Multiple phenotypic traits as triggers of host attacks towards ant symbionts: body size, morphological gestalt, and chemical mimicry accuracy

Background Ant colonies are plagued by a diversity of arthropod guests, which adopt various strategies to avoid or to withstand host attacks. Chemical mimicry of host recognition cues is, for example, a common integration strategy of ant guests. The morphological gestalt and body size of ant guests have long been argued to also affect host hostility, but quantitative studies testing these predictions are largely missing. We here evaluated three guest traits as triggers of host aggression—body size, morphological gestalt, and accuracy in chemical mimicry—in a community of six Eciton army ant species and 29 guest species. We quantified ant aggression towards 314 guests in behavioral assays and, for the same individuals, determined their body size and their accuracy in mimicking ant cuticular hydrocarbon (CHC) profiles. We classified guests into the following gestalts: protective, myrmecoid, staphylinid-like, phorid-like, and larval-shaped. We expected that (1) guests with lower CHC mimicry accuracy are more frequently attacked; (2) larger guests are more frequently attacked; (3) guests of different morphological gestalt receive differing host aggression levels. Results Army ant species had distinct CHC profiles and accuracy of mimicking these profiles was variable among guests, with many species showing high mimicry accuracy. Unexpectedly, we did not find a clear relationship between chemical host similarity and host aggression, suggesting that other symbiont traits need to be considered. We detected a relationship between the guests’ body size and the received host aggression, in that diminutive forms were rarely attacked. Our data also indicated that morphological gestalt might be a valuable predictor of host aggression. While most ant-guest encounters remained peaceful, host behavior still differed towards guests in that ant aggression was primarily directed towards those guests possessing a protective or a staphylinid-like gestalt. Conclusion We demonstrate that CHC mimicry accuracy does not necessarily predict host aggression towards ant symbionts. Exploitation mechanisms are diverse, and we conclude that, besides chemical mimicry, other factors such as the guests’ morphological gestalt and especially their body size might be important, yet underrated traits shaping the level of host hostility against social insect symbionts.

Colony expansions underlie the evolution of army ant mass raiding

The mass raids of army ants are an iconic collective phenomenon, in which many thousands of ants spontaneously leave their nest to hunt for food, mostly other arthropods. While the structure and ecology of these raids have been relatively well studied, how army ants evolved such complex cooperative behavior is not understood. Here, we show that army ant mass raiding has evolved from a different form of cooperative hunting called group raiding, in which a scout directs a small group of ants to a specific target through chemical communication. We describe the structure of group raids in the clonal raider ant, a close relative of army ants in the subfamily Dorylinae. We find evidence that the coarse structure of group raids and mass raids is highly conserved and that all doryline ants likely follow similar behavioral rules for raiding. We also find that the evolution of army ant mass raiding occurred concurrently with expansions in colony size. By experimentally increasing colony size in the clonal raider ant, we show that mass raiding gradually emerges from group raiding without altering individual behavioral rules. This suggests that increasing colony size can explain the evolution of army ant mass raids and supports the idea that complex social behaviors may evolve via mechanisms that need not alter the behavioral interaction rules that immediately underlie the collective behavior of interest.

Temporary prey storage along swarm columns of army ants: an adaptive strategy for successful raiding?

While pillaging brood of other social insects, Eciton army ants often accumulate prey in piles (or caches) along their foraging trails. Descriptions scattered throughout the past 100 years link this behavior to foraging-related migration. However, no empirical work has yet investigated its adaptive value. Here we asked whether caches facilitate prey flow from foraging fronts to temporary nests (or bivouacs) in the hook-jawed army ant, Eciton hamatum. We counted workers arriving at caches with prey from foraging fronts and departing caches towards the bivouac, quantifying their prey loads. While more workers carrying single-item prey loads arrived at rather than left caches towards the bivouac, ants carrying multiple-item prey loads arrived at and departed at the same rate. This probably resulted from raiders depositing prey in safe locations and rapidly returning to the foraging front, while other workers safely transported prey to the bivouac in multiple-item loads. This cache-mediated traffic partitioning probably allows maximizing the prey collection rate, and may be a counter-adaptation to the strategies prey colonies deploy to defend their brood from army ants.

Eciton Army Ants—Umbrella Species for Conservation in Neotropical Forests

Identification of priority areas for conservation is crucial for the maintenance and protection of biodiversity, particularly in tropical forests where biodiversity continues to be lost at alarming rates. Surveys and research on umbrella species can provide efficient and effective approaches to identify potential areas for conservation at small geographical scales. Army ants of the genus Eciton are keystone species in neotropical forests due to their major role as top predators and due to the numerous vertebrate- and invertebrate associated species that depend upon their colonies for survival. These associates range from the iconic army ant-following birds to a wide range of arthropod groups, some of which have evolved intricate morphological, behavioural and/or chemical strategies to conceal their presence and integrate into the colony life. Furthermore, Eciton colonies require large forested areas that support a diverse leaf litter prey community and several field-based and genetic studies have demonstrated the negative consequences of forest fragmentation for the long-term maintenance of these colonies. Therefore, Eciton species will not only act as umbrella for their associates but also for many other species in neotropical forests, in particular for those that require a large extent of forest. This review summarises past and recent accounts of the main taxonomic groups found associated with Eciton colonies, as well research assessing the impact of forest fragmentation on this army ant, to encourage the adoption of Eciton army ants as umbrella species for the identification of priority areas for conservation and assessments of the effect of disturbance in neotropical forests.

Correction to: Widespread army ant aversion among East African jumping spiders (Salticidae)

The article Widespread army ant aversion among East African jumping spiders (Salticidae)

Army-ant following in Neotropical birds: A review and prospectus

Army-ants (particularly swarm-raiding species Eciton burchellii and Labidus praedator) are keystone predators in Neotropical forests. Hundreds of associated species from diverse taxa depend on them for survival, the most conspicuous of which are the ant-following birds. These birds forage on cryptic arthropods and vertebrates as they attempt to escape raiding army-ants. Despite capturing the attention of tropical biologists for centuries, research on ant-following birds has been largely limited to natural history observations until more recently. Here, we argue that this complex system provides unique and underappreciated opportunities for testing hypotheses in 5 main areas of interest: (1) competitive interactions among attendant birds, (2) cognitive and behavioral adaptations for locating swarms, (3) evolution of ant-following behavior, (4) biogeography of ant-following behavior, and (5) conservation of ant-following birds. For each research area, we review the current state of knowledge and make suggestions for fruitful research avenues that we believe will help address important questions in the fields of ecology, evolution, and behavior.