Ant activity in an ant-plant mutualims

Abstract

The Optimal Defense Hypothesis (ODH) posits that investment in defense is proportional to the value of what is defended, predicting that the most valuable structures should have the strongest defenses. In this study, we determined whether an obligate ant-plant mutualism fit the predictions of the ODH and the role of sporadic rain in this process. Sampling was performed in a tropical deciduous forest on the Yucatán Peninsula, Mexico, in Vachellia globulifera plants and their obligate mutualist ant, Pseudomyrmex ferruginous. We sampled ant activity before, during, and after an experimental rain event in branches that offer housing and food rewards (spines and leaves with nectaries and food bodies) and in branches with only housing rewards (spines). We found that: a) the number of patrolling events was 100% higher in branches with spines + leaves compared with branches with only spines, b) during a sporadic rain event, ants on branches with spines + leaves stayed on the branch at a higher rate compared to branches with only spines, and c) right after the rain, while branches with the two types of resources reach levels of ant activity almost identical to those observed before the rain (90%), branches with only spines achieve only 20%. All these findings support ODH and provide experimental evidence on the effect of rain events on ant-plant mutualisms, demonstrating that the ant activity on the most valuable resources for ants and plants can be resilient to small climatic events such as sporadic rains.