Venom flow in rattlesnakes

most specialized feeding mechanisms among vertebrates. The biomechanics and kinematics of prey ingestion have been documented from several snake taxa, although the full diversity of ingestive mechanics has only recently been appreciated (Cundall and Greene, 2000). Less attention has been paid to prey capture and, in particular, the mechanics of venom injection (Savitzky, 1980; Kochva, 1987). Venom is expelled when the skeletal musculature surrounding the venom gland contracts, causing an increase in pressure within the venom gland and forcing venom out (Rosenberg, 1967). This explanation, commonly referred to as the intraglandular pressure hypothesis, has been supported, directly and indirectly, by a number of investigations (Freyvogel and Honegger, 1965; Kardong and Lavin-Murcio, 1993; Young et al., 2000). To date, no study has directly recorded the flow of venom or directly linked venom flow with the mechanics and kinematics of the strike. In recent years, the behavioral ecology of snake venoms, particularly rattlesnake (Crotalus spp.) venom, has received considerable attention. Using techniques such as enzymelinked immunosorbent assay and protein assay, researchers have quantified the amount of venom injected into, or onto, a target and compared these results with theoretical arguments for venom use based on optimization and energy conservation (for a review, see Hayes et al., 2001). Although causal evidence is lacking, these studies frequently attribute differential venom flow to differences in the kinematics of the strike or to the mechanics of venom injection (Gennaro et al., 1961; Kardong, 1986a,b; Rowe and Owings, 1990; Hayes, 1991, 1992). The present study reports on a new technique for the study of venom mechanics. Transonic flow probes, surgically implanted onto the venom duct, use ultrasound to directly quantify venom flow through the venom-delivery system. The output of these flow probes is synchronized with a high-speed digital video camera, which permits both venom flow and strike kinematics to be quantified with a temporal resolution of 2 ms. The primary purpose of this study was to explore the mechanics of venom expulsion in western diamondback rattlesnakes, Crotalus atrox. Venom flow was recorded during strikes directed at different-sized biological targets and under different behavioral contexts, thus enabling a direct quantitative test for the regulation of venom flow.