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.     

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.     

Spatial Variation in Army Ant Swarm Raiding and its Potential Effect on Biodiversity

Invertebrate communities of the tropical rain forest floor are highly diverse, characterized by patchy species distribution patterns and high variation in species density. Spatial variation in the foraging activity of swarm raiding army ants, prime invertebrate predators in tropical rain forests, is discussed as a mechanism contributing to these patterns, but highly resolved long‐term data on army ant raiding on the local and landscape scale are hitherto lacking. In this study, 196 positions in 11 study sites in a tropical rain forest in western Kenya were continuously monitored over ~4 mo for the occurrence of swarm raids of army ants. Using population simulation analyses, the consequences of army ant raiding for prey communities were assessed. We found an unexpectedly high variation in raid rates at the study site and landscape scale. The weekly chance of communities to become raided by army ants was on average 0.11, but ranged from 0 to 0.50 among the 196 positions. Simulating population developments of two Lotka–Volterra species—showing slight trade‐offs between competitive strength and resistance to army ant raids—in the real raiding landscapes showed that the observed spatial variation in raid rates may produce high prey diversity at larger spatial scales (due to high β‐diversity) and strong variation in species density. Our results indicate that high spatial variation in army ant swarm raiding is a mechanism capable of generating patchy species distribution patterns and maintaining the high biodiversity of invertebrate communities of the tropical rain forest floor.     

Evidence for facilitation among avian army‐ant attendants: specialization and species associations across elevations

Mixed‐species assemblages can involve positive and negative interactions, but uncertainty about high‐value patchy resources can increase the value of information sharing among heterospecific co‐foragers. I sampled species composition of bird‐flocks attending army‐ant raids in three adjacent elevation zones in Costa Rica, across multiple years, to test for positive and negative associations among raid‐attending bird species. My goal was to test whether the most frequent and specialized raid‐attending species showed evidence of facilitating or excluding other bird species. I quantified elevational variation in avian community composition at raids, then asked whether species composition was associated with variation in flock characteristics (flock size and species richness). I identified the most frequent raid‐attending species (those that attended raids most frequently relative to their mist‐net capture rates), and bird species that performed specialized army ant‐following behavior (bivouac‐checking, which allows birds to memorize and track mobile army‐ant colonies). There was significant turnover of bird species among zones (including the frequent and specialized attendants); patterns of species overlap suggested a gradual transition from a Pacific‐slope to an Atlantic‐slope raid‐attending bird fauna. Raid‐attendance frequency was positively correlated with bivouac‐checking behavior. With few exceptions, the most frequent raid‐attending bird species, and the bivouac‐checking species, also participated in the most species‐rich flocks. High species‐gregariousness suggests many of the frequently attending and/or bivouac‐checking species functioned as core flock members. However, some bird species pairs were significantly negatively associated at raids. Despite species turnover, per‐flock numbers of birds at raids did not differ among geographic zones, but flocks on the Pacific‐slope were heavier because larger bodied bird species attended raids. Previous studies showed that the size (biomass) of bird‐flocks corresponds to the amount of food the birds kleptoparasitize from ant raids, and the heavier Pacific‐slope bird‐flocks could have greater negative kleptoparasitic impacts.     

Species‐level predation network uncovers high prey specificity in a Neotropical army ant community

Army ants are among the top arthropod predators and considered keystone species in tropical ecosystems. During daily mass raids with many thousand workers, army ants hunt live prey, likely exerting strong top‐down control on prey species. Many tropical sites exhibit a high army ant species diversity (>20 species), suggesting that sympatric species partition the available prey niches. However, whether and to what extent this is achieved has not been intensively studied yet. We therefore conducted a large‐scale diet survey of a community of surface‐raiding army ants at La Selva Biological Station in Costa Rica. We systematically collected 3,262 prey items from eleven army ant species (genera Eciton, Nomamyrmex and Neivamyrmex). Prey items were classified as ant prey or non‐ant prey. The prey nearly exclusively consisted of other ants (98%), and most booty was ant brood (87%). Using morphological characters and DNA barcoding, we identified a total of 1,103 ant prey specimens to the species level. One hundred twenty‐nine ant species were detected among the army ant prey, representing about 30% of the known local ant diversity. Using weighted bipartite network analyses, we show that prey specialization in army ants is unexpectedly high and prey niche overlap very small. Besides food niche differentiation, we uncovered a spatiotemporal niche differentiation in army ant raid activity. We discuss competition‐driven multidimensional niche differentiation and predator–prey arms races as possible mechanisms underlying prey specialization in army ants. By combining systematic prey sampling with species‐level prey identification and network analyses, our integrative approach can guide future research by portraying how predator–prey interactions in complex communities can be reliably studied, even in cases where morphological prey identification is infeasible.     

Elevational and geographic variation in army ant swarm raid rates

Geographic and elevational variation in the local abundance of swarm-raiding army ants has implications for the population dynamics of their prey, as well as affecting the profitability of army-ant-following behavior for birds. Here, we analyze systematically collected data on E. burchellii and L. praedator raid rates from geographically and elevationally wide-ranging sites, from lowland to montane forests. We show that raids of each species, and of both species pooled, reach peak densities at intermediate (premontane) elevations. These patterns suggest that army ant swarm raids are relatively abundant in Neotropical montane forests. Therefore, a paucity of ant raids does not explain the absence of obligate ant-following bird species, particularly true antbirds (Thamnophilidae), from montane forests. As army ant raids are relatively common at middle elevations, opportunities exist for other montane bird taxa to exploit army ant raids as a food source.     

The blind leading the blind in army ant raid patterns: Testing a model of self-organization (Hymenoptera: Formicidae)

We present field experiments and analyses that test both the assumptions and the predictions of a model that showed how the swarm raids of the army ant Eciton burchellimight be self-organizing, i.e., based on hundreds of thousands of interactions among the foraging workers rather than a central administration or hierarchical control. We use circular mill experiments to show that the running velocity of the ants is a sigmoidal function of the strength of their trail pheromones and provide evidence that the swarm raid is structured by the interaction between outbound and inbound forager traffic mediated by the pheromones produced by both of these sets of ants. Inbound traffic is also affected by the distribution of prey, and hence, sites of prey capture alter the geometry of the raid. By manipulating the prey distributions for E. burchelliswarms, we have made them raid in a form more typical of other army ant species. Such self-organization of raids based on an interaction between the ants and their environment has profound consequences for interpretations of the evolution of army ant species.     

How much do army ants eat? On the prey intake of a neotropical top-predator

New World army ants (Ecitoninae) are nomadic group-predators that are widely thought to have a substantial impact on their prey. Nevertheless, quantitative data on prey intake by army ants is scarce and mostly limited to chance encounters. Here, I quantify the prey intake of the army ant Eciton hamatum at the contrasting scales of raid, colony (sum of simultaneous raids), and population. Like most army ants, E. hamatum conducts narrow ‘column raids’ and has a specialized diet of ant prey. I show that individual raids often had periods of no prey intake, and raid intake rates, calculated in g/min, differed significantly among colonies. Moreover, neither mean nor peak raid intake rates were correlated with colony size. Similarly, colony intake rates differed significantly among colonies, and mean colony intake rates were not correlated with colony size. However, mean colony intake rates were significantly higher than mean raid intake rates, and peak colony intake rate was correlated with colony size. Having multiple raids thus improves colony-level intake rates, and larger colonies can harvest more prey per unit time. Mean colony intake rate across colonies was 0.067 g/min dry weight and mean daily colony intake was calculated at 38.2 g. This intake is comparable to that of Eciton burchellii, which has a more generalized diet and conducts spectacular ‘swarm raids’ that are seen as having a greater impact on prey than column raids. Population size on Barro Colorado Island, Panama, was estimated to be 57 colonies, which extrapolates to a daily population intake of nearly 2 kg of prey dry weight, or 120 g/km². Broadly, these findings demonstrate that column raiding army ants experience considerable variation in prey intake for individual raids, but can still achieve notable impact at the larger scales of colony and population. Furthermore, they challenge the idea that swarm-raiding species necessarily have greater intake and thus impact on prey. Instead, I suggest that conducting multiple column raids may be a strategy that allows for comparable intake from a more specialized diet.     

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.     

Army ant males lose seasonality at a site on the equator

Army ants are keystone predators in the tropics and subtropics. During reproduction, males fly between colonies to mate with unmated, wingless queens. The males of most species are attracted to lights, and thus their presence and the timing of reproduction can be monitored using light traps. Previous studies examined the seasonality of army ant male reproduction and its relationship to climate factors at individual sites, but less is known about variation among sites. We examined army ant male flight seasonality at three sites: (1) La Selva Biological Station in Costa Rica, a site with weak temperature seasonality and moderate rainfall and day length seasonality, (2) Yasuní National Park, Ecuador, a site with no temperature or day length seasonality and very weak rainfall seasonality, and (3) the state of Paraná in southern Brazil, a site with very strong temperature, rainfall, and day length seasonality. Army ants showed strong seasonality at the La Selva and Paraná sites, and very weak to no seasonality at the Yasuní site. At La Selva and Paraná, flight times varied among species, but were very predictable from year to year, which suggests day length or temperature as predictable cues rather than rainfall. Lack of seasonal cues near the equator may be a challenge for army ant species that need to synchronize colony reproduction, and thus may have conservation implications for minimum population sizes needed to ensure stable populations.     

Litter ant patchiness at the 1-m2 scale: disturbance dynamics in three Neotropical forests

Large-scale (>100 m²/day) raids by tropical army ants have been linked to litter ant patchiness and diversity. In three Neotropical forests, densities of litter ants vary 10–20 fold at the 1-m² scale. A survey of Barro Colorado Island. Panama, revealed that most army ant raids also occur on a 1-m² scale with fronts 1 m wide. To explore the role that such small scale disturbance may play in creating litter ant patchiness, all litter ant nests were removed from 1-m² plots. Control and disturbance plots were resampled 3 months later. In contrast to a previous study of large litter gaps, ant foundresses did not appear to prefer these smaller gaps. Nest densities, species richness, and species composition differed most from controls in a dry hilltop forest in Panama, second most in a wetter ravine forest nearby, and least in a Costa Rican wet forest. Disturbance may not leave a lasting signature in the wetter forests due to higher background levels of disturbance, faster recovery, or both.     

Coordination of Raiding and Emigration in the Ponerine Army Ant Leptogenys distinguenda (Hymenoptera: Formicidae: Ponerinae): A Signal Analysis

Several glandular sources of trail pheromones have been discovered in army ants in general. Nevertheless, at present the understanding of the highly coordinated behavior of these ants is far from complete. The importance of trail pheromone communication for the coordination of raids and emigrations in the ponerine army ant Leptogenys distinguenda was examined, and its ecological function is discussed. The secretions of at least two glands organize the swarming activities of L. distinguenda. The pygidial gland is the source of an orientation pheromone holding the group of raiding workers together. The same pheromone guides emigrations to new nest sites. In addition, the poison sac contains two further components: one with a weak orientation effect and another which produces strong, but short-term attraction and excitement. The latter component is important in prey recruitment and characterizes raid trails. This highly volatile recruitment pheromone allows the extreme swarm dynamic characteristic of this species. Emigration trails lack the poison gland secretion. Due to their different chemical compositions, the ants are thus able to distinguish between raid and emigration trails. Nest emigration is not induced chemically, but mechanically, by the jerking movements of stimulating workers.     

Predation and patchiness in the tropical litter: do swarm-raiding army ants skim the cream or drain the bottle?

1. Swarm-raiding army ants have long been considered as episodic, catastrophic agents of disturbance in the tropical litter, but few quantitative data exist on their diets, preferences, and, critically, their ability to deplete prey. 2. Here, we provide such data for two common species of swarm raiders broadly sympatric throughout the Neotropics: the iconic Eciton burchellii and the more secretive, less studied Labidus praedator. In Ecuador, Costa Rica, Venezuela and Panama, patches of forest floor were sampled for litter invertebrates immediately before and after army ant raids. These invertebrates have been shown to regulate decomposition and vary 100-fold in local densities across the forest floor. 3. Contrary to Eciton's popular image, only Labidus consistently reduced the biomass of litter invertebrates and only then by an average of 25%. Eciton's impacts were concentrated on rich patches of invertebrates, while Labidus prey depletion showed no such density dependence. Labidus reduced the biomass of some invertebrates-isopods, larviforms and coleoptera-by up to 75%; Eciton showed no such prey preferences. 4. Our results suggest that Eciton specializes on high biomass patches, while Labidus feeds profitably from any litter patch. Combined, these swarm raiders sum to be chronic, but not catastrophic, predators of common litter invertebrates of the brown food web.     

Effect of trail pheromones and weather on the moving behaviour of the army ant <Emphasis Type="Italic">Eciton burchellii</Emphasis>

Most of what we know about the moving behaviour of the nomadic army ant Eciton burchellii comes from Barro Colorado Island (BCI) in Panama. Nomadic colonies raid roughly in straight line during the day and relocate their nests along this path in the evening. At BCI, nomadic colonies raid roughly in the same compass bearing of the previous day, presumably using their pheromone-marked raiding trails as cues to pick directions. Deviations from this direction occur when a nomadic colony fails to move, possibly due to environmental conditions. The generality of these results has been questioned. We studied nomadic colonies of E. burchellii at La Selva Biological Station, Costa Rica to evaluate the generality of the results obtained from BCI. We measured the angle between consecutive raids, manipulated the distribution of previous day’s raid pheromones around nests to evaluate the effect of raid pheromone on foraging direction, and evaluated the effect of rainfall on the probability of moving and on deviation from the previous day’s raid. Colonies did not follow the same compass bearing of the previous day and formed new raids on areas with previous day’s raid pheromones or without them. Rainfall can explain when nomadic colonies move, but did not explain deviation from the previous day’s raid direction. Our results suggest that caution should be taken when generalizing the insightful results obtained from the BCI population.     

Combat between large derived societies: A subterranean army ant established as a predator of mature leaf-cutting ant colonies

Summary. Mature colonies of Atta leaf-cutting ants are dominant herbivores throughout the Neotropics. Although young colonies have natural enemies, mature colonies, which live in extensive nests containing millions of workers, currently have no recognised predators. New World army ants (Ecitoninae) are specialist social predators of other ants, and the army ant Nomamyrmex esenbeckii, a primarily subterranean species, is known to prey upon young Atta colonies. Here we present the results of the first long-term study of the predator-prey interaction between N. esenbeckii and Atta. Our study establishes the army ant N. esenbeckii as the only known predator capable of successfully attacking and killing mature as well as young colonies of Atta leaf-cutting ants. In natural raids, and experimental tests, Atta rapidly recruited their largest workers (majors) as a specific defensive response to N. esenbeckii raiders and both taxa used their largest individuals in the frontline of battles. The deployment and behaviour of these large workers demonstrates a size-related division of labour and agrees with the predictions of Lanchesters Linear Law of Combat. Both taxa also used cooperative combat teams to overwhelm large combatants from the other side. The success of N. esenbeckii raids varied greatly, such that they were prevented from entering Atta nests in the least successful raids, and completely overran Atta colonies in the most successful raids. The speed and magnitude of the defensive response of mature Atta colonies was key in determining the level of success of N. esenbeckii raids.Received 12 December 2003; revised 25 March 2004; accepted 1 April 2004.Work conducted at the Smithsonian Tropical Research Institute, Apartado 2072, Balboa, Ancon, Republic of Panama     

Foraging dynamics in the group-hunting myrmicine ant,Pheidologeton diversus

Pheidologeton diversus workers group-hunt (that is, search for food in raiding groups) and are in this way remarkably convergent with army ants (Dorylinae and Ecitoninae). Raids appear usually to take independent courses and are capable of tracking areas of high food density. However, raid advance is not dependent on continual food discovery at the raid front, since raids can advance over areas without food. Most raids extend from trunk trails, which originate when the basal trail of a raid remains in use even after the original raid has ceased. Trunk trails can last at least as long as 10 weeks, with the terrain and the distance to the nest influencing the trail stability. Territories are limited to the trail systems, with rich food items in particular being vigorously defended. Group hunting permits P. diversus to quickly harvest booty, usurp foods from competing species, and capture large prey. This strategy is compared with the raiding strategies of other ants. I hypothesize that group hunting originated from an ancestor which hunted solitarily from trunk trails through the acceleration of trail production and reduction in worker autonomy.     

Raiding and emigration dynamics in the ponerine army ant Leptogenys distinguenda (Hymenoptera, Formicidae)

Summary: Field and laboratory observations demonstrate that Leptogenys distinguenda is characterized by typical army ant behavior. Like in the "classical" army ants from the subfamilies Ecitoninae, Dorylinae and Aenictinae, raiding and emigration behavior are closely linked. The direction of raids can be altered in field experiments to a wide extent by offering ample food, suggesting it is highly influenced by the patchiness of prey. The sum of recruitments coming from one direction and the recruitment overrun are primarily responsible for the spatial development and the extension of raids. Emigration frequency can be suppressed by overfeeding a colony in the field. This result is interpreted as a secondary effect of reduced swarming activity, which gets suppressed as well in the same experiment. The discovery of a suitable nest site is considered the proximate stimulus for emigration, and the best explored areas are those that have been most thoroughly raided. As a result, emigrations are likely to lead into areas with high prey densities.     

The foraging ecology of the army ant Eciton rapax: an ergonomic enigma?

Abstract. 1. The army ant Eciton rapax (F. Smith) produces longer raid systems than any other member of its genus and it is a specialist predator of forest floor and understory ants such as species of Camponotus, Odontomachus and Pachycondyla.
2. Allometrical analysis confirms that E. rapax is the only member of its genus without distinct physical castes among its workers: its foraging population is entirely monomorphic and there are no majors.
3. The workers of E. rapax are distributed over a considerable size range, and there are distinct divisions of labour within these colonies: small workers tend to stay in the nests and among the larger foraging workers those retrieving prey items are significantly bigger than the rest.
4. An analysis of foraging efficiency and worker performance in E. rapax suggests that transport costs, resulting from the great distances that workers travel during raids and emigrations, are one of the selection pressures that have favoured the evolution of large monomorphic workers in this species.     

The raiding success of Pheidole megacephala on other ants in both its native and introduced ranges

We studied the behaviour of the invasive African myrmicine ant, Pheidole megacephala, when confronted with colonies of other common ant species in Cameroon, a part of its native range, and in Mexico, where it has been introduced. P. megacephala raided the nests of the other ants in both cases. Eleven species out of 12 put up a rather strong resistance to raiding P. megacephala workers in Cameroon compared to only three species out of 11 in Mexico, where only colonies of Solenopsis geminata, Dorymyrmex pyramicus and Dolichoderus bispinosus resisted these raids. We conclude that P. megacephala's heightened ability to successfully raid colonies of competing ants may help explain its success and the decline of native ants in areas where it has been introduced.     

Wood-nesting ants and their parasites in forests and coffee agroecosystems

Agricultural intensification is linked to reduced species richness and may limit the effectiveness of predators in agricultural systems. We studied the abundance, diversity, and species composition of wood-nesting ants and frequency of parasitism of poneromorph ants in coffee agroeco systems and a forest fragment in Chiapas, Mexico. In three farms differing in shade management and in a nearby forest fragment, we surveyed ants nesting in rotten wood. We collected pupae of all poneromorph ants encountered, and incubated pupae for 15 d to recover emerging ant parasites. If no parasites emerged, we dissected pupae to examine for parasitism. Overall, we found 63 ant morphospecies, 29 genera, and 7 subfamilies from 520 colonies. There were no significant differences in ant richness or abundance between the different sites. However, there were significant differences in the species composition of ants sampled in the four different sites. The parasitism rates of ants differed according to site; in the forest 77.7% of species were parasitized, and this number declined with increasing intensification in traditional polyculture (40%),commercial polyculture (25%), and shade monoculture (16.6%). For three of four poneromorph species found in >1 habitat, parasitism rates were higher in the more vegetatively complex sites. The result that both ant species composition and ant parasitism differed among by site indicates that coffee management intensification affects wood-nesting ant communities. Further, coffee intensification may significantly alter interactions between ants and their parasites, with possible implications for biological control in coffee agroecosystems.