Phylogeny, scaling, and the generation of extreme forces in trap-jaw ants

Abstract

Trap-jaw ants of the genus Odontomachus produce remarkably fast predatory strikes. The closing mandibles of Odontomachus bauri, for example, can reach speeds of over 60 m s(-1). They use these jaw strikes for both prey capture and locomotion - by striking hard surfaces, they can launch themselves into the air. We tested the hypothesis that morphological variation across the genus is correlated with differences in jaw speeds and accelerations. We video-recorded jaw-strikes at 70 000-100 000 frames s(-1) to measure these parameters and to model force production. Differences in mean speeds ranged from 35.9+/-7.7 m s(-1) for O. chelifer, to 48.8+/-8.9 m s(-1) for O. clarus desertorum. Differences in species' accelerations and jaw sizes resulted in maximum strike forces in the largest ants (O. chelifer) that were four times those generated by the smallest ants (O. ruginodis). To evaluate phylogenetic effects and make statistically valid comparisons, we developed a phylogeny of all sampled Odontomachus species and seven outgroup species (19 species total) using four genetic loci. Jaw acceleration and jaw-scaling factors showed significant phylogenetic non-independence, whereas jaw speed and force did not. Independent contrast (IC) values were used to calculate scaling relationships for jaw length, jaw mass and body mass, which did not deviate significantly from isometry. IC regression of angular acceleration and body size show an inverse relationship, but combined with the isometric increase in jaw length and mass results in greater maximum strike forces for the largest Odontomachus species. Relatively small differences (3%) between IC and species-mean based models suggest that any deviation from isometry in species' force production may be the result of recent selective evolution, rather than deep phylogenetic signal.