ROWE, A.H.*; ROWE, M.P.; North Carolina State Univ.; Appalachian State Univ.: Coevolution between grasshopper mice (Onychomys spp.) and bark and striped scorpions (Centruroides spp.)

Asymmetrical selection has been proposed as the strongest argument for rejecting hypotheses of coevolutionary arms races between predators and prey. While predators exert strong selection against prey, prey exert only weak selection against predators. However, potentially lethal prey may increase selection intensities on their predators. This study employs grasshopper mice and bark and striped scorpions as a model to test the hypothesis that interaction between a predator and a potentially lethal prey will result in reciprocal selection (i.e., coevolution). Bark scorpions (Centruroides exilicauda) and striped scorpions (C. vittatus) produce vertebrate-specific neurotoxins potentially lethal to mammals. Southern grasshopper mice (Onychomys torridus), broadly sympatric with bark scorpions in the Sonoran Desert, and Mearns� grasshopper mice (O. arenicola), broadly sympatric with striped scorpions in the Chihuahuan Desert, are known to be voracious predators of scorpions. Northern grasshopper mice (O. leucogaster) are broadly allopatric with Centruroides spp. We evaluated venom resistance in all three species of grasshopper mice and venom toxicity in bark and striped scorpions. Venom resistance analyses demonstrated that while all three species of grasshopper mice have evolved some resistance to the neurotoxins produced by Centruroides spp, resistance levels were significantly higher among species and populations of mice sympatric with these scorpions. Concomitantly, venom toxicity tests demonstrated that Centruroides sympatric with resistant populations of mice possess significantly more toxic venom. Such co-variation between venom toxicity in Centruroides and venom resistance in Onychomys is consistent with a coevolutionary arms-race hypothesis.