<p>The first response to a changing climate is often behavioral. The responses of social insects occur at both the individual and colony levels. Insects’ behaviors to escape heat stress are especially important in tropical species because ambient temperatures are typically closer to their physiological tolerance limits. We studied the effect of experimentally elevated temperatures on the tropical ant <i>Pseudomyrmex spinicola</i>, a mutualist of the bullhorn acacia plant, <i>Vachellia collinsii</i>. The plant provides shelter and food to the ants, and the ants defend the plant from herbivores. We performed in situ whole-plant warming experiments and compared ant behavior and physiology on heated and control plants. The ants were less active at experimentally elevated temperatures, which reduced their aggressive plant defense behavior. We measured the ants’ maximum critical thermal limit and observed that they experienced temperatures close to their physiological tolerance. Further long-term studies incorporating plant and herbivore responses to elevated temperatures should be pursued to examine whether this mutualism will persist with a changing climate.</p>