Just as rainy days can affect our moods, rain fluctuations can stress out tropical forests. Plants for example may invest less in growth during dry periods. However, scientists at the Smithsonian Tropical Research Institute (STRI) in Panama recently discovered that swollen-thorn acacias (Vachellia collinsii) plants, commonly known as cachito or cornizuelo in Spanish, actually invested more in some plant resources during a drought, keeping their mutualistic ants happy.

In the obligate, mutualistic relationship between neotropical swollen-thorn acacias and Pseudomyrmex ants, hundreds of fearless ants defend the plant against hungry herbivores in exchange for shelter and food. The plant’s thorns have evolved to be hollow as homes for ant colonies. Moreover, the plant produces specific food resources that have evolved exclusively for their ants: nectar from specialized glands called extrafloral nectaries, and protein-rich structures called Beltian bodies that the adult ants harvest for their larvae. In this obligate mutualism, the survival of one species depends on its association with the other. Interestingly, the size and quantity of both housing and food rewards can vary at different times or depending on the ant species. For example, swollen-thorn acacias can also be colonized by another tenant: Crematogaster ants, which, because they do not do as good a job defending, usually receive fewer rewards from the host plant.

In a new study published in the journal Oecologia, researchers in Panama measured ant rewards in swollen-thorn acacias colonized by high-quality (Pseudomyrmex) and poor-quality defender (Crematogaster) ants: they recorded the size of the thorns, the number of extrafloral nectaries, and the number of Beltian bodies produced per leaf at the end of the wet season in 2019 - during a period of regular precipitation, and at the end of the wet season in 2023 - during a drought driven by the El Niño Southern Oscillation (ENSO) phenomenon. Unexpectedly, they found that acacia plants produced larger food and housing rewards for the ants during the drought in 2023, irrespective of the ant defender species, evidencing the importance of their relationship with their ant bodyguards. Probably, increased light availability during El Niño may have played a role in the production of rewards. However, “the more interesting possibility to me, which has support from other ant-plant systems also, is that plants may need ants more when there is less water: if I'm a plant with limited resources to make a new leaf, I better make sure my ants protect what I have,” explains STRI fellow and first author of the paper Cristian Molina.

“In coevolved mutualistic interactions, especially obligate mutualisms, the organisms involved have limited flexibility to respond to changes, as their response depends on another species. With this study, we demonstrate the ability of a neotropical plant to maintain its mutualistic host during a water-limited period, which may be critical to its survival,” stated STRI staff scientist and author of the paper Sabrina Amador. However, “we still face the task of investigating how global change may affect other vital aspects of the plant’s reproductive success, such as the production of flowers.” Understanding the evolution and flexibility of mutualistic relationships to respond to changes in the environment and the effects on overall ecosystems, especially in the tropics, is key to unraveling the future of tropical biodiversity under the pressure of a changing climate.

Reference: Molina, C. E., Naugle, J. M., Guevara, M., Zeilstra, E. B., González, Y., & Amador Vargas, S. 2025. Swollen-thorn acacias increased their investments in resident ants during an El Niño event. Oecologia, 207(7), 1-12.