Division of foraging labor in ants can mediate demands for food and safety

Solitary foragers can balance demands for food and safety by varying their relative use of foraging patches and their level of vigilance. Here, we investigate whether colonies of the ant, Formica perpilosa, can balance these demands by dividing labor among workers. We show that foragers collecting nectar in vegetation near their nest are smaller than are those collecting nectar at sites away from the nest. We then use performance tests to show that smaller workers are more likely to succumb to attack from conspecifics but feed on nectar more efficiently than larger workers, suggesting a size-related trade-off between risk susceptibility and harvesting ability. Because foragers that travel away from the nest are probably more likely to encounter ants from neighboring colonies, this trade-off could explain the benefits of dividing foraging labor among workers. In a laboratory experiment, we show that contact with aggressive workers results in an increase in the mean size of recruits to a foraging site: this increase was not the result of more large recruits, but rather because fewer smaller ants traveled to the site. These results suggest that workers particularly susceptible to risk avoid dangerous sites, and suggest that variation in worker size can allow colonies to exploit profitably both hazardous and resource-poor patches.Communicated by L. Sundström     

The organization of work in Polybia occidentalis: costs and benefits of specialization in a social wasp

Summary Nest construction, a complex social activity requiring the coordination of 3 tasks (Fig.2), was compared in large (<350 adults) and small (<50 adults) colonies of Polybia occidentalis. The 3 tasks—water foraging, pulp foraging, and building—are performed by 3 separate groups of workers (Fig.4). Of the 8 acts comprising the 3 tasks, 5 regularly involve the transfer of water or pulp from one worker to another on the nest.Small colonies required nearly twice as long (35.4 worker-min) as large colonies (20.1 workermin) to complete a unit amount of construction work. Behavioral acts involving material transfer among workers were responsible for most of the increase in small colonies. In other words, the waiting times experienced by material donors and recipients were greater in small colonies. In small colonies workers switched among the three tasks more frequently than in large colonies (Fig. 4). This was the result of more frequent switching by generalists (workers that performed 2 or 3 of the tasks), rather than by a decrease in the proportion of specialists (workers performing only 1 task type) (Fig. 3).The series-parallel system by which Polybia occidentalis organizes nest construction has a major advantage over the series operation of solitary wasps. Pulp foragers collect and carry loads that are 6.1 times as large as builders can work with at the nest, and water foragers bring in loads that appear to be limited only by crop capacity and that provide all the moisture necessary for the complete processing of 0.74 of a foraged pulp load. As a result P. occidentalis can collect and process a given amount of nest material using 2.6 times fewer foraging trips than would be required by the series system. This in turn means that P. occidentalis not only achieves an energy saving that probably more than offsets the increased costs of material handling at the nest, but it reduces the exposure of its foragers to predators in the field.     

Global optimization from suboptimal parts: foraging sensu lato by leaf-cutting ants

Central-place foraging theory has been unable to explain the load selection behavior of leaf-cutting ants (Atta spp., Attini: Formicidae). We suggest that this is due to incomplete consideration of the sequence of behaviors involved in resource acquisition by these ants. Unlike most central-place foragers, leaf-cutting ants do not return to their nests with food. Instead, the leaf fragments they gather must be processed within the nest to convert them to substrate for fungal gardens. We have shown previously that leaf fragment size affects the rate of distribution and processing of leaf tissue inside laboratory nests of Atta colombica. Including these tasks in the calculation of foraging rate may help explain load selection and other features of central-place foraging by Atta colonies. Here we develop a mathematical model of the complete sequence of external and internal tasks that lead to addition of substrate to fungal gardens. Using realistic parameter values, the leaf fragment sizes predicted to maximize a colony's rate of foraging in this broad sense correspond well with the mean fragment sizes actually collected by Atta colonies in the field. The optimal fragment size for global performance in the model is below the size that would maximize the delivery rate by above-ground foragers. The globally optimal size also fails to maximize the rate of either fragment distribution or fragment processing within the nest. Our results show how maximum collective performance of an ensemble of linked tasks may require behavior that would appear suboptimal in a piecemeal analysis of tasks.     

High social density increases foraging and scouting rates and induces polydomy in Temnothorax ants

Many organisms live in crowded groups where social density affects behavior and fitness. Social insects inhabit nests that contain many individuals where physical interactions facilitate information flow and organize collective behaviors such as foraging, colony defense, and nest emigration. Changes in nest space and intranidal crowding can alter social interactions and affect worker behavior. Here, I examined the effects of social density on foraging, scouting, and polydomy behavior in ant colonies—using the species Temnothorax rugatulus. First, I analyzed field colonies and determined that nest area scaled isometrically with colony mass—this indicates that nest area changes proportionally with colony size and suggests that ants actively control intranidal density. Second, laboratory experiments showed that colonies maintained under crowded conditions had greater foraging and scouting activities compared to the same colonies maintained at a lower density. Moreover, crowded colonies were significantly more likely to become polydomous. Polydomous colonies divided evenly based on mass between two nests but distributed fewer, heavier workers and brood to the new nests. Polydomous colonies also showed different foraging and scouting rates compared to the same colonies under monodomous conditions. Combined, the results indicate that social density is an important colony phenotype that affects individual and collective behavior in ants. I discuss the function of social density in affecting communication and the organization of labor in social insects and hypothesize that the collective management of social density is a group level adaptation in social insects.     

Manuscript in preparation for <Emphasis Type="Italic">Behavioral Ecology and Sociobiology</Emphasis> Bumble bee pollen foraging regulation: role of pollen quality,storage levels,and odor

The regulation of protein collection through pollen foraging plays an important role in pollination and in the life of bee colonies that adjust their foraging to natural variation in pollen protein quality and temporal availability. Bumble bees occupy a wide range of habitats from the Nearctic to the Tropics in which they play an important role as pollinators. However, little is known about how a bumble bee colony regulates pollen collection. We manipulated protein quality and colony pollen stores in lab-reared colonies of the native North American bumble bee, Bombus impatiens. We debut evidence that bumble bee colony foraging levels and pollen storage behavior are tuned to the protein quality (range tested: 17–30% protein by dry mass) of pollen collected by foragers and to the amount of stored pollen inside the colony. Pollen foraging levels (number of bees exiting the nest) significantly increased by 55%, and the frequency with which foragers stored pollen in pots significantly increased by 233% for pollen with higher compared to lower protein quality. The number of foragers exiting the nest significantly decreased (by 28%) when we added one pollen load equivalent each 5 min to already high intranidal pollen stores. In addition, pollen odor pumped into the nest is sufficient to increase the number of exiting foragers by 27%. Foragers directly inspected pollen pots at a constant rate over 24 h, presumably to assess pollen levels. Thus, pollen stores can act as an information center regulating colony-level foraging according to pollen protein quality and colony need. An erratum to this article can be found at     

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 South East Asian ponerine ant of the genus Leptogenys (Hym., Form.) with army ant life habits

Summary The emigration and raiding behavior of the SE Asian ponerine ant Leptogenys sp. 1, which resembles L. mutabilis, were observed in the field (Ulu Gombak, Malaysia). The ants formed monogynous colonies that consisted of up to 52 100 workers. The bivouac sites of this species were found in leaf litter, rotten logs, ground cavities, etc., and were rarely modified by the ants. The colonies stayed in these temporary nests for several hours to 10 days; afterwards, they moved to a new nest site. The emigration distances ranged from 5–58 m. Since nest changing takes place at irregular intervals, and pupae and larvae are always present in the nest relocations of Leptogenys sp. 1, the emigration behavior is not linked to a synchronized brood development. Leptogenys sp. 1 is a nocturnal forager; in our study, up to 42 600 workers participated in each raid. The ants move forward on a broad front; behind the swarm a fan-shaped network of foraging columns converges to form a main trunk trail. A new system of foraging trails is developed in each raid. The workers search for their prey collectively; they attack and retrieve the booty together. The diet of Leptogenys sp. 1 consists mainly of arthropods. Army ant behavior is characterized by (1) formation of large monogynous colonies, (2) frequent emigrations, and (3) mass raids in which all foraging activities are carried out collectively. Since Leptogenys sp. 1 performs these typical army ant behavior patterns, this species represents the army ant ecotype. However, this species differs considerably from army ant species that have synchronized broods and huge colonies with dichthadiiform queens.Dedicated to Professor Dr. M. Lindauer on the occasion of his 70th birthday     

Forager specialization and the control of nest repair in Polybia occidentalis Olivier (Hymenoptera : Vespidae)

We measured patterns of individual forager specialization and colony-wide rates of material input during periods of response to experimental nest damage and during control periods in three colonies of the tropical social wasp Polybia occidentalis.

RAPD markers suggest genotypic effects on forager specialization in a eusocial wasp

Genetic variability within insect societies may provide a mechanism for increasing behavioral diversity among workers, thereby augmenting colony efficiency or flexibility. In order to assess the possibility that division of labor has a genetic component in the eusocial wasp Polybia aequatorialis, I asked whether the genotypes of workers within colonies correlated with behavioral specialization. Workers specialized by foraging for one of the four materials (wood pulp, insect prey, nectar, or water) gathered by their colonies. I collected foragers on 2 days from each of three colonies and identified the material the foragers were carrying when collected. I produced random amplified polymorphic DNA (RAPD) markers from the genomic DNA of these foragers and estimated genotypic similarity of foragers based on sharing of variable RAPD marker bands. Contingency tests on 20 variable loci per colony showed statistically significant (P <0.05) biases in RAPD marker frequencies among forager types in the three colonies. Patterns of association of RAPD marker bands with specializations were constant in two colonies, but changed between collection days in one colony. RAPD marker biases suggest that division of labor among workers includes a genetic component in P. aequatorialis. Colony-level selection on variation in division of labor is a possible factor favoring the evolutionary maintenance of high genotypic variability (low relatedness) in epiponine wasp colonies and in other eusocial insects. Received: 18 July 1995/Accepted after revision: 1 October 1995     

Demography, demand, death, and the seasonal allocation of labor in the Florida harvester ant (Pogonomyrmex badius)

As a self-organizing entity, an ant colony must divide a limited number of workers among numerous competing functions. Adaptive patterns of labor allocation should vary with colony need across each annual cycle, but remain almost entirely undescribed in ants. Allocation to foraging in 55 field colonies of the Florida harvester ant (Pogonomyrmex badius) followed a consistent annual pattern over 4 years. Foragers preceded larvae in spring and peaked during maximal larval production in summer (0.37). In spring, proportion foraging increased due to an increase in forager number and reduction in colony size, and in late summer, it decreased as colony size increased through new worker birth and a loss of ～3 % of foragers per day. The removal of 50 % of the forager population revealed that, at the expense of larval survival, colonies did not draw workers from other castes to fill labor gaps. To determine if labor allocation was age specific, whole colonies were marked with cuticle color-specific wire belts and released, and each cohort's time to first foraging was noted. Workers that eclosed in summer alongside sexual alates darkened quickly and became foragers at ～43 days of age, whereas autumn-born workers required 200 or more days to do so. Following colony reproduction, these long-lived individuals foraged alongside short-lived, summer-born sisters during the next calendar year. Therefore, the large-scale, predictable patterns of labor allocation in P. badius appear to be driven by bimodal worker development rate and age structure, rather than worker responsiveness to changes in colony demand.     

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     

Colony nutritional status modulates worker responses to foraging recruitment pheromone in the bumblebee Bombus terrestris

Foraging activity in social insects should be regulated by colony nutritional status and food availability, such that both the emission of, and response to, recruitment signals depend on current conditions. Using fully automatic radio-frequency identification (RFID) technology to follow the foraging activity of tagged bumblebees (Bombus terrestris) during 16,000 foraging bouts, we tested whether the cue provided by stored food (the number of full honeypots) could modulate the response of workers to the recruitment pheromone signal. Artificial foraging pheromones were applied to colonies with varied levels of food reserves. The response to recruitment pheromones was stronger in colonies with low food, resulting in more workers becoming active and more foraging bouts being performed. In addition to previous reports showing that in colonies with low food successful foragers perform more excited runs during which they release recruitment pheromone and inactive workers are more prone to leave the nest following nectar influx, our results indicate that evolution has shaped a third pathway that modulates bumblebee foraging activity, thus preventing needless energy expenditure and exposure to risk when food stores are already high. This new feedback loop is intriguing since it involves context-dependent response to a signal. It highlights the integration of information from both forager-released pheromones (signal) and nutritional status (cue) that occurs within individual workers before making the decision to start foraging. Our results support the emerging view that responses to pheromones may be less hardwired than commonly acknowledged. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Encounter rate and task allocation in harvester ants

As conditions change, social insect colonies adjust the numbers of workers engaged in various tasks, such as foraging and nest work. This process of task allocation operates without central control; individuals respond to simple, local cues. This study investigates one such cue, the pattern of an ant's interactions with other workers. We examined how an ant's tendency to perform midden work, carrying objects to and sorting the refuse pile of the colony, is related to the recent history of the ant's brief antennal contacts, in laboratory colonies of the red harvester ant, Pogonomyrmex barbatus. The probability that an ant performed midden work was related to its recent interactions in two ways. First, the time an ant spent performing midden work was positively correlated with the number of midden workers that ant had met while it was away from the midden. Second, ants engaged in a task other than midden work were more likely to begin to do midden work when their rate of encounter per minute with midden workers was high. Cues based on interaction rate may enable ants to respond to changes in worker numbers even though ants cannot count or assess total numbers engaged in a task. Received: 1 July 1998 / Accepted: 15 November 1998     

The role of per-capita productivity in the evolution of small colony sizes in ants

The evolution of colony size in social insects is influenced by both extrinsic and colony-intrinsic factors. An important intrinsic trait, per-capita productivity, often declines in larger colonies. This pattern, known as Michener’s paradox, can limit the growth of insect societies. In this study, we first describe this problem, survey its occurrence across different ant species, and present a case study of eight cavity-dwelling ants with very small colony sizes. In these species, colonies might never reach sizes at which per-capita productivity decreases. However, in six out of the eight focal species, per-capita productivity did decline with increasing size, in accordance with other studies on per-capita productivity in ants. Several mechanisms, such as resource availability or nest-site limitation, may explain the decrease in per-capita productivity with increases in colony size in our focal species. In these central-place foragers, the individual foraging mode is expected to lead to an increase in travel time as colonies grow. We suggest that polydomy, the concomitant occupation of several nest sites, could serve as a potential strategy to overcome this limitation. Indeed, for one species, we show that polydomy can help to circumvent the reduction in productivity with increasing colony size, suggesting that limited resource availability causes the observed decrease in per-capita productivity. Finally, we discuss the influence of other factors, such as the nesting ecology and colony homeostasis, on the evolution of colony size in these cavity-dwelling ants.     

The migrating herdsman Dolichoderus (Diabolus) cuspidatus: an ant with a novel mode of life

Summary The Malayan ant Dolichoderus cuspidatus lives in obligatory symbiosis with the pseudococcid Malaicoccus formicarii and other species of the same genus. The assemblies, which may be encountered up to 25 m away from the nest, are constantly covered with a great number of worker ants who protect them and receive honeydew. In the event of heavy rain the workers from a dense protective cluster, clinging to each other on top of the mealybugs. Neither hunting behavior nor active search for protein sources was observed in D. cuspidatus, although dead insects were accepted as food. When not searching for new plants, the activity of the ants outside the colony is limited to visiting the mealybugs. During the night and parts of the day the ants stay in their nest. Ant colonies deprived of their mealybugs are not viable due to their dependence on the symbiosis and because of the competition of other ants. Antless M. formicarii are likewise not viable. The mealybugs are extremely polyphagous and feed on many different monocotylous and dicotylous angiosperms. They feed exclusively on the phloem sap of young plant parts which are rich in amino acids. Dolichoderus cuspidatus workers carry the mealybugs to such locations. During the picking up and carrying process both partners display typical behavioral patterns. The colonization of new feeding sites takes place in well organized mass processions. During the foundation or disintegration of large feeding complexes, provisional depots with waiting mealybugs and ants are set up. The pseudococcids are carried not only while shifting the feeding sites, but also whenever the colony leaves its former nesting site and especially when any kind of disturbance occurs. They are even carried about without any apparent external cause, which leads to the fact that, at all times of trail activity, on average more than 10% of all ants using the trails carry mealybugs. Mealybugs are also present within the nest, especially adult females which are viviparous and give birth to their offspring there. Censused colonies each consisted of over 10 000 workers, about 4000 larvae and pupae, more than 5000 mealybugs and one ergatoid queen. Male winged ants were observed in large numbers during the dry season (January–February) and during the rainy season (September–October). The colonies form typical clumplike bivouac nests consisting of clusters of workers clinging to each other, thereby covering the brood and the mealybugs. The nesting site is in no way altered by constructive measures and is mostly found close to the ground. The preferred nesting sites are clusters of leaves, and cavities in wood or soil, although a freely hanging bivouac between a few branches may be set up as well. As soon as the distance between the nest and the feeding site is too great the colony moves to the feeding site, whereby the brood and the mealybugs are carried along in a well organized manner. During such nest-moving the establishment of intermediate depots can be observed. A shift of nest sites can also be induced by disturbances or by a change in the microclimate in the vicinity of the nest. Colonies multiply by budding. The tropical rain forest continuously offers different sprouting plants, the utilization of which requires extreme mobility on the part of the consumer. The unique behavioral strategy of D. cuspidatus, to carry constantly their polyphagous mealybug partners to new feeding sites and to take the whole colony there has enabled this ant and its symbiont to occupy this rich food niche. Dolichoderus cuspidatus is the first true nomad found in ants.     

Social foraging in stingless bees: how colonies of Melipona fasciata choose among nectar sources

In an experimental set-up, a colony of the stingless bee Melipona fasciata demonstrated its ability to choose the better of two nectar sources. This colony pattern was a result of the following individual behavioural decisions: continue foraging, abandon the feeder, restart foraging and initiate foraging. Only very rarely did individuals switch from one feeder to the other. With the first combination of a rich (2.7 M) and a poor (0.8 M) feeder M. fasciata behaved differently from Apis mellifera. Recruitment occurred to both feeders and the poor feeder was not abandoned completely. When the poor feeder was set to 0.4 M, M. fasciata abandoned the poor feeder rapidly and allocated more foragers to the rich feeder. These patterns were similar to those reported for A. mellifera with the first combination of feeders. Over a sequence of 4 days, experienced bees increasingly determined the colony patterns, and the major function of communication between workers became the reactivation of experienced foragers. The foragers modulated their behaviour not only according to the profitability of the feeder, but also according to previous experience with profitability switches. Thus, experience and communication together regulated colony foraging behaviour. These findings and the results of studies with honeybees suggest that M. fasciata and honeybees use similar decision-making mechanisms and only partly different tools. Received: 21 December 1998 / Accepted: 5 January 1999     

Social context influences cue-mediated recruitment in an invasive social wasp

Social insects often serve as model systems for communication and recruitment studies, and yet, it remains controversial whether social vespid wasps can reliably communicate resource information to nestmates. In this study, I present empirical evidence that foraging strategies depend on the initial assessment of resource size and potential competition by foraging yellowjackets. The context dependent foraging behavior of Vespula pensylvanica provides a potential explanation for the inconsistent reports of the existence of recruitment communication in vespid wasps. Furthermore, life history traits may influence yellowjacket foraging behavior; annual V. pensylvanica colonies, whose foragers routinely patrol near the nest, exhibited increased bait visitation in response to the return of successful foragers, whereas perennial colonies did not. These behavioral disparities provide insight into how foraging strategies and search patterns may shift with colony size and longevity. In experiments that investigate the effects of visual cues of conspecifics and bait dispersion, foraging decisions corresponded with expectations of yellowjackets integrating resource quantity and access into a perception of demand. When resource competition could be assessed as high, V. pensylvanica foragers quickly exploited the bait closest to their colony regardless of occupation by other wasps; however, foragers preferred visiting unoccupied baits in situations where competition could be perceived as low. Moreover, a meta-analysis revealed that context-dependent, cue-mediated recruitment was widespread in Vespidae, where such foraging behaviors changed with habitat and the potential for resource competition. Such plastic foraging strategies may contribute to the invasion success of some vespid wasps.     

Limited flexibility in the temporal caste system of the honey bee

Caste theory predicts that social insect colonies are organized into stable groups of workers specialized on particular task sets. Alternative concepts of organization of work suggest that colonies are composed of extremely flexible workers able to perform any task as demand necessitates. I explored the flexibility of workers in temporal castes of the honey bee Apis mellifera by determining the ability of colonies to reorganize labor after a major demographic disturbance. I evaluated the flexibility of temporal castes by comparing the foraging rates of colonies having just lost their foragers with colonies having also lost their foragers but having been given a week to reorganize. The population sizes and contents of the colonies in each group were equalized and foraging rates were recorded for one week. Colonies given a weeks initial recovery time after the loss of their foragers were found to forage at significantly higher rates than those colonies given no initial recovery time. This result was consistent for nectar and pollen foraging. These results suggest that honeybee workers lack sufficient flexibility to reorganize labor without compromising foraging. This finding is consistent with the caste concept model of organization of work in insect societies.     

The function of polydomy: the ant <Emphasis Type="Italic">Crematogaster torosa</Emphasis> preferentially forms new nests near food sources and fortifies outstations

Many ant species are polydomous, forming multiple spatially segregated nests that exchange workers and brood. However, why polydomy occurs is still uncertain. We investigated whether colonies of Crematogaster torosa form new polydomous nests to better exploit temporally stable food resources. Specifically, we tested the effect of food presence or absence and distance on the likelihood that colonies would form a new nest. Because this species also forms little-known structures that house only workers without brood (outstations), we also compared the function of this structure with true nests. Laboratory-reared colonies were connected to a new foraging arena containing potential nest sites with or without food for 4 months. When food was present, most colonies formed polydomous nests nearby and the remainder formed outstations. When food was absent, the behavior of colonies differed significantly, frequently forming outstations but never polydomous nests. Distance had no effect on the type of structure formed, but when food was present, a larger proportion of the workforce moved shorter distances. Workers often fortified the entrances to both structures and used them for storage of dried insect tissue (“jerky”). In an investigation of spatial fidelity, we found that workers on the between-nest trail were associated with the original nest, whereas workers collecting food were more likely to be associated with the new nest or outstation. C. torosa appears to have a flexible colony structure, forming both outstations and polydomous nests. Polydomous nests in this species were associated with foraging and were only formed near food resources.