Evidence of behavioral co-option from context-dependent variation in mandible use in trap-jaw ants (Odontomachus spp.)
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Evolutionary co-option of existing structures for new functions is a powerful yet understudied mechanism for generating novelty. Trap-jaw ants of the predatory genus Odontomachus are capable of some of the fastest self-propelled appendage movements ever recorded; their devastating strikes are not only used to disable and capture prey, but produce enough force to launch the ants into the air. We tested four Odontomachus species in a variety of behavioral contexts to examine if their mandibles have been co-opted for an escape mechanism through ballistic propulsion. We found that nest proximity makes no difference in interactions with prey, but that prey size has a strong influence on the suite of behaviors employed by the ants. In trials involving a potential threat (another trap-jaw ant species), vertical jumps were significantly more common in ants acting as intruders than in residents (i.e. a dangerous context), while horizontal jumps occurred at the same rate in both contexts. Additionally, horizontal jump trajectories were heavily influenced by the angle at which the substrate was struck and appear to be under little control by the ant. We conclude that while horizontal jumps may be accidental side-effects of strikes against hard surfaces, vertical escape jumps are likely intentional defensive behaviors that have been co-opted from the original prey-gathering and food-processing functions of Odontomachus jaws.
KeywordsForaging behavior Locomotion Competition Prey capture Defense Formicidae
The authors thank Brian Fisher of the California Academy of Sciences, Chris Smith of the University of Illinois, and Mark Deyrup of the Archbold Field Station for assistance collecting and maintaining ant colonies. We also thank Kevan Citta and Rebecca Schield for assistance with data collection. For permission to collect and import ants, we thank the Ministry of Environment and Energy (Permit 122-2004-OFAU) of Costa Rica, the Ministerio de Salud y Ambiente (Permit 20202/05) of Argentina, the Administracion de Parques Nacionales (Permit 002870-2) of Argentina, James Cook University, Australia for the loan of O. cephalotes specimens, and the United States Department of Agriculture (APHIS import permit 69963). This work was supported by a seed grant from the Beckman Institute for Advanced Science and Technology.
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