Abstract

Despite contradictory evidence, it is widely believed that venomous snakes carefully control, or 'meter', the quantities of venom expended when feeding upon rodent prey. The major purpose of this study was to clarify experimentally whether juvenile prairie rattlesnakes inject more venom into larger mice than smaller mice. The subjects ( N=7) were videotaped as they struck at small, medium and large mice in each of two separate trials. The quantity of venom expended was measured by enzyme-linked immunosorbent assay (ELISA) of whole-animal homogenate. In the first ('naive') trial, the snakes injected similar quantities of venom into all size classes of prey. But in the second ('experienced') trial, the snakes injected significantly more venom into larger prey. No other aspect of striking varied among prey sizes or changed in the two trials. Thus, venom expenditure was probably not a consequence of, or constrained by, some extrinsic aspect of striking, such as duration of fang contact. More likely, the rattlesnakes, with experience, attempted to inject (or meter) more venom into larger prey through intrinsic control of venom delivery. Both natural (snakebite) and artificial (syringe) envenomations indicated that mice injected with larger quantities of venom died more quickly. Because larger prey succumb less rapidly to venom, metering more venom into larger prey may be an adaptive strategy for immobilizing and killing large prey more quickly.