Anyone have tips on ID'ing garter snakes?

I’m from western Montana in the United States and I’ve been regularly going through Montana observations to identify those I can. Recently there have been a lot of lovely observations of garter snakes in the state, including many with pretty high-quality photos, but when I try to verify the ID I find myself totally stumped!

The two main ID’s I see on MT observations in the genus are common garter snakes and western terrestrial garter snakes. I’ve tried reading detailed physical descriptions, googling, using the compare tool, etc. but with the variety of colors/patterns and the overlap in observed ranges I always end up at a loss to confirm one or the other. I love snakes and have a fair amount of casual knowledge about them, but no formal scientific training.

Do any experienced herpers have tips for the key differentiating factors I should look for? What makes you confident in a garter snake ID?

ETA: If you have links to/suggestions of any more general snake ID resources I’d also love those, not sure where to start on expanding my knowledge on other local reptiles either!

I’m not in Montana but know there are one or two field guides to herps of that state. I think I have one of those somewhere in my library. I think you have 2species of Thamnophis to deal with which isn’t too bad. Western Terrestrial can be pretty variable in mountain west. The position of the lateral stripe on the scale rows is often useful for garter snake ID but seeing that in photos is often not possible. Unfortunately I don’t have any really good advice other than getting a good field guide for your state.

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I see you have T. radix also but perhaps not in your area of interest.

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I’m not a herp expert, but I believe that a key feature for garter snake ID is the number of “upper labials”, and which of these touch the eye. That is, the scales along the “upper lip” on one side. A good photo of the head from the side is needed to show this feature.

Here’s a typical Western Terrestrial Garter Snake showing eight upper labials, with the fourth the first one to contact the eye.

Here’s a typical Common Garter Snake showing seven upper labials, with the third the first one to contact the eye.

Of course, many photos aren’t good enough to show this level of detail, but I would be most comfortable identifying observations that do, and skipping others. More info here that might be helpful: https://fieldguide.mt.gov/displaySpecies.aspx?family=Colubridae

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Yes, I forgot that the labial count – along with the position of the lateral body stripe in relation to scale rows – is a useful characteristic. We have 8 species of Thamnophis down here in NM, but most of the time I don’t need those scale characteristics to ID – the color pattern, location, and habitat where found are usually sufficient to ID to species. I have no idea what T. sirtalis does up in Montana, but down here it doesn’t overlap hardly at all with T. elegans; their habitat preferences are quite different and they really don’t resemble each other. But IDing someone else’s photos vs. having a snake in hand are different challenges.

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This is exactly the sort of thing I was looking for, thank you! I think at least one observation I was puzzled by has good enough pictures for this, so it’s good to know.

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That’s a great suggestion–I haven’t taken the plunge of actually getting field guides yet (except for a Sibley’s bird book I’ve had for ages), but I should do that soon.

I’m interested in the whole state, mainly because since there aren’t that many Montanans overall I feel obligated to improve our data. But it’s true that since I’m a western MT person I don’t have nearly as much experience with plains species.

This is the field guide I have (but not at my current location). I think it’s been revised but not sure. And I didn’t spend $900 to buy it (as this Amazon page suggests), more like $20 !

https://www.amazon.com/Amphibians-Reptiles-Montana-Kirwin-Werner/dp/0878425004

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Yeah, you definitely want to count and compare scales. California Herps’ Garter Snake Key is really great, and it has scale photos and counts. The key has T. elegans and T. sirtalis scale info, which applies to the subspecies found in Montana. It’s not possible to ID to species in California based on pattern alone, in some areas.

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I found my paperback copy of the Werner et al. field guide for Montana herps. I bought it in 2004 for $16, but it might be out of print which would explain the ridiculous prices I’ve seen online. It looks like something you might want to get if you can find a used copy at a reasonable price – has pretty good info on the 3 garter snakes in the state and other MT species.

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