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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. [ABSTRACT FROM AUTHOR]

Over 40 years ago, the dacetine ant Strumigenys arizonica was discovered in a nest of the fungus-growing ant Trachymyrmex arizonensis at Madera Canyon in the Santa Rita Mountains of the southwestern United States. This discovery suggested that the two species form compound nests, but this hypothesis has not been investigated. Here, we characterize this symbiosis through an analysis of collection records supplemented by recent field and laboratory observations. Our observations show that S. arizonica and T. arizonensis form compound nests that are a type of commensalistic symbiosis. Individuals of S. arizonica forage in galleries and tunnels of T. arizonensis nests but do not steal fungus or brood. Instead, individuals of S. arizonica hunt collembolans in the internal refuse piles of T. arizonensis nests. Interestingly, S. arizonica was never found independent of its host T. arizonensis over a significant portion of the geographic range of T. arizonensis. These results suggest a tight but asymmetric association where compound nesting is obligate for S. arizonica and facultative for T. arizonensis. [ABSTRACT FROM AUTHOR]

Fungus-farming 'attine' ant agriculture consists of five distinct agricultural systems characterized by a remarkable symbiont fidelity in which five phylogenetic groups of ants faithfully cultivate five phylogenetic groups of fungi. Across-system garden switching experiments result in colony decline and death, indicating that attine ant-fungus symbiont fidelity is enforced by poorly understood biological constraints. The most dramatic violation of this pattern of symbiont fidelity occurs in the relict species Apterostigma megacephala, the only lower-attine ant known to cultivate a higher-attine fungus. Apterostigma megacephala is the sole surviving representative of an ancient lineage that diverged from all other Apterostigma fungus-farming ants ~39 million years ago, yet it cultivates Leucoagaricus gongylophorus, a highly domesticated fungal species that originated in the gardens of the recently evolved leaf-cutting ants 8-11 million years ago. Understanding the biology of A. megacephala, therefore, may provide important clues about the biological mechanisms that constrain the otherwise seemingly obligate ant-fungus associations that characterize attine ant agriculture. Here, based on field work in the Floresta Nacional de Carajás in the state of Pará in Brazil, we report the previously unknown biology of A. megacephala, including nest architecture, colony demography, foraging behavior, and the morphologies of the previously undescribed gyne, male, and larva. [ABSTRACT FROM AUTHOR]

Social parasites exploit the colony resources of social species to secure their own survival and reproduction. Social parasites are frequently studied as models for conflict and cooperation as well as for speciation. The eusocial Hymenoptera harbor a diverse array of socially parasitic species with idiosyncratic life history strategies, but it is probably in the ants where social parasites are most speciose and have evolved the highest degrees of morphological and behavioral specialization. In the fungus-growing ants, a total of five obligate social parasites are known: four species are parasites of leaf-cutting ants and one species parasitizes a primitive fungus-growing ant species in the genus Mycocepurus. Here we describe a new species of socially parasitic leaf-cutting ant, Acromyrmex charruanus sp. nov., from Uruguay, and we report initial observations on the parasite's life history as well as on the morphological and behavioral adaptations related to the inquiline syndrome. Our observations suggest that Acromyrmex charruanus is an obligate inquiline social parasite of the thatch-mound-building, leaf-cutting ant Acromyrmex heyeri. Acromyrmex charruanus appears to be tolerant of the host, producing sexual offspring in the presence of the A. heyeri host queen. Queens of A. charruanus appear to reproduce semelparously and sexual offspring are produced during the austral fall (February), which differs significantly from the mating biology of the host species, which reproduces during the southern-hemisphere spring (October-December). We suggest that the diametrically opposed mating seasons of parasite and host might be adaptive, allowing the parasite to avoid competition for resources with the host sexual brood. The morphological and behavioral adaptations of A. charruanus accord with characters observed to arise early during the evolution of other ant inquiline parasite species, and so far we have no evidence for the existence of a worker caste in A. charruanus. Further field studies and behavioral experiments need to confirm our first observations and explore A. charruanus's behavioral ecology, evolution, and life history in more detail. [ABSTRACT FROM AUTHOR]