Group recruitment in a thermophilic desert ant, Ocymyrmex robustior

Thermophilic desert ants—Cataglyphis, Ocymyrmex, and Melophorus species inhabiting the arid zones of the Palaearctic region, southern Africa and central Australia, respectively—are solitary foragers, which have been considered to lack any kind of chemical recruitment. Here we show that besides mainly employing the solitary mode of food retrieval Ocymyrmex robustior regularly exhibits group recruitment to food patches that cannot be exploited individually. Running at high speed to recruitment sites that may be more than 60 m apart from the nest a leading ant, the recruiter, is followed by a loose and often quite dispersed group of usually 2–7 recruits, which often overtake the leader, or may lose contact, fall back and return to the nest. As video recordings show the leader, while continually keeping her gaster in a downward position, intermittently touches the surface of the ground with the tip of the gaster most likely depositing a volatile pheromone signal. These recruitment events occur during the entire diurnal activity period of the Ocymyrmex foragers, that is, even at surface temperatures of more than 60 °C. They may provide promising experimental paradigms for studying the interplay of orientation by chemical signals and path integration as well as other visual guidance routines.     

Orientation and Communication in the Neotropical Ant Odontomachus bauri Emery (Hymenoptera,Formicidae, Ponerinae)

The Neotropical species Odontomachus bauri employs canopy orientation during foraging and homing. An artificial canopy pattern above the ants is much more effective as an orientation cue than horizontal landmarks or chemical marks. However, both horizontal visual cues and chemical marks on the ground can serve in localizing the nest entrance. Successful O. bauri foragers recruit nestmates to leave the nest and search for food. However, the recruitment signals do not contain directional information. Antennation bouts and pheromones from the pygidial gland most likely serve as stimulating recruitment signals. Secretions from the mandibular and poison gland elicit alarm and attack behavior.     

Soil temperature, digging behaviour, and the adaptive value of nest depth in South American species of Acromyrmex leaf-cutting ants

In leaf-cutting ants, workers are expected to excavate the nest at a soil depth that provides suitable temperatures, since the symbiotic fungus cultivated inside nest chambers is highly dependent on temperature for proper growth. We hypothesize that the different nesting habits observed in Acromyrmex leaf-cutting ants in the South American continent, i.e. superficial and subterranean nests, depend on the occurrence, across the soil profile, of the temperature range preferred by workers for digging. To test this hypothesis, we first explored whether the nesting habits in the genus Acromyrmex are correlated with the prevailing soil temperature regimes at the reported nest locations. Second, we experimentally investigated whether Acromyrmex workers engaged in digging use soil temperature as a cue to decide where to excavate the nest. A bibliographic survey of nesting habits of 21 South American Acromyrmex species indicated that nesting habits are correlated with the soil temperature regimes: the warmer the soil at the nesting site, the higher the number of species inhabiting subterranean nests, as compared to superficial nests. For those species showing nesting plasticity, subterranean nests occurred in hot soils, and superficial nests in cold ones. Experimental results indicated that Acromyrmex lundi workers use soil temperature as an orientation cue to decide where to start digging, and respond to rising and falling soil temperatures by moving to alternative digging places, or by stopping digging, respectively. The soil temperature range preferred for digging, between 20°C and maximally 30.6°C, matched the range at which colony growth would be maximized. It is suggested that temperature-sensitive digging guides digging workers towards their preferred range of soil temperature. Workers’ thermopreferences lead to a concentration of digging activity at the soil layers where the preferred range occurs, and therefore, to the construction of superficial nests in cold soils, and subterranean ones in hot soils. The adaptive value of the temperature-related nesting habits, and the temperature-sensitive digging, is further discussed.     

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.     

Quorum sensing by encounter rates in the ant Temnothorax albipennis

Emigrating colonies of the ant Temnothorax (formerly Leptothorax)albipennis can choose the best of several nest sites, even whenthe active ants organizing the move do not compare sites. Thiscollective ability depends on a quorum rule used by ants assessinga candidate site. Only when the site's population has surpasseda threshold do they switch from slow recruitment of fellow activeants by tandem runs to rapid transport of the majority of thecolony. Here, I show that ants perceive the achievement of aquorum through their rate of direct encounters with nest matesat the site. When ants in a crowded site were prevented fromtactile contact with nest mates, they recruited by tandem runs,as though to an empty nest. Furthermore, when the encounterrate was raised independent of population, by reducing the sizeof the candidate nest, ants started to transport at a significantlylower population. The switch occurred at the same encounterrate regardless of nest size, whether the rate was measuredas the mean over the entire visit or as the inverse of the latencyuntil the first encounter. Because encounter rate reflects thedensity of nest mates and thus varies with nest size as wellas population, the ants' collective decision-making algorithmmay be robust to the exact population at which the switch totransport occurs. Ants cease monitoring quorum presence afterswitching to transport, coincident with an abrupt shorteningof visit duration by approximately 2 min, which may be interpretedas the time required for quorum detection.     

Spatial organization in a dimorphic ant: caste specificity of clustering patterns and area marking

Living in groups constitutes the root of social organizationin animals. Likewise, the spatial aggregation between membersof insects societies plays a crucial role in social cohesionand division of labor, namely, in polymorphic ant species. Inthe present paper, we show caste-specific aggregation patternsin the strictly dimorphic Pheidole pallidula ant species. Weinvestigate the influence on the clustering of ants exertedby direct contacts between nest mates as well as by indirectcues through chemical marking. In a homogeneous environmentdeprived of chemical cues, majors show a higher aggregationlevel than minors and a centripetal behavior. By contrast, minorsare more scattered in the experimental arena and display a centrifugalbehavior. In addition, area marking laid by minors enhancestheir own aggregative behavior while contributing to the localizationof the spontaneously aggregating majors. Such differences inaggregative patterns as well as their adaptive value have tobe coupled with the mobility level and the task performanceefficiency of each worker caste. Contrary to majors that arelikely to aggregate, highly mobile minors, scattered insideand outside the nest colony, can detect colony needs and cancarry out most of the daily tasks for which they are more efficientthan majors.     

Respective contributions of leader and trail during recruitment to food inTetramorium bicarinatum (Hymenoptera: Formicidae)

Summary This paper describes the food recruitment strategy of the antTetramorium bicarinatum, at both the individual and collective levels. The general organisation of recruitment used by this species during the exploitation of sucrose solutions shows similarities with group recruitment described for other species. However, our experiments demonstrate that inT. bicarinatum, the first trail laid by a recruiter during its return trip to the nest is more efficient than in other species using group recruitment, for exampleTetramorium impurum. Moreover, the efficiency of the first trail of aT. bicarinatum recruiter is comparable with that ofTapinoma erraticum, a species that uses mass recruitment. Despite the efficiency of the trail, choice experiments show that the recruited workers prefer to follow the leader rather than the first trail, suggesting the emission of a more attractive signal by the leader on its way back to the food. The function of the leader in this strategy is discussed in terms of collective decisions.     

Simple rules based on pile slope are used in the self organization of sand pile formation by <Emphasis Type="Italic">Pheidole oxyops</Emphasis> ants

We tested the hypothesis that slope influences where worker ants deposit excavated soil on piles near the nest entrance. We predicted that ants will deposit their load near the top of a pile where the slope changes from upward to downward, to prevent material rolling back towards the entrance. We tested this hypothesis by studying five natural colonies of Pheidole oxyops ants at a field site at S?o Sim?o, Brazil. At this site, each colony was dumping sandy soil excavated from its underground nest in a crescent-shaped pile c. 13 cm from and perpendicular to the nest entrance. Each nest was given an experimental sand pile of symmetrical curved cross section on a plywood platform that could be tilted 15 degrees up or down. From videos, the locations where individual ants dumped their soil loads were measured in relation to the inner (position = 0) and outer (position = 1) edges of the pile. When the platform was tilted down the ants deposited their loads significantly closer to the inner edge (0.458 ± 0.007) than when not tilted (0.530 ± 0.006). When the platform was tilted up the ants deposited their loads significantly further from the inner edge (0.626 ± 0.006) than when not tilted (0.522 ± 0.006). These results support the hypothesis that ants use pile slope in deciding where to dump their load. A similar rule is probably used in other ant species that place excavated soil in steep piles near the nest entrance. Received 5 February 2007; revised 10 June 2007; accepted 9 October 2007.     

The Role of the Vibration Signal during Nest‐Site Selection by Honey Bee Swarms

The vibration signal may influence nest‐site selection by honey bee swarms by enhancing scouting and recruitment. We investigated this hypothesis by comparing (1) the number of nest sites and the distances communicated by nest‐site dancers on swarms from which vibrators were and were not removed and (2) the behavior of scouts visiting higher‐quality (HQ) and lower‐quality (LQ) nest boxes. The removal of vibrators from swarms did not alter the number of nest sites investigated, the distances traveled to nest sites, or the time required to select a new nest cavity. In contrast, vibrator removal tripled the time required for swarms to achieve liftoff after a cavity had been selected, although all swarm eventually became airborne and moved to a new site. About 14% of the scouts that visited the HQ and LQ nest boxes performed vibration signals; however, nest‐box quality did not influence the tendency to produce the signal or intermix vibration signals and recruitment dances. However, we did find a significant, positive correlation between overall levels of vibration signal activity and nest‐site recruitment during the house‐hunting process. When viewed in concert, our results suggest that the vibration signal contributes to the house‐hunting process by operating in a non‐specific manner that may enhance scouting and recruitment in general during nest‐site selection and facilitate rapid swarm liftoff after a new nest site has been chosen. The vibration signal is therefore a component in the cascade of communication signals that orchestrate house‐hunting and colony relocation decisions.     

Recruitment in starved nests: the role of direct and indirect interactions between scouts and nestmates in the ant <Emphasis Type="Italic">Lasius niger</Emphasis>

In social insects, the foraging activity usually increases with the length of food deprivation. In Lasius niger, a mass-recruiting ant species, the foraging adjustment to the level of food deprivation is regulated by the scout that fed at the food source and by the response of the nestmates to signals performed by the scout inside the nest. In this study, we look at the role of these direct interactions (antennations or trophallaxes) and indirect interactions (pheromonal emission) and how they are influenced by the level of food deprivation. At the beginning of recruitment, the relative number of nestmates leaving the nest to forage increases with the level of deprivation. The nestmates’ propensity to exit the nest is not influenced by a previous trophallactic and/or antennal contact with a scout. Our results strongly suggest that the exit of nestmates is triggered by a chemical signal emitted by a scout. Deprivation lowers the response threshold of nestmates to this chemical signal resulting in a more important exit from the nest. Surprisingly, 27% of starved nestmates that receive food from the scout relay the information by depositing a chemical signal before having discovered and drunk the food source. Both phenomena boost the recruitment process. Though successful foragers returning to the nest have a significant role in the recruitment to the food source, we observed that the response of the nestmates inside the nest also greatly influence regulation of the foraging activity.     

Behavioral mechanisms of collective nest-site choice by the ant Temnothorax curvispinosus

This paper examines the individual behavior underlying collective choice among nest sites by the ant Temnothorax (formerly Leptothorax) curvispinosus. Colonies can actively compare options, rejecting a mediocre site when it is paired with a good one, but accepting the same mediocre design if it is instead paired with a worse site. This ability emerges from the behavior of an active minority of workers who organize emigrations. When one of these finds a promising site, she recruits nest mates to it, but only after a delay that varies inversely with site quality. Ants first recruit fellow active ants via slow tandem runs, but eventually switch to speedier transports of the colony’s passive majority. Later transports grow faster still, as ants improve their speed with experience. An ant’s choice of recruitment type is governed by a quorum rule, such that her likelihood of starting to transport increases with the population of the new site. The size of the quorum depends on experience, with ants demanding a larger population to launch immediately into transport than they do to switch to transport after first leading a few tandem runs. Perception of quorum attainment requires direct contact between ants. The ants’ behavior qualitatively matches that of T. albipennis, where models have shown that decentralized choice of the best site depends on quality-dependent recruitment delays, amplified by a quorum rule for initiating transport. Parameter estimates for an agent-based model show significant quantitative differences between the species, and suggest that T. albipennis may place relatively greater emphasis on emigration speed. Received 11 February 2005; revised 10 May 2005; accepted 20 May 2005.     

Recruitment and Other Communication Behavior in the Ponerine Ant Ectatomma ruidum

The ponerine ant Ectatomma ruidum, though previously reported to possess only rudimentary recruitment ability, was found to lay chemical trails for mass recruitment to rich or difficult food sources. The pheromone originates from the Dufour's gland, a new source of trail pheromones in the primitive ant subfamily Ponerinae. During nest emigrations, E. ruidum practices stereotyped social carrying in the myrmicine mode. The discovery of this form of social carrying and of a recruitment pheromone in the Dufour's gland secretions support the hypothesis that the subfamily Myrmicinae is derived from an ectatommine ancestor. Other communication behaviors exhibited by E. ruidum include exchange of liquid food carried between the mandibles, chemical alarm communication, nest entrance marking, and an additional social carrying posture previously unknown in ants.     

Nestmate recognition in the unicolonial ant Formica paralugubris

In unicolonial populations of ants, individuals can mix freelywithin large networks of nests that contain many queens. Ithas been proposed that the absence of aggression in unicolonialpopulations stems from a loss of nest mate recognition, butfew studies have tested this hypothesis. We investigated patternsof aggression and nest mate recognition in the unicolonial woodant, Formica paralugubris. Little aggression occurred, evenbetween workers from nests separated by up to 5 km. However,when aggression took place, it was directed toward non–nestmates rather than nest mates. Trophallaxis (exchange of liquidfood) occurred very frequently, and surprisingly, workers performedsignificantly more trophallaxis with non–nest mates thanwith nest mates (bias 2.4:1). Hence, workers are able to discriminatenest mates from non–nest mates. Higher rates of trophallaxisbetween non–nest mates may serve to homogenize the colonyodor or may be an appeasement mechanism. Trophallaxis rate andaggression level were not correlated with geographical distanceand did not differ within and between two populations separatedby several kilometers. Hence, these populations do not representdifferentiated supercolonies with clear-cut behavioral boundaries.Overall, the data demonstrate that unicoloniality can evolvedespite well-developed nest mate recognition. Reduced levelsof aggression might have been favored by the low rate of interactionswith foreign workers, high cost of erroneously rejecting nestmates, and low cost of accepting foreign workers.     

Seedling recruitment in a semi‐arid Patagonian steppe: Facilitative effects of refuse dumps of leaf‐cutting ants

Question: What is the influence of refuse dumps of leaf‐cutting ants on seedling recruitment under contrasting moisture conditions in a semi‐arid steppe? Location: Northwestern Patagonia, Argentina. Methods: In a greenhouse experiment, we monitored seedling recruitment in soil samples from refuse dumps of nests of the leaf‐cutting ant Acromyrmex lobicornis and non‐nest sites, under contrasting moisture conditions simulating wet and dry growing seasons. Results: The mean number of seedling species and individuals were higher in wet than in dry plots, and higher in refuse dump plots than in non‐nest soil plots. The positive effect of refuse dumps on seedling recruitment was greater under low moisture conditions. Both the accumulation of discarded seeds by leaf‐cutting ants and the passive trapping of blowing‐seeds seems not explain the increased number of seeds in refuse dumps. Conversely, refuse dumps have higher water retention capacity and nutrient content than adjacent non‐nest soils, allowing the recruitment of a greater number of species and individual seedlings. Conclusions: Nests of A. lobicornis may play an important role in plant recruitment in the study area, allowing a greater number of seedlings and species to be present, hence resulting in a more diverse community. Moreover, leaf‐cutting ant nests may function as nurse elements, generating safe sites that enhance the performance of neighbouring seedlings mainly during the driest, stressful periods.     

Record Dynamics in Ants

The success of social animals (including ourselves) can be attributed to efficiencies that arise from a division of labour. Many animal societies have a communal nest which certain individuals must leave to perform external tasks, for example foraging or patrolling. Staying at home to care for young or leaving to find food is one of the most fundamental divisions of labour. It is also often a choice between safety and danger. Here we explore the regulation of departures from ant nests. We consider the extreme situation in which no one returns and show experimentally that exiting decisions seem to be governed by fluctuating record signals and ant-ant interactions. A record signal is a new ‘high water mark’ in the history of a system. An ant exiting the nest only when the record signal reaches a level it has never perceived before could be a very effective mechanism to postpone, until the last possible moment, a potentially fatal decision. We also show that record dynamics may be involved in first exits by individually tagged ants even when their nest mates are allowed to re-enter the nest. So record dynamics may play a role in allocating individuals to tasks, both in emergencies and in everyday life. The dynamics of several complex but purely physical systems are also based on record signals but this is the first time they have been experimentally shown in a biological system.     

Chemical warfare between fungus-growing ants and their pathogens

Fungus-growing attine ants are under constant threat from fungal pathogens such as the specialized mycoparasite Escovopsis, which uses combined physical and chemical attack strategies to prey on the fungal gardens of the ants. In defence, some species assemble protective microbiomes on their exoskeletons that contain antimicrobial-producing Actinobacteria. Underlying this network of mutualistic and antagonistic interactions are an array of chemical signals. Escovopsis weberi produces the shearinine terpene-indole alkaloids, which affect ant behaviour, diketopiperazines to combat defensive bacteria, and other small molecules that inhibit the fungal cultivar. Pseudonocardia and Streptomyces mutualist bacteria produce depsipeptide and polyene macrolide antifungals active against Escovopsis spp. The ant nest metabolome is further complicated by competition between defensive bacteria, which produce antibacterials active against even closely related species.     

Wax on, wax off: nest soil facilitates indirect transfer of recognition cues between ant nestmates

Social animals use recognition cues to discriminate between group members and non-members. These recognition cues may be conceptualized as a label, which is compared to a neural representation of acceptable cue combinations termed the template. In ants and other social insects, the label consists of a waxy layer of colony-specific hydrocarbons on the body surface. Genetic and environmental differences between colony members may confound recognition and social cohesion, so many species perform behaviors that homogenize the odor label, such as mouth-to-mouth feeding and allogrooming. Here, we test for another mechanism of cue exchange: indirect transfer of cuticular hydrocarbons via the nest material. Using a combination of chemical analysis and behavioral experiments with Camponotus aethiops ants, we show that nest soil indirectly transfers hydrocarbons between ants and affects recognition behavior. We also found evidence that olfactory cues on the nest soil influence nestmate recognition, but this effect was not observed in all colonies. These results demonstrate that cuticular hydrocarbons deposited on the nest soil are important in creating uniformity in the odor label and may also contribute to the template.     

Nest mounds of red wood ants (Formicaaquilonia): hot spots for litter-dwelling earthworms

A previously undocumented association between earthworms and red wood ants (Formicaaquilonia Yarr.) was found during an investigation of the influence of wood ants on the distribution and abundance of soil animals in boreal forest soil. Ant nest mounds and the surrounding soil of the ant territories were sampled. The ant nest mound surface (the uppermost 5-cm layer) harboured a much more abundant earthworm community than the surrounding soil; the biomass of the earthworms was about 7 times higher in the nests than in the soil. Dendrodrilusrubidus dominated the earthworm community in the nests, while in soils Dendrobaenaoctaedra was more abundant. Favorable temperature, moisture and pH (Ca content), together with abundant food supply (microbes and decomposing litter) are likely to make a nest mound a preferred habitat for earthworms, provided that they are not preyed upon by the ants. We also conducted laboratory experiments to study antipredation mechanisms of earthworms against ants. The experiments showed that earthworms do not escape predation by avoiding contact with ants in their nests. The earthworm mucus repelled the ants, suggesting a chemical defence against predation. Earthworms probably prevent the nest mounds from becoming overgrown by moulds and fungi, indicating possible mutualistic relationships between the earthworms and the ants. Received: 21 November 1996 / Accepted: 3 April 1997     

Beyond temperature coupling: Effects of temperature on ectotherm signaling and mate choice and the implications for communication in multispecies assemblages

Many organisms share communication channels, generating complex signaling environments that increase the risk of signal interference. Variation in abiotic conditions, such as temperature, may further exacerbate signal interference, particularly in ectotherms. We tested the effects of temperature on the pulse rate of male signals in a community of Oecanthus tree crickets, and for one focal species we also assessed its effect on female pulse rate preferences and motivation to seek mates. We confirm prior findings of temperature‐dependent signals that result in increasing signal similarity at lower temperatures. Temperature also affected several aspects of female preferences: The preferred pulse rate value was temperature dependent, and nearly perfectly coupled with signal pulse rate; the range of pulse rate values that females found attractive also increased with temperature. By contrast, the motivation of females to perform phonotaxis was unaffected by temperature. Thus, at lower temperatures the signals of closely related species were more similar and females more discriminating. However, because signal similarity increased more strongly than female discrimination, signal interference and the likelihood of mismating may increase as temperatures drop. We suggest that a community approach will be useful for understanding the role of environmental variability in the evolution of communication systems.     

Tandem Recruitment and Trail Laying in the Ponerine Ant Diacamma rugosum: Signal Analysis1

During nest emigration, Diacamma rugosum scouts recruit female nestmates to the new nest site by tandem running. ♂♂, brood and prey objects are carried. The tandem leaders stimulate their nestmates to follow them by pushing or, more rarely, pulling them with the mandibles. The signals releasing the following behaviour are mainly chemical. Only 15–30% of motivated followers react positively to an odourless paper or wax dummy. The ♀♀ follow cut-off thoraces, legs and gasters plugged at the end or without hind gut and sting apparatus, just as well as a leading ant. The ants even follow a dummy consisting of body parts if they are not in direct mechanical contact with it. The leading effect of the body surface is not reduced by intensive cleaning with water or by extraction in solvents of different polarity. Only maceration in potash lye eliminates the leading effectiveness of the cuticula. None of the secretions of the tested abdominal glands release following behavior. Only the hind gut fluid possesses leading signal qualities. No distribution of faeces over the bodies of the leading ants can be observed before or during tandem running. The faecal pheromone responsible for this leading effect does not seep into the superficial layer of a leader dummy made from bee wax but it can be washed off with water. These results show that the actual signal releasing tandem following is an unspecific odour substance of the cuticula surface. The presence of the hind gut fluid is not necessary for the tandem following response. It may be an additional signal. During nest emigration the hind gut fluid is scattered patchily in the form of small droplets by the nest finders on their way to the new nest. No increased deposition of faecal droplets could be observed on the way to repeatedly visited rich food sources. The hind gut fluid serves as a long lasting trail substance and has an exclusively orienting effect. The secretions of the other abdominal glands are not trail pheromones. The nest entrance is marked colony-specifically.