How does food distance influence foraging in the ant Lasius niger: the importance of home-range marking

We study the influence of food distance on the individual foraging behaviour of Lasius niger scouts and we investigate which cue they use to assess their distance from the nest and accordingly tune their recruiting behaviour. Globally, the number of U-turns made by scouts increases with distance resulting in longer travel times and duration of the foraging cycle. However, over familiar areas, home-range marking reduces the frequency and thereby the impact of U-turns on foraging times leading to a quicker exploitation of food sources than over unmarked set-ups. Regarding information transfer, the intensity of the recruitment trail reaching the nest decreases with increasing food distance for all set-ups and is even more reduced in the absence of home-range marking. Hence, the probability of a scout continuing to lay a trail changes along the homeward journey but in a different way according to home-range marking. Over unexplored setups, at a given distance from the food source, the percentage of returning trail-laying ants remains unchanged for all tested nest-feeder distances. Hence, the tuning of the trail recruiting signal by scouts was not influenced by an odometric estimate of the distance already travelled by the ants during their outward journey to the food. By contrast, over previously explored set-ups, a distance-related factor – that is the intensity of home-range marking – strongly influences their recruiting behaviour. In fact, over a home-range marked bridge, the probability of returning ants maintaining their trail-laying behaviour increases with decreasing food distance while the gradient of home-range marks even induces ants which have stopped laying a trail to resume this behaviour in the nest vicinity. We suggest that home-range marking laid passively by walking ants is a relevant cue for scouts to indirectly assess distance from the nest but also local activity level or foraging risks in order to adaptively tune trail recruitment and colony foraging dynamics. Received 13 July 2004; revised 26 January and 20 May 2005; accepted 2 July 2005.     

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.     

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.     

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.     

Ants move to improve: colonies of Leptothorax albipennis emigrate whenever they find a superior nest site

A high-quality home can be a major factor determining fitness. However, when house hunting becomes necessary, animals might often face a speed-versus-accuracy trade-off and therefore be unable to survey their environment extensively for the optimum site. We found that the ant Leptothorax albipennis was able to correct errors made in such a hurried decision by continuing to survey even after a colony had settled in a nest site. Colonies moved from intact undisturbed nests to another nest site whenever the new nest site presented a sufficient improvement in nest quality. Thus, scout ants must be able to judge and compare the quality of the new, empty nest site with the one currently inhabited by the colony. Emigrations from intact nests were initiated by high numbers of ants recruited by tandem runs. This evidence may explain how a small number of scouts can motivate an entire colony to move when there is no immediate need to do so. Compared with their behaviour in emigrations from destroyed nests, the ants favoured even more strongly accuracy over speed, because they waited for a larger number of scouts to agree on one site before starting the emigration. They could do this without increased risk because the rest of the colony remained safely in the old nest.     

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.     

Individual variation in parental workload and breeding productivity in female European starlings: is the effort worth it?

We analyzed individual variation in work load (nest visit rate) during chick‐rearing, and the consequences of this variation in terms of breeding productivity, in a highly synchronous breeder, the European starling (Sturnus vulgaris) focusing on female birds. There was marked (10‐ to 16‐fold) variation in total, female and male nest visit rates, among individuals, but individual variation in female nest visit rate was independent of environment (rainfall, temperature) and metrics of individual quality (laying date, clutch size, amount of male provisioning help), and was only weakly associated with chick demand (i.e., day 6 brood size). Female nest visit rate was independent of date and experimentally delayed birds provisioned at the same rate as peak‐nesting birds; supporting a lack of effect of date per se. Brood size at fledging was positively but weakly related to total nest visit rate (male + female), with >fivefold variation in nest visit rate for any given brood size, and in females brood size at fledging and chick mass at fledging were independent of female nest visit rate, that is, individual variation in workload was not associated with higher productivity. Nevertheless, nest visit rate in females was repeatable among consecutive days (6–8 posthatching), and between peak (first) and second broods, but not among years. Our data suggest that individual females behave as if committed to a certain level of parental care at the outset of their annual breeding attempt, but this varies among years, that is, behavior is not fixed throughout an individual's life but represents an annually variable decision. We suggest females are making predictable decisions about their workload during provisioning that maximizes their overall fitness based on an integration of information on their current environment (although these cues currently remain unidentified).     

Ant Colonies Prefer Infected over Uninfected Nest Sites

During colony relocation, the selection of a new nest involves exploration and assessment of potential sites followed by colony movement on the basis of a collective decision making process. Hygiene and pathogen load of the potential nest sites are factors worker scouts might evaluate, given the high risk of epidemics in group-living animals. Choosing nest sites free of pathogens is hypothesized to be highly efficient in invasive ants as each of their introduced populations is often an open network of nests exchanging individuals (unicolonial) with frequent relocation into new nest sites and low genetic diversity, likely making these species particularly vulnerable to parasites and diseases. We investigated the nest site preference of the invasive pharaoh ant, Monomorium pharaonis, through binary choice tests between three nest types: nests containing dead nestmates overgrown with sporulating mycelium of the entomopathogenic fungus Metarhizium brunneum (infected nests), nests containing nestmates killed by freezing (uninfected nests), and empty nests. In contrast to the expectation pharaoh ant colonies preferentially (84%) moved into the infected nest when presented with the choice of an infected and an uninfected nest. The ants had an intermediate preference for empty nests. Pharaoh ants display an overall preference for infected nests during colony relocation. While we cannot rule out that the ants are actually manipulated by the pathogen, we propose that this preference might be an adaptive strategy by the host to “immunize” the colony against future exposure to the same pathogenic fungus.     

Influence of colony associated factors on nest selection in an Indian queenless ant

1. Organisms face the difficult task of selecting an optimal new nest from the available options during relocation. Studies on honeybees and ants in their natural habitat indicate that scouts encounter multiple options that vary in their physical and biotic characteristics. 2. Architectural features, location, odour, and the presence of nest mates impact their choice of nest site selection. In order to examine the influence of diverse parameters on final nest site selection we conducted choice experiments on ants in the context of relocation. 3. After controlling for any influence by physical characteristics, we found that the presence of brood, adults, and colony odour acted as attractants with more colonies relocating into these new nests than expected by chance alone. In contrast, the presence of a reproductive female, or familiarity of location had no influence on the choice. New nests containing dead ants evoked cleaning responses from scouts, which may interfere with relocation into these nests. 4. Even although colonies consist of hundreds of adults and brood, colony integrity was maintained in 98.7% of colonies. Furthermore, we found that none of the eight studied colonies relocated when faced with minor flooding in their natural habitat, indicating that the cost of relocation is non‐trivial and that this species is capable of minor damage repairs. 5. These observations highlight the complexity of relocation in general, allow the characterisation of desirable nest attributes in this species, and highlight the need for similar exploration in other social insects.     

Searching for a new home--scouting behavior of honeybee swarms

Honeybee scouting, where individual bees search the environmentwithout prior information about the possible location of foodsources or nest sites, is notoriously difficult to study. Yet,understanding scouting behavior is important as it providesinsights into how social insects trade-off exploitation withexploration. The use of simulation models is an ideal way toinvestigate the possible mechanisms behind the regulation ofscouting at the group level as well as the ways in which theswarm searches its environment. We used an individual-basedsimulation model to study the scouting behavior of honeybeeswarms. In our model, we implemented a simple decision rulethat regulates the number of scouts: individual bees first attemptto find a dance to follow but become scouts if they fail todo so. We show that this rule neatly allows the swarm to adjustthe number of scouts depending on the quality of the nest sitesknown to the swarm. We further explored different search strategiesthat allow the swarm to select good-quality nest sites independentof their distance from the swarm. Assuming that it is costlyto move to a site that is far away, the best search strategywould be to give precedence to nearby sites while still allowingthe discovery of better sites at distances farther away.     

Nest‐Site Defense by Competing Honey Bee Swarms During House‐Hunting

Cavity‐nesting animals must often defend their homes against intruders, especially when the availability of suitable cavities is limited. Competition for nest sites is particularly strong when multiple groups of the same species migrate synchronously to found a new home. This may be the case for honey bees during the reproductive season, because neighboring colonies often cast swarms simultaneously, leading to potential competition for high‐quality nesting cavities. To test the idea that honey bee swarms may compete for and defend potential nest sites as they search for a new home, we observed pairs of artificial swarms that were house‐hunting concurrently. Workers from one swarm in each pair carried a gene influencing body color, so that the bees from the two swarms were easily distinguished. We set up a high‐quality nest box and waited for nest‐site scouts from each swarm to explore and recruit swarm mates to it. We recorded all the interactions between competing scouts at the nest box and found that when scouts from both swarms explored the box simultaneously they behaved agonistically toward bees from the other swarm. The level of aggression depended on the number of scouts from each swarm present at the nest box. When only one to three scouts from each swarm were at the box, they rarely fought. But when the scouts from one swarm outnumbered those from the other swarm (4–20 vs. one to three bees), those in the majority advertised their presence with a buzzing behavior at the entrance opening, and started mobbing and killing those in the minority. When one swarm gained clear control of the nest box (20+ vs. zero to one bees), some of its scouts guarded the box’s entrance, preventing entry by foreign scouts. Our study exemplifies how cavity‐nesting animals may compete for and defend suitable nesting sites.     

Quorum sensing by encounter rates in the ant Temnothorax albipennis

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

Ants use pheromone markings in emigrations to move closer to food-rich areas

Nest site quality can affect survival and reproduction, and thus many animals have evolved behaviors which facilitate nest site assessment and selection. Ants of the genus Temnothorax have been shown to include an array of nest site attributes when choosing such a site. Here, we show that they also include traits of the habitat surrounding nest sites. In particular, we found that during emigration, ants preferred to move to nests located close to a previously explored food-rich area. We also determined that scent markings played a role in this choice and that scouts and transporting ants may have tracked scent marks laid in foraging, and this behavior could have biased emigration toward nests located near previously foraged areas. These results indicate that pheromones play a bigger role in Temnothorax foraging and decision making in emigration than previously thought. Overall, this work provides new insights into the mechanisms involved in habitat selection in ants and contributes to our understanding of collective behavior in social insects in general.     

Not everything that counts can be counted: ants use multiple metrics for a single nest trait

There are claims in the literature that certain insects can count. We question the generality of these claims and suggest that summation rather than counting (sensu stricto) is a more likely explanation. We show that Temnothorax albipennis ant colonies can discriminate between potential nest sites with different numbers of entrances. However, our experiments suggest that the ants use ambient light levels within the nest cavity to assess the abundance of nest entrances rather than counting per se. Intriguingly, Weber's Law cannot explain the ants' inaccuracy. The ants also use a second metric, independent of light, to assess and discriminate against wide entrances. Thus, these ants use at least two metrics to evaluate one nest trait: the configuration of the portals to their potential homes.     

Task-related variation of postpharyngeal and cuticular hydrocarbon compositions in the ant Myrmicaria eumenoides

In the ant Myrmicaria eumenoides we investigated postpharyngeal and cuticular hydrocarbons. At eclosion the glands contained almost no hydrocarbons and there were no lipid inclusions in the glandular epithelium. During the first 3 weeks of adult life the amount of hydrocarbons in the gland increased until day 5, and then remained constant while the lipid content in the epithelium increased steadily. Intracolonial hydrocarbon compositions were not uniform. Compositions of post-pharyngeal and cuticular hydrocarbons in individual ants varied simultaneously, but in different manner depending on the tasks of the ant (brood-tenders, foragers, scouts). Variations on the cuticle were greater than in the gland, but they were strongly correlated. Independent of ants' age and task, cuticular hydrocarbon compositions were dominated by alkenes and alkadienes. Task-specific differences in cuticular compositions were mainly in the amount of alkenes (high in foragers) and alkadienes (high in brood-tenders). Variation of hydrocarbons was low in ants up to 10 weeks old. Thereafter, ants fell into two groups: (1) ants that did not change their hydrocarbons and remained in the nest, and (2) ants that changed their hydrocarbon compositions and became foragers. These results contribute to an ongoing discussion of the dynamic relationship between post-pharyngeal and cuticular hydrocarbons.     

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.     

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.     

The ‘truck‐driver’ effect in leaf‐cutting ants: how individual load influences the walking speed of nest‐mates

The foraging behaviour of social insects is highly flexible because it depends on the interplay between individual and collective decisions. In ants that use foraging trails, high ant flow may entail traffic problems if different workers vary widely in their walking speed. Slow ants carrying extra‐large loads in the leaf‐cutting ant Atta cephalotes L. (Hymenoptera: Formicidae) are characterized as ‘highly‐laden’ ants, and their effect on delaying other laden ants is analyzed. Highly‐laden ants carry loads that are 100% larger and show a 50% greater load‐carrying capacity (i.e. load size/body size) than ‘ordinary‐laden’ ants. Field manipulations reveal that these slow ants carrying extra‐large loads can reduce the walking speed of the laden ants behind them by up to 50%. Moreover, the percentage of highly‐laden ants decreases at high ant flow. Because the delaying effect of highly‐laden ants on nest‐mates is enhanced at high traffic levels, these results suggest that load size might be adjusted to reduce the negative effect on the rate of foraging input to the colony. Several causes have been proposed to explain why leaf‐cutting ants cut and carry leaf fragments of sizes below their individual capacities. The avoidance of delay in laden nest‐mates is suggested as another novel factor related to traffic flow that also might affect load size selection The results of the presennt study illustrate how leaf‐cutting ants are able to reduce their individual carrying performance to maximize the overall colony performance.     

Size matters: nest colonization patterns for twig‐nesting ants

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