How do ants assess food volume?

By comparing the behaviour of Lasius niger scouts at sucrose droplets of different volumes, we empirically identified the criterion used by each scout to assess the amount of food available as well as the rules governing its decision to lay a recruitment trail. When scouts discovered food volumes exceeding the capacity of their crop (3 or 6 μl), 90% immediately returned to the nest laying a recruitment trail. In contrast, when smaller food droplets (0.3, 0.7 or 1 μl) were offered, several scouts stayed on the foraging area, presumably exploring it for additional food. If unsuccessful, they returned to the nest without laying a trail. The droplet volume determined the percentage of trail-laying ants but had no influence on the intensity of marking when this was initiated. The key criterion that regulated the recruiting behaviour of scouts was their ability to ingest their own desired volume. This volume acted as a threshold triggering the trail-laying response of foragers. Collective regulation of foraging according to food size resulted from the interplay between the distribution of these desired volume thresholds among colony members and the food volume available. We relate some aspects of the foraging ecology of aphid-tending ants to this decision-making process. Copyright 2000 The Association for the Study of Animal Behaviour.     

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.     

Triggering and persistence of trail-laying in foragers of the ant Lasius niger

In the ant Lasius niger, the ability to ingest their own desired volume is the key criterion that rules the recruiting behaviour of scouts. This volume acts as a threshold triggering the trail-laying response of foragers. In this paper, we show that this desired volume is specific to each individual and is kept constant over successive trips to a food source. This individual specificity contrasts with the variability of all individual desired volumes within the colony. In this study, it is also shown that, among L. niger foragers, 14% never participate in the formation of the chemical pathway and never lay a trail over successive trips. Among the others foragers, interindividual differences in the persistence of trail-laying behaviour over successive trips are observed but do not rely on an individual specialisation, in which some ants would lay a trail more frequently and persistently than other scouts. We discuss how an individual in the foraging behaviour can play an essential role in the regulation of food retrieval dynamics.     

How food type and brood influence foraging decisions of Lasius niger scouts

We investigated how the type of food (sucrose or protein) and the presence of brood influence foraging decisions of Lasius niger L. scouts. In particular, we studied whether and how these parameters alter the drinking behaviour of scouts and the allocation of workers to food retrieving and recruiting tasks. We analysed drinking and recruiting behaviour of single scouts from nests with or without brood that encountered a proteinaceous or sucrose droplet. A substantial fraction of scouts encountering a proteinaceous droplet did not ingest it and did not then return to the nest whereas nearly all drank at sugar droplets; brood presence did not influence this decision. Once an ant started drinking, it needed to drink a critical volume before returning to the nest; this critical volume did not depend on the type of food and the presence of brood. Scouts laid a trail only if they returned to the colony. Food type and brood presence altered the proportion of individuals that laid a trail but not the individual trail-laying intensity. We discuss the consequences of this decision system through simple individual assessments and decision rules, with regard to the self-organized foraging patterns of this species and the efficient collective exploitation of natural resources.     

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.     

Individual flexibility and choice of foraging strategy in Polyrhachis laboriosa F. Smith (Hymenoptera, Formicidae)

Summary: We report in this study that the tree-dwelling African ant Polyrhachis laboriosa (Formicinae) uses different foraging strategies according to the size of the available food sources. We demonstrate that a recruitment behaviour can be induced with a 125 7l alimentary reward and that foraging remains solitary when rewards are smaller. Small rewards do not elicit trail-laying behaviour, and exploration behaviour is considerable. With large permanent food sources, scouts use group recruitment and there is less exploration around the reward. The choice of the foraging strategy is determined by the first forager, which modifies its behaviour according to the volume of the food supply. Independently of the size of the reward, the forager shows many exploratory displays during the first visit to the source, and contrary to most ants, it never lays a trail during its first return to the nest. Visual cues remain mainly used for individual orientation; information collected during the first trips are then transmitted to nestmates thanks to temporary trail laying behaviour.     

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.     

Influence of seed size and seed nature on recruitment in the polymorphic harvester ant Messor barbarus

In the polymorphic harvester ant Messor barbarus, the colony modulates response during foraging according to seed characteristics. The easiness of picking up small seeds (oat fragments or canaryseeds) shortens the time lapse between food discovery and the return of scouts to the nest, favouring the onset of recruitment and higher mobilisation rates than bigger seeds (whole oat). However, size is not the only criterion, as canaryseeds are preferred among small ones. A modulation of the trail laid by the first scouts also plays a role in the shaping of harvesting patterns and in the enhancement of recruitment towards small and/or preferred seed. This flexibility extends to the relative participation of the three worker sizes classes, which differs according to seed. Media are the most numerous at the foraging arena and largely responsible for trail-laying. Minor participate in seed harvesting and trail-laying, mostly when they can be efficient in carrying seeds, such as oat fragments. Major scarcely participate in harvesting and trail-laying, but are involved in the exploitation of bigger and/or preferred seed species. When faced with different seed species, M. barbarus seems thus to collectively adopt a “foraging time minimisation” rather than an “energetic gain per item harvested maximisation” strategy. Dynamics of foraging and division of labour between foragers of different sizes are related to both the workers’ ability to carry seed items, and the transfer of information to nestmates by trail-laying, all parameters that vary according to seed characteristics.     

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.     

Just follow your nose: homing by olfactory cues in ants

How is an ant-equipped with a brain that barely exceeds the size of a pinhead-capable of achieving navigational marvels? Even though evidences suggest that navigation is a multimodal process, ants heavily depend on olfactory cues-of pheromonal and non-pheromonal nature-for foraging and orientation. Recent studies have directed their attention to the efficiency of pheromone trail networks. Advances in neurophysiological techniques make it possible to investigate trail pheromone processing in the ant's brain. In addition to relying on pheromone odours, ants also make use of volatiles emanating from the nest surroundings. Deposited in the vicinity of the nest, these home-range markings help the ants to home after a foraging run. Furthermore, olfactory landmarks associated with the nest enhance ants' homing abilities.     

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.     

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.     

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.     

The self-organizing exploratory pattern of the argentine ant

Workers of the Argentine ant, Iridomyrmex humilis,start to explore a chemically unmarked territory randomly. As the exploratory front advances, other explorers are recruited and a trail extends from it to the nest. Whereas recruitment trails are generally constructed between two points, these exploratory trails have no fixed destination, and strongly resemble the foraging patterns of army ants. A minimal model shows how the exploratory pattern may be generated by the individual workers' simple trail-laying and -following behavior, illustrating how complex collective structures in insect colonies may be based on self-organization.     

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.     

Host-plant selection,diet diversity,and optimal foraging in a tropical leafcutting ant

Summary A month-long study was conducted on the comparative foraging behavior of 20 colonies of the leafcutting ant, Atta cephalotes L. in Santa Rosa National Park, Guanacaste Province, Costa Rica. The study was conducted during the middle of the wet season, when trees had mature foliage and the ants were maximally selective among species of potential host plants. The colonies always gathered leaves from more than a single tree species but on average one species constituted almost half the diet with the remaining species being of geometrically decreasing importance. Colonies exhibited greater diversity in their choice of leaves and lower constancy of foraging when the average quality of resource trees was lower, as predicted by elementary optimal foraging theory. Furthermore, the ants were more selective of the species they attacked at greater distances from the nest. However, the ants sometimes did not attack apparently palatable species, and often did not attack nearby individuals of species they were exploiting at greater distances.A classical explanation for why leafcutting ants exploit distant host trees when apparently equally good trees are nearer, is that the ants are pursuing a strategy of conserving resources to avoid long-term overgrazing pressure on nearby trees. We prefer a simpler hypothesis: (1) Trees of exploited species exhibit individual variation in the acceptability of their leaves to the ants. (2) The abundance of a species will generally increase with area and radial distance from the nest, so the probability that at least one tree of the species will be acceptable to the ants also increases with distance. (3) The ants forage using a system of trunk-trails cleared of leaf litter, which significantly reduces their travel time to previously discovered, high-quality resource trees (by a factor of 4- to 10-fold). (4) Foragers are unware of the total pool of resources available to the colony. Therefore once scouts have chanced upon a tree which is acceptable, the colony will concentrate on harvesting from that tree rather than searching for additional sources of leaves distant from the established trail.     

Context-dependent navigation in a collectively foraging species of ant, Messor cephalotes

More than 100 years of scientific research has provided evidence for sophisticated navigational mechanisms in social insects. One key role for navigation in ants is the orientation of workers between food sources and the nest. The focus of recent work has been restricted to navigation in individually foraging ant species, yet many species do not forage entirely independently, instead relying on collectively maintained information such as persistent trail networks and/or pheromones. Harvester ants use such networks, but additionally, foragers often search individually for food either side of trails. In the absence of a trail, these ‘off-trail’ foragers must navigate independently to relocate the trail and return to the nest. To investigate the strategies used by ants on and off the main trails, we conducted field experiments with a harvester ant species, Messor cephalotes, by transferring on-trail and off-trail foragers to an experimental arena. We employed custom-built software to track and analyse ant trajectories in the arena and to quantitatively compare behaviour. Our results indicate that foragers navigate using different cues depending on whether they are travelling on or off the main trails. We argue that navigation in collectively foraging ants deserves more attention due to the potential for behavioural flexibility arising from the relative complexity of journeys between food and the nest.     

Maintenance of foraging trails by the giant tropical antParaponera clavata (Insecta: Formicidae: Ponerinae)

Summary Establishment and maintenance of foraging trails to an artificial nectar source by ten colonies ofParaponera davata (Fabr.) in Panama is reported. The first forager to locate the artificial nectar source was responsible for recruiting additional foragers and for marking trails to orient these foragers. More than half of the trail marking was performed by the first two ants to mark the path back to the colony, although up to 11 ants per colony per hour marked trails. The number of trail marks and the number of marking ants decreased through time, presumably as foragers learned the location of the artificial nectar source. Four categories of recruits were noted: markers, foragers, patrollers, and visitors.