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A Quick Guide on grasshopper mice which, contrary to the great majority of mouse species, are obligate carnivores., (Copyright © 2015 Elsevier Ltd. All rights reserved.)

Painful venoms are used to deter predators. Pain itself, however, can signal damage and thus serves an important adaptive function. Evolution to reduce general pain responses, although valuable for preying on venomous species, is rare, likely because it comes with the risk of reduced response to tissue damage. Bark scorpions capitalize on the protective pain pathway of predators by inflicting intensely painful stings. However, grasshopper mice regularly attack and consume bark scorpions, grooming only briefly when stung. Bark scorpion venom induces pain in many mammals (house mice, rats, humans) by activating the voltage-gated Na(+) channel Nav1.7, but has no effect on Nav1.8. Grasshopper mice Nav1.8 has amino acid variants that bind bark scorpion toxins and inhibit Na(+) currents, blocking action potential propagation and inducing analgesia. Thus, grasshopper mice have solved the predator-pain problem by using a toxin bound to a nontarget channel to block transmission of the pain signals the venom itself is initiating.

Predators feeding on toxic prey may evolve physiological resistance to the preys' toxins. Grasshopper mice (Onychomys spp.) are voracious predators of scorpions in North American deserts. Two species of grasshopper mice (Onychomys torridus and Onychomys arenicola) are broadly sympatric with two species of potentially lethal bark scorpion (Centruroides exilicauda and Centruroides vittatus) in the Sonoran and Chihuahuan deserts, respectively. Bark scorpions produce toxins that selectively bind sodium (Na(+)) and potassium (K(+)) ion channels in vertebrate nerve and muscle tissue. We previously reported that grasshopper mice showed no effects of bark scorpion envenomation following natural stings. Here we conducted a series of toxicity tests to determine whether grasshopper mice have evolved resistance to bark scorpion neurotoxins. Five populations of grasshopper mice, either sympatric with or allopatric to bark scorpions, were injected with bark scorpion venom; LD50s were estimated for each population. All five populations of grasshopper mice demonstrated levels of venom resistance greater than that reported for non-resistant Mus musculus. Moreover, venom resistance in the mice showed intra- and interspecific variability that covaried with bark scorpion sympatry and allopatry, patterns consistent with the hypothesis that venom resistance in grasshopper mice is an adaptive response to feeding on their neurotoxic prey.