Chapter 2 - Differences between buccal gland secretion and associated venom delivery systems of front-fanged snakes and non-front-fanged snakes: Low- versus high-pressure gland function and canaliculated versus solid dentition

Abstract

An unknown number of non-front-fanged colubroid snakes possess Duvernoy's venom glands that produce venom released under low pressure (due to a lack of muscular compression of the glands). This is in marked contrast to the highly muscular and thus pressurized glands of front-fanged colubroids such as viperids, elapids, and atractaspidids. While pressurized venom glands release a bolus of stored venom under high pressure (that may exceed 30 psi), non-front-fanged colubroids inoculate or introduce their venom and/or other oral products into wounds produced by maxillary teeth that may be enlarged, posterior or mid-maxillary (in a few studied examples these teeth are just posterior to the anterior-most maxillary teeth), and may or may not be grooved; but unlike those of front-fanged colubroids, they are never canaliculated (e.g., never hollow, or with a lumen). Dispholidus typus (boomslang) and possibly other members of the tribe Dispholidini have limited striated muscle insertion into the gland and thus may be considered to have a partially pressurized venom delivery system. Several hypotheses have been considered that attempt to address the evolution of the venom apparatus and the selection for venom delivery systems. The definition and use of the term “venom” is indicative of the biological use for prey subjugation and/or possibly defense. The clinical effects of ophidian oral secretions do not constitute criteria for use of the term ‘venom’, and terms such as “mildly venomous” are misleading for the same reason. Experimental investigation of a relatively small number of non-front-fanged snake venoms and observations of prey handling in a small variety of species has demonstrated notable prey-specific venom in several species (e.g., brown tree snake, Boiga irregularis, mangrove or gold-ringed cat eye snake, Boiga dendrophila, and Amazon puffing snake, Spilotes sulphureus). While most studied non-front-fanged species have little or no medical importance, others such as Dispholidus typus, the African twig, bird, or vine snakes, Thelotornis spp., three species of keelbacks, Rhabdophis spp., the Montpellier snake or hooded malpolon, Malpolon monspessulanus, and Lichtensteins' green racer (Philodryas olfersii) have medical importance, although serious envenoming by some of these (e.g., M. monspessulanus, and the flower keelback, Rhabdophis ceylonensis) is rare and there is so far only one well-documented case of each.