Plant killing by Neotropical acacia ants: ecology, decision-making, and head morphology

Abstract

Mutualistic species often associate with several partners that vary in the benefits provided. In some protective ant–plant mutualisms, ants vary in the extent at which they kill neighboring vegetation. Particularly, in acacia ants (Pseudomyrmex), the area around the host tree that ants keep free from vegetation (“clearings”) vary depending on the species. This study assessed whether interspecific variation in clearing size corresponds to workers biting on plant tissue of different thickness. As expected, workers from species making the largest clearings bit more often on thicker plant tissues than workers from species making smaller clearings. Because head shape affects mandible force, I also assessed whether pruning on thick tissue in mutualistic ant species or being a predator in non-mutualistic species correlated with broader heads, which yield stronger mandible force. The species with the broader heads were non-mutualistic predators or mutualistic pruners of thick tissues, which suggest that pruning neighboring vegetation in non-predatory species demands force even when the ants do not kill prey with their mandibles. The findings reveal that clearing size variation in mutualistic ant partners of plants can also be observed at the level of individual decision-making processes among workers, and suggest that head morphology could be a trait under selection in protective ant–plant mutualisms. Abstract in Spanish is available with online material.