Does group foraging promote efficient exploitation of resources?

Increased avoidance of food patches previously exploited by other companions has been proposed as one adaptive benefit of group foraging. However, does group foraging really represent the most efficient way to exploit non- or slowly-renewing resources? Here, I used simulations to explore the costs and benefits of exploiting non-renewing resources by foragers searching for food patches independently or in groups in habitats with different types of resource distribution. Group foragers exploited resources in a patch more quickly and therefore spent proportionately more time locating new patches. Reduced avoidance of areas already exploited by others failed to overcome the increased time cost of searching for new food patches and group foragers thus obtained food at a lower rate than solitary foragers. Group foraging provided one advantage in terms of a reduction in the variance of food intake rate. On its own, reduced avoidance of exploitation competition through group foraging appears unlikely to increase mean food intake rate when exploiting non-renewing patches but may provide a way to reduce the risk of an energy shortfall.     

An exploration of the relationship between recruitment communication and foraging in stingless bees

Social information is widely used in the animal kingdom and can be highly adaptive. In social insects, foragers can use social information to find food, avoid danger, or choose a new nest site. Copying others allows individuals to obtain information without having to sample the environment. When foragers communicate information they will often only advertise high-quality food sources, thereby filtering out less adaptive information. Stingless bees, a large pantropical group of highly eusocial bees, face intense inter- and intra-specific competition for limited resources, yet display disparate foraging strategies. Within the same environment there are species that communicate the location of food resources to nest-mates and species that do not. Our current understanding of why some species communicate foraging sites while others do not is limited. Studying freely foraging colonies of several co-existing stingless bee species in Brazil, we investigated if recruitment to specific food locations is linked to 1) the sugar content of forage, 2) the duration of foraging trips, and 3) the variation in activity of a colony from 1 day to another and the variation in activity in a species over a day. We found that, contrary to our expectations, species with recruitment communication did not return with higher quality forage than species that do not recruit nestmates. Furthermore, foragers from recruiting species did not have shorter foraging trip durations than those from weakly recruiting species. Given the intense inter- and intraspecific competition for resources in these environments, it may be that recruiting species favor food resources that can be monopolized by the colony rather than food sources that offer high-quality rewards.     

Effect of predation risk on selectivity in heteromyid rodents

Variations in predation risk affect the costs of foraging and may therefore warrant different foraging decisions. One class of models ("higher requisite profit") predicts that foragers should become more selective when predation risk increases, as low-profitability items that do not cover the increased costs are dropped from the diet. An alternative class of models ("reduced finickiness") predicts that foragers should become less selective when predation risk increases, because selectivity requires more extensive assessment and/or search behaviour, prolonging exposure to risk. We assessed the selectivity of foraging heteromyid rodents (Merriam's kangaroo rats, Dipodomys merriami, and pocket mice, Chaetodipus spp.) by comparing differences in "giving up densities" (GUD: the quantity of cryptic food left in a patch by animals for whom the diminishing marginal gains from foraging have dropped below the threshold for continued search) for foods of different value as a measure of selectivity in patches varying in predation risk. Data collected over two field seasons revealed that heteromyids were more selective when predation risk was highest; away from the protective cover of shrubs during the full moon. These findings support the predictions of higher requisite profit models.     

Deactivation of dietary wariness through experience of novel food

When a forager encounters an unfamiliar type of food, it mustdecide whether to eat it and risk being poisoned or avoid eatingit and risk forfeiting a potentially valuable resource. Birdstypically respond to such situations with "dietary wariness";they show a transient aversion to approaching new food (neophobia),and many individuals also show a much longer lasting reluctanceto consume the new food (dietary conservatism), even once neophobiahas waned. Very little is known about how these processes, togethertermed "wariness," are controlled. We therefore present a seriesof experiments investigating how wariness of novel foods indomestic chicks, Gallus gallus domesticus, can be deactivatedand reactivated by different experiences of colored foods, varyingin their degree of novelty and palatability. We found that priorexperience of a single novel color of palatable chick crumbswas sufficient to deactivate both neophobia and dietary conservatismof any other novel color of crumbs tested. Relatively littleprior experience of a novel training food was needed to deactivateneophobia, after which the birds would peck at any other novelfood. In contrast, much more extensive experience of eatinga novel training food was needed before the birds would incorporateother novel foods into their diet. Chicks needed direct physicalcontact with the training food before they overcame their warinessto eat another novel food. However, observational learning wassufficient to encourage them to peck at the food (overcomingtheir neophobia). Reinstating wariness was much more easilyachieved than its deactivation. We discuss these surprisingresults in relation to the foraging behavior of wild and domesticbirds.     

Accounting for interspecific competition and age structure in demographic analyses of density dependence improves predictions of fluctuations in population size

Understanding species coexistence has long been a major goal of ecology. Coexistence theory for two competing species posits that intraspecific density dependence should be stronger than interspecific density dependence. Great tits and blue tits are two bird species that compete for food resources and nesting cavities. On the basis of long‐term monitoring of these two competing species at sites across Europe, combining observational and manipulative approaches, we show that the strength of density regulation is similar for both species, and that individuals have contrasting abilities to compete depending on their age. For great tits, density regulation is driven mainly by intraspecific competition. In contrast, for blue tits, interspecific competition contributes as much as intraspecific competition, consistent with asymmetric competition between the two species. In addition, including age‐specific effects of intra‐ and interspecific competition in density‐dependence models improves predictions of fluctuations in population size by up to three times.     

Effects of group size and group density on trade-offs in resource selection by a group-territorial central-place foraging woodpecker

Trade-offs in resource selection by central-place foragers are driven by the need to balance the benefits of selecting resources against the costs of travel from the central place. For group-territorial central-place foraging birds, trade-offs in resource selection are likely to be complicated by a competitive advantage for larger groups at high group density that may limit accessibility of high-quality distant resources to small groups. We used the group-territorial, central-place foraging Red-cockaded Woodpecker Leuconotopicus borealis (RCW) as a case study to test predictions that increases in group density lead to differences in foraging distances and resource selection for groups of different sizes. We used GPS tracking and LiDAR-derived habitat data to model effects of group size on foraging distances and selection for high-quality pines (≥ 35.6 cm diameter at breast height (dbh)) and lower quality pines (25.4–35.6 cm dbh) by RCW groups across low (n = 14), moderate (n = 10) and high group density (n = 10) conditions. At low and moderate group density, all RCW groups selected distant high-quality pines in addition to those near the central place because competition for resources was low. In contrast, at high group density, larger groups travelled further to select high-quality pines, whereas smaller groups selected high-quality pines only when they were close to the central place and, conversely, were more likely to select lower quality pines at greater distances from the central place. Selection for high-quality pines only when close to the cavity tree cluster at high group density is important to long-term fitness of small RCW groups because it allows them to maximize benefits from both territorial defence and selecting high-quality resources while minimizing costs of competition. These relationships suggest that intraspecific competition at high group density entails substantive costs to smaller groups of territorial central-place foragers by limiting accessibility of distant high-quality foraging resources.     

The foraging behavior of the central mudminnow and yellow perch: the influence of foraging site,intraspecific and interspecific competition

Summary The central mudminnow (Umbra limi) and yellow perch (Perca flavescens) are two species of fishes that commonly co-occur in small bog lakes in the Great Lakes region of North America. Both species are dietary generalists with a high degree of dietary overlap, and perch populations have been shown to have a strong negative effect on mudminnow populations. I compared the foraging rates of the two species in a series of laboratory experiments in which four foraging sites were simulated (open bottom substrate, water column, submerged aquatic macrophytes, and water surface). Fish were observed as they foraged singly, in monospecific pairs, and in mixed species pairs to evaluate the effects of intra- and interspecific competition on foraging success and the potential for resource partitioning based on foraging site. Single species trials showed that each species had similar foraging rates at each of the four sites. Across all sites combined, interspecific competition had a greater negative effect on the foraging success of mudminnows than did intraspecific competition. Conversely, intraspecific competition had a greater negative effect on the foraging success of perch than did interspecific competition. In mixed species trials perch took more food items from each of the four foraging sites than did mudminnows and caused shifts in site-use patterns of mudminnows. Differences in the foraging success of the two species reflected the superior ability of perch to discover the presence of food and to search rapidly for additional items, but were unrelated to handling times. These differences were linked to the species' foraging modes. The competitive superiority of perch, demonstrated in these experiments, could contribute to the negative impact of perch on mudminnow populations under natural conditions.     

Intraspecific Thievery in the Ant Ectatomma ruidum is Mediated by Food Availability

Animals modify their foraging strategies in response to environmental changes that affect foraging performance. In some species, cleptobiosis represents an alternative strategy for resource access. The environmental factors that favor the incidence or prevalence of cleptobiosis, however, are poorly described. The cleptobiotic Neotropical ant Ectatomma ruidum is characterized by a high frequency of thievery behavior, a specific type of intraspecific cleptobiosis, in which specialized thief workers insinuate themselves into nests of neighboring colonies and intercept food items brought into these nests. Here, we evaluate how colonies adjust thievery behavior in response to food availability. We supplemented food availability and measured how the incidence and intensity of thievery responded to resource availability. We found that the incidence and intensity of thievery decline in response to supplemental food, suggesting that thievery behavior is a response to resource limitation at the population scale. This finding indicates that the phenomenon of intraspecific thievery, although a rare strategy in among colonies of social animals, is a viable alternative foraging tactic in the context of competition and food limitation.     

Optimal Foraging and Hominid Evolution: Labor and Reciprocity

We argue that cooperative foraging incorporating information exchange may have preceded tool use during the course of hominid evolution. In moving to the savanna, early hominids must have faced increasingly dispersed but sometimes more profitable food sources. The problem is finding such foods. Search costs can be reduced for each individual if a number of foragers cooperate by ranging over different parts of the habitat and by exchanging information about encountered food items. Given the probability of encountering a given food item and the return per individual for that item, it is possible to specify the optimal group size. Thus, in the patchy savanna environment, selection would have favored increased gregariousness and cooperation on the part of early hominids, setting the stage for the emergence of reciprocal exchanges of information and resources. However, such a system of reciprocity is open to manipulation. Outside the foraging context, the tension between reciprocity and manipulation would shape other social interactions. Communication and information exchange may have been more critical than labor and technology in evolving hominids from hominoids. Human sociality may find its origins in a shift in primate foraging tactics.     

The possibility of independent foraging in the rain forest of Peninsular Malaysia

This paper examines the question of whether hunter-gatherers could live in the tropical rain forest of Peninsular Malaysia without access to cultivated foods. It considers the wild food sources used by the Batek De', a contemporary foraging-trading group of Kelantan state, historical and ethnohistorical evidence concerning the Batek economy in the past, and archeological evidence for independent foraging in the Pleistocene and early Holocene. The conclusion reached is that small nomadic groups of foragers can live off wild resources alone in that environment and have done so in the past, although trade for agricultural produce makes life much easier. This suggests that nonseasonal tropical rain forests vary in their potential for supporting human foraging, depending upon the particular species of plants and animals present.     

Pulsed resource availability changes dietary niche breadth and partitioning between generalist rodent consumers

Identifying the mechanisms that structure niche breadth and overlap between species is important for determining how species interact and assessing their functional role in an ecosystem. Without manipulative experiments, assessing the role of foraging ecology and interspecific competition in structuring diet is challenging. Systems with regular pulses of resources act as a natural experiment to investigate the factors that influence the dietary niches of consumers. We used natural pulses of mast‐fruiting of American beech (Fagus grandifolia) to test whether optimal foraging or competition structure the dietary niche breadth and overlap between two congener rodent species (Peromyscus leucopus and P. maniculatus), both of which are generalist consumers. We reconstructed diets seasonally over a 2‐year period using stable isotope analysis (δ¹³C, δ¹⁵N) of hair and of potential dietary items and measured niche dynamics using standard ellipse area calculated within a Bayesian framework. Changes in niche breadth were generally consistent with predictions of optimal foraging theory, with both species consuming more beechnuts (a high‐quality food resource) and having a narrower niche breadth during masting seasons compared to nonmasting seasons when dietary niches expanded and more fungi (a low‐quality food source) were consumed. In contrast, changes in dietary niche overlap were consistent with competition theory, with higher diet overlap during masting seasons than during nonmasting seasons. Overall, dietary niche dynamics were closely tied to beech masting, underscoring that food availability influences competition. Diet plasticity and niche partitioning between the two Peromyscus species may reflect differences in foraging strategies, thereby reducing competition when food availability is low. Such dietary shifts may have important implications for changes in ecosystem function, including the dispersal of fungal spores.     

Age-Related Microhabitat Segregation in Willow Tit Parus montanus Winter Flocks

It is expected that through flexibility in behaviour, flock living birds respond to the asymmetries in resource access derived from dominance relationships. We analysed the microhabitat use of willow tits in winter flocks and assessed possible factors which shape habitat segregation between adults and juveniles in different temperature regimes. When foraging in mild conditions (ambient temperature > 0°C), flocks split up into subgroups with adults foraging in inner parts of trees more often than juveniles. However, no differences were recorded in the vertical position occupied in trees. In harsh conditions (< ? 4°C), flocks re‐united and juveniles further moved to outer parts of trees, increasing horizontal segregation between age classes. In mild conditions, vigilance behaviour was not related to the position of birds in trees, but in harsh conditions, scanning frequency was higher in outer parts of trees only for adults. In mild weather, juvenile position in trees was associated with body size and mass. The foraging microhabitat segregation detected in harsh conditions fits the age‐related hoarding distribution previously described in the same population. This supports the hypothesis that hoarded food is important in determining future foraging habitat use. Adult preference and intraspecific competition for safer or richer inner parts of trees as foraging sites during harsh conditions seems to determine the habitat segregation between adults and juveniles. Furthermore, we suggest that in mild weather, when foraging in the absence of adults, juveniles balance the costs of using a potentially dangerous microhabitat with the benefits of building energetically cheap and large food reserves through hoarding. The expected patterns of microhabitat segregation may differ in parids, depending on whether predation risk or other factors such as food availability are the main factors controlling habitat quality.     

Social effects on foraging behavior and success depend on local environmental conditions

In social groups, individuals' dominance rank, social bonds, and kinship with other group members have been shown to influence their foraging behavior. However, there is growing evidence that the particular effects of these social traits may also depend on local environmental conditions. We investigated this by comparing the foraging behavior of wild chacma baboons, Papio ursinus, under natural conditions and in a field experiment where food was spatially clumped. Data were collected from 55 animals across two troops over a 5‐month period, including over 900 agonistic foraging interactions and over 600 food patch visits in each condition. In both conditions, low‐ranked individuals received more agonism, but this only translated into reduced foraging performances for low‐ranked individuals in the high‐competition experimental conditions. Our results suggest one possible reason for this pattern may be low‐ranked individuals strategically investing social effort to negotiate foraging tolerance, but the rank‐offsetting effect of this investment being overwhelmed in the higher‐competition experimental environment. Our results also suggest that individuals may use imbalances in their social bonds to negotiate tolerance from others under a wider range of environmental conditions, but utilize the overall strength of their social bonds in more extreme environments where feeding competition is more intense. These findings highlight that behavioral tactics such as the strategic investment of social effort may allow foragers to mitigate the costs of low rank, but that the effectiveness of these tactics is likely to be limited in certain environments.     

Competition for nectar resources does not affect bee foraging tactic constancy

1. Competition alters animal foraging, including promoting the use of alternative resources. It may also impact how animals feed when they are able to handle the same food with more than one tactic. Competition likely impacts both consumers and their resources through its effects on food handling, but this topic has received little attention. 2. Bees often use two tactics for extracting nectar from flowers: they can visit at the flower opening, or rob nectar from holes at the base of flowers. Exploitative competition for nectar is thought to promote nectar robbing. If so, higher competition among floral visitors should reduce constancy to a single foraging tactic as foragers will seek food using all possible tactics. To test this prediction, field observations and two experiments involving bumble bees visiting three montane Colorado plant species (Mertensia ciliata, Linaria vulgaris, Corydalis caseana) were used under various levels of inter- and intra-specific competition for nectar. 3. In general, individual bumble bees remained constant to a single foraging tactic, independent of competition levels. However, bees that visited M. ciliata in field observations decreased their constancy and increased nectar robbing rates as visitation rates by co-visitors increased. 4. While tactic constancy was high overall regardless of competition intensity, this study highlights some intriguing instances in which competition and tactic constancy may be linked. Further studies investigating the cognitive underpinnings of tactic constancy should provide insight on the ways in which animals use alternative foraging tactics to exploit resources.     

Flexible expression of a food-processing behaviour: determinants of dunking rates in wild Carib grackles of Barbados

Dunking, the softening of dry food in water to speed up consumption time, is normally a very rare behaviour in wild Carib grackles (Quiscalus lugubris) of Barbados. Its frequency can be experimentally increased when large numbers of dry items are repeatedly placed near a standing source of water in conditions that minimize intraspecific competition and risk of theft. To reconcile the normally low frequency of the behaviour in the wild with the high rates obtained in previous experiments, we tested three conditions where dunking varied between 0 and 70%. Dunking was very rare when it had been made unnecessary by pre-soaking the food, water was far from the dry items offered and only one food item was given, focusing all competitive interactions and theft attempts on a single individual. In contrast, dunking rate was high when food was not pre-soaked, water was close to dry food and more than one item (and hence target for competition and theft) was given. These experiments confirm that dunking rates, like other proto-tool-like food-processing techniques, depend on the costs and benefits of the situation where they are used.     

Scrounging Tactics in Free-Ranging Ravens, Corvus corax

Social foraging allows individuals to scrounge, i.e. to exploit the food others have made available. The conditions promoting scrounging as an alternative foraging tactic have yet received limited attention. We presently examine whether ravens, as opportunistic scavengers, adjust their foraging tactics according to the potential costs involved in accessing a particular food source. We observed wild ravens foraging in a game park, at the enclosures of wolves, Canis lupus, and wild boars, Sus scrofa. Wolves may aggressively defend their food and even kill ravens, whereas wild boars do not. When co‐feeding with wolves, the ravens showed higher scrounging rates than with wild boars. Only at the wolves, they tended to specialize either on scrounging or on getting food directly from the site. However, scrounging techniques differed in relation to the state of food depletion. Early on, after food became available, the ravens most frequently displaced others from food, whereas towards the end, stealing, solicited sharing, and cache raiding became prevalent. These techniques differed in their profitability and their use was related to the scroungers’ age, social status and affiliative relationships. This suggests that ecological conditions, such as co‐feeding with potential predators, may influence the individuals’ decision whether or not to scrounge in competition for food. Social conditions, on the other hand, may affect the way how to get at food possessed by others and may thus, to a large extent, determine the profitability of scrounging.     

Neophobia affects choice of food-item size in group-foraging common ravens (Corvus corax)

Individuals foraging in groups should develop behavioural tactics to optimise their gain. In novel feeding situations, predation risk and pressure of kleptoparasites may be particularly high and hence may constrain optimal foraging. To create a novel feeding situation, we offered common ravens (Corvus corax) equal numbers of either small (40 g) or large (160 g) pieces of meat on successive days, always in combination with the same novel object. During the first weeks, when ravens were still neophobic, small pieces were taken in larger numbers than large pieces. Intraspecific kleptoparasitism was more likely to occur when ravens carried large food items. It seems that initiating foragers were mainly innovative subdominants. Preference for small items might have decreased with increasing habituation because more dominants were then feeding directly at the source and hence were less likely to resort to kleptoparasitism as an alternative foraging tactic. Electronic Publication     

Contrasting responses of bumble bees to feeding conspecifics on their familiar and unfamiliar flowers

Animals exploiting their familiar food items often avoid spatio-temporal aggregation with others by avoiding scents, less rewarding areas or visual contacts, thereby minimizing competition or interference when resources are replenished slowly in patches. When animals are searching or assessing available food sources, however, they may benefit from reducing sampling costs by following others at food sites. Therefore, animals may adjust their responses to others depending on their familiarity with foraging situations. Here, we conducted field experiments to test whether nectar-collecting bumble bees make this adjustment. We allowed free-foraging bees to choose between two inflorescences, one occupied by a conspecific bee and another unoccupied. When bees were presented with flowers of a familiar type, they avoided occupied inflorescences. In contrast, bees visited an occupied inflorescence when the flower type was unfamiliar. To our knowledge, this is the first report suggesting that animals adjust their responses to feeding conspecifics depending on their familiarity with food sources. Such behavioural flexibilities should allow foragers to both explore and exploit their environments efficiently.     

Intra- and interspecific dominance hierarchies and variation in foraging tactics of two species of stream-dwelling chars

Aggressive interactions, foraging behavior, habitat use and diet were studied in sympatric populations of white-sported char,Salvelinus leucomaenis, and Dolly Varden,Salvelinus malma, in a Japanese mountain stream. Underwater observations on individuals of both species revealed two distinct behavioral regimes: aggressive drift foragers and non-aggressive benthos foragers. Aggressive drift foragers defended partial territories around focal points from which they made forays to capture invertebrates drifting in the water column. Non-aggressive benthos foragers cruised around and beneath cobble in large foraging ranges that overlapped each other. Intra- and interspecific, size-dependent dominance hierarchies were recognized among aggressive drift foragers, whereas non-aggressive benthos foragers showed no such relationships. Terrestrial invertebrates were the most abundant prey in the diets of drift foragers, whereas a very small proportion of the diet of benthos foragers was made up of these taxa. Benthos foragers showed more complex diet composition than drift foragers. These results suggest that non-aggressive benthos foragers may avoid not only interference but also exploitative competition by using alternative foraging tactics. The proportion of drift foragers to benthos foragers among white-spotted char was more than 35 times that among Dolly Varden. The significant difference in the proportion of each species using the two types of foraging strategy results in interspecific food segregation in sympatric populations.     

Influence of environmental and colony factors on the initial commodity choice of foragers of the stingless bee Plebeia tobagoensis (Hymenoptera, Meliponini)

Novice foragers of social bees have to decide what food commodity to collect when they start foraging for the first time. In this decision making process two types of factors are involved: internal factors (the response threshold) and external factors (environmental and colony conditions). In this study we will focus on the importance of two external factors, pollen storage level and information from experienced foragers about food availability in the field, on the initial commodity choice of foragers of the stingless bee species Plebeia tobagoensis. We also studied the effect of the initial choice of individuals on their subsequent foraging career. This study was performed in a closed greenhouse compartment, where food availability and colony condition could be controlled. Information on food availability in the field from experienced foragers and pollen storage level both greatly influenced the initial commodity choice of individuals, with more choices for the commodity communicated by experienced foragers or lacking in storage. The initial choice of foragers is of importance for their future foraging career, although a substantial proportion of foragers did switch between food commodities. Because of the ability of novice foragers to become flexibly distributed over foraging tasks, social bees are able to react to changes in their environment without directly having to decrease foraging effort devoted to other foraging tasks. This, in combination with individual flexibility during foraging careers makes it possible for colonies of P. tobagoensis to forage efficiently in an ever-changing environment. Received 7 November 2005; revised 12 January 2006; accepted 16 February 2006.