Gregs correct that they all do it. My B. cauldalis does it when she is opaque and does not want to be messed with. I have just reciently noticed that my Squam (Atheris)is doing it too.
My Rhinos do it also when it doesn't want to be disturbed. As Klaus said in the other thread you started, they really stress out quickly, so they do best if disturbed little.
One more point about Bitis; They tend to hold onto their feces for a very long time, so don't sweat it if they don't go on a regular basis. My Gabby goes about every 11 weeks. Keep good notes on your specimens recording all the important information.
Good Luck,
Al Coritz
www.neavb.org

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Take nothing but pictures, leave nothing but footprints.