Allocation of Colony‐Level Foraging Effort in Vespula germanica in Response to Food Resource Quantity,Quality, and Associated Olfactory Cues

In social insects, selection takes place primarily at the level of the colony. Therefore, unlike solitary insects, social species are expected to forage at rates that maximize colony fitness rather than individual fitness. Workers can increase the net benefit of foraging by responding to increased resource availability, by responding more strongly to higher‐quality resources, and by decreasing the uncertainty with which nestmates find resources. Unlike many ants and social bees, no social wasp is known to utilize a nest‐based recruitment signal to inform nestmates of food location. On the other hand, wasps do learn the odor of food brought to the nest and use this cue to locate the food source outside the nest. Here, we quantify the effects of three food‐associated variables on the allocation of foraging effort in the yellowjacket Vespula germanica. We used an experimental approach to assess whether resource quantity, quality, or associated olfactory information affect the probability that a forager will leave the nest on a foraging trip. We addressed these questions by inserting a known amount of sucrose solution directly into nests and recording foraging effort (departure rate) over the subsequent hour‐long observation period. No differences were found in foraging effort because of the presence/absence of olfactory cues, but there was strong evidence that foraging effort increased in response to resource influx and resource quality. Thus, while olfactory cues are learned in the nest, only resource quality and the cue of increased amount of food in the nest factor into a forager's decision of whether or not to depart on a foraging trip. However, as prior work has shown, once a wasp forager leaves the nest, it uses the learned olfactory cues to aid in finding resources.     

Spatial foraging patterns and colony energy status in the African honey bee,Apis mellifera scutellata

The relationship between changes in foraging patterns (inferred from waggle dance activity) and colony energy status (inferred from brood rearing activity, food storage, and colony weight) was examined for the African honey bee during a period of relative resource abundance and resource dearth. When resources were more abundant mean foraging distances (about 400 m) and foraging areas (4–5 km²) were small, and colonies recruited to 12–19 different sites per day. Colony foraging ranges and sites visited increased slightly during the dearth period, yet foraging continued to be concentrated within less than 10 km². The degree to which fluctuations in foraging patterns were correlated with colony energy status varied with the availability of floral resources. During periods of relative forage abundance, increases in foraging range and number of sites visited were significantly correlated with increases in brood rearing and colony weight. In contrast, colonies examined during periods of resource dearth exhibited no correlations between foraging areas, foraging distances, and fluctuations in brood rearing, food storage, or colony weight. Thus, during dearth periods colonies may not be able to coordinate foraging patterns with changes in colony energy status.     

The effects of spatial food distribution and group size on foraging behaviour in a benthic fish

Animals foraging in heterogeneous environments benefit from information on local resource density because it allows allocation of foraging effort to rich patches. In foraging groups, this information may be obtained by individuals through sampling or by observing the foraging behaviour of group members. We studied the foraging behaviour of goldfish (Carassius auratus) groups feeding in pools on resources distributed in patches. First, we determined if goldfish use sampling information to distinguish between patches of different qualities, and if this allowed goldfish to benefit from a heterogeneous resource distribution. Then, we tested if group size affected the time dedicated to food searching and ultimately foraging success. The decision of goldfish to leave a patch was affected by whether or not they found food, indicating that goldfish use an assessment rule. Giving-up density was higher when resources were highly heterogeneous, but overall gain was not affected by resource distribution. We did not observe any foraging benefits of larger groups, which indicate that grouping behaviour was driven by risk dilution. In larger groups the proportion searching for food was lower, which suggests interactions among group members. We conclude that competition between group members affects individual investments in food searching by introducing the possibility for alternative strategies, such as scrounging or resource monopolisation.     

羊草无性系植物种群觅养生长格局与资源分配的研究

本文按照理论种群生态学的研究方法,从生长参数的形态功能变化和发育参数的时间变化入手,系统地研究了羊草无性系植物种群觅养生长格局与资源分配的基本规律。研究结果表明,羊草无性系种群适合于用ψ＝0.5、μ=1.0、Tr=7和Ts＝2的分配函数模型描述其觅养生长格局与资源分配。文中建立了大小—制约分配模型,并用此模型预测了与觅养生长格局有关的资源分配。这一研究深刻地揭示了较为动物种群搜索途径复杂得多的无性系植物种群的觅养行为。     

Foraging responses ofFormica truncorum (Hymenoptera; Formicidae); exploiting stable vs spatially and temporally variable resources

Summary Social organization allows a division of labor between reproduction and foraging, as well as a task allocation among foraging individuals. Therefore, a colony simultaneously exploiting various resources can use different ways of getting them. This work analyzes wood ant foraging tactics both at the collective and the individual level. The main issues are: (1) How does a wood ant colony respond to stable vs temporally and spatially variable resources in terms of worker force allocation? (2) How is retrieval of ephemeral but rich resources effected at the individual level? The results show a correspondence between resource stability and behavior. Ants visited stable resources continually and recruited to ephemeral ones. They also visited empty food sites at a low frequency suggesting territoriality and constant scanning. The results suggest that recruitment may involve defence of the resource, since only a small fraction of all the workers available carry the food home. Additional ants arriving at the resource are recruited from inside the nest, not by reallocating outdoor workers. The foragers also form two separate groups, one site-persistent and another site-flexible. These two forager groups may reflect specialization on two different kinds of diet: honeydew or insect prey (spatially and temporally stable vs unstable resources, respectively).Formica truncorum is thus an example of how sociality allows the use of two different foraging strategies simultaneously: one that is based on recruitment to ephemeral resources and another that is based on search persistance and memory.     

Dispersed central place foraging in Australian meat ants

Summary Australian meat ants often inhabit colonies with widely dispersed nest holes, and this study examines how resource is harvested and distributed in a colony ofIridomyrmex sanguineus Smith (Formicidae: Dolichoderinae). The three principal types of foragers (tenders, honeydew transporters, scavengers) exhibited nest hole fidelity, where harvested resource was consistently delivered to the same nest hole by each foraging individual. Australian meat ants thus use a harvesting system based on dispersed central place foraging. Evidence of frequent larval transport among nest holes, age polyethism developing in the direction of foraging, and the tendency for nest-associated workers to accept new nest holes more readily than foragers, suggests that workers develop fidelity to the particular nest hole in which they eclose. Coupled with larval transport, nest hole fidelity may allow a colony with widely dispersed nest holes to adjust its structure to more efficiently harvest a resource distributed unevenly in space or time.     

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.     

Foraging trip duration of bumblebees in relation to landscape-wide resource availability

Abstract.  1. The study tested the hypotheses that bumblebees have shorter foraging trips in environments that provide abundant resources than in environments that provide sparse resources, and that shorter foraging trips translate into greater colony growth.
2. Six even-aged Bombus terrestris colonies were established in contrasting resource environments. Three colonies had access to abundant resources ( Phacelia tanacetifolia fields with high flower densities), and three colonies were placed in an environment with sparse resources (scattered semi-natural habitats with food plants at lower densities).
3. A total of 870 foraging trips of 220 marked B. terrestris foragers were observed using automated camcorder recordings.
4. The duration of foraging trips was shorter in environments with abundant resources (66 ± 4.6 min) than in environments with sparse resources (82 ± 3.7 min). Within 34 days colonies that had access to abundant resources gained significantly more weight (129 ± 40 g) than colonies foraging on sparse resources (19 ± 7 g).
5. Thus, the spatial distribution and quality of resources at landscape level affected the duration of foraging trips and the colony growth. It was concluded that future conservation schemes need to improve the spatial and temporal availability of resources in agricultural landscapes to counteract the ongoing decline of bumblebees.     

Bumblebees experience landscapes at different spatial scales: possible implications for coexistence

Coexistence in bumblebee communities has largely been investigated at local spatial scales. However, local resource partitioning does not fully explain the species diversity of bumblebee communities. Theoretical studies provide new evidence that partitioning of space can promote species coexistence, when species interact with their environment at different spatial scales. If bumblebee species possess specific foraging ranges, different spatial resource utilisation patterns might operate as an additional mechanism of coexistence in bumblebee communities. We investigated the effects of the landscape-wide availability of different resources (mass flowering crops and semi-natural habitats) on the local densities of four bumblebee species at 12 spatial scales (landscape sectors with 250–3,000 m radius) to indirectly identify the spatial scales at which the bumblebees perceive their environment. The densities of all bumblebee species were enhanced in landscapes with high proportions of mass flowering crops (mainly oilseed rape). We found the strongest effects for Bombus terrestris agg. and Bombus lapidarius at large spatial scales, implying foraging distances of 3,000 and 2,750 m, respectively. The densities of Bombus pascuorum were most strongly influenced at a medium spatial scale (1,000 m), and of Bombus pratorum (with marginal significance) at a small spatial scale (250 m). The estimated foraging ranges tended to be related to body and colony sizes, indicating that larger species travel over larger distances than smaller species, presumably enabling them to build up larger colonies through a better exploitation of food resources. We conclude that coexistence in bumblebee communities could potentially be mediated by species-specific differences in the spatial resource utilisation patterns, which should be considered in conservation schemes.     

Colony size does not predict foraging distance in the ant Temnothorax rugatulus: a puzzle for standard scaling models

Body size is often positively correlated with ecologically relevant traits such as fecundity, survival, resource requirements, and home range size. Ant colonies, in some respects, behave like organisms, and their colony size is thought to be a significant predictor of many behavioral and ecological traits similar to body size in unitary organisms. In this study, we test the relationship between colony size and field foraging distance in the ant species Temnothorax rugatulus. These ants forage in the leaf litter presumably for small arthropod prey. We found colonies did not differ significantly in their foraging distances, and colony size is not a significant predictor of foraging distance. This suggests that large colonies may not exhaust local resources or that foraging trips are not optimized for minimal distance, and thus that food may not be the limiting resource in this species. This study shows T. rugatulus are behaving in ways that differ from existing models of scaling.     

When Parasitoid Males Make Decisions: Information Used when Foraging for Females

Optimal foraging models predict how an organism allocates its time and energy while foraging for aggregated resources. These models have been successfully applied to organisms such as predators looking for prey, female parasitoids looking for hosts, or herbivorous searching for food. In this study, information use and patch time allocation were investigated using male parasitoids looking for mates. The influence of the former presence of females in absence of mates and the occurrence of mating and other reproductive behaviours on the patch leaving tendency was investigated for the larval parasitoid Asobara tabida. Although males do not modify their patch residence time based on the number of females that visited the patch, they do show an increase in the patch residence time after mating a virgin female and performing courtship behaviour such as opening their wings. These results are in concordance with an incremental mechanism, as it has been described for females of the same species while foraging for hosts. The similarities between males and females of the same species, and the conditions under which such a patch-leaving decision rule is fitted are discussed. This is the first study describing an incremental effect of mating on patch residence time in males, thus suggesting that similar information use are probably driving different organisms foraging for resource, regardless of its nature.     

Fitness consequences of cooperative colony founding in the desert leaf-cutter ant Acromyrmex versicolor

The evolution of cooperative colony foundation (pleometrosis)in ants has been attributed to conversion of extra resourcesinto increased competitive ability. Most cooperative foundingspecies provide these additional resources from internal fatstores; however, in those species that forage for resources, theextent and type of individual investment in multiqueen coloniesis not well understood. We compared singly- and group-foundedlaboratory colonies of the desert leaf-cutter ant Acromyrmexversicolor to investigate how cooperation affects colony survival,foraging success, and worker production. Under laboratory conditions,single foundresses were significantly less likely to initiatea successful symbiotic fungus garden, which inevitably led to colonystarvation and death. If gardens were initiated successfully,however, there was no difference in the growth trajectoriesand foraging patterns between colony types. Cooperation in thisspecies may more likely be maintained by survival benefits thanby growth rate differences, which may be constrained in groupsby individual and colony-level costs.     

Behavioural Correlations May Cause Partial Support for the Risk Allocation Hypothesis in Damselfly Larvae

Prey animals are often confronted with situations that differ in predation risk. According to the risk allocation hypothesis, prey animals should adaptively allocate antipredator behaviour in accordance with the magnitude and frequency of those risk situations. According to the first prediction prey animals should increase foraging in the safe situations and decrease foraging in the dangerous situations as these situations become relatively more dangerous. The second prediction is that with increased time spent in the dangerous situations, progressively more foraging effort is shown in both the dangerous and safe situations, especially in the safer ones. Prey animals may, however, show maladaptive behaviour due to behavioural correlations across risk situations. Here we test for the first time both predictions generated by the risk allocation hypothesis while considering behavioural correlations. We reared larvae of the damselfly Ischnura elegans, from the egg stage, under five rearing risk conditions: (i) in isolation, (ii) in the presence of conspecific larvae, (iii) in the presence of one fish, (iv) in the presence of two fish, and (v) in the presence of two fish for 50% of the time. For each rearing risk condition, we scored their behaviour in the absence and in the presence of fish. In accordance with the first prediction, in the absence of a predator, larvae reared under increasing risk conditions increased their level of foraging. In accordance with the second prediction, in the absence of a predator, larvae that were more frequently exposed to fish during rearing, increased foraging. However, opposite to the predictions from the risk allocation hypothesis, foraging increased both with increasing rearing risk, and with increased predator exposure frequency. The observed positive behavioural correlation of foraging activity across test situations with and without fish, may generate the combination of adaptive patterns in the absence of fish and the maladaptive patterns in the presence of fish. Former studies of the risk allocation hypothesis also found, at best, mixed support, and we hypothesize that behavioural correlations across risk situations, if present, will likely cause partial deviations from model predictions.     

Ant Colonies Do Not Trade-Off Reproduction against Maintenance

The question on how individuals allocate resources into maintenance and reproduction is one of the central questions in life history theory. Yet, resource allocation into maintenance on the organismic level can only be measured indirectly. This is different in a social insect colony, a “superorganism” where workers represent the soma and the queen the germ line of the colony. Here, we investigate whether trade-offs exist between maintenance and reproduction on two levels of biological organization, queens and colonies, by following single-queen colonies of the ant Cardiocondyla obscurior throughout the entire lifespan of the queen. Our results show that maintenance and reproduction are positively correlated on the colony level, and we confirm results of an earlier study that found no trade-off on the individual (queen) level. We attribute this unexpected outcome to the existence of a positive feedback loop where investment into maintenance (workers) increases the rate of resource acquisition under laboratory conditions. Even though food was provided ad libitum, variation in productivity among the colonies suggests that resources can only be utilized and invested into additional maintenance and reproduction by the colony if enough workers are available. The resulting relationship between per-capita and colony productivity in our study fits well with other studies conducted in the field, where decreasing per-capita productivity and the leveling off of colony productivity have been linked to density dependent effects due to competition among colonies. This suggests that the absence of trade-offs in our laboratory study might also be prevalent under natural conditions, leading to a positive association of maintenance, (= growth) and reproduction. In this respect, insect colonies resemble indeterminate growing organisms.     

Variation in resource size distribution around colonies changes ant–parasitoid interactions

The distribution of resources within habitats affects species abundance, richness and composition, but the role of resource distribution in species interactions is rarely studied. In ant communities, changes in resource distribution within habitats may influence behavioral interactions because many ant species are specialized to efficiently harvest a subset of available resources. This study investigates whether interactions between the behaviorally dominant host ant Pheidole diversipilosa and its specialist parasitoid (Phoridae: Apocephalus orthocladus) depend on resource size distribution around the colony. Using in situ foraging arenas to manipulate parasitoid abundance and resource size distribution around colonies, we tested whether variation in resource size distribution allows P. diversipilosa to alter its foraging behavior in ways that lessen the impact of parasitoid attack. P. diversipilosa colonies do not lower the impact of parasitoid attack by increasing the number of workers foraging individually on small and widely dispersed resources. However, the presence of multiple large resources allows colonies to temporarily redistribute soldier ants from resources patrolled by parasitoids to other resources not patrolled by parasitoids, and to maintain soldier abundance at levels found in the absence of parasitoids. These results highlight the importance of placing behavioral interactions within the context of variation in resource distribution.     

The Effect of Food Size and Dispersion Pattern on Retrieval Rate by the Argentine Ant, Linepithema humile (Hymenoptera: Formicidae)

Food acquisition in central-place foraging animals demands efficient detection and retrieval of resources. Most ant species rely on a mass recruitment foraging strategy, which requires that some potential foragers remain at the nest where they can be recruited to food once resources are found. Because this strategy reduces the number of workers initially looking for food, it may reduce the food detection rate while increasing the postdiscovery food retrieval rate. In previous studies this tradeoff has been analyzed by computer simulation and mathematical models. Both kinds of models show that food acquisition rate is greatly influenced by food distribution and resource patch size: as food is condensed into fewer patches, the maximal acquisition rate is achieved by a shift to fewer initial searchers and more potential recruits. In general, these models show that a mass recruitment strategy is most effective when resources are clumped. We tested this prediction in two experiments by letting laboratory colonies of the Argentine ant (Linepithema humile) forage for resources placed in different distributions. When all prey were small, retrieval rate increased with increasing resource patch size, in support of foraging models. When prey were large, however, the mass of prey returned to the colony over time was much lower than when prey were small and widely distributed. As more ants reached a large prey item, the distance the prey item was transported decreased due to a greater emphasis on feeding rather than transport. Because Argentine ants can transport more biomass externally than they can ingest, food retrieval that depends only on ingestion can depress the biomass retrieval rate. Thus, our results generally support theoretical foraging models, but we show how prey size, through differential prey-handling behavior, can produce an outcome greatly different from that predicted only on the distribution of resources.     

Interpopulation resource partitioning of Lesser Frigatebirds and the influence of environmental context

Conspecific individuals inhabiting nearby breeding colonies are expected to compete strongly for food resources owing to the constraints imposed by shared morphology, physiology, and behavior on foraging strategy. Consequently, colony‐specific foraging patterns that effectively partition the available resources may be displayed. This study aimed to determine whether intraspecific resource partitioning occurs in two nearby colonies of Lesser Frigatebirds (Fregata ariel). A combination of stable isotope analysis and GPS tracking was used to assess dietary and spatial partitioning of foraging resources during the 2013 and 2014 breeding seasons. These results were compared to vessel‐derived estimates of prey availability, local primary productivity, and estimates of reproductive output to suggest potential drivers and implications of any observed partitioning. Isotopic data indicated a more neritic source of provisioned resources for near‐fledged chicks at an inshore colony, whereas their offshore counterparts were provisioned with resources with a more pelagic signal. Deep pelagic waters (>200 m) had higher availability of a preferred prey type despite a trend for lower primary productivity. Differences in foraging ecology between the two populations may have contributed to markedly different reproductive outputs. These findings suggest environmental context influences dietary and spatial aspects of foraging ecology. Furthermore, the effect of colony‐specific foraging patterns on population demography warrants further research.     

Optimizing time and resource allocation trade-offs for investment into morphological and behavioral defense

Prey organisms are confronted with time and resource allocation trade-offs. Time allocation trade-offs partition time, for example, between foraging effort to acquire resources and behavioral defense. Resource allocation trade-offs partition the acquired resources between multiple traits, such as growth or morphological defense. We develop a mathematical model for prey organisms that comprise time and resource allocation trade-offs for multiple defense traits. Fitness is determined by growth and survival during ontogeny. We determine optimal defense strategies for environments that differ in their resource abundance, predation risk, and defense effectiveness. We compare the results with results of simplified models where single defense traits are optimized. Our results indicate that selection acts in favor of integrated traits. The selective advantage of expressing multiple defense traits is most pronounced at intermediate environmental conditions. Optimizing single traits generally leads to a more pronounced response of the defense traits, which implies that studying single traits leads to an overestimation of their response to predation. Behavioral defense and morphological defense compensate for and augment each other depending on predator densities and the effectiveness of the defense mechanisms. In the presence of time constraints, the model shows peak investment into morphological and behavioral defense at intermediate resource levels.     

Behavioural syndrome in a solitary predator is independent of body size and growth rate

Models explaining behavioural syndromes often focus on state-dependency, linking behavioural variation to individual differences in other phenotypic features. Empirical studies are, however, rare. Here, we tested for a size and growth-dependent stable behavioural syndrome in the juvenile-stages of a solitary apex predator (pike, Esox lucius), shown as repeatable foraging behaviour across risk. Pike swimming activity, latency to prey attack, number of successful and unsuccessful prey attacks was measured during the presence/absence of visual contact with a competitor or predator. Foraging behaviour across risks was considered an appropriate indicator of boldness in this solitary predator where a trade-off between foraging behaviour and threat avoidance has been reported. Support was found for a behavioural syndrome, where the rank order differences in the foraging behaviour between individuals were maintained across time and risk situation. However, individual behaviour was independent of body size and growth in conditions of high food availability, showing no evidence to support the state-dependent personality hypothesis. The importance of a combination of spatial and temporal environmental variation for generating growth differences is highlighted.     

Nutritional requirements for web synthesis in the tetragnathid spider Nephila clavipes

Trap-constructing organisms provide a unique opportunity for the study of resource allocation, because an observer can unambiguously determine the allocation to foraging. In species that synthesize a trap from physiologically important compounds, there is the further advantage that there may be direct trade-offs between allocation of resources to foraging and physiological functions. We examined the ability of the spider Nephila clavipes (L.; Araneae: Tetragnathidae) to synthesize resources that are known to be used for both web synthesis and non-foraging physiological functions. We found that choline, required for both web function and physiological function, is an essential nutrient: it is not synthesized by this spider. Under laboratory conditions with a diet of fruit flies, choline is limiting, and the spiders make allocation trade-offs between investing choline in foraging (the web) or in their body.