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.

中文翻译：

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相思树与寄生蚂蚁的刺相比越来越少

专为蚂蚁防御的植物提供食物和庇护所，以换取对草食动物的保护。中美洲相思树具有专一的蚂蚁共生关系，但寄生的非防御性蚂蚁也可以在树上筑巢。我们评估了奖励是否对应于植物物种内的蚂蚁防御。正如我们预期的那样，我们发现寄生虫居住的树木比蚂蚁捍卫的树木具有更少的肿胀刺。在寄生虫植物中，脊柱直径较小，但脊柱长度没有差异，这表明脊柱可作为机械性的食草保护。寄生虫植物可能因建立时的植物差异，对有限资源的可塑性响应或在感知到缺乏防御时的能量分配差异等原因而减少了回报。