Combined use of pheromone trails and visual landmarks by the common garden ant Lasius niger

This study investigated the relative importance of pheromone trails and visual landmarks on the ability of Lasius niger foragers to relocate a previously used food source. Colonies formed foraging trails to a 1-M sucrose feeder. Sections of this trail were then presented back to the same colony after variable time intervals. Individual outgoing foragers were observed to determine if they walked for 15 cm in the direction of the feeder or not. On newly established pheromone trails formed by 500 ant passages, 77% of the foragers walked in the correct direction vs 31% for control foragers (no trail pheromone). Pheromone trails decayed to the control levels in 20–24 h. Trails formed with fewer ant passages (125 or 30) decayed quicker. The use of visual landmarks was investigated by using trails with outgoing foragers from the colony that established the trail, either in the same room or in a different room, with different visual landmarks, to that used during trail establishment. Approximately 20% more ants walked in the correct direction in the same room vs the different room. This difference decreased to around 10% 2 h after trail establishment, indicating that the ants in the different room were learning the new visual cues to navigate by. Our results show that visual landmarks and pheromone trails are approximately equally useful in initially guiding L. niger foragers to food locations and that these two information sources have a complementary function.     

Decision making in ant foragers (<Emphasis Type="Italic">Lasius niger</Emphasis>) facing conflicting private and social information

Foragers of many ant species use pheromone trails to guide nestmates to food sources. During foraging, individual workers can also learn the route to a food source. Foragers of the mass-recruiting ant Lasius niger use both pheromone trails and memory to locate a food source. As a result, an experienced forager can have a conflict between social information (trail pheromones) and private information (route memory) at trail bifurcations. We tested decision making in L. niger foragers facing such an informational conflict in situations where both the strength of the pheromone trail and the number of previous visits to the food source varied. Foragers quickly learned the branch at a T bifurcation that leads to a food source, with 74.6% choosing correctly after one previous visit and 95.3% after three visits. Pheromone trails had a weaker effect on choice behaviour of naïve ants, with only 61.6% and 70.2% choosing the branch that had been marked by one or 20 foragers versus an unmarked branch. When there was a conflict between private and social information, memory overrides pheromone after just one previous visit to a food source. Most ants, 82–100%, chose the branch where they had collected food during previous foraging trips, with the proportion depending on the number of previous trips (1 v. 3) but not on the strength of the pheromone trail (1 v. 20). In addition, the presence of a pheromone trail at one branch in a bifurcation had no effect on the time it took an experienced ant to choose the correct branch (the branch without pheromone). These results suggest that private information (navigational memory) dominates over social information (chemical tail) in orientation decisions during foraging activities in experienced L. niger foragers.     

Learning,retention and coding of nest-associated visual cues by the Australian desert ant, <Emphasis Type="Italic">Melophorus bagoti</Emphasis>

A variety of social insects use visual cues for homing. In this study, we examine the possible factors affecting the learning and retention of nest-associated visual cues by the Australian desert ant Melophorus bagoti and the manner in which such cues are encoded by foraging ants. We placed four prominent cylindrical landmarks around a nest and trained foragers from that nest to a food source. Ants were tested with the landmark array in a distant testing field after (1) a known number of exposures to the landmarks (1, 3, 7 or 15 trials, spread over a period of 1 day, 2 days or ≥3 days) and (2) after a known period of delay (0, 24, 48, 96 or 192 h). The results show that a combination of an increase in training trials and an increase in number of training days affected the acquisition of landmark memory. Moreover, once the landmarks were learnt, they became a part of long-term memory and lasted throughout the ants’ foraging lifetime. To examine visual cue encoding behaviour, ants trained under similar conditions for 4 days were tested with (1) an identical landmark array, (2) landmarks of the same size used in training, but placed at twice the distance from each other, and (3) landmarks whose dimensions were doubled and placed at twice the distance from each other. In conditions (1) and (3), the ants searched extensively at the centre of the four landmarks, suggesting that, similar to the Saharan ant (genus Cataglyphis) and the honeybee, M. bagoti too uses a snapshot to match the view of the landmarks around the nest. But contrary to the snapshot model, in condition (2), the ants did not search extensively at the centre of the landmarks, but searched primarily 0.5 m from the landmark, the distance from each landmark to the nest during training. We discuss how various search models fare in accounting for these findings.     

Polyethism and the importance of context in the alarm reaction of the grass-cutting ant, Atta capiguara

Leaf-cutting ants exhibit an aggressive alarm response. Yet in most alarm reactions, not all of the ants encountering a disturbance will respond. This variability in behaviour was investigated using field colonies of Atta capiguara, a grass-cutting species. Crushed ant heads were applied near foraging trails to stimulate alarm reactions. We found that minor workers were disproportionately likely to respond. Only 34.7DŽ.8% of ants travelling along foraging trails were minor workers, but 82.1Lj.1% of ants that responded were minors. Workers transporting grass did not respond at all. The alarm response was strongest at the position and time where minors were most abundant. Ants were more likely to respond when they were travelling along trails with low rather than high traffic. Minor workers followed a meandering route along the trail, compared with the direct route taken by foragers. We argue that an important function of minor workers on foraging trails is to patrol the trail area for threats, and that they then play the key role in the alarm reaction.     

Energetic and time costs of foraging in harvester ants,Pogonomyrmex occidentalis

Summary Western harvester ants, Pogonomyrmex occidentalis, preferentially utilize low vegetational cover pathways. Energetic costs for foraging ants were less than 0.1% of caloric rewards of harvested seeds, suggesting that reduction of energetic cost is not a major benefit of this preference. Walking speed was significantly faster on lower cover routes, increasing net return rates from equidistant artificial food sources. Undisturbed foragers on low cover routes traveled farther, increasing their total foraging area without increasing foraging time. These results suggest that in animals with low costs of locomotion relative to energetic rewards, time costs are more important than direct energetic costs in influencing foraging decisions. In baited experiments with equidistant food sources, preferential use of low cover routes resulted in a large increase in net energetic gain rate, but only a slight increase in energetic efficiency. Under natural conditions, net energetic gain rates were constant for foragers using low and high vegetational cover routes, but foragers using low cover paths had lower efficiencies. This suggests that net energetic gain rate is a more important currency than energetic efficiency for foraging harvester ants.     

Flexible seed selection by individual harvester ants, Pogonomyrmex occidentalis

Summary Individual seed harvester ants (Pogonomyrmex) have been shown to specialize on specific seed types. We examined possible mechanisms for seed specialization and tested whether fidelity to food type limits the foraging decisions of individual western harvester ants, Pogonomyrmex occidentalis. The seed selection regimes of individually marked ants foraging at piles of two seed types were described and related to differences in seed quality and colonial dietary history. Individual foraging choices were affected by multiple factors, including seed caloric rewards, the previous seed selected, and the dietary history of the colony. Individual seed choices generally converged on the most energetically profitable species, suggesting that foragers exhibit labile preference. However, for a portion of the foragers, seed specialization was also partially due to constancy, defined as a tendency to select seed species that were previously collected. When colonies were presented with one seed type for 1 h and then were offered a mix of that seed and a novel seed type, individuals showed a strong preference for the novel seeds. Such rapid changes in seed preference argue strongly that individual P. occidentalis ants are highly flexible in seed choice and that resource assessment by these ants is more complex than simple maximization of net energetic return.Offprint requests to: J.H. Fewell at the current address     

Exploitation of carbohydrate food sources in <Emphasis Type="Italic">Polybia occidentalis</Emphasis>: social cues influence foraging decisions in swarm-founding wasps

An efficient exploitation of carbohydrate food sources would be beneficial for social wasp species that store nectar within their nest. In the swarm-founding polistine wasp Polybia occidentalis, we now demonstrate that the decisions of when and where to forage are influenced by information from conspecifics. Only when foragers had been trained to collect at artificial carbohydrate feeders did newcomers (food-source-naive individuals) continuously arrive at these feeders during 2 h of experiment. In control tests, in which no forager had been trained, not a single newcomer alighted at any of the offered carbohydrate food sources. This indicates that, during the foraging process, a nest-based input provided by successful foragers must have stimulated nestmates to search for food. Once activated, the newcomers’ choice on where to collect was strongly influenced by field-based social information. The mere visual presence of accumulated conspecifics (wasp dummies placed on one of the feeders) attracted newcomers to the food sources. Interestingly, however, visual enhancement was not the only decision-biasing factor at the feeding site. In an experimental series where searching wasps had to choose between the experimental feeder at which 3 foragers continuously collected and the control feeder with nine wasp dummies, only 40% of the wasps chose the visually enhanced feeder. This points to the existence of additional mechanisms of local enhancement. The possibility that, in social wasps, recruitment is involved in the exploitation of carbohydrate food sources is discussed.     

Blinkered teaching: tandem running by visually impaired ants

During tandem runs, one ant worker recruits another to an important resource. Here, we begin to investigate how dependent are tandem leaders and followers on visual cues by painting over their compound eyes to impair their vision. There are two ways in which Temnothorax albipennis might use vision during tandem running. First, the follower might track the movements of the leader by keeping it in sight. Our results suggest that the ants do not use vision in this way. For example, in all four classes of tandem run (those with either leader or follower, both, or neither of their participants with visual impairments) progress was most smooth at about 3 mm/s. This suggests that communication between leaders and followers during tandem runs is not based on vision and is purely tactile and pheromonal. Second, the leader and the follower might be using vision to navigate and our results support this possibility but also suggest that these ants have other methods of navigation. Ants with visual impairments were more likely to follow than to lead, but could occupy either role, even though they had many fully sighted nestmates. This might help to explain why the ants did not focus grooming on their most visually impaired nestmates. Wild-type tandem runs, with both participants fully sighted and presumably taking time to learn landmarks, were overall significantly slower, smoother, and a little less tortuous, than the other treatments. All four classes of tandem run significantly increased mean instantaneous speeds and mean absolute changes in instantaneous acceleration over their journeys. Moreover, tandems with sighted followers increased their speed with time more than the other treatments. In general, our findings suggest that eyesight is used for navigation during tandem running but that these ants also probably use other orientation systems during such recruitment and to learn how to get to new nest sites. Our results suggest that the ants’ methods of teaching and learning are very robust and flexible.     

Recency preference of odour memory retrieval in honeybees

Free-flying honeybees (Apis mellifera) were trained on tasks in which they had to choose one of three odours for a reward of sugar water. In acquisition, the bees learned this task in about five trials of training. Unrewarded retention tests showed that the odour memory was retained after 24-h delay. These findings are unsurprising. Integration experiments were then performed in which the bees had to learn two successive tasks of odour discrimination with conflicting demands. In task 1 (20 trials), one of three odours provided sugar water while the other two provided tap water. In task 2 (ten trials), which followed task 1 immediately, a different odour provided the reward. The bees were given unrewarded tests immediately after training on task 2 and then tested again after 10 min, 22 h or 24 h. The 22-h delay coincided with the circadian time for the start of task 1 training, while the 24 h coincided with the circadian time for the end of the task 2 training. Bees strongly preferred the rewarded odour for task 2 on immediate testing and after a 10-min delay. After delays of 22 and 24 h, they still preferred the rewarded odour for task 2. We conclude that the most recently acquired odour memory dominates behaviour in honeybees. The close association between floral odour and reward availability under natural circumstances may predispose honeybees to rely more on the most recently rewarded odour cue rather than on circadian time.     

Increased grooming after repeated brood care provides sanitary benefits in a clonal ant

Repeated pathogen exposure is a common threat in colonies of social insects, posing selection pressures on colony members to respond with improved disease-defense performance. We here tested whether experience gained by repeated tending of low-level fungus-exposed (Metarhizium robertsii) larvae may alter the performance of sanitary brood care in the clonal ant, Platythyrea punctata. We trained ants individually over nine consecutive trials to either sham-treated or fungus-exposed larvae. We then compared the larval grooming behavior of naive and trained ants and measured how effectively they removed infectious fungal conidiospores from the fungus-exposed larvae. We found that the ants changed the duration of larval grooming in response to both, larval treatment and their level of experience: (1) sham-treated larvae received longer grooming than the fungus-exposed larvae and (2) trained ants performed less self-grooming but longer larval grooming than naive ants, which was true for both, ants trained to fungus-exposed and also to sham-treated larvae. Ants that groomed the fungus-exposed larvae for longer periods removed a higher number of fungal conidiospores from the surface of the fungus-exposed larvae. As experienced ants performed longer larval grooming, they were more effective in fungal removal, thus making them better caretakers under pathogen attack of the colony. By studying this clonal ant, we can thus conclude that even in the absence of genetic variation between colony members, differences in experience levels of brood care may affect performance of sanitary brood care in social insects.     

Preferences and tradeoffs in nectar temperature and nectar concentration in the Asian hive bee Apis cerana

Honey bee foragers need to asses and make trade-offs between a number of potentially conflicting floral attributes. Here, we investigate multi-attribute decision making in the eastern honey bee, Apis cerana, when foraging on food sources that varied in warmth and sucrose concentration. We show that foragers prefer warm (30 °C) sucrose solution over cool (10 °C) sucrose solution and concentrated (30 % w/w) sucrose solution over dilute (15 % w/w) sucrose solution. When we offered the preferred sucrose concentration (30 % w/w) at the less-preferred temperature (10 °C), and the less-preferred sucrose concentration (15 % w/w) at the preferred temperature (30 °C), foragers prioritized warmth by choosing the warmer, but lower concentration solution. When the temperature difference was less extreme, bees preferred more concentrated cooler syrup (30 % ww at 15 °C over 15 % 30 °C). However, the addition of a decoy item to the choice set had a significant effect on the bees' preferences. Our results highlight the critical importance of considering context effects when measuring the foraging preferences of animals.     

Paying for information: partial loads in central place foragers

Information about food sources can be crucial to the success of a foraging animal. We predict that this will influence foraging decisions by group-living foragers, which may sacrifice short-term foraging efficiency to collect information more frequently. This result emerges from a model of a central-place forager that can potentially receive information on newly available superior food sources at the central place. Such foragers are expected to return early from food sources, even with just partial loads, if information about the presence of sufficiently valuable food sources is likely to become available. Returning with an incomplete load implies that the forager is at that point not achieving the maximum possible food delivery rate. However, such partial loading can be more than compensated for by an earlier exploitation of a superior food source. Our model does not assume cooperative foraging and could thus be used to investigate this effect for any social central-place forager. We illustrate the approach using numerical calculations for honeybees and leafcutter ants, which do forage cooperatively. For these examples, however, our results indicate that reducing load confers minimal benefits in terms of receiving information. Moreover, the hypothesis that foragers reduce load to give information more quickly (rather than to receive it) fits empirical data from social insects better. Thus, we can conclude that in these two cases of social-insect foraging, efficient distribution of information by successful foragers may be more important than efficient collection of information by unsuccessful ones.     

Balancing between mutualism and exploitation: the symbiotic interaction between Lasius ants and aphids

Nests of Lasius niger (L.) ants were given varied food regimens to test whether their behaviour towards an aphid partner, Aphis fabae (Scop.), changed with alternative food supplies. Honeydew collection and predation on aphids were measured by video monitoring the movement of ants between their nest and an aphid aggregation. Data collected from the aphid aggregations enabled comparisons between remaining aphid biomass and between the tending intensities of the ants. I tested how ant behaviour was influenced by their access to alternative prey and sugar. The results showed that ants accepted a honey solution as a substitute for the honeydew produced by aphids. Ants not only attended their aphid partners, but also preyed on them. The average predation rate increased eightfold when ants were offered the alternative of sugar, whereas alternative prey had no significant effect. In contrast, ant-tending intensity decreased with alternative sugar whereas alternative prey elicited no effect.     

Effect of substrate roughness on load selection in the seed-harvester ant Messor barbarus L. (Hymenoptera, Formicidae)

We investigated the effect of substrate roughness on load selection in the seed-harvester ant Messor barbarus. Ants were forced to travel either on sand or on gravel to reach a seed patch containing seed fragments of different weights. We hypothesized that foragers travelling on a rough substrate could either increase their load as a result of the increased distance travelled (due to a more sinuous path and an increase in the vertical component of the path) or decrease their load because of the anticipated difficulty of moving with a heavy load on a rough surface. Our results were consistent with neither of these hypotheses: Load selection by ants did not depend on the roughness of the substrate encountered during their outbound trip. The main effect of substrate roughness was to slow down the progression of the ants and increase their probability of dropping or transferring heavy seeds on their way back to the nest, thus resulting in an overall reduction of the rate of seed return to the nest.     

Ontogenetic differences in chemical alarm cue production determine antipredator responses and learned predator recognition

How individuals assess, respond and subsequently learn from alarm cues is crucial to their survival and future fitness. Yet this information is not constant through time; many individuals are exposed to different predators throughout their life as they outgrow some predators or move to habitats containing different predators. To maximise overall fitness, individuals should discriminate between different cues and respond and learn from only those that are relevant to their current ontogenetic stage. We tested whether juvenile spiny chromis, Acanthochromis polyacanthus, could distinguish between chemical alarm cues from conspecific donors of different ontogenetic stages and whether the cue ontogenetic stage of the cue donor affected the efficacy of learning about predators. Juveniles displayed a significant antipredator response when conditioned with juvenile chemical alarm cues paired with predator odour but failed to respond when conditioned with predator odour paired with either adult alarm cues or with saltwater. Subsequently, individuals only recognised the predator odour alone as a threat when conditioned with juvenile alarm cues. This demonstrates that prey may be highly specific in how they use information from conspecific alarm cues, selectively responding to and learning from only those cues that are relevant to their developmental stage.     

The effect of colony size and starvation on food flow in the fire ant,Solenopsis invicta (Hymenoptera: Formicidae)

Summary Food-sharing experiments were performed with laboratory colonies of Solenopsis invicta containing 1000, 10,000, or 20,000 workers and starved for 0, 3, 7, or 14 days. The effect of these variables was measured on the uptake of radioactive sugar water (1 M) by 1% of the colony's workers and on the trophallactic flow of food from these foragers to the remainder of the colony.Patterns of food distribution in small colonies differed significantly from those in larger nests. In 1000-ant nests, small workers more frequently received food than large workers, but in bigger colonies the opposite occurred.Fire ants were adept at distributing sugar water, with food from a few workers rapidly reaching the majority of the colony as foragers donate their crop contents to groups of recipients and these recipients may themselves act as donors.Foragers respond to colony starvation by individually taking up more food and sharing this fluid with a greater proportion of nestmates. Even foragers from satiated colonies can retrieve at least small amounts of liquid.The forager's state of hunger plays an important role in regulating food distribution. In sugar-satiated nests, previously starved foragers are highly successful at passing on labelled sugar whereas prviously fed foragers are not.     

A stingless bee (Melipona panamica) indicates food location without using a scent trail

The study of location specification in recruitment communication by bees has focused on two dimensions: direction and distance from the nest. Yet the third dimension, height above ground, may be significant in the tall and dense forest habitats of stingless bees. Foragers of the stingless bee Scaptotrigona postica recruit to a specific three-dimensional location by laying a scent trail. Stingless bees in the genus Melipona are thought to have a more sophisticated recruitment system that communicates distance through sounds inside the nest and direction through pointing zig-zag flights outside the nest. However, prior research on Melipona has not examined height communication or even established that foragers can recruit newcomers to a specific location. We used identical paired feeders to investigate recruitment to food in M panamica on Barro Colorado Island, Panama. We trained foragers from an observation hive to one feeder and monitored both feeders for the subsequent arrival of newcomers. We changed the relative positions of the feeders to test for correct direction, distance, and canopy-level communication. A 40-m canopy tower located inside the forest enabled us to examine canopy-level communication. We found that M. panamica foragers can recruit to a specific (1) direction, (2) distance, and (3) canopy level. To test the possibility that foragers accomplish this by means of a scent trail, we placed the colony on one shore of a small cove and trained bees over 116 m of open water to a feeder located on the opposite shore. We also placed a second feeder on this shore, equidistant from the colony but 20 m from the first feeder. Significantly more newcomers consistently arrived at the feeder visited by the foragers. Thus foragers evidently do not need a scent trail to communicate direction. Inside the nest, a forager produces pulsed sounds while visibly vibrating her wings after returning from a good food source. She is attended by other bees who cluster and hold their antennae around her, following her as she rapidly spins clockwise and counterclockwise. Locational information may be encoded in this behavior. However, foragers may also directly lead newcomers to the food source. Further experiments are planned to test for such piloting and other communication mechanisms.     

Recruitment to food by the German yellowjacket, Vespula germanica

The hypothesis that Vespula germanica foragers can recruit nestmates to food resources was tested using a protocol that controlled for the biasing effects of social factors at the resource, including local enhancement and food-site marking substances. Foragers from an observation colony in the field were trained to visit a dish of scented corn syrup solution 15?m east of the nest. A second feeding station, 22?m northeast of the nest, offered incoming foragers a choice between food with the training scent and food with a control scent. Significantly more naive foragers arriving at that station chose the food with the training scent. We conclude that the German yellowjacket is able to recruit nestmates to carbohydrate food sources, and that recruits use food odor to locate the source of food being brought into the nest.