Biology Reference
In-Depth Information
5
Summary and Conclusions
The taxonomically artificial ophidian assemblage, family Colubridae, is under dynamic
revision. The previous uniform grouping of these snakes was phylogenetically incor-
rect and misleading as many are unrelated, and are gradually being reassigned to
different families and subfamilies. An indeterminate number of non-front-fanged col-
ubroid species produce secretions of varying toxicity from Duvernoy's gland; a low-
pressure system that functions by supplying the secretion as needed, as in almost all
studied to date there is no significant reservoir of secretion. None of these snakes has
canaliculate fangs as in elapids, viperids, or atractaspidids. Some species have enlarged
posterior or mid-maxillary teeth that may or may not be grooved associated with the
Duvernoy's glands, others do not. Three-finger-fold neurotoxins, prothrombin activa-
tors, proteolytic fibrinogenases, and several other components have been characterized
from a few taxa, but the secretion properties of the majority of these snakes remain
poorly studied. Transcriptome and/or proteomic analyses of several non-front-fanged
colubroid taxa have identified multiple classes of toxins, and other biologically
active components shared with front-fanged species (Ching et al., 2006; Estrella et
al, 2010; Fry et al., 2003, 2008; Weldon and Mackessy, 2010). Most of the char-
acterized postsynaptic neurotoxins exhibit prey specificity (for birds, lizards, etc.).
Several species produce highly toxic secretions that are medically important. These
species (the Dispholidini,
D. typus
,
T. kirtlandii
,
T. capensis
, and the natricids,
R.
tigrinus
and
R. subminiatus
) meet the criteria for the term “venomous” (Mebs, 2002;
Russell, 1980), as they actively use their venom in prey capture and/or antipreda-
tor defense, as well as exhibit marked toxicity to other animal species. Prey capture/
subjugation functions and toxicity have been poorly documented in most other spe-
cies. Therefore, it is premature to use the term “venomous” for the secretion prop-
erties of many of these snakes, as their Duvernoy's glands and associated products
have unproven or unverified roles. This is accentuated by the marked differences in
functional morphology between glands of “true” venomous snakes (elapids, viperids,
and atractaspidids) and those of other colubroids formerly termed colubrids (Greene,
1997; Jackson, 2003; Kardong, 1996; Taub, 1967; Weinstein and Kardong, 1994;
Weinstein et al., 2010). Thus, the Duvernoy's secretions of a relatively small number
of non-front-fanged colubroids have convincingly met the criteria defining the term
“venom” (see p. 32).
Most Duvernoy's secretions studied to date have low toxicity, but several dispho-
lidines and natricids (as noted earlier) produce venoms with marked lethal potency
for prey species as well as humans. Characterization of avian or saurian prey-specific
toxins present in some of these secretions (
B. dendrophila
,
B. irregularis
) supports
their classification as “venoms.” However, regardless of the need for a careful bio-
logical as well as pharmacological-biomedical definition and use of this term, the
word “venom” carries a medical inference suggesting clinical importance. As has
