Head bobbing...

[Zephyr]

Hmm... Perhaps I should do some experiments with this this summer. Maybe do something along the lines of taking two wild garters, introducing them directly to eachother and observing, and then trying it with two other garters and put some sort of barrier between them that would allow their "smell" to pass through, but no visual cues that another snake is in the area.

[aSnakeLovinBabe]

usually their tongue needs to directly hit another garter for them to give a good twitch.