Scrounger Tactic Research Articles

Advantages of social foraging in crab spiders: Groups capture more and larger prey despite the absence of a web

AbstractAmong group‐living spiders, subsocial representatives in the family of crab spiders (Thomisidae) are a special case, as they build protective communal leaf nests instead of extensive communal capture webs. It could thus be inferred that antipredator benefits (e.g., enhanced protection in larger nests) rather than foraging‐related advantages (e.g., capture of more and larger prey) promote sociality in this family. Nonetheless, subsocial crab spiders do share prey, and if this behaviour does not reflect mere food scramble but has a cooperative character, crab spiders may offer insights into the evolution of social foraging applicable to many other cooperative predators that hunt without traps. Here, we performed a comparative laboratory feeding experiment on three of the four subsocial crab spider species—Australomisidia ergandros, Australomisidia socialis and Xysticus bimaculatus—to determine if crab spiders derive advantages from foraging in groups. In particular, we tested artificially composed groups of five sibling spiderlings vs. single siblings in terms of prey capture success and prey size preference. Across species, groups had higher prey capture success (measured in terms of capture rates and capture latency) and were more likely to attack large, sharable prey—dynamics leading to reduced food competition among group members in favour of living and foraging in groups. Within groups, we further compared prey extraction efficiency among the three applied social foraging tactics: producing, scrounging and feeding alone. In A. ergandros, individuals were exceptionally efficient when using the non‐cooperative scrounger tactic, which entails feeding on the prey provided by others. Thus, our multispecies comparison confirms foraging advantages in maintaining a cooperative lifestyle for crab spiders, but also demonstrates the relevance of research into exploitation of cooperative foraging in this family.

Zebra finches in poor condition produce more and consume more food in a producer–scrounger game

When they forage in groups, animals can search for their own food (producer tactic) or exploit the discoveries of others (scrounger tactic). Previous experimental inquiries have demonstrated that individuals vary in their tendency to play either tactic but the extent to which individual factors influence variation in foraging behavior are little studied. In the present study, we have assessed the influence of natural variation in body condition on the differential use of social foraging tactics and their resulting payoffs in zebra finches (Taeniopygia guttata). The producer tactic was found to yield more consistent and predictable rewards across trials than the scrounger tactic. The use of producer was related to reduced variation in food intake and an increased amount of food consumed. We found that poor-condition birds were more likely to produce and so consumed more seeds than good-condition birds. The results are consistent with theoretical models of variance-sensitive social foraging but suggest that scrounging may not represent a variance-averse option in all situations. We propose that in a producer‐scrounger context, the variance-averse option may depend on group size and food clumping. Finally, we discuss our results in relation to interindividual differences in metabolism and behavior. Key words: bootstrap, decision rules, food intake, residual body mass, scramble competition, variance-sensitive foraging. [Behav Ecol]

Family-related differences in social foraging tactic use in the zebra finch (Taeniopygia guttata)

When animals forage in groups, they can search for food themselves (producer tactic), or they can search for opportunities to exploit the food discoveries of others (scrounger tactic). Both theoretical and empirical work have shown that group-level use of these alternative tactics is influenced by environmental conditions including group size and food distribution, and individual tactic use can be influenced by several measures of individual state, including body condition. Because body condition has been shown to be heritable for various species, social foraging tactics may also be heritable. We looked for evidence of heritability in social foraging tactic use in the zebra finch (Taeniopygia guttata) by testing whether: (1) natural variation in body condition correlates with tactic use, (2) there are family-related differences in body condition, and (3) there are family-related differences in observed tactic use. Tactic use in the zebra finch was significantly related to body condition; individuals with lower body condition scores had a significantly higher use of the scrounger tactic as predicted from variance-sensitive producer-scrounger models. Body-condition scores differed significantly between families, suggesting that this aspect of individual state may have a heritable component. Finally, we recorded significant family-related differences in the use of producer and scrounger alternatives. These results are consistent with heritability in observed tactic use resulting from an inheritance of individual state, in this case body condition, which itself influences tactic use. Understanding how and why individuals differ in their use of alternative tactics is fundamental as it may provide important insights into inter-individual variation in fitness.

Predator inadvertent social information use favours reduced clumping of its prey

When animals forage socially, individuals can obtain prey from their own searching (producer tactic) or by using the behaviour of others (scrounger tactic) when it provides inadvertent social information (ISI) that food has been located. This ISI may either indicate the location of food (social information, SI), or it may indicate the quality of the resource (public information, PI). To date, few studies have explored the selective consequences for prey of being exploited by predators that use ISI. Prey exploited by such predators should evolve traits that favour high levels of ISI use (scrounging) because this would result in lower predator search efficiency given that fewer predators would be searching directly for the prey. Our simulations confirm that ISI‐using predators should increase their use of ISI when their prey form larger clumps resulting in higher prey survival. Our objective therefore is to explore whether prey will evolve towards higher clumpiness when their predators use ISI, using genetic algorithm simulation. The prey were subjected to one of three types of predators for over 500 prey generations. The predators either used: (1) no social information (NS), (2) SI only, or (3) PI. Surprisingly, the prey evolved the highest clumpiness for NS predators. Prey evolved towards smaller clump sizes with SI predators and the clumps were marginally larger when predators used PI. The result is due to the prey evolving the minimum clumpiness required to cause maximal ISI use by their predators. We discuss how this response by prey may favour the use of PI over SI in their ISI‐using predators.

Testing dynamic variance-sensitive foraging using individual differences in basal metabolic rates of zebra finches

Social foragers can alternate between searching for food (producer tactic), and searching for other individuals that have located food in order to join them (scrounger tactic). Both tactics yield equal rewards on average, but the rewards generated by producer are more variable. A dynamic variance-sensitive foraging model predicts that social foragers should increase their use of scrounger with increasing energy requirements and/or decreased food availability early in the foraging period. We tested whether natural variation in minimum energy requirements (basal metabolic rate or BMR) is associated with differences in the use of producer–scrounger foraging tactics in female zebra finches Taeniopygia guttata. As predicted by the dynamic variance-sensitive model, high BMR individuals had significantly greater use of the scrounger tactic compared with low BMR individuals. However, we observed no effect of food availability on tactic use, indicating that female zebra finches were not variance-sensitive foragers under our experimental conditions. This study is the first to report that variation in BMR within a species is associated with differences in foraging behaviour. BMR-related differences in scrounger tactic use are consistent with phenotype-dependent tactic use decisions. We suggest that BMR is correlated with another phenotypic trait which itself influences tactic use decisions.

Testing dynamic variance‐sensitive foraging using individual differences in basal metabolic rates of zebra finches

Social foragers can alternate between searching for food (producer tactic), and searching for other individuals that have located food in order to join them (scrounger tactic). Both tactics yield equal rewards on average, but the rewards generated by producer are more variable. A dynamic variance‐sensitive foraging model predicts that social foragers should increase their use of scrounger with increasing energy requirements and/or decreased food availability early in the foraging period. We tested whether natural variation in minimum energy requirements (basal metabolic rate or BMR) is associated with differences in the use of producer–scrounger foraging tactics in female zebra finches Taeniopygia guttata. As predicted by the dynamic variance‐sensitive model, high BMR individuals had significantly greater use of the scrounger tactic compared with low BMR individuals. However, we observed no effect of food availability on tactic use, indicating that female zebra finches were not variance‐sensitive foragers under our experimental conditions. This study is the first to report that variation in BMR within a species is associated with differences in foraging behaviour. BMR‐related differences in scrounger tactic use are consistent with phenotype‐dependent tactic use decisions. We suggest that BMR is correlated with another phenotypic trait which itself influences tactic use decisions.

Increasing vulnerability to predation increases preference for the scrounger foraging tactic

When animals forage in groups, individuals can search for food themselves (producer tactic) or they can search for and join other individuals that have located food (scrounger tactic). The scrounger tactic may provide greater antipredator benefits than the producer tactic because “scroungers” hop with their heads up and tend to occupy central positions in a group, whereas “producers” hop with their heads down and tend to occupy edge positions. We tested whether increasing an individual's vulnerability to predation (using wing-loading manipulations) causes an increased preference for the scrounger tactic in zebra finches (Taeniopygia guttata). Wing-loading manipulations were effective at increasing focal individuals' perception of vulnerability to predation; treatment individuals increased their total time allocated to vigilance, whereas control individuals did not. Treatment individuals also increased their use of the scrounger tactic (proportion of hops with head up) and scrounged a greater proportion of patches, whereas control individuals exhibited no changes. Our results are consistent with the hypothesis that the scrounger tactic confers greater antipredator benefits than the producer tactic, although whether antipredator benefits are achieved through differences in head orientation, spatial position, or both, remains unclear. Our finding that individuals adjust their use of the scrounger tactic according to changes in their phenotype provides evidence for phenotype-limited allocation strategies in producer–scrounger games.

Early experience affects producer–scrounger foraging tendencies in the house sparrow

Group foragers can use a ‘producer’ tactic which involves searching for food or a ‘scrounger’ tactic which involves joining others who have discovered food. While these alternative behaviours are well documented, it is not clear to what extent an individual's tendency to forage independently or to follow others is under genetic control or rather is affected by experience. To examine whether hand-reared juveniles can learn to prefer using a producer or a scrounger tactic, we hand-reared house sparrow, Passer domesticus, nestlings that upon fledging were assigned to one of two training groups; the first was expected to enhance joining (scrounging) behaviour and the second to enhance searching (producing) behaviour. In the first group, fledglings were imprinted on a parent model (stuffed female sparrow) that visited locations containing food. In the second group, fledglings were imprinted on a parent model that visited locations containing no food, while food was available in different locations. At the end of a 5-day training phase, all fledglings were released into a shared aviary, and their social foraging tendencies were measured. We found that fledglings from the first group used significantly more joining behaviour than fledglings from the second group, suggesting that an individual whose early experience positively reinforced joining behaviour is more likely to later become a joiner. To our knowledge, this is the first experimental evidence for the effect of learning on the choice between social foraging strategies in the context of the producer–scrounger game.

Phenotypic Correlates of Scrounging Behavior in Zebra Finches: Role of Foraging Efficiency and Dominance

AbstractIn flocks, individuals can search for their own food using the producer tactic or exploit the discoveries of companions using the scrounger tactic. Models of the producer–scrounger game usually assume that tactic payoffs are independent of individual phenotypic traits. However, factors such as dominance status or foraging efficiency may constrain the use of tactics and lead to asymmetric tactic use among individuals. For instance, in flocks composed of foragers with unequal foraging efficiency, foragers that are less efficient at obtaining food are expected to rely on the scrounger tactic to a greater extent. I examined the role of foraging efficiency and dominance status as potential correlates of scrounging behavior in small aviary flocks of zebra finches (Taenopygia guttata). Individual foraging efficiency was documented in each flock in a treatment that prevented scrounging. In a subsequent treatment that allowed scrounging, higher levels of scrounging occurred as predicted in foragers with lower foraging efficiency. Dominance status was a poor predictor of tactic choice. Birds that arrived later on the foraging grid foraged less efficiently when scrounging was prevented and used scrounging to a greater extent when allowed, suggesting a link between boldness, foraging efficiency and the choice of foraging tactics in small flocks of zebra finches.

The effects of predation risk on the use of social foraging tactics

The effects of predation on the use of social foraging tactics, such as producing and scrounging, are poorly known in animals. On the one hand, recent theoretical models predict increased use of scrounging with increasing predation risk, when scroungers seeking feeding opportunities also have a higher chance of detecting predators. On the other hand, there may be no relation between tactic use and predation when antipredator vigilance is not compatible with scanning flockmates. We investigated experimentally the effects of predation risk on social foraging tactic use in tree sparrows, Passer montanus. We manipulated predation risk in the field by changing the distance between shelter and a feeder. Birds visited the feeder in smaller flocks, spent less time on it and were somewhat more vigilant far from shelter than close to it. Increased predation risk strongly affected the social foraging tactic used: birds used the scrounger tactic 30% more often far from cover than close to it. Between-flock variability in scrounging frequency was not related to the average vigilance level of the flock members, and within-flock variability in the use of scrounging was negatively related to the vigilance of birds. Our results suggest that in tree sparrows, the increased frequency of scrounging during high predation risk cannot simply be explained by an additional advantage of increasing antipredator vigilance. We propose alternative mechanisms (e.g. increased stochasticity in food supply, and that riskier places are used by individuals with lower reserves) that may explain increased scrounging when animals forage under high predation risk.

Incompatibility between antipredatory vigilance and scrounger tactic in nutmeg mannikins, Lonchura punctulata

Most social foragers must search for food while avoiding predators. Group-foraging nutmeg mannikins engaged in a producer–scrounger game search for their own food (play producer) by hopping with the head down and search for others' food discoveries (play scrounger) by hopping with the head up. If the scrounger tactic is compatible with antipredatory vigilance, then an increase in antipredatory vigilance should lead to the detection of more joining opportunities, and hence to more joining by foragers. We tested this prediction as well as the extent to which stationary birds use head up exclusively for antipredatory purposes and hopping birds use head up for foraging purposes only. We observed three flocks of nutmeg mannikins searching for hidden clumps of food in an indoor aviary. We used a 2×2 factorial design in which both the distance to a safe refuge and the food distribution were manipulated. The use of head up by stationary and eating birds increased significantly with increased distance to cover. Distance to cover, however, had no effect on the use of the scrounger tactic or on the level of joining. We found no evidence of compatibility between the scrounger tactic and antipredatory vigilance. Our results provide the first unambiguous evidence for the existence of two distinct and incompatible patterns of vigilance for predators and for conspecifics. Copyright 2003 Published by Elsevier Ltd on behalf of The Association for the Study of Animal Behaviour.

Increasing foraging group size increases scrounger use and reduces searching efficiency in nutmeg mannikins ( Lonchura punctulata )

Social foragers can benefit from others' success by joining and sharing their food discoveries. In a producer-scrounger (PS) system, foragers can either search for food themselves (play producer) or search for joining opportunities (play scrounger), but not both at the same time. Empirical evidence is accumulating to show that the joining decision of ground-feeding birds like nutmeg mannikins (Lonchura punctulata) can be modeled by a PS game. However some predictions remain to be tested. For instance, foragers are predicted to increase their use of the scrounger tactic as group size increases. Also, one consequence of the incompatibility between producer and scrounger tactics is that the per capita searching efficiency should decrease as the use of scrounger increases. I tested these predictions in an indoor aviary using four flocks of nutmeg mannikins. I manipulated the stable equilibrium frequency (SEF) of the scrounger tactic by varying group size and the finder's share. As predicted by PS games, birds increased their use of scrounger as group size increased. Also, the per capita interval between patch discoveries increased and the per capita finding rate decreased as conditions called for a higher SEF of scrounger. I discuss why the decreased searching efficiency observed likely follows from the incompatibility between producer and scrounger tactics rather than from artifacts of the conditions used or from any form of interference.

Head position as an indicator of producer and scrounger tactics in a ground-feeding bird

The benefits of group foraging depend on the frequency of feeding from food uncovered by companions (joining) versus self-discovered food (finding). Information-sharing (IS) and producer–scrounger (PS) games predict different joining frequencies because they make distinct assumptions about food searching. IS games assume individuals can search concurrently for finding and joining opportunities while PS games assume incompatible search modes; individuals search either as a producer, detecting only finding opportunities, or as a scrounger, detecting only joining opportunities. To determine the search assumption for flocks of ground-feeding granivores we studied the behaviour of spice finches, Lonchura punctulata, foraging in indoor aviaries for clumps of hidden millet seed. We looked for behaviour patterns preceding finding and joining events. An analysis of covariance showed that the frequencies of hopping with the head pointing up and down were statistically associated with the frequencies of a bird's joining and finding, respectively. When the expected stable frequency of the scrounger tactic was altered by changing the seed distribution, the birds' relative frequency of hopping with the head up changed accordingly. When the seed distribution made any use of the scrounger tactic unprofitable, the frequency of hopping with the head up declined to zero. Consequently, in ground-feeding birds such as spice finches, finding and joining behaviour conform more closely to the assumptions of a PS rather than an IS game.

The effect of prior residence and pair bond on scrounging choices in flocks of zebra finches, Taenopygia guttata

In groups, animals can use the producer tactic to locate food patches and the scrounger tactic to join the food discoveries of other companions. At equilibrium, models predict a mixture of the two tactics with equal payoffs. Several factors may constrain the use of tactics and lead to biases in scrounging choices. I explored the effect of prior residence and pair bond as potential constraints on scrounging choices in flocks of zebra finches ( Taenopygia guttata). Experimental flocks contained two birds already established in an aviary (prior residents) and two birds recently released in the aviary for the first time (new residents). All birds were previously trained to find food on a foraging grid. In the aviary, new residents followed prior residents from perches to the grid and relied heavily on prior residents to locate food patches. Low initial success by new residents probably favoured heavy reliance on the scrounger tactic. New residents that formed pair bonds with prior residents foraged closer to their mates and scrounged selectively from their mates in some cases. Prior residence, and pair bond to a lesser extent, influenced scrounging choices in zebra finches and could lead to deviation from the expected use of foraging tactics.

Experimental evidence that group foragers can converge on predicted producer–scrounger equilibria

When foraging together, animals are often observed to feed from food discoveries of others. The producer–scrounger (PS) game predicts how frequently this phenomenon of food parasitism should occur. The game assumes: (1) at any moment all individuals can unambiguously be categorized as either playing producer (searching for undiscovered food resources) or scrounger (searching for exploitation opportunities), and (2) the payoffs received from the scrounger tactic are negatively frequency dependent; a scrounger does better than a producer when the scrounger tactic is rare, but worse when it is common. No study to date has shown that the payoffs of producer and scrounger conform to the game's assumptions or that groups of foragers reach the predicted stable equilibrium frequency (SEF) of scrounger, whereby both tactics obtain the same payoff. The current study of three captive flocks of spice finches, Lonchura punctulata, provides the first test of the PS game using an apparatus in which both assumptions of the PS game are met. The payoffs to the scrounger, measured as feeding rate (seeds/s), were highly negatively frequency dependent on the frequency of scrounger. The feeding rate for scrounger declined linearly while the rate for producer either declined only slightly or not at all with increasing scrounger frequency. When given the opportunity to alternate between tactics, the birds changed their use of each, such that the group converged on the predicted SEF of scrounger after 5–8 days of testing. Individuals in this study, therefore, demonstrated sufficient plasticity in tactic use such that the flock foraged at the SEF of scrounger.

Attraction Toward Feeding Conspecifics when Food Patches are Exhaustible

Animals that forage in groups commonly share patches of food discovered by one or a few individuals.