Metering

Venom metering refers to the hypothesis that some snakes may be able to regulate the amount of venom they inject with regard to the context of the envenomation (Hayes, 1993; Hayes et al., 1995).  For example, the northern Pacific rattlesnake was found to inject more venom into larger prey items (Hayes et al., 1995).  This would seem to be an adaptive strategy in order to subdue larger prey more quickly, and the authors suggested that these differences were due to the snake’s ability to tightly regulate its venom delivery system (Hayes et al., 1995).  Other studies, however, do not find such strong evidence for metering.  A previous study with the same species of rattlesnake did not find that they meter venom according to prey size (Hayes, 1992b). Western diamondback rattlesnakes also showed a lack of metering behavior with regard to prey size (Young and Zahn, 2001).  A study on the effects of hunger on envenomation behavior found that hungry snakes injected less venom, seemingly the opposite of what one would expect if snakes could finely regulate their envenomation behaviors (Hayes, 1993). Furthermore, it seems unlikely that snakes meter their venom during defensive strikes since may humans are envenomated each year (Greene, 1997).       

A western diamondback rattlesnake (Crotalus atrox) with the fangs and fang sheaths visible. Photo by John D. Willson.

Young et al. (2002) review the literature on venom metering and offer an alternative hypothesis to explain the findings of such studies.  They note that metering studies are difficult to compare because they vary greatly in the sizes of prey used; some studies only find that snakes meter venom if prey sizes differ by over 300 g while others suggest that snakes can differentiate between prey items of approximately 11 g difference (Young et al., 2002).  Instead of assuming that snakes are able to make complex decisions about how to use their venom, Young et al. (2002) hypothesize that the differences in venom injection are probably the result of physiological and morphological constraints.  For example, they suggest that as the fang sheath (see image above) moves up the leading edge of the fang during a strike it causes changes in the shape of the venom chamber which results in differential amounts of venom output (Young et al., 2002).  In this way, the amount of venom injected by a snake may vary considerably between prey types (because different surfaces like skin or feathers may displace the fang sheath differently) or simply because of the unique events during that particular strike (where and how the fangs are introduced to the prey, etc.).  This hypothesis is useful in explaining the large variation often seen in venom expulsion in snakes without attributing those results to decision-making capabilities on the snake’s part (Young et al., 2002).