Habitat loss, land-use transformation, climate change, and biological invasions all elevate the importance of plasticity in food selection for the continued persistence of dietary specialists. Horned lizards (Phrynosoma spp.) are myrmecophagic specialists and the abundance of ant prey make their populations vulnerable to habitat loss, as well as invasive ants and associated pest control programs. We studied ant use by Phrynosoma cornutum (Texas Horned Lizards) on an insular urban reserve in central Oklahoma that was bereft of harvester ants (Pogonomyrmex spp.), presumed to be their chief prey. The five most commonly available ant genera based on bait station captures were Monomorium (69%), Forelius (11%), Pheidole (10%), Crematogaster (7%), and Tapinoma (2%). Based on the examination of 124 scat samples from adult and juvenile P. cornutum, Crematogaster (81%), Pheidole (12%), Formica (6%), and Monomorium (1%) were used as prey. Consumption of prey in several ant genera by P. cornutum disproportionately to their availability was related to ant mass and presumed nutritional value. Among juveniles, gape size did not influence Pheidole use but may influence Formica use. We suggest that P. cornutum are adaptive ant specialists whose populations might be maintained in habitat fragments without harvester ants as long as abundant medium- and large-sized native ant communities are present. Therefore, urban reserves, when effectively managed for native fauna, can conserve declining native species by serving as habitat havens in an otherwise unsuitable landscape.