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.     

Communication in the migratory termite-hunting ant Pachycondyla (= Termitopone) marginata (Formicidae, Ponerinae)

The Neotropical species Pachycondyla marginata conducts well-organized predatory raids on the termite species Neocapritermes opacus and frequently emigrates to new nest sites. During both activities the ants employ chemical trail communication. The trail pheromone orginates from the pygidial gland. Among the substances identified in the pygidial gland secretions, only citronellal was effective as a trail pheromone. Isopulegol elicited an increase in locomotory activity in the ants and may function as a synergist recruitment signal. The chemical signal is enhanced by a shaking display performed by the recruiting ant.     

Distributed leadership and adaptive decision-making in the ant Tetramorium caespitum

In the ant species Tetramorium caespitum, communication and foraging patterns rely on group-mass recruitment. Scouts having discovered food recruit nestmates and behave as leaders by guiding groups of recruits to the food location. After a while, a mass recruitment takes place in which foragers follow a chemical trail. Since group recruitment is crucial to the whole foraging process, we investigated whether food characteristics induce a tuning of recruiting stimuli by leaders that act upon the dynamics and size of recruited groups. High sucrose concentration triggers the exit of a higher number of groups that contain twice as many ants and reach the food source twice as fast than towards a weakly concentrated one. Similar trends were found depending on food accessibility: for a cut mealworm, accessibility to haemolymph results in a faster formation of larger groups than for an entire mealworm. These data provide the background for developing a stochastic model accounting for exploitation patterns by group-mass recruiting species. This model demonstrates how the modulations performed by leaders drive the colony to select the most profitable food source among several ones. Our results highlight how a minority of individuals can influence collective decisions in societies based on a distributed leadership.     

Out of sight but not out of mind: modulation of recruitment according to home range marking in ants

Animals can acquire a global knowledge about their environment that exceeds their individual capacities by estimating the local density and activity of nestmates in an area. In ants, home range marking can indicate the density and activity of nestmates, allowing scouts to assess the potential interest of the area as a foraging site. We investigated how home range marking through footprints influences the foraging behaviour of Lasius niger scouts at a sugary food source (1 M, 1.5 ml). Over a marked apparatus the discovery time of food sources decreased while the probability of scouts recruiting nestmates and of continuing to lay a trail increased. For ants making U turns on their return to the nest, home range marking helped them to resume laying a trail after the U turn and delayed the occurrence of the U turn. As a result, the trail intensity and the rate at which information about food was conveyed by scouts to nestmates depended on home range marking. Such modulation of information reduces the number of foragers mobilized to less frequented areas that are potentially dangerous and promotes recruitment and exploitation of food sources to better known sites.     

How does food distance influence foraging in the ant Lasius niger: the importance of home-range marking

We study the influence of food distance on the individual foraging behaviour of Lasius niger scouts and we investigate which cue they use to assess their distance from the nest and accordingly tune their recruiting behaviour. Globally, the number of U-turns made by scouts increases with distance resulting in longer travel times and duration of the foraging cycle. However, over familiar areas, home-range marking reduces the frequency and thereby the impact of U-turns on foraging times leading to a quicker exploitation of food sources than over unmarked set-ups. Regarding information transfer, the intensity of the recruitment trail reaching the nest decreases with increasing food distance for all set-ups and is even more reduced in the absence of home-range marking. Hence, the probability of a scout continuing to lay a trail changes along the homeward journey but in a different way according to home-range marking. Over unexplored setups, at a given distance from the food source, the percentage of returning trail-laying ants remains unchanged for all tested nest-feeder distances. Hence, the tuning of the trail recruiting signal by scouts was not influenced by an odometric estimate of the distance already travelled by the ants during their outward journey to the food. By contrast, over previously explored set-ups, a distance-related factor – that is the intensity of home-range marking – strongly influences their recruiting behaviour. In fact, over a home-range marked bridge, the probability of returning ants maintaining their trail-laying behaviour increases with decreasing food distance while the gradient of home-range marks even induces ants which have stopped laying a trail to resume this behaviour in the nest vicinity. We suggest that home-range marking laid passively by walking ants is a relevant cue for scouts to indirectly assess distance from the nest but also local activity level or foraging risks in order to adaptively tune trail recruitment and colony foraging dynamics. Received 13 July 2004; revised 26 January and 20 May 2005; accepted 2 July 2005.     

Chemical communication during foraging in the harvesting ants Messor pergandei and Messor andrei

We combined behavioral analyses in the laboratory and field to investigate chemical communication in the formation of foraging columns in two Nearctic seed harvesting ants, Messor pergandei and Messor andrei. We demonstrate that both species use poison gland secretions to lay recruitment trails. In M. pergandei, the recruitment effect of the poison gland is enhanced by adding pygidial gland secretions. The poison glands of both species contain 1-phenyl ethanol. Minute quantities (3 μl of a 0.1 ppm solution) of 1-phenyl ethanol drawn out along a 40 cm long trail released trail following behavior in M. pergandei, while M. andrei required higher concentrations (0.5–1 ppm). Messor pergandei workers showed weak trail following to 5 ppm trails of the pyrazines 2,5-dimethylpyrazine and 2,3,5-trimethylpyrazine, whereas M. andrei workers showed no behavioral response. Minute quantities of pyrazines were detected in M. pergandei but not in M. andrei poison glands using single ion monitoring gas chromatography–mass spectrometry.     

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.     

Regulation of ants' foraging to resource productivity

We investigate the behavioural rule used by ant societies to adjust their foraging response to the honeydew productivity of aphids. When a scout finds a single food source, the decision to lay a recruitment trail is an all-or-none response based on the opportunity for this scout to ingest a desired volume acting as a threshold. Here, we demonstrate, through experimental and theoretical approaches, the generic value of this recruitment rule that remains valid when ants have to forage on multiple small sugar feeders to reach their desired volume. Moreover, our experiments show that when ants decide to recruit nest-mates they lay trail marks of equal intensity, whatever the number of food sources visited. A model based on the 'desired volume' rule of recruitment as well as on experimentally validated parameter values was built to investigate how ant societies adjust their foraging response to the honeydew productivity profile of aphids. Simulations predict that, with such recruiting rules, the percentage of recruiting ants is directly related to the total production of honeydew. Moreover, an optimal number of foragers exists that maximizes the strength of recruitment, this number being linearly related to the total production of honeydew by the aphid colony. The 'desired volume' recruitment rule that should be generic for all ant species is enough to explain how ants optimize trail recruitment and select aphid colonies or other liquid food resources according to their productivity profile.     

Trail Pheromone of the Argentine Ant,Linepithema humile (Mayr) (Hymenoptera: Formicidae)

The Argentine ant (Linepithema humile) is recognized as one of the world''s most damaging invasive species. One reason for the ecological dominance of introduced Argentine ant populations is their ability to dominate food and habitat resources through the rapid mobilization and recruitment of thousands of workers. More than 30 years ago, studies showed that (Z)-9-hexadecenal strongly attracted Argentine ant workers in a multi-choice olfactometer, suggesting that (Z)-9-hexadecenal might be the trail pheromone, or a component of a trail pheromone mixture. Since then, numerous studies have considered (Z)-9-hexadecenal as the key component of the Argentine ant trails. Here, we report the first chemical analyses of the trails laid by living Argentine ants and find that (Z)-9-hexadecenal is not present in a detectible quantity. Instead, two iridoids, dolichodial and iridomyrmecin, appear to be the primary chemical constituents of the trails. Laboratory choice tests confirmed that Argentine ants were attracted to artificial trails comprised of these two chemicals significantly more often than control trails. Although (Z)-9-hexadecenal was not detected in natural trails, supplementation of artificial dolichodial+iridomyrmecin trails with an extremely low concentraion of (Z)-9-hexadecenal did increase the efficacy of the trail-following behavior. In stark contrast with previous dogma, our study suggests that dolichodial and iridomyrmecin are major components of the Argentine ant trail pheromone. (Z)-9-hexadecenal may act in an additive manner with these iridoids, but it does not occur in detectable quantities in Argentine ant recruitment trails.     

Noise improves collective decision-making by ants in dynamic environments

Recruitment via pheromone trails by ants is arguably one of the best-studied examples of self-organization in animal societies. Yet it is still unclear if and how trail recruitment allows a colony to adapt to changes in its foraging environment. We study foraging decisions by colonies of the ant Pheidole megacephala under dynamic conditions. Our experiments show that P. megacephala, unlike many other mass recruiting species, can make a collective decision for the better of two food sources even when the environment changes dynamically. We developed a stochastic differential equation model that explains our data qualitatively and quantitatively. Analysing this model reveals that both deterministic and stochastic effects (noise) work together to allow colonies to efficiently track changes in the environment. Our study thus suggests that a certain level of noise is not a disturbance in self-organized decision-making but rather serves an important functional role.     

Stingless bees (<Emphasis Type="Italic">Scaptotrigona pectoralis</Emphasis>) learn foreign trail pheromones and use them to find food

Foragers of several species of stingless bees (Hymenoptera, Apidae and Meliponini) deposit pheromone marks in the vegetation to guide nestmates to new food sources. These pheromones are produced in the labial glands and are nest and species specific. Thus, an important question is how recruited foragers recognize their nestmates’ pheromone in the field. We tested whether naïve workers learn a specific trail pheromone composition while being recruited by nestmates inside the hive in the species Scaptotrigona pectoralis. We installed artificial scent trails branching off from trails deposited by recruiting foragers and registered whether newly recruited bees follow these trails. The artificial trails were baited with trail pheromones of workers collected from foreign S. pectoralis colonies. When the same foreign trail pheromone was presented inside the experimental hives while recruitment took place a significant higher number of bees followed the artificial trails than in experiments without intranidal presentation. Our results demonstrate that recruits of S. pectoralis can learn the composition of specific trail pheromone bouquets inside the nest and subsequently follow this pheromone in the field. We, therefore, suggest that trail pheromone recognition in S. pectoralis is based on a flexible learning process rather than being a genetically fixed behaviour.     

Memory and chemical communication in the orientation of two mass-recruiting ant species

Summary The relative contribution of visual and chemical components in the orientation ofLasius niger andIridomyrmex humilis (Argentine ant) workers during mass recruitment to newly discovered food sources is analyzed over short time intervals. While both species orient in response to the trail pheromone, a large number ofL. niger foragers rapidly switch to a more individual orientation, based on their memory of environmental cues.I. humilis workers, on the other hand, predominantly use collective chemical cues. The effect of the number of reinforcements on visual learning and its interference with chemical communication show that olfactory cues always prevail in the Argentine ant. InL. niger, the proportion of ants orienting to visual cues is independent of the trail concentration. Detailed observations of the trail-laying behavior of individually marked foragers show that nearly all theI. humilis workers initially lay a trail, whereas only half theL. niger foragers do so. This proportion decreases considerably with the number of trips performed byL. niger workers, while remaining constant for the Argentine ants. These results are interpreted with respect to the species' behavioral ecology.     

Trail pheromones of ants

Abstract The study of trail laying, recruitment of workers and trail‐following by worker ants comprises a co‐operative study of entomologists and chemists that has resulted in the identification of the chemical nature of such pheromones in many species of five subfamilies of ants. These pheromones may comprise a single compound or, in one exceptional case, a blend of as many as 14 compounds, they may come from a single gland, or in some cases, a combination of two glands. They may be peculiar to a single species or may be shared by a number of species. They exist in the glandular secretion in nanogram to picogram quantities and are detected by workers in minute amounts on a trail. The present state of knowledge of these pheromones and their chemical structures is reviewed. Suitable bioassays and odour perception are discussed and the stereobiology of a few examples is considered.     

Pheromone trail decay rates on different substrates in the Pharaoh's ant, Monomorium pharaonis

Abstract. Many ants use pheromone trails to organize collective foraging. This study investigated the rate at which a well‐established Pharaoh's ant, Monomorium pharaonis (L.), trail breaks down on two substrates (polycarbonate plastic, newspaper). Workers were allowed to feed on sucrose solution from a feeder 30 cm from the nest. Between the nest and the feeder, the trail had a Y‐shaped bifurcation. Initially, while recruiting to and exploiting the feeder, workers could only deposit pheromone on the branch leading to the feeder. Once the trail was established (by approximately 60 ants per min for 20 min), the ants were not allowed to reinforce the trail and were given a choice between the marked and unmarked branches. The numbers of ants choosing each branch were counted for 30 min. Initially, most went to the side on which pheromone had been deposited (80% and 70% on the plastic and paper substrates, respectively). However, this decayed to 50% within 25 min for plastic and 8 min for paper. From these data, the half‐life times of the pheromone are estimated as approximately 9 min and 3 min on plastic and paper, respectively. The results show that, for M. pharaonis, trail decay is rapid and is affected strongly by trail substrate.     

Worker size polymorphism and ethological role of sting associated glands in the harvester ant Messor barbarus

Summary: Though harvester ants are closely similar in ecology, species differ in their worker size polymorphism as well as in the glandular source of their trail pheromones and defensive compounds. In the harvester ant Messor barbarus, we find that the recruitment trail pheromone is located in the Dufour gland, while defence-alarm substances are produced in the poison gland. We also investigated how the glandular development and the ethological response to these abdominal glands are related to worker body size. For both glands, M. barbarus workers show monophasic and nonisometric growths with slopes of allometric regression lines lower than 1. The highest trail-following response is elicited by the Dufour gland secretion from media workers, responsible for most foraging activities in M. barbarus. Aggressive behaviour is more frequently observed in the presence of poison gland secretions from medium and large-sized workers. Differences between species and between worker size classes in the ethological role of sting associated glands are discussed in relation to the foraging ecology and defensive characteristics of harvester ants.     

From the Western Alps across Central Europe: Postglacial recolonisation of the tufa stream specialist Rhyacophila pubescens (Insecta,Trichoptera)

Background

Hybridization can have complex effects on evolutionary dynamics in ants because of the combination of haplodiploid sex-determination and eusociality. While hybrid non-reproductive workers have been found in a range of species, examples of gene-flow via hybrid queens and males are rare. We studied hybridization in East African army ants (Dorylus subgenus Anomma) using morphology, mitochondrial DNA sequences, and nuclear microsatellites.

Results

While the mitochondrial phylogeny had a strong geographic signal, different species were not recovered as monophyletic. At our main study site at Kakamega Forest, a mitochondrial haplotype was shared between a "Dorylus molestus-like" and a "Dorylus wilverthi-like" form. This pattern is best explained by introgression following hybridization between D. molestus and D. wilverthi. Microsatellite data from workers showed that the two morphological forms correspond to two distinct genetic clusters, with a significant proportion of individuals being classified as hybrids.

Conclusions

We conclude that hybridization and gene-flow between the two army ant species D. molestus and D. wilverthi has occurred, and that mating between the two forms continues to regularly produce hybrid workers. Hybridization is particularly surprising in army ants because workers have control over which males are allowed to mate with a young virgin queen inside the colony.     

The Behavior of <Emphasis Type="Italic">Acromyrmex crassispinus</Emphasis> (Hymenoptera: Formicidae) on Trail Bifurcations and the Influence of Ant Flow on Error Rates of Nestbound Laden Workers

Ants are ordinarily faced with a succession of bifurcations along their foraging networks. Given that there is no directionality in pheromone trails, each bifurcation is potentially an opportunity for error in the trajectory of laden workers to the nest, which could entail considerable inefficiencies in the transportation of food to the colony. Leaf-cutting ants (Atta and Acromyrmex) commonly show intense traffic and complex foraging trail systems, which make them ideal organisms to study worker behavior in trail bifurcations. The behavior of leaf-cutting ants of the genus Acromyrmex in trail bifurcations is still largely unexplored. Thus, this study aimed to assess the behavior of Acromyrmex crassispinus workers on trail bifurcations and to investigate whether differences in ant flow on foraging trails influence the error rate of nestbound laden workers at trail bifurcation. There was a negative relationship between ant flow and error rate of nestbound laden workers. Most workers walked in the central part of the foraging trails but occupied a broader area of the foraging trail when the ant flow was high. The results of this study provide valuable insight into the organization of traffic flow in A. crassispinus and its impacts on the foraging strategy of the species.     

Transgenerational effects and the cost of ant tending in aphids

Ant protection of extrafloral nectar (EFN)-secreting plants is a common form of mutualism found in most habitats around the world. However, very few studies have considered these mutualisms from the ant, rather than the plant, perspective. In particular, a whole-colony perspective that takes into account the spatial structure and nest arrangement of the ant colonies that visit these plants has been lacking, obscuring when and how colony-level foraging decisions might affect tending rates on individual plants. Here, we experimentally demonstrate that recruitment of Crematogaster opuntiae (Buren) ant workers to the EFN-secreting cactus Ferocactus wislizeni (Englem) is not independent between plants up to 5 m apart. Colony territories of C. opuntiae are large, covering areas of up to 5,000 m², and workers visit between five and 34 EFN-secreting barrel cacti within the territories. These ants are highly polydomous, with up to 20 nest entrances dispersed throughout the territory and interconnected by trail networks. Our study demonstrates that worker recruitment is not independent within large polydomous ant colonies, highlighting the importance of considering colonies rather than individual workers as the relevant study unit within ant/plant protection mutualisms.     

Convergent evolution,superefficient teams and tempo in Old and New World army ants

Swarm raiding army ants, with hundreds of thousands or millions of workers per colony, have evolved convergently in the Old World and New World tropics. Here we demonstrate for the first time, to our knowledge, superefficient foraging teams in Old World army ants and we compare them quantitatively with such teams in New World army ants. Colonies of Dorylus wilverthi in the Old World and Eciton burchelli in the New World retrieve almost identical sizes of prey item and the overall size range of their workers is very similar. However, 98% of D. wilverthi workers are within the size range of the smallest 25% of E. burchelli workers. In E. burchelli larger workers specialize in prey retrieval, whereas in D. wilverthi workers form many more teams than in E. burchelli. Such teams compensate for the relative rarity of larger workers in Dorylus. The proportions of prey items retrieved by teams in Dorylus and Eciton are 39% and 5%, respectively. The percentages of all prey biomass retrieved by teams in Dorylus and Eciton are 64% and 13%, respectively. Working either as single porters or teams, Dorylus carry more per unit ant weight than do Eciton, but Eciton are swifter. However, these different ergonomic factors counterbalance one another, so that performance at the colony level is remarkably, although by no means completely, similar between the Old and New World species. The remaining differences are attributable to adaptations in worker and colony tempo associated with the recovery dynamics of their prey populations. Our comparative analysis provides a unique perspective on worker-level and colony-level adaptations and is a special test of the theory of worker caste distributions.