Commitment Issues: Acacia-Ant Mutualism

Josh Schauer (L) and Brady Hartman (R) record responses of acacia ants

Study by Alex Morrison, Sophia Kruger, Brady Hartman, & Josh Schauer

In nature, mutualistic relationships are rare, and therefore they are well studied. One of the best studied relationships of this kind in Costa Rica—and in Palo Verde National Park—is between the Acacia tree and the Acacia ants. While many aspects of this relationship have been studied, we questioned what the extent of this relationship was, specifically what conditions could impact ants’ behaviors of defense. We asked how defense response time of Pseudomyrmex spinicola varies in relation to different temperatures throughout the day. We hypothesized that the response time to a threat would lengthen as temperature increases, likely due to the ants’ preference to conserve energy during hotter temperatures. To test this, we sampled 30 different Acacia trees of species Vachellia collinsii, all similar in stem diameter (0.6-1.5cm), throughout the day: 8 am, 9:30 am, 11 am, 1 pm, 2:30 pm, 5 pm, and 8 pm. To determine response time to an external stimulus, we tapped a branch of the tree with a stick three times measuring the time elapsed from the initial stimulus to the time an ant arrives at the area of impact. The temperature of the thorn the ants come out of was also measured. After running an ANCOVA, we found that ants responded slower to threats in the morning and when temperatures were colder. While these findings did not support our initial hypothesis, they contribute to the idea that external conditions may affect the threshold to which mutualistic relationships are beneficial for contributing species, this being temperature for the Acacia-ant relationship investigated here. With the changing climate, it is possible that these benefits can decline if the changing conditions are outside of a mutualistic threshold.