Army ants in four forests: geographic variation in raid rates and species composition

1. The New World army ants are top predators in the litter of tropical forest, but no comprehensive studies exist on variation in assemblage-wide activity and species composition. We used standardized protocols to estimate foraging raid rates and species composition of army ant communities in four Neotropical forests. The study sites spanned approximately 10 degrees latitude, with two sites each in Central and South America. 2. We recorded a total of 22 species of army ants. The four sites varied in observed and estimated species richness. Species overlap was highest between the Central American sites, and lowest between the South American sites. 3. Raid activity varied significantly among sites. Raid activity per kilometre of trail walks was over four times higher at the most active site (Sta. Maria, Venezuela) than at the least active site (Barro Colorado Island, Panama). Furthermore, each site showed a different diel pattern of activity. For example, raid activity was higher during daylight hours in Costa Rica, and higher at night in Venezuela. Raid activity relationships with ambient temperature also varied significantly among sites. 4. The overall rate of army ant raids passing through 1 m(2) plots was 0.73 raids per day, but varied among sites, from 0 raids per day (Panama) to 1.2 raids per day (Venezuela). 5. Primarily subterranean species were significantly more abundant in Venezuela, and above-ground foragers that form large swarm fronts were least abundant in Panama. The site heterogeneity in species abundance and diel activity patterns has implications for army ant symbionts, including ant-following birds, and for the animals hunted by these top predators.     

Characterization of 12 new microsatellite loci in Aenictus and Neivamyrmex army ants

Here we describe 12 polymorphic microsatellite loci that were cloned and characterized for three species of army ants: the North American Neivamyrmex nigrescens, and the Asian Aenictus laeviceps and Aenictus dentatus. Observed and expected heterozygosities ranged from 0.37 to 0.97 (mean 0.70), and from 0.48 to 0.95 (mean 0.72), respectively. We observed 2–30 (mean 12) alleles per locus. These new genetic markers will be useful for studies of overall population structure and the complex pedigrees in colonies of army ants.     

Species and site differences in Neotropical army ant emigration behaviour

Abstract 1. Along with large wingless queens and group foraging raids, periodic colony movements or emigrations are a defining feature of army ant behaviour. Emigrations are important because they regulate spacing among colonies, and provide access to new foraging sites. 2. Raid and emigration activity of army ant communities was measured at four Neotropical sites using a standardised sampling protocol. Sampling of emigration activity was carried out throughout the diel cycle. 3. Both raid and emigration activity occurred throughout the day and night. Emigration activity was highest under cover of darkness, but sites and species differed in the diel timing of emigrations. These site and species differences have implications for the evolution of emigration behaviour, as well as for the community ecology of army ants.     

Extraordinary Predation by the Neotropical Army Ant Cheliomyrmex andicola: Implications for the Evolution of the Army Ant Syndrome1

Workers of the genus Cheliomyrmex are unique among the New world army ants (subfamily Ecitoninae) in that their mandibles are armed with elongate, spine‐like teeth. We present the first prey records for this genus. Cheliomyrmex andicola prey on large‐bodied ground dwelling invertebrates and, possibly, on vertebrates. Unlike other army ants, C. andicola workers use their sting during prey capture. The workers' unusual mandibles and potent stings may be adapted for piercing and gripping the integument of nonarthropod prey animals, and for rapidly subduing large‐bodied prey, respectively. The genus Cheliomyrmex may be the sister taxon to other Neotropical army ants (Ecitoninae), and Cheliomyrmex shares features of mandibular morphology and prey selection with Old World driver ants in the genus Dorylus. Mass cooperative foraging, an important element of army ant behavior, may have arisen in part as an adaptation for exploiting large‐bodied prey.     

The evolution of multiple mating in army ants

The evolution of mating systems in eusocial Hymenoptera is constrained because females mate only during a brief period early in life, whereas inseminated queens and their stored sperm may live for decades. Considerable research effort during recent years has firmly established that obligate multiple mating has evolved only a few times: in Apis honeybees, Vespula wasps, Pogonomyrmex harvester ants, Atta and Acromyrmex leaf-cutting ants, the ant Cataglyphis cursor, and in at least some army ants. Here we provide estimates of queen-mating frequency for New World Neivamyrmex and Old World Aenictus species, which, compared to other army ants, have relatively small colonies and little size polymorphism among workers. To provide the first overall comparative analysis of the evolution of army ant mating systems, we combine these new results with previous estimates for African Dorylus and New World Eciton army ants, which have very large colonies and considerable worker polymorphism. We show that queens of Neivamyrmex and Aenictus mate with the same high numbers of males (usually ca. 10-20) as do queens of army ant species with very large colony sizes. We infer that multiple queen mating is ancestral in army ants and has evolved over 100 million years ago as part of the army ant adaptive syndrome. A comparison of army ants and honeybees suggests that mating systems in these two distantly related groups may have been convergently shaped by strikingly similar selective pressures.     

Microclimatic factors associated with elevational changes in army ant density in tropical montane forest

Abstract.  1. The density (rate of encountering foraging raids) and species richness of army ants (Formicidae: Ecitoninae, and behaviourally convergent Ponerinae) was measured in montane tropical forest. Above-ground and subterranean army ant raids were sampled using standard protocols at four sites across an elevational gradient (1200–1650 m above mean sea level) in and near cloud forest in the area of Monteverde, Costa Rica.
2. Mean ambient temperature differed among sites, and decreased with elevation. For the above-ground foraging army ant species, raid rates also declined with elevation. Surface army ant raid rates, however, were not affected by day to day weather variation within sites (temperature, cloud cover, or precipitation).
3. For the underground foraging army ant species, raid rates did not vary directionally with elevation, and subterranean raid rates were not affected by day to day weather variation within sites.
4. Army ant species richness was not directionally related to elevation, and species sharing among sites was generally high.
5. Army ant community structure changes with elevation in Neotropical montane forest, and the results suggest that the strongest effects are of temperature regimes on the density of raids. These findings provide a baseline against which to detect changes in army ant communities that may accompany directional climate change in tropical cloud forests.     

Complex Nesting Behavior by Two Neotropical Species of the Ant Genus Stenamma (Hymenoptera: Formicidae)1

Two Neotropical ant species, Stenamma expolitum Smith and S. alas new species (described here), exhibit three unusual nesting behaviors: (1) they build architecturally sophisticated nest entrances that elevate the nest opening away from the surface; (2) they maintain multiple identical nests but occupy only one of them; and (3) they keep a round “door pebble” at the nest entrance, with which they plug the opening in response to army ants. Adaptive hypotheses for these behaviors are discussed, including the possibility that there are multiple lines of defense against army ant predation.     

First Record of the Army Ant Cheliomyrmex morosus in Panama and its High Associate Diversity

The occurrence and distribution of hypogaeic army ants is still poorly known despite their potential importance for tropical soil ecosystems. We present the first record of one of the most cryptic of army ants, Cheliomyrmex morosus , in Panama and list a diverse array of associated arthropods. The late discovery of these ants and high diversity of their associates indicates that much remains to be discovered even in one of the most intensely studied research areas.     

Specializations of birds that attend army ant raids: An ecological approach to cognitive and behavioral studies

Tropical birds forage at army ant raids on several continents. Obligate foraging at army ant raids evolved several times in the Neotropical true antbird family (Thamnophilidae), and recent evidence suggests a diversity of bird species from other families specialize to varying degrees on army ant exploitation. Army ant raids offer access to high prey densities, but the ant colonies are mobile and widely spaced. Successful army ant exploitation requires solving a complex foraging problem because army ant raids are unpredictable in space and time. Birds can counteract the challenges posed by the ants by using strategies that raise their chances of detecting army ant raids, and birds can use additional strategies to track army ant colonies they have located. Some features of army ant biology, such as their conspicuous swarms and columns, above-ground activity, and regular cycles of behavior, provide opportunities for birds to increase their effectiveness at exploiting raids. Changes in sensory, cognitive and behavioral systems may all contribute to specialized army ant exploitation in a bird population. The combination of specializations that are employed may vary independently among bird species and populations. The degree of army ant exploitation by birds varies geographically with latitude and elevation, and with historical patterns such as centers of distribution of obligate thamnophilid antbirds. We predict the set of specializations a given bird population exhibits will depend on local ecology, as well as phylogenetic history. Comparative approaches that focus on these patterns may indicate ecological and evolutionary factors that have shaped the costs and benefits of this foraging strategy. The development of army ant exploitation in individual birds is poorly understood, and individual expression of these specializations may depend on a combination of genetic adaptation with cognitive plasticity, possibly including social and experiential learning. Future studies that measure developmental changes and quantify individual differences in army ant exploitation are needed to establish the mechanisms underlying this behavior.