SCHWENK, K.*; SHERBROOKE, W.C.; Univ. of Connecticut; Southwestern Research Station, Amer. Museum of Natural History: Mucus-Binding of Dangerous Prey by Horned Lizards

Predator-prey evolutionary arms races are rarer than supposed owing to asymmetry in the strength of selection, and the sensitivity of selection patterns to population dynamics and adaptive trade-offs. Two factors � predator dietary specialization and dangerous prey � are believed to increase the likelihood of an arms race by promoting balance and reciprocity in predator-prey selection. We present evidence suggesting that horned lizards (Phrynosoma) and their prey of harvester ants (Pogonomyrmex) are involved in such a race: the lizards are ant-specialists that favor Pogonomyrmex species endowed with formidable defensive weapons, including an exceptionally venomous sting and a powerful bite � weapons believed to have evolved in response to horned lizard predation. Indeed, one commonly eaten prey species, Pogonmyrmex maricopa, possesses the most toxic venom of any invertebrate known. Nevertheless, horned lizards consume dozens of these noxious ants daily without harm, capturing them with the soft tissues of the tongue and swallowing them directly with neither biting nor chewing. High speed films of horned lizards feeding, gross and microscopic anatomy of the mouth and pharynx, and stomach content analyses of wild specimens show that horned lizards incapacitate prey immediately upon capture by binding them with mucus secreted by long papillae within the pharynx. Comparative studies of closely related lizards demonstrate that morphological and behavioral components of mucus-binding are uniquely derived in Phrynosoma, suggesting an adaptive response to the ants� weaponry consistent with a predator-prey arms race.