Resource Assessment, Recruitment Behavior, and Organization of Cooperative Prey Retrieval in the Ant Formica schaufussi (Hymenoptera: Formicidae)

Foragers of the ant Formica schaufussi recruit nestmates to large anthropod prey and cooperatively transport the prey to the nest. The size of the group of ants retrieving prey is significantly correlated with the prey mass at the point at which the retrieval group reaches the nest entrance. To understand the mechanism involved in this size matching process, the regulation of retrieval group size was investigated by examining the modulatory role of trail pheromones in recruitment communication and the behavioral processes that might adjust retrieval group size to prey mass. Laboratory studies of hindgut, poison, and Dufour's gland extracts showed that the contents of the hindgut, which was determined to be the source of trail pheromone, induced recruitment and orientation behavior in ants and regulated the recruitment response of ants in the absence of any other communication signal. However, chemical mass communication alone did not appear to explain the regulation of retrieval group size. Scout ants assess whether to collect prey individually or recruit nestmates to group-retrieve 100-, 200-, or 400-mg prey but did not vary group size in relation to either the prey mass or the presence of interspecific competitors once the decision to initiate group retrieval was made. The number of recruits leaving the nest was independent of these factors and first matched prey mass during prey transport, possibly through a process of differential individual response to immobile versus mobile prey items. Unpredictable factors such as prey resistance to movement and rapidly changing degrees of interspecific competition may preclude scouts from fine-tuning the retrieval group size before it reaches the prey.     

Foraging,recruitment and predation by Decamorium uelense (Sanstchi) (Formicidae: Myrmicinae) on termites in Southern Guinea Savanna,Nigeria

Summary The myrmicine ant Decamorium uelense is a predator of small termites which feed inside their food sources of roots and plant debris. The ant uses a specialized recruitment regime to secure its prey. A scout ant searches for foraging termites and returns to the nest where it recruits a column of 10–30 ants which attack and immobilize the termites. A mass recruitment phase is then instigated, with larger numbers of ants retrieving the prey. The major prey item is Microtermes (consumed at an annual rate of 632 termites m^-2), with other small termites accounting for 0.5% of the total predation. The annual predation of Microtermes in primary savanna woodland by D. uelense removes 74% of the standing population.     

Observation of group recruitment to prey in a primitive ponerine ant,Amblyopone sp. (reclinata group) (Hymenoptera: Formicidae)

Summary Group recruitment during foraging was observed in a primitive ponerine ant,Amblyopone sp. (reclinata group) under laboratory condition. Workers searched for prey singly; however, if a item of prey was stung by a worker, other ants joined the attack. After the prey became immobile, one of the workers laid a trail directly toward the nest. This scout worker recruited additional workers (between 3 and 33). They formed a single file procession to the point of prey capture, and cooperatively transported the prey. A scout worker could stimulate nest workers to leave the nest without direct contact, and the recruited workers could trace the trail without guidance by the scout worker. This is the first report of recruitment behavior during foraging in the primitive antAmblyopone.     

The Effect of Food Size and Dispersion Pattern on Retrieval Rate by the Argentine Ant, Linepithema humile (Hymenoptera: Formicidae)

Food acquisition in central-place foraging animals demands efficient detection and retrieval of resources. Most ant species rely on a mass recruitment foraging strategy, which requires that some potential foragers remain at the nest where they can be recruited to food once resources are found. Because this strategy reduces the number of workers initially looking for food, it may reduce the food detection rate while increasing the postdiscovery food retrieval rate. In previous studies this tradeoff has been analyzed by computer simulation and mathematical models. Both kinds of models show that food acquisition rate is greatly influenced by food distribution and resource patch size: as food is condensed into fewer patches, the maximal acquisition rate is achieved by a shift to fewer initial searchers and more potential recruits. In general, these models show that a mass recruitment strategy is most effective when resources are clumped. We tested this prediction in two experiments by letting laboratory colonies of the Argentine ant (Linepithema humile) forage for resources placed in different distributions. When all prey were small, retrieval rate increased with increasing resource patch size, in support of foraging models. When prey were large, however, the mass of prey returned to the colony over time was much lower than when prey were small and widely distributed. As more ants reached a large prey item, the distance the prey item was transported decreased due to a greater emphasis on feeding rather than transport. Because Argentine ants can transport more biomass externally than they can ingest, food retrieval that depends only on ingestion can depress the biomass retrieval rate. Thus, our results generally support theoretical foraging models, but we show how prey size, through differential prey-handling behavior, can produce an outcome greatly different from that predicted only on the distribution of resources.     

Group hunting in a ponerine ant,Leptogenys nitida Smith

Field observations on the emigration and foraging behaviours of the southern African ponerine ant, Leptogenys nitida, were undertaken at Mtunzini, Natal, South Africa. These colonies have a single ergatoid queen and 200–1000 workers. The nest sites are found in the leaf litter and these nests are moved frequently over distances ranging from 0.5 to 5 m. Leptogenys nitida is a diurnal predator of arthropods dwelling in the leaf litter. Up to 500 workers participate in each foraging trail, and are not led by definite scouts. Ants form clear trunk trails and fan out at various intervals to search for prey. The prey is searched for and retrieved cooperatively. From laboratory tests it was determined that ants will follow pygidial gland extracts, with the poison gland extract eliciting a limited response. The type of army ant behaviour observed in L. nitida seems to be different to that observed in other ponerine ants.     

Predatory behavior of a seed-eating ant:Brachyponera senaarensis

The great flexibility of the feeding strategies exhibited by the ponerine ant Brachyponera senaarensis (Mayr) allows it to exploit either seeds or animal prey items as food resources. Predation is generally limited to small prey and is very similar to scavenging behavior. In laboratory conditions, the predatory behavior of B. senaarensis is not different in structure from that known in other carnivorous ants species. The workers forage individually and return to the nest using a series of cues involving light, a chemical graduated marking system near the nest entrance, and memory. During nest-moving, recruitment by tandem running was observed. However, in colonies where the food supply is regular, workers that discover food do not recruit nestmates, but make repeated trips between the nest and the food source. On the contrary, in starved colonies, the introduction of prey may produce a massive exit of foragers, corresponding to a primitive form of mass recruitment similar to that observed in some other ant species.     

Recruitment Behavior During Foraging in the Neotropical Ant Gnamptogenys moelleri (Formicidae: Ponerinae): Does the Type of Food Matter?

Gnamptogenys moelleri nests in bromeliads and feeds on an array of food items, including dead and live animals, and nectar. Field data in Brazilian forests indicate that G. moelleri hunts solitarily, while retrieving is performed both by solitary workers for small items, or by a group of recruited workers for large items. This flexible foraging strategy was investigated in the laboratory through a series of experiments to assess the context in which recruitment is elicited. Three types of food were used: 50% honey solution, large insect prey, and cluster of small insects. For all food types the first encounter by a scout resulted in increased numbers of ants leaving the nest and finding the food in the arena. After finding liquid food or large prey, the forager returns to the nest and transmits information to nestmates about food location on the substrate. The successful scout repeatedly taps the sting on the ground, and recruited ants collectively retrieve the large insect to the nest. On the other hand, there is no transmission of information to nestmates about the location of small clumped prey, although the returning scout induces nestmates to leave the nest and hunt. Because foraging in G. moelleri is restricted mostly to the nest bromeliad, and small worker size (0.5 cm) precludes capturing large prey solitarily, recruitment behavior widens the spectrum of food items consumed by this ant species. Although recruitment behavior in ponerines has already been reported to vary with the type and size of a food source, this study also shows that the transmission of information about food location depends on the type of food found (large prey or liquid food versus cluster of small prey).     

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.     

Trail following and stowaway behaviour of the myrmecophilous staphylinid beetle,Homoeusa acuminata,during foraging trips of its hostLasius fuliginosus (Hymenoptera: Formicidae)

Summary The behaviour of adultHomoeusa acuminata on trails of its hostLasius fuliginosus was investigated both in the field and in the laboratory. The beetles were active from May to September, accurately following the foraging trails of their hosts up to 20 metres from the nest. Most of the time, they were ignored by the ants, but if attacked they raised their abdomen as a possible appeasement or defensive behaviour. On trails the beetles most probably act as food robbers, feeding on prey collected by ants. The following method, called stowaway behaviour, was used by the beetles: when a beetle encountered an ant carrying a prey back to the nest it jumped on the prey, probably feeding on it while being transported.Laboratory experiments on circular artificial trails demonstrated thatH. acuminata follows a water extract of hindguts of the ants, the source of the trail pheromone. Both beetles and ants responded to an artificial trail of 0.03 hindgut equivalent per cm, but the mean distance followed by the beetles was about twelve times higher than that covered by the ants themselves. In contrast, experiments with solutions of the six fatty acids reported as the active components of the trail pheromone showed that the beetles did not respond at all, and that the ants only respond to the fatty acids at a very high concentration.     

Hunting behaviour,prey capture and ant avoidance in the tropical social waspPolybia sevicea (Hymenoptera: Vespidae)

Summary Polybia sericea (Hymenoptera: Vespidae) prey foraging was studied by following individual foragers as they hunted in the field, by observing how wasps handled prey once they had captured it, and by observing wasps as they returned to the nest with prey. Wasps were most likely to forage for prey between 0700 and 1300 hours and between 1600 and 1700 hours. The prey foraging sequence consisted of the behaviours high flight, search, touch, land, groom, walk, bite and malaxate. Captured small prey were malaxated and carried to the nest. Wasps removed the gut from large prey and dragged the meat up a twig or grass stem. A load of the meat was then bitten off and malaxated; the remainder was cached while the wasp made an orientation flight and returned to the nest. The forager returned within minutes for the remainder of the prey. Experiments demonstrated that caching the prey remains above the ground rather than close to the ground, where the prey are generally captured, reduces the chance that the prey will be found and expropriated by ants.     

Food robbing in ants,a form of interference competition

Summary Food robbing is a special form of interference competition in ants. It has been frequently observed in Myrmecocystus mimicus, which waylay returning foragers of several Pogonomyrmex species at their nests and take insect prey, particularly termites, away from them. Myrmecocystus more successfully robs prey from P. desertorum and P. maricopa than from P. barbatus. Usually only those Pogonomyrmex nests are affected by Myrmecocystus prey robbing that are not farther away than 10 m from the Myrmecocystus nest. Some M. mimicus workers show a high specialization in prey robbing. Other cases of food robbing in ants are described.     

Solar Elevation Triggers Foraging Activity in a Thermophilic Ant

Nycthemeral rhythm is an important biological trait that allows animals to escape predation and competition and, conversely, to coincide with mutualists. Although laboratory studies have shown that the rhythm depends on both endogenous factors and cyclic environmental cues, the latter is often poorly understood, particularly in the wild. Because insects are mostly ectothermal organisms, their activity rhythm is often thought to depend directly on ground temperature. In Mediterranean habitats, Cataglyphis ants are well known for their unusual thermoresistance, allowing them to forage in summer at the central hours of the day when the ground reaches temperatures that are lethal to their competitors. However, we show that the rhythm of Cataglyphis floricola in south‐western Spain is governed by light cues rather than by temperature. First, variations in ant traffic at the nest entrance were better explained by solar elevation angle than by ground temperature on both seasonal and daily scales. Second, if ants waited for the ground to reach a threshold temperature to start their activity, we would expect similar temperatures regardless of the opening hour. However, we found a significant increase in ground temperature as opening hour got later in the day. Third, by using a simple experimental set‐up that increased the apparent solar elevation over the nest entrance, we provoked a delay of nest closure time. We discuss the relevance of these results with respect to the life history of Cataglyphis species and their possible consequences in relation to global warming.     

ANT DISPERSAL OF CALATHEA (MARANTACEAE) SEEDS BY CARNIVOROUS PONERINES (FORMICIDAE) IN A TROPICAL RAIN FOREST

Two tropical herbaceous plant species, Calathea microcephala and Calathea ovandensis, exhibit morphological adaptations for seed dispersal by ants. In the field, 21 ant species are attracted to the arils of the seeds. Previously known as predatory carnivores, the ponerine ants Odontomachus laticeps, O. minutus, Pachycondyla harpax, and P. apicalis carry the seeds toward their nests, behaving as though they were carrying prey. These ants remove the aril in the nest, and seeds without arils germinate more readily than seeds with arils. Ant foraging distances can account for seedling distributions. Myrmecochory may be far more common in tropical rain forests than has been previously suspected.     

Frequent colony orphaning triggers the production of replacement queens via worker thelytoky in a desert-dwelling ant

Although related, Cataglyphis floricola and Cataglyphis tartessica show very different responses to colony orphaning. In the laboratory, under queenless conditions, C. tartessica workers produced male offspring via arrhenotoky, while C. floricola workers produced female offspring, including new queens, via thelytoky. Both species have workers with active ovaries that produce trophic eggs. In the field, in the late spring, C. floricola colonies were more likely to be orphaned than were C. tartessica colonies, probably due to differences in how they performed fission, their colony foundation system. The combined action of these two features could explain the presence of thelytoky in C. floricola and its absence in C. tartessica.

     

Field study on the foraging characteristics of a ponerine ant,Hagensia havilandi Forel

Summary A field study of the foraging strategy used by the ponerine ant,Hagensia havilandi is reported. They have permanent nests in the leaf litter of coastal forests.H. havilandi is a diurnal forager and collects a variety of live and dead arthropods. These predatory ants exhibit individual foraging with no cooperation in the search for or retrieval of food items. Three colonies were observed and showed similar temporal and spatial foraging patterns. The paths of individual ants were followed and the results showed that the foragers exhibit area fidelity, and return to the nest via a direct route on finding on prey item. Several foragers did not return to the nest at dusk but returned the following morning. Occasionally a limited amount of tandem recruitment was displayed.     

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.     

Secondary seed dispersal by ants in Neotropical cerrado savanna: species‐specific effects on seeds and seedlings of Siparuna guianensis (Siparunaceae)

1. Most woody plant species in tropical habitats are primarily vertebrate‐dispersed, but interactions between ants and fallen seeds and fruits are frequent. This study assesses the species‐specific services provided by ants to fallen arillate seeds of Siparuna guianensis, a primarily bird‐dispersed tree in cerrado savanna. The questions of which species interact with fallen seeds, their relative contribution (versus vertebrates) to seed removal, and the potential effects on seedling establishment are investigated. 2. Seeds are removed in similar quantities in caged and control treatments, suggesting that ants are the main dispersers on the ground. Five ant species attended seeds. Pheidole megacephala (≈0.4 cm) cooperatively transported seeds, whereas the smaller Pheidole sp. removed the seed aril on spot. Large (> 1.0 cm) Odontomachus chelifer, Pachycondyla striata, and Ectatomma edentatum individually carried seeds up to 4 m. Bits of aril are fed to larvae and intact seeds are discarded near the nest entrance. 3. Overall, greater numbers of seedlings were recorded near ant nests than in control plots without nests. This effect, however, was only detected near P. megacephala and P. striata nests, where soil penetrability was greater compared with controls. Soil nutrients did not differ between paired plots. 4. This study confirms the prevalence of ant–seed interactions in cerrado and shows that ant‐derived benefits are species‐specific. Ant services range from seed cleaning on the spot to seed displacement promoting non‐random spatial seedling recruitment. Although seed dispersal distances by ants are likely to be shorter than those by birds, our study of S. guianensis shows that fine‐scale ant‐induced seed movements may ultimately enhance plant regeneration in cerrado.     

Defense of termitaria by termitophilous ants

Summary Mounds of Amitermes laurensis are frequently faided by meat ants Iridomyrmex sanguineus. Of eight ant species which often cohabit with the termites, Camponotus sp. B and C were considerably dependent on the termintaria for their nest sites and effectively protected it from the attacks by meat ants. Many termite colonies cohabiting with those two ant species were vigorous, suggesting that this ant-termite relationship is mutualistic; thus, the ants were provided nest sites and probably even food and the termites were protected from destructive natural enemies.     

Individual Flexibility and Tempo in the Ant, Pheidole dentata, the Influence of Group Size

This study investigates individual flexibility of foraging ants (Pheidole dentata) when the number of nestmates is altered by establishing broodless and queenless colony fragments all originating from a single big colony. Scouts from small groups (5 to 15 ants) behave like solitary foragers. They feed for long periods of time, they return slowly into the nest, and they recruit weakly. The ingested food is distributed by trophallaxis. Scouts from larger (20- to 30-ant) fragments forage more socially. Feeding and return times are short and recruitment is strong. Later the food is always transported into the nest. Two alternative mechanisms are discussed to explain the differences in individual foraging behavior. For the first—individual flexibility—assumptions have to be made about the capabilities of the individual, its work repertoire, and decision making outside the nest. The second mechanism takes into account that ants are capable of perceiving CO ₂ concentration differences and that ant groups are more active at higher CO ₂ concentrations. The organizational differences at the group level are explained simply by tempo differences in individual ants without making assumptions about individual capabilities.     

Prey weight and overwhelming difficulty impact the choice of retrieval strategy in the Neotropical ant <Emphasis Type="Italic">Gnamptogenys sulcata</Emphasis> (F. Smith)

By regulating both the choice between solitary or collective retrieval and the level of nestmate investment for heavy prey transport, individual hunters of the ant Gnamptogenys sulcata are able to increase the foraging efficiency of the whole society, despite the extremely small size of this elite group (1 –4 hunters per colony). Their predatory behavior changes according to the static (weight, size, shape) and dynamic (mobility, escape behaviors) characteristics of the prey. The behavioral sequences resemble those of other poneromorph ants, but palpation after the approach phase is absent, probably because of the swiftness of the attack. Hunting is always performed by solitary workers but, independently of prey type and mobility, small, light prey trigger solitary retrieval whereas large, heavy prey trigger collective retrieval. For intermediate prey weights (8.5 to 21 times the hunter’s weight), some variability in the strategy choice is encountered. Both static and dynamic factors are involved in assessing the difficulty of handling living prey as shown by the number of stings delivered by the hunter for prey immobilization. However, the lack of any stinging against already dead prey indicates that these factors must be uncoupled when selecting retrieval strategies. Prey weight has a major role in this choice. For a given prey size always triggering solitary retrieval, a two-fold increase in prey weight is sufficient to trigger collective transport. Conversely, for a given prey size always triggering collective retrieval, a decrease in prey weight can trigger solitary transport. Moreover, presenting hunters with prey of “infinite” weight triggers several waves of recruitment, so that the number of recruited workers compensates for the “apparent” prey weight. Despite the inability to finely match the number of recruits to the weight of retrieved prey, recruitment strategies adopted by G. sulcata may reflect an evolutionary primitive step towards the finely graded recruitment behavior observed in closely related poneromorph species. Received 5 February 2007; revised 25 May 2007; accepted 15 June 2007.