Acromyrmex fowleri: a new inquiline social parasite species of leaf-cutting ants from South America, with a discussion of social parasite biogeography in the Neotropical region

Ant inquiline social parasites obligately depend on their hosts for survival and reproduction. Because of their shift from a eusocial to a socially parasitic life history, inquiline social parasites are interesting study systems for exploring the dynamics between conflict and cooperation in eusocial insect colonies. In addition, inquiline social parasites are of interest to evolutionary biology, because some species evolved directly from their hosts via sympatric speciation. With five described species, inquiline social parasites are relatively diverse in the fungus-growing ants. So far, four species have been reported from the leaf-cutting ant genus Acromyrmex and its closely affiliated social parasite genus Pseudoatta. In contrast, only a single parasite species was described from the lower attine genus Mycocepurus. Here, we describe a new species of inquiline social parasite, Acromyrmex fowleri sp. nov., which was discovered 27 years ago in the tropical region of Brazil (State of Bahia), living inside the colonies of its host Acromyrmex rugosus. We also report observations on the behavioral ecology and natural history of A. fowleri and its host. Our study suggests that A. fowleri is an obligate, queen-tolerant, workerless inquiline social parasite of A. rugosus and that A. fowleri represents some but not all morphological and life history characters of the inquiline syndrome, supporting the hypothesis that the complex traits of the inquiline syndrome evolve in a mosaic fashion. Considering that A. fowleri is a new social parasite species from tropical Brazil, we discuss the paradoxical biogeographic distribution of ant social parasites, which we refer to as the “Kutter–Wilson Paradox”, and conclude that the Kutter–Wilson Paradox is a genuine biogeographical pattern, instead of being a mere sampling artifact.