Acacia trees with parasitic ants have fewer and less spacious spines than trees with mutualistic ants.

Obligate ant-defended plants provide food and shelter in exchange for protection against herbivores. Mesoamerican acacia trees have an obligate ant mutualism, but parasitic non-defending ants can also nest on the tree. We assessed whether rewards corresponded to ant defense within a plant species. As we expected, we found that parasite-inhabited trees had fewer swollen spines than ant-defended trees. Spine diameter was smaller in parasite-inhabited plants, but there were no differences in spine length, suggesting that spines serve as mechanical protection against herbivory. Parasite-inhabited plants may have reduced rewards because of plant differences when establishing, a plastic response to limited resources, or differential energy allocation when sensing the lack of defense.