ROWE, M.P.*; ROWE, A.H.; Appalachian State Univ.; North Carolina State Univ.: Risk assessment by grasshopper mice feeding on potentially lethal prey

Predators should benefit from assessing the risks posed by prey that differ in their dangerousness. Assessment, coupled with adaptive adjustment of the predator�s behavior, could come at any phase of the predatory sequence: search/recognition; pursuit/attack; and handling/subjugation. Grasshopper mice (Onychomys spp.) are voracious carnivores of North American deserts, feeding almost exclusively on arthropods. In certain regions, grasshopper mice co-occur with extremely neurotoxic bark scorpions (Centruroides spp.), a genus whose sting can prove lethal to humans. Given the density of these scorpions and the large home ranges of the mice, encounters between the two are a certainty. We presented three different prey items to wild-caught grasshopper mice at several field sites in the southwestern U.S. The three items, listed in increasing order of dangerousness, were: lab crickets, possessing few if any defenses; Vaejovis spinigerus, a noxious but non-lethal scorpion; and two species of toxic Centruroides. Grasshopper mice made no distinctions among the prey in either the recognition or pursuit phases of the encounter, attacking crickets and both genera of scorpions with little hesitation. There were, however, significant differences in how the mice handled the three different prey types, with Centruroides requiring dramatically more time and effort to subdue. The difficulties mice had in dispatching Centruroides were not related to the neurotoxic components of these scorpions� venoms, to which the mice are extremely resistant. Instead, the difficulties appear to be the result of other constituents of Centruroides� venom that cause intense, short-term pain. Results are consistent with an arms-race analogy for the co-evolution between venom resistance in the mice and venom diversification by the scorpions.