Directional fidelity as a foraging constraint in the western harvester ant,Pogonomyrmex occidentalis

Summary Understanding the foraging behavior of an animal is critically dependent upon knowledge of the constraints on that animal. In this study, I tested whether fidelity to foraging direction acts as a behavioral constraint to foraging western harvester ants, Pogonomyrmex occidentalis. Individual P. occidentalis foragers showed strong fidelity to foraging route and direction. Directional fidelity in this population was not related to trunk trail use, food specialization, colony activity levels, or mortality risks. Directional fidelity constrained individual foraging decisions; when colonies were offered seeds of different quality in 2 directions, individuals did not switch directions to obtain the energetically more rewarding seeds. Colony-level recruitment was increased for energetically more profitable seeds, indicating that colonial responses may compensate for the constraints of directional fidelity on individual foragers.     

Food and reproductive strategies of the myrmecophilous staphylinid beetleHomoeusa acuminata (aleocharinae) on the foraging trails of its host antLasius fuliginosus (Hymenoptera: Formicidae)

Summary The trail following behaviour of the adult myrmecophilous beetleHomoeusa acuminata on the foraging trails on the host antLasius fuliginosus was examined in June and all the observed behaviours were quantified. The beetle appears as a food robber, using the stowaway behaviour which consists of attaching itself to prey transported by ants to the nest. Foraging trails are also used by the beetles as a meeting place for mating. However, the beetle does not enter into the nest and appears as a symbiont poorly integrated in the social life of its host.     

Persistent Intercolonial Trunkroute-marking in the African Weaver Ant Oecophylla longinoda Latreille (Hymenoptera,Formicidae): Tom Thumb's versus Ariadne's Orienting Strategies

The territorial African weaver ant Oecophylla longinoda forages in a 3-dimensional system when attending honeydew secreted by coccid colonies. The orienting strategy allowing workers to reach a static food resource was studied in laboratory conditions. Contrary to other studies devoted to territoriality in O. longinoda, our experiments show that, when moving off the nest, territorial faecal marking by the major workers is not randomly placed all over the home-range area. The chemical trail leading from the nest to a given food site is reinforced by faecal materials both on the horizontal and the vertical planes. In addition to visual cues and chemical trails laid by the major workers (Holldobler & Wilson 1978), foragers use the territorial marking both to localize the food site and to come back to the nest. Thus, anal-drop deposition in O. longinoda also has a clear dual-purpose function: territorial and orientationai marking. The ecological value of such a discrete Tom Thumb's orienting strategy enables workers to quickly reach a food location even after more than 5 months of inactivity on this given site. The persistent marked trail also has an intercolonial effect. This could allow the keeping of exploitation of a definite food site by alien colonies of the same species by reducing time costs for new exploring phases or for learning new spatial cues when foraging on an unknown environment.     

Division of labor in post-emergence colonies of the primitively eusocial waspPolistes instabilis de Saussure (Hymenoptera: Vespidae)

Polyethism was quantified in post-emergence colonies of the primitively eusocial wasp,Polistes instabilis, and compared to polyethism in a sympatric advanced eusocial wasp,Polybia occidentalis. Like P.occidentalis, P. instabilis foragers collected food (nectar and prey) and nest materials (wood pulp and water).P. instabilis foragers showed some evidence of specialization with respect to which materials they gathered, but most foragers, divided their effort among food and nest materials, a pattern that is rarely seen inP. occidentalis. In colonies of both species, more foragers collected nectar than any other material; in contrast, most water foraging was performed by one or two workers. Upon returning to the nest,P. instabilis foragers gave up part or all of most nectar, prey, and pulp loads to nestmates, while water was rarely partitioned. Prey loads were most likely to be given up entirely.P. instabilis workers show evidence of conflict over the handling of materials at the nest. The frequency with which workers took portions of nectar loads from forgers was positively correlated with their frequency of aggressive dominant behavior, and with their frequency of taking other foraged materials. Compared to polyethism inP. occidentalis P. instabilis showed less individual specialization on foraging tasks and less partitioning of foraged materials with nestmates, suggesting that these characteristics of polyethism have been modified during the evolution of advanced insect societies.     

The significance of visual landmarks for navigation of the giant tropical ant,Paraponera clavata (Formicidae,Ponerinae)

Summary Workers of the giant tropical ant,Paraponera clavata, use trail pheromones for orientation and recruitment of nestmates. However, chemical markings may not always be sufficient for successful navigation in complex three-dimensional terrain, and additional orientation cues may be required. Behavioral field experiments were performed to investigate the significance of visual landmarks for homing foragers. Animals which were prevented from seeing the canopy were unable to navigate back to the nest, even though trail pheromones were still present. In contrast, foragers found their way back to the nest after their trail pheromones had been abolished but their visual scenes remained unchanged. This emphasizes the important role of visual landmarks during spatial orientation in homingP. clavata foragers. Individually foraging scouts were discovered in the understory of the forest floor up to 30 m away from their nest. They were rewarded, and displaced between 0.8 m and 13.6 m. Fifteen out of 16 animals had no difficulties in finding the nest entrance despite the altered appearance of local and distant landmarks at the release site. Apparently the scouts were able to recognize the visual scenes at the release site, and used them for reference to locate the nest entrance. In contrast, ants displaced from their nest to sites around 4 m away had more difficulties to re-find the nest.     

Trail laying behaviour during food recruitment in the antLasius niger (L.)

Summary The trail-laying behaviour of foragers of the antLasius niger was observed in the laboratory on a 20 cm bridge between the nest and the food source. We measured both the frequency of trail laying, as defined by the proportion of trips during which trail laying occurred, and its intensity, as defined by the number of marks laid during one bridge crossing.Foragers do not exhibit trail-laying behaviour until a food source is discovered. Trail laying then occurs more or less equally both to and from the nest, and both its frequency and intensity decrease as the recruitment proceeds. Foragers from very small colonies less than a year old appear to have quantitatively the same trail laying behaviour as those from older and much larger colonies.Groups of recruiters and recruits were individually marked. Their trail laying intensity was similar, both for trips to and from the nest, and for an ant's first, second, third and fourth trip. The frequency diminished rapidly with the number of trips made by each individual, and was 2–3 times higher for recruiters than for recruits, for trips both to and from the nest. Even though foragers stop marking after a variable number of passages, they continue to move between the nest and the food source, and other ants start marking. Different foragers appear to have widely different levels of trail laying, although we cannot say whether these differences are stable between different recruitments.Trail laying is strongly affected by the foragers' position on the bridge, especially for ants returning to the nest which lay up to five times more on the segment closest to the source than that closest to the nest. Foragers on a weakly marked trail appear to mark more than those on a well-marked trail. However, this effect is weak and could partly be attributed to their lower speed.Finally, a model using the experimental data gathered on the individuals' trail-laying behaviour reproduced satisfactorily the colony's overall trail laying.     

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.     

The allocation of foragers in red wood ants

Abstract. 1. We studied how colonies of the red wood ant, Formica polyctena , adjust the numbers of foragers allocated to different foraging trails. In a series of field experiments, foragers were marked and transferred from one nest to another, related nest, where they joined the foraging force. Transferred workers acted as a reserve of uncommitted, available foragers.
2. Previous work shows that each individual forager habitually uses one trail. We found that for an uncommitted forager, the influence of recruitment initially is stronger than that of directional fidelity. Transferred workers were likely to use trails leading to new food sources. When transferred to a new nest, foragers were not likely to use a trail in the same direction as their original trail in the donor nest.
3. After a week, transferred foragers tended to develop route fidelity. Even after bait was no longer present, they continued to use the trail that had formerly led to a bait source.
4. We examined how colonies adjust numbers on a trail by experimentally depleting some trails. Colonies usually did not compensate for depletion: foragers were not recruited to depleted trails.
5. In general, the dynamics of foraging in this species facilitate a consistent foraging effort rather than rapid adjustments of forager allocation.     

Memory of Location and Site Recognition in the Ant Formica uralensis (Hymenoptera: Formicidae)

Spatial recognition cues used in site fidelity in the ant Formica uralensis Ruzsky were studied using outdoor and laboratory arenas. Ant workers visiting symmetrically spaced feeders were colour-marked corresponding to the initial feeder visited during sampling. The effect of manipulating environmental cues on the mean 'spatial specialization' of the population was measured. Site recognition appears to be based on visual landmark/canopy cues. However, ants maintained some fidelity when shielded from these cues, suggesting the involvement of additional cues. When ridding our experimental device of olfactory deposits and shielding visual cues, site fidelity was lost. Idiothetic and/or geomagnetic cues are thought to provide spatial references to visual or olfactory landmarks. Altering nest position relative to the arena and changing the geomagnetic field within the arena in our study, however, did nothing to the site fidelity of visually deprived and non-deprived foragers.
We conclude that site fidelity is developed in a visually structured environment but supplemented by an olfactory backup system that is probably based on discrete home range markings rather than radial odour trails. We demonstrate furthermore that the visual component involved in site location can be stored in the memory of individual F. uralensis foragers during a 6-month hibernation period.     

Dominance order and resting site fidelity in the intertidal pulmonate limpetSiphonaria sirius (Pilsbry)

Consequences of ‘pushing contests’ between pairs of the intertidal pulmonate limpetSiphonaria sirius were analyzed, focusing on the relationship between dominance order and resting site fidelity. The pushing contests occurred near patches ofEnteromorpha sp., their favorite algal food resource. Three or four dominance ranks were determined from the consequences of the contests in different seasons. Individuals in lower dominance ranks tended to shift their resting sites significantly more than those in higher ranks. The dominance order of limpets is considered to play an important role in migration by its effect on resting site fidelity. Density dependent dispersal, from the lower to the upper part of limpets' habitats, was observed in late summer to winter when desiccation was not a problem and food resources became abundant in less populated areas. In late spring to summer, however, migration was directed downwards, from less to more populated areas, where desiccation pressure was relatively weaker. This migration is considered to have a survival advantage, and the behavior of the limpets may have masked density-dependent dispersal during the two seasons. Distribution of home site was also considered affected by aggressive social behavior during the two periods.     

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.     

Oviposition behavior of an ant-parasitizing fly,Neodohrniphora curvinervis (Diptera: Phoridae), and defense behavior by its leaf-cutting ant hostAtta cephalotes (Hymenoptera: Formicidae)

This study examines the oviposition behavior of the phorid parasitoid Neodohrniphora curvinervisand the antiparasitoid defense behavior of its leafcutting ant host Atta cephalotes. N. curvinervisfemales are diurnal sit- and- wait parasitoids that attack only outbound foragers of head width 1.6 mm or greater. Females deposit a single egg through the foramen magnum of each host successfully parasitized. Pursuit of hosts is usually initiated when an outbound forager of acceptable size passes by a parasitoid perch site. Individual foragers defend themselves against pursuing parasitoids by outrunning them along the foraging trail or by standing their ground and fending them off with their legs,antennae, and mandibles. At the colony level, susceptible foragers are protected against parasitism by a shift in the forager size distribution toward smaller unsusceptible sizes during the day when parasitoids are active and toward larger sizes at night when parasitoids are inactive. The frequency of parasitism of susceptible foragers was 15%, which is more than five times the frequency found in another system involving the phorid parasitoid Apocephalus attophilusand the leafcutting ant host Atta colombica.We offer several possible explanations for such differences in the frequency of parasitism and also examine reasons for the high incidence of superparasitism (19%) observed in the system studied.     

Aphid–ant mutualism: how honeydew sugars influence the behaviour of ant scouts

Honeydew is the keystone on which ant–aphid mutualism is built. The present study investigates how each sugar identified in Aphis fabae Scopoli honeydew acts upon the feeding and the laying of a recruitment trail by scouts of the aphid‐tending ant Lasius niger Linnaeus, and thus may enhance collective exploitation by the ant mutualists. The feeding preferences shown by L. niger for honeydew sugars are: melezitose = sucrose = raffinose > glucose = fructose > maltose = trehalose = melibiose = xylose. Although feeding is a prerequisite to the launching of trail recruitment, the reverse is not necessarily true: not all ingested sugar solutions elicit a trail‐laying behaviour among fed scouts. Trail mark laying is only triggered by raffinose, sucrose or melezitose, with the latter sugar being specific to honeydew. By comparing gustatory and recruitment responses of ant foragers to sugar food sources, the present study clarifies the role of honeydew composition both as a source of energy and as a mediator in ant–aphid interactions. Lasius niger feeding preferences can be related to the physiological suitability of each sugar (i.e. their detection by gustatory receptors as well as their ability to be digested and converted into energy). Regarding recruitment, the aphid‐synthesized oligosaccharide (melezitose) could be used by ant scouts as a cue indicative of a long‐lasting productive resource that is worthy of collective exploitation and defence against competitors or aphid predators.     

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.     

Factors affecting individual variation in resting site fidelity in the patellid limpet,Cellana toreuma (Reeve)

An investigation was made of individual variation in resting site fidelity in the limpet,Cellana toreuma, with special attention to the adaptiveness of its homing behaviour. Limpets resting in the higher part of the study site showed strong resting site fidelity while those in the middle part showed weaker fidelity and tended to shift their resting sites to higher parts. Limpets resting in the lower part where there were an abundant supply of food and high predation intensity, had the strongest resting site fidelity and always returned to fixed resting sites with mucous sheets. The resting site fidelity varied continuously among individuals. Four causes for shift of resting site were detected: fleeing from starfish,Coscinasterias acutispina, pushing contests for resting site, non-returning excursions following and without following other's trail. The frequency of shift of resting sites due to each of these causes varied among individuals. Estimation of energy budget during about half a month showed that the location of resting site and frequency of shift of the site considerably affected the growth of limpets through energy expenditure by respiration and stationary mucus secretion, and energy intake by foraging. These variations were proposed as being a result not of density dependent dispersal nor adaptation to a single factor such as desiccation, heavy wave impacts or predation, but being due to individual difference in a compromise between two conflicting factors, susceptibility to predation and accessibility to food resources.     

Spatial organization in ants’ nests: does starvation modify the aggregative behaviour of Lasius niger species?

Ant colonies that undergo long starvation periods have to tune their exploratory and foraging responses to face their food needs. Although the number of foragers is known to increase with food deprivation in the ant Lasius niger, such enhanced food exploitation is not related to a more intense recruitment by successful scouts. We thus suggest that the colony’s response to a food shortage could result from changes at the level of the ant recruits, in particular from changes in their spatial organization inside the nest. Since aggregation plays a key role in the social organization of ants, we assume that the colony’s response to starvation could be due to changes in the aggregative behaviour of L. niger nestmates.We thus compared the aggregation dynamics of inner-nest workers and foragers having undergone either a short or a long-lasting starvation period. Whatever the ethological group (foragers or inner-nest workers), there was no significant influence of starvation on the aggregation dynamics nor on any feature of the observed clusters. This result shows that an increased foraging response to food shortage cannot be explained by changes in the tendency of nestmates to aggregate within the nest. Finally, we discuss other behavioural mechanisms, in particular changes in behavioural thresholds that could underlie the adaptive changes seen in colony foraging after long starvation periods. Received 25 June 2007; revised 21 January 2008; accepted 24 January 2008.     

How patrollers set foraging direction in harvester ants

Recruitment to food or nest sites is well known in ants; the recruiting ants lay a chemical trail that other ants follow to the target site, or they walk with other ants to the target site. Here we report that a different process determines foraging direction in the harvester ant Pogonomyrmex barbatus. Each day, the colony chooses from among up to eight distinct foraging trails; colonies use different trails on different days. Here we show that the patrollers regulate the direction taken by foragers each day by depositing Dufour's secretions onto a sector of the nest mound about 20 cm long and leading to the beginning of a foraging trail. The patrollers do not recruit foragers all the way to food sources, which may be up to 20 m away. Fewer foragers traveled along a trail if patrollers had no access to the sector of the nest mound leading to that trail. Adding Dufour's gland extract to patroller-free sectors of the nest mound rescued foraging in that direction, while poison gland extract did not. We also found that in the absence of patrollers, most foragers used the direction they had used on the previous day. Thus, the colony's 30-50 patrollers act as gatekeepers for thousands of foragers and choose a foraging direction, but they do not recruit and lead foragers all the way to a food source.     

To be or not to be faithful: flexible fidelity to foraging trails in the leaf‐cutting ant Acromyrmex lobicornis

1. Ants using trails to forage have to select between two alternative routes at bifurcations, using two, potentially conflicting, sources of information to make their decision: individual experience to return to a previous successful foraging site (i.e. fidelity) and ant traffic. In the field, we investigated which of these two types of information individuals of the leaf‐cutting ant Acromyrmex lobicornis Emery use to decide which foraging route to take. 2. We measured the proportion of foraging ants returning to each trail of bifurcations the following day, and for 4–7 consecutive days. We then experimentally increased ant traffic on one trail of the bifurcation by adding additional food sources to examine the effect of increased ant traffic on the decision that ants make. 3. Binomial tests showed that for 62% of the trails, ant fidelity was relatively more important than ant traffic in deciding which bifurcation to follow, suggesting the importance of previous experience. 4. When information conflict was generated by experimentally increasing ant traffic along the trail with less foraging activity, most ants relied on ant traffic to decide. However, in 33% of these bifurcations, ants were still faithful to their trail. Thus, there is some degree of flexibility in the decisions that A. lobicornis make to access food resources. 5. This flexible fidelity results in individual variation in the response of workers to different levels of ant traffic, and allows the colony to simultaneously exploit both established and recently discovered food patches, aiding efficient food gathering.     

Influence of Illuminance and Forager Experience on Use of Orientation Cues in Social Wasps (Vespinae)

We tested the influence of illuminance and level of forager experience on nest orientation behavior of the social wasps Vespula vulgaris, Vespa crabro, and Dolichovespula saxonica in an artificial laboratory tunnel system. The number of wasps which oriented themselves chemically via a terrestrial trail or used visual orientation were determined at different illuminance levels for foragers which were naïve or experienced with the tunnel system. In V. vulgaris and D. saxonica, mainly the young and naïve foragers used the chemical trail for orientation in brightness. Experienced foragers used visual cues for nest orientation. In V. crabro, naïve and experienced foragers followed the chemical trail in a similar intensity. In darkness, when visual orientation was limited, the relative importance of the chemical trail increased dramatically in all species and all experience classes.     

Use of terrestrial chemical trails for nest orientation in an open nesting wasp, <Emphasis Type="Italic">Dolichovespula saxonica</Emphasis> F.

Summary The long-cheeked wasp Dolichovespula saxonica typically constructs exposed nests which can be reached by flying. Usually foragers do not walk on substrates in the close vicinity of the nests as cavity breeding wasps do (Steinmetz et al., 2002). Unexpectedly, when forced to walk outside the nest instead of flying in an artificial tunnel system, D. saxonica foragers lay a terrestrial trail and use it for orientation in the nest area in our experiments. 41% of the foragers followed the trail in a direction they were not accustomed to. We suggest that the foragers have employed the same orientation cues normally used for orientation in the close vicinity of the nest when approaching a free-hanging nest by flying, for example nest odour. Nest odour substances may have been transferred to the substrate as a trail as a consequence of foragers walking through the tunnels.Received 23 August 2002; revised 27 January 2003; accepted 30 April 2003.