MEYERS, J.J.*; HERREL, A.; NISHIKAWA, K.C.; Northern Arizona University, Flagstaff; University of Antwerp, Belgium; Northern Arizona University, Flagstaff: Modelling of bite force in horned lizards (Phrynosoma): functional adaptations in a specialized clade

The North American horned lizards (Phrynosoma) have long been noted for their myrmecophagous (ant-eating) behavior. However, the degree of dietary fidelity varies dramatically among the 13 species, with many species feeding opportunistically on a variety of insect prey. While many myrmecophagous animals demonstrate a reduction in robustness of the feeding apparatus in response to diet, the presence of non-ant prey in Phrynosoma implies a feeding system unconstrained by diet. To examine the feeding morphology in this group, we measured characters on the head and dentary that are thought to be important in prey processing. Additionally, we removed the jaw musculature of 12 species of Phrynosoma. Muscle coordinates and physiological cross section of each jaw adductor muscle were used as input into a static bite force model to determine bite performance of each species. Phylogenetic independent contrasts were used to address whether ant eating was associated with changes in morphology and bite force. Morphological analysis revealed few head shape differences related to diet, yet suggested that a higher percentage of ants in the diet is associated with a reduction in dentition. Modelling of bite performance revealed a gradation in bite force that is inversely related to the number of ants eaten, possibly constraining poor biters to a diet of softer prey. Interestingly, the hardest-biting species are capable of biting more forcefully than closely related dietary generalists. Closer scrutiny of diet in these hard biters reveals not only harder prey items in the diet but also distinct evidence of processing. It appears that within this specialized group of ant-eating lizards, a specialized group of hard-biting, durophagous species has evolved.