In some species the consequences of self or cagemate envenomation seem to be fairly trivial most of the time. I do not know whether this is a behavioral issue where these species do not meter out much venom on these kinds of bites, or a physiological issue of significant resistance to their own venom.
To generalize, most of the pit viper self and cagemate envenomations I have seen have been of minimal consequence. I just saw a terciopelo cagemate envenomation to the head last week, two gigantic B. asper arguing over a food item. There was some swelling but it resolved over a few days with no further incident. I have seen cantil, cottonmouth, copperhead and Western diamondback cagemate envenomations with similarly trivial effects.
Elapid self and cagemate envenomations have gotten a bit nastier. A few days ago I treated a severe intramuscular abscess caused by cagemate envenomation in a Suphan cobra, and I've seen similarly nasty messes in an Egyptian banded, a monocled and some spitters. My shieldnose cobras (Aspidelaps) have bitten each other without apparent effect, but so far every such incident I've seen in the genus Naja has been fairly serious with long term health consequences for the animal.
What I do not tend to see is systemic effects from this kind of envenomation. The effects appear to be local. When I have seen cases of same species cagemates that actually kill each other, it has always been a direct head bite with enough physical damage from the fangs alone to warrant the effects. I have heard a number of anecdotal reports that cross species cagemate envenomations tend to result in rapid death from systemic effects.
I would hesitate to generalize rules for all snake to snake envenomations based on the small sample I have personally seen. I don't know how much formal research has been done on the subject, if any, but a good place to ask would be Dr. Bryan Fry's Venomdoc forums (
http://www.venomdoc.com)
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