and south, you will not likely find an Aquatic Garter Snake (though they do occur in Clark Creek which feeds Lake Britton from the north). Between those creeks, very careful attention must be paid to determine the species, especially in Rock Creek and in Deep Creek where both species are known to occur.

In addition to county wide distribution map for the Sierra Garter Snake and Aquatic Garter Snake, a dot locality map of the contact zone is provided: /map/pitContact.svg. The dot locality contact zone map does not include any museum records, it only includes records in our database for which I am confident on identification and have specific permission to reveal the dot locality. Right now, that is not very many. It does however provide a good map of the contact zone and streams within the contact zone.

Examine The Upper Labials

The upper labials are probably the most consistant identification key. Common Garter Snakes are the most likely to differ in that they sometimes have 8 upper labials instead of 7. However, they are the only Shasta County gartersnake to have red blotches on the side, which is a dead giveaway to the species regardless of their upper labials.

Terrestrial Garter Snake Aquatic Garter Snake Sierra Garter Snake Common Garter Snake
Thamnophis elegans Thamnophis atratus Thamnophis couchii Thamnophis sirtalis
Typically 8, 6th and 7th grossly enlarged compared to 5th Typically 8, 6th and 7th not grossly enlarged, 6th usually shorter than 7th Typically 8, 6th and 7th not grossly enlarged, 6th usually longer than 7th Typically 7, occasionally has 8 on one or both sides
[Upper Labials of the Terrestrial Garter Snake] [Upper Labials of Pacific Coast Aquatic Garter Snake] [Upper Labials of the Sierra Garter Snake] [Upper Labials of the Common Garter Snake]

When examining upper labials, the first upper labial is the one closest to nose. The seventh or eighth is the one farthest from the nose.

Chin Shields

Every Terrestrial Garter Snake I have ever examined, and I have examined quite a few, matches the identification key presented here. I have not examined very many Aquatic Garter Snakes (and none yet in Shasta County), but they too have all matched the identification key. Common Garter Snakes usually match the identification key, but I have examined one specimen from Burney Creek that did not, at least not to a significant extent (see Record 331). With respect to Sierra Garter Snakes, I can not find published data with respect to this key. I have only examined one live specimen and one photograph, both Shasta County specimens and in both cases the front and rear chin shields were about equal in length. However until I find published data or have examined enough specimens to know what the trend is, if there is one, chin shields should not be used as a key for Sierra Garter Snakes.

Terrestrial Garter Snake Aquatic Garter Snake Sierra Garter Snake Common Garter Snake
Thamnophis elegans Thamnophis atratus Thamnophis couchii Thamnophis sirtalis
Front and Rear pairs equal in length Rear pair longer than front May not be reliable key Rear pair longer than front
[Chin Shields of the Terrestrial Garter Snake] [Chin Shields of the Pacific Coast Aquatic Garter Snakes] [Chin Shields of the Sierra Garter Snake] [Chin Shields of the Common Garter Snake]

Internasals

Terrestrial Garter Snake Aquatic Garter Snake Sierra Garter Snake Common Garter Snake
Thamnophis elegans Thamnophis atratus Thamnophis couchii Thamnophis sirtalis
Wider than long, rounded in front Elongated, pointed in front Tend to be Longer than wide, pointed in front May not be reliable key
[Internasals of the Terrestrial Garter Snake] [Internasals of the Pacific Coast Aquatic Garter Snake] [Internasals of the Sierra Garter Snake] [Internasals of the Common Garter Snake]

The internasals are the pair of front pair most scales on the top of the head, directly between the nostrils.

Pit River Aquatics

In the Pit River Drainage, the Sierra Garter Snake and the Aquatic Garter Snake both occur and can be difficult to distinguish, and some specimens may be hybrids.

In the study published by Lousiana State University (Rossman and Stewart 1987 pg. 2-9), numerous characteristics were found that can be used to distinguish the two species from each other. Some of them are difficult to measure on a young and/or live specimen, but the following characteristics should not be too difficult to measure in the field without harming the specimen:

  1. 6th and 7th Upper Labial. In the Sierra Garter Snake the 6th upper labial is longer than the 7th. In the Aquatic Garter Snake the 7th is longer than the 6th.
  2. Head and Ventral Pigment. In the Pit River drainage, the Sierra Garter Snake tends to have a lot of black pigment on the head and venter. The Aquatic Garter Snake does not.
  3. Ventral Scale Count. The Sierra Garter Snake tends to have 169 to 187 ventral scales. The Aquatic Garter Snake tends to have 156 to 168 ventral scales. In both species, males tend to be higher in the ventral scale count than female, so probing a snake to determine sex can be useful if the ventral scale count is near 16{8,9}. While not impossible, this characteristic may be difficult to measure on a live specimen in the field.
  4. Dorsal Scale Rows at Midbody. This key is less reliable. The Sierra Garter Snake usually has 21 scale rows at midbody. The Aquatic Garter Snake usually has 19, though some (most often female) do have 21.

Please note that with the exception of the upper labial diagnostic, the above keys only apply to the Pit River drainage. They may not hold true in other localities. Fortunately for us, the two species only have contact in the Pit River drainage.

Aquatic Garter Snake Sierra Garter Snake
Thamnophis atratus Thamnophis couchii
Usually lacks black pigment on head and upper labials, venter. 7th upper labial usually longer than 6th. Usually has black pigment on head, especially upper labials, and venter. 6th upper labial noticably longer than 7th.
[Head of the Aquatic Garter Snake] [Head of the Sierra Garter Snake]
[Venter of the Pit River Aquatic Garter Snake] [Venter of the Sierra Garter Snake]  

When entering Pit River Aquatic gartersnakes from the contact zone into the database, if both ventral blotching and sixth upper labial point to the same species, use that species for the ID but note any other keys you measured. If those two keys differ, use your judgement but note the results of every key you can.

 
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