Playing to their evolutionary strengths; heteromyid rodents provide opposite snake evasion strategies in the face of known and novel snakes

Abstract

We evaluated the response of two competing heteromyid rodents, Desert Pocket Mice (Chaetodipus penicillatus) and Merriam's Kangaroo Rat (Dipodomys merriami) towards risk from morphologically convergent snakes, the Sidewinder Rattlesnake (Crotalus cerastes), and the Saharan Horned Viper (Cerastes cerastes). The sidewinder and these rodents originated in the Mojave Desert and, thus, they share an evolutionary history. The Horned Viper originated in the Sahara and is an unknown and novel predator. Each snake poses a different risk for which the optimal strategy of the rodents will differ. The sidewinder belongs to a lineage that evolved heat sensing pits, allowing it to detect rodents on nights without ambient moon light, while the Horned Vipers lack this adaptation. We used Giving-up Densities as a measure of risk perception and compared their responses in relationship to microhabitat, moonlight, time and risk from other predators. Pocket mice feared both snakes and kangaroo rats acted in a relatively fearless manner. The differences between the rodents can be attributed to their snake-evasion adaptations. Kangaroo rats used their saltatorial agility which allows them to take risk while pocket mice climb out of harm's-way and enter torpor to reduce energetic costs when risk prevents foraging.