NORTHERN PACIFIC RATTLESNAKES (CROTALUS-VIRIDIS-OREGANUS) METER VENOM WHEN FEEDING ON PREY OF DIFFERENT SIZES

The purpose of this study was to clarify experimentally whether northern Pacific rattlesnakes (Crotalus viridis oreganus) inject more venom into larger prey than into smaller prey. Medium-sized and large rattlesnakes were videotaped as they each struck small and large laboratory mice (Mus musculus). Venom measurements were made by enzyme-linked immunosorbent assay (ELISA) of whole-animal homogenate. Although medium and large snakes expended similar quantities of venom, both groups injected significantly more venom into large mice than into small mice. Slow-motion videotape analyses indicated that behavioral aspects of striking were similar for medium and large snakes and did not differ between size classes of prey. Likewise, no behavioral aspect of striking was correlated with the quantity of venom expended. Thus, venom expenditure was not a consequence of or constrained by some extrinsic aspect of striking such as duration of fang contact. Instead, the rattlesnakes metered more venom into larger prey through intrinsic control of venom delivery. Because of the predatory advantages for quick death and efficient digestion of larger prey, injection of more venom into larger prey is likely an adaptive strategy.