Rear-fanged colubrids are actually quite widespread, both
phylogenetically and geographically. Only a handful
have venom that is medically significant to humans, but the
few that do really pack a wallop (twig snake, boomslang, a
few others). Interestingly, despite the convergent
nature of opisthoglyphy in the various venomous colubrid
groups, many (most?) of them seem to share a tendency to
utilize blood vessel wall damage and anticoagulant effects
as a primary attack form. Very few, if any, are
substantially neurotoxic. Human fatalities from
boomslang bites, for example, occur as a result of massive
internal bleeding. Even those species that have little
effect on humans and are not considered "medically
significant" seem to produce somewhat accelerated bleeding
from the bit wound. To the best of my knowledge, no
one has determined a functional, phylogenetic, or
developmental underpinning to this trend (but someone else
here may know something I don't).
Cheers,
--Mike Habib
Michael Habib
Assistant Professor of Biology
Chatham University
Woodland Road, Pittsburgh PA 15232
Buhl Hall, Room 226A
mhabib@chatha