ABSTRACT
True venom systems evolved at least twice in extant reptiles – first, early in helodermatid lizards and second, much later in advanced snakes (non-front-fanged colubroids and front-fanged colubroids – viperids, elapids and the lamprophiid genus Atractaspis). In helodermatids, the venom gland lies along the lower jaw and empties near grooved, multiple teeth within the mouth. As these slow-moving lizards feed largely on eggs and nestlings, this venom system probably comprises part of a defensive strategy. Within venomous snakes, the venom gland lies in the temporal region. In viperids and elapids, it generally consists of a main venom gland, pressurized by the contraction of directly attached striated muscles and thereby facilitating the forceful ejection of a pre-stored venom bolus during a delivered strike, and an accessory gland with connecting ducts eventually emptying into a hollow fang. Gland morphology is variable, and some smaller fossorial (burrowing) species have less luminal storage space for pre-synthesized venom. The burrowing asps or mole vipers (Atractaspis spp., Lamprophiidae, Atractaspidinae) possess only a main venom gland, although it too is pressurized by striated muscles. Several genera of front-fanged fossorial or semi-fossorial species, including some species of Atractaspis spp., some night adders (Causus spp., 7 species; Viperidae, Viperinae) and long-glanded coral snakes (Calliophis spp., 11 species; Elapidae), as well as Buerger’s forest snake (Toxicocalamus buergersi, Elapidae), have unusually elongated venom glands. As mentioned, most front-fanged venom systems are closed systems producing a sudden, high-pressure discharge of the venom bolus drawn from a reservoir within the gland. The pressure generated in some studied crotaline viperid venom glands may exceed 30 psi, comparable to automobile tire pressure. In contrast, many non-front-fanged colubroid snakes possess a low-pressure system based on a gland lacking a large reservoir, which releases secretion (venom) more slowly into oral epithelium adjacent to teeth that are sometimes deeply grooved and may be partly enclosed, e.g., have a partially formed lumen but are never fully enclosed or hollow. Consequently, prey acquisition/subjugation systems based on a “Duvernoy’s” or “low-pressure venom gland” system may employ an adaptive strategy different from that of front-fanged venomous snakes. In viperids, elapids and Atractaspis, the venom system 100discharges a bolus of venom quickly, in many cases (but not all) dispatching the prey (or perhaps thwarting a predator). Such differences in deployment of these oral glands, in an adaptive context, account for variation in gland structure and in the composition of their products. Although extensive research has focused on the toxic properties of these oral secretions, it is now clear that venom components perform multiple biological functions. However, assigned biological roles must be based on either experimental or carefully obtained observational evidence, not conjecture or assumption, whereby it is shown that the oral secretions in fact are injected at levels capable of producing favorable prey capture/subjugation results. However, some prey-specific toxins may compensate for minimal venom volumes that are ultimately delivered to vertebrate and invertebrate species that account for a large proportion of ingested prey items. Elucidating the intricacies of reptile venom, as well as other oral products, and their delivery will significantly improve our understanding of the evolution of the complexity of composition and function of these secretions and their delivery to prey and/or possibly predators.
