Can Specialists Generalize? Diets of Texas Horned Lizards (Phrynosoma cornutum) Living in Small Texas Towns
Department of Biology
Texas Christian University
Fort Worth, TX
The Texas horned lizard (Phrynosoma cornutum) is considered a species of conservation concern in Texas and Oklahoma, due to substantial range declines over the past 50 years. Horned lizards are traditionally considered harvester ant (Pogonomyrmex spp.) specialists, increasing their vulnerability to reductions in harvester ant availability from urbanization, pesticide use, and the spread of invasive fire ants (Solenopsis invicta). However, numerous studies have suggested horned lizards will opportunistically exploit alternative ant and arthropod species. Using the hypothesis that horned lizards are harvester ant specialists, we tested the following predictions in horned lizard populations in small towns in south Texas: 1) horned lizard density is positively related to harvester ant density, 2) harvester ants account for more than 60% of horned lizard diets, and 3) the proportion of harvester ants in horned lizard diets is independent of harvester ant availability, space, and time. We used visual identification of exoskeletons and DNA barcoding to identify prey items found in 133 fecal pellets. Our results failed to support the hypothesis that Texas horned lizards are harvester ant specialists. There was no relationship between horned lizard and harvester ant densities, and harvester ants accounted for less than 8% of all prey consumed. Big-headed ants (Pheidole spp.) and harvester termites (Tenuirostritermes cinereus) accounted for over 70% of prey. There were significant differences in average dietary proportion of harvester ants between sites, which were strongly correlated with harvester ant availability. These results suggest harvester ants are not essential to the survival of Texas horned lizards. The relative importance of harvester ants in the diet of Texas horned lizards is likely dictated by other environmental factors including alternative prey quality and abundance, predation risk, soil type, and vegetation community.