Anim. Behav., 1995, 50, 33–40 Venom metering by juvenile prairie rattlesnakes, Crotalus v. viridis:effects of prey size and experience WILLIAM K. HAYES* Department of Z oology and Physiology, University of W yoming, Laramie, W Y 82071, U.S.A. ( R eceived 10 A ugust 1993; initial acceptance 12 N ovember 1993; final acceptance 12 August 1994; M S. number: 6776) 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.  1995 The Association for the Study of Animal Behaviour As opportunistic predators, rattlesnakes consume prey of diverse body sizes (e.g. Macartney 1989; Brown 1990; Wallace & Diller 1990). They pro- cure their food by means of an envenomating strike, during which variable quantities of venom are injected into prey through a pair of hollow fangs (Hayes 1991a, 1992a, b, 1993; Hayes et al. 1992). Adult mice and larger rodents are usually released immediately after envenomation to avoid retaliatory injury, but smaller rodents may be held (Radcliffe et al. 1980; Kardong 1986a). Envenom- ated rodents that are released may subsequently wander several metres or more before succumbing to the venom (Kuhn et al. 1991; Hayes 1992a). The lifeless prey must then be relocated by strike- induced chemosensory searching (SICS), which allows the snake to follow the trail of and relocate dispatched prey through use of its tongue- vomeronasal organ system (reviewed by Chiszar et al. 1983, 1992). The quantity of venom expended during striking may significantly influence the success of predation for several reasons. First, larger prey are less susceptible to the effects of venom (Russell 1980), and thus more likely to wander beyond recovery range after envenomation (Klauber 1972). Consequently, snakes should attempt to inject more venom into larger prey. Second, the lower surface-to-mass ratio of larger prey offers less surface area on which digestive enzymes of the snake’s gut can work, and putre- faction of large prey before digestion is com- pleted poses a serious risk for snakes (Pough & Groves 1983). Because venom has proteolytic properties that accelerate prey digestion from within (Thomas & Pough 1979; Rodriguez- Robles & Thomas 1992), rattlesnakes might obtain digestive benefits by delivering more venom into larger prey. Finally, injection of too much venom into smaller prey could be meta- bolically wasteful and may deplete venom reserves, leaving the snake vulnerable to predation or unable to secure additional prey encountered soon thereafter. *Present address: Biology Department, Southern College, P.O. Box 370, Collegedale, TN 37315-0370, U.S.A. 0003–3472/95/070033+ 08 $12.00/0  1995 The Association for the Study of Animal Behaviour 33