Food Clumps Research Articles

Food distribution influences social organization and population growth in a small rodent

In polygynous mammals, the spatial clumping and predictability of food should influence spacing behavior of females whose reproductive success depends to a great extent on food availability, which would in turn affect male spacing behavior. Changes in the social and mating systems can then influence individual fitness and population dynamics. To test these hypotheses, we manipulated food distribution and predictability in enclosed populations of bank voles (Myodes glareolus) and monitored spacing behavior, survival, and reproduction of adult females and males over 3 months. Food was either spread out (dispersed treatment), spatially clumped and highly predictable (clumped treatment) or spatially clumped but less predictable (variable treatment). We found that females in the clumped treatment were more aggregated and had more overlapping home ranges compared with females in the dispersed and variable treatments. Male spacing behavior followed the same patterns. Despite different social organizations between treatments, no differences in home range size and mating systems were found in females and males. In addition, we found that females in the clumped food treatment had a higher probability of successfully producing weaned offspring, likely due to lower infanticide rates. This led to higher population growth compared with the other 2 treatments. These results suggest a tight relationship between the spatiotemporal distribution of food, social organization, and population dynamics.

Food divisibility and interference competition among captive ruddy turnstones, Arenaria interpres

Interference competition among foraging animals arises from agonistic interactions among foragers. Interactions can concern single food items but also clumps of food. Food clumps consist of multiple food items, and are therefore easier to divide among foragers than food items. Theoretical studies indicate that differences in divisibility can be essential to the interference process. Empirically, however, little is known about effects of resource divisibility on interference competition. Therefore, we performed an experiment with captive ruddy turnstones. Turnstones foraged either alone or together with a competitor. We offered food at two so-called food pits and varied the divisibility of food in these pits by burying a fixed number of food items either in several layers (divisible) or in a single layer (indivisible). Additionally, we varied the distance between food pits. We accounted for differences in the social dominance status of foragers by using pairs of foragers as our experimental unit: each pair had both a dominant and a subordinate member. We found a strong asymmetry in the intake of birds of different dominance status. The strength of this asymmetry depended on both the divisibility of food and on the distance between food pits. Only when food was divisible did subordinate foragers get a share of the food; only when food pits were close to each other could dominant foragers monopolize food pits. These findings imply that to understand and predict interference competition we need to consider both the detailed characteristics of resources, and the determinants of dominance status.

Food sharing among retaliators: sequential arrivals and information asymmetries

Many animals share food, that is, to tolerate competitors at a defensible clump. Most accounts of resource sharing invoke special evolutionary processes or ecological circumstances that reduce their generality. Surprisingly, the Hawk–Dove game has been unable to address in a simple and general way why so many group foraging animals share food. We modify the Hawk–Dove game by allowing a finder the opportunity of retaliating if joiners escalate and by considering the consequences of information asymmetries concerning resource value among players. Introducing the first change, the retaliator strategy was sufficient to predict widespread sharing in habitats where food clumps are of intermediate richness. When information asymmetry between finder and joiner is created by allowing the quality of clumps to vary, we show that the conditions for sharing are even more easily met and apply to a wider range of resource qualities. Our model therefore offers one of the most parsimonious and potentially general evolutionary accounts of the origin of non-aggressive resource sharing.

Spatial clumping of food and social dominance affect interference competition among ruddy turnstones

In studying the success of foraging animals, studies of interference competition have put emphasis on effects of competitor density, whereas studies of resource defense have focused on the effects of the spatial distribution of food within patches. Very few studies have looked at both factors simultaneously, that is, determined whether the effects of competitor density on foraging success depend on the spatial distribution of food. We studied the behavior and the foraging success of ruddy turnstones (Arenaria interpres) using an experiment in which we varied both the presence of a competitor and the food distribution. Because turnstones may differ strongly in their relative dominance status, we also experimentally varied the foragers’ relative dominance status. We found that the presence of a competitor only reduced the foraging success of subordinate birds foraging at the clumped food distribution. At this condition, dominant and subordinate birds differed markedly in their foraging success. Contrary to our expectations, we did not observe more agonistic behavior at the clumped food distribution. This indicates that the amount of agonistic behavior observed may be a bad indicator of interference effects. These findings have specific implications for models of interference competition. Most notably they show that the effects of competitor density on agonistic behavior and foraging success may well depend on the spatial distribution of food and the foragers’ relative dominance status. Additionally, our results suggest that social dominance will not be fully understood without considering long-term processes such as the formation and maintenance of social dominance hierarchies. Key words: agonistic interactions, Arenaria interpres, density dependence, foraging behavior, foraging experiment, resource defense. [Behav Ecol]

Effects of competitor-to-resource ratio on aggression and size variation within groups of convict cichlids

Resource defence theory predicts that the intensity of competitive aggression, degree of resource monopolization, and variation in fitness will be highest at intermediate levels of the spatial clumping of resources. We tested for this predicted dome-shaped relationship by manipulating the spatial clumping of food via competitor-to-resource ratio (CRR), the number of potential competitors divided by the number of resource units. Groups of 10 convict cichlids, Archocentrus nigrofasciatus, were allowed to compete for a fixed amount of food in one of five CRR treatments (1, 1.43, 2, 5 and 10), created by varying the number of patches in which the food appeared (i.e. 10, 7, 5, 2 and 1 patch, respectively). As predicted, both the frequency of aggression and the coefficient of variation of body mass (a measure of the consequences of food monopolization and variation in a component of fitness within groups) increased significantly as CRR increased from 1 to 2, and then decreased significantly at CRRs of 5 and 10. In addition, mean growth rate decreased in groups with high rates of aggression, suggesting an important cost of aggression. Our study provides the first quantitative support for the predicted dome-shaped relationship and suggests that CRR is a useful measure of the degree of spatial clumping of resources.

The Forager’s Dilemma: Food Sharing and Food Defense as Risk‐Sensitive Foraging Options

Although many variants of the hawk-dove game predict the frequency at which group foraging animals should compete aggressively, none of them can explain why a large number of group foraging animals share food clumps without any overt aggression. One reason for this shortcoming is that hawk-dove games typically consider only a single contest, while most group foraging situations involve opponents that interact repeatedly over discovered food clumps. The present iterated hawk-dove game predicts that in situations that are analogous to a prisoner's dilemma, animals should share the resources without aggression, provided that the number of simultaneously available food clumps is sufficiently large and the number of competitors is relatively small. However, given that the expected gain of an aggressive animal is more variable than the gain expected by nonaggressive individuals, the predicted effect of the number of food items in a clump-clump richness-depends on whether only the mean or both the mean and variability associated with payoffs are considered. More precisely, the deterministic game predicts that aggression should increase with clump richness, whereas the stochastic risk-sensitive game predicts that the frequency of encounters resulting in aggression should peak at intermediate clump richnesses or decrease with increasing clump richness if animals show sensitivity to the variance or coefficient of variation, respectively.

Resource defense in a group-foraging context

When foraging in groups, animals frequently use either scramble or contest tactics to obtain food at clumps found by others. The question of which competitive tactic should be used has been addressed from two different perspectives: a simple optimality approach and a game theoretic approach. Surprisingly, both approaches make strikingly different predictions about how percapita frequency of aggression within groups should change as a function of food abundance and competitor density. Resource defense theory typically predicts dome-shaped relationships between the per-capita frequency of aggression and both food abundance and competitor density, whereas game theoretic models predict an increase in aggression with competitor density and a decline in aggression with increased food abundance. We developed a game theoretic model to explore whether the predictions of resource defense theory and the game theoretic approach can be reconciled. Our model assumes that players have different competitive abilities and can adopt roles of either finder or joiner that affect the quantity of food that can be gained from a food clump. In accordance with earlier game theoretic models, we predict an increase in aggression with competitor density when animals compete by pair-wise contests. However, when food clumps can be challenged by more than one competitor, both the costs and benefits of defending increase with competitor density, which results in a dome-shaped relationship between the two variables. Our model predicts that aggression should always decrease as the density of food clumps increases. Key words: aggression, competitor asymmetry, evolutionarily stable strategy model, finder’s advantage, foraging groups, resource defense. [Behav Ecol 14:2‐9 (2003)]

THE EFFECTS OF DOMINANCE ON SOCIAL FORAGING TACTIC USE IN HOUSE SPARROWS

We investigated whether social foraging tactic use (producing and scrounging) in birds is affected by the dominance rank of individuals as predicted by a phenotype limited producer-scrounger game. In a captive flock of house sparrows, we observed the behaviour of the birds when they were foraging on a grid containing clumps of seeds. We measured the fighting success of the birds, determined the method by which they found food clumps (finding or joining), and measured their feeding rate. Joining were frequently observed and usually involved aggressive interactions. Most birds used both finding and joining to obtain food. We found that foraging method was related to dominance: the frequency of joining gradually increased with increasing dominance rank, as predicted by the phenotype limited model for flocks where there are moderate competitive asymmetries among the birds. Food intake rate of individuals was not related to either their dominance rank or foraging method. Similar weak relationships were predicted by the model among these variables for flocks with moderate competitive asymmetries. Behavioural variability among sparrows in locomotion frequency and vigilance was not related to their foraging method, but the rate of investigating potential food caches strongly decreased with increasing frequency of joining. We conclude that the phenotype limited model successfully predicts the relationship between dominance and joining frequency in house sparrows feeding on concentrated food sources, and we suggest that the distribution of food used in tests of the model may crucially affect their results.

Effects of the temporal predictability and spatial clumping of food on the intensity of competitive aggression in the Zenaida dove

The spatial and temporal clumping of food influence an animal's aggressiveness during competition. No studies, however, have investigated the effects of the temporal predictability of food and few studies have tested for interactions between the effects of two components of resource distribution on the rates of competitive aggression. We simultaneously manipulated the temporal predictability and the spatial clumping of food to test whether aggression increases as food becomes more predictable in time and more clumped in space. We tested these predictions using wild Zenaida doves (Zenaida aurita) in Barbados because previous work showed marked differences in social behavior between two populations, apparently related to differences in the distribution of food in space and time. There was a significant interaction between the effects of the temporal predictability and spatial clumping of food. As predicted, the rate of aggression increased as the temporal predictability of food increased, but only significantly in the spatially clumped condition. Similarly, as predicted, aggression increased as the spatial clumping of food increased, but only significantly in the temporally predictable condition. In addition, the per capita rate of aggression peaked at intermediate competitor densities in the spatially clumped condition. Differences in rates of aggression observed during experimental manipulations and between the two populations during baseline observations were generally consistent with predictions of resource defense theory.

The effect of depletion and predictability of distinct food patches on the timing of aggression in red deer stags

For group‐foraging ungulates, forage is generally widely and relatively evenly dispersed. However, for free‐ranging red deer Cervus elaphus supplementary winter feeding provides distinct patches of predictable food. These patches differ in size, but also in temporal distribution and depletion rate. Interference competition is known to increase with increasing spatial clumping (decreasing patch size), but the influence of temporal clumping and the predictability of food occurrence has received much less attention. Therefore in this study we investigated the effects of different degrees of spatial and temporal clumping of food on interference competition during feeding. Patch size was the main parameter influencing participation in feeding as well as interference competition during feeding on the respective patch. Temporal dispersion and the predictability of food occurrence were however, important parameters for the timing of aggressive interactions. Generally, aggression occurs during feeding and increases with decreasing patch size. But when depletion rate was high food availability was predictably short and the patch occurred predictably (such as hay), middle ranking stags increased aggression already prior to feeding at the respective patch. We suggest that in this way they confirmed hierarchy outside feeding on the quickly depleted patch and as a result gained actual feeding time when feeding on the respective patch. With the patch occurring predictably but varying in size the number of participating subordinates varied concomitantly with variation in patch size. Subordinates assessed patch profitability and left without having fed when patch size was too small for an efficient participation. When patch size was predictably small enough to be defended exclusively (feed blocks), subordinate stags did not assess profitability each time but did not participate at all in feeding at the respective patch. The relative importance of these various food‐related parameters (patch size depletion rate, predictability) influencing feeding competition and the timing of aggression will vary with group size, rank, alternative food sources, physical characteristics of the food as well as different hierarchy systems of the feeding animals.

Interactions between the spatial and temporal clumping of food affect the intensity of aggression in Japanese medaka

Previous studies have largely supported the predictions that an animal's aggressiveness increases as the clumping of resources increases in space but decreases in time. Such tests of resource defence theory, however, have manipulated only one aspect of resource distribution at a time. We tested for an interaction between the effects of the spatial and temporal clumping of food by allowing groups of 10 Japanese medaka fish,Oryzias latipes, to compete for food in a 2×2 factorial design. There was a significant interaction between the effects of the spatial and temporal clumping of food on both the total rate of aggression in tanks and on the per capita rate of aggression by fish in the feeding patch. Despite these interactions, all trends in per capita rate of aggression in the feeding patch were consistent with the predictions. In contrast, total rate of aggression in the tank, which included fish that were not competing for food, apparently contradicted a key prediction. Our results suggest that powerful tests of resource defence theory should take into account an animal's opportunity to be aggressive. In addition, it may be difficult to extrapolate the results of single-factor tests of resource defence theory to multi-factor or field situations.

Altered search speed and growth:social versus independent foraging in two pelagic juvenile fishes

Prior studies have demonstrated that juvenile walleye pollock Theragra cl?alcogramma forage socially in schools for spatially and temporally clumped food, but forage more independently for spatially and temporally dispersed food. One advantage of social foraging is that fish in schools may be able to locate more food clumps than fish foraging individually. However, data also indicate that walleye pollock swim faster when foraging socially. CVe conducted laboratory experiments to evaluate the effect of food distribution upon the energetic foraglng costs lncurred by juvenile walleye pollock and sablefish Anaplopoma fimbna. We predicted that when given identical rations, fish receiving clumped food would swim faster, expending more energy, and therefore grow more slowly than fish receiving dispersed food After 2 wk under these 2 foraging regimes, juvenile walleye pollock receiving clumped food swam 50% faster, but experienced 19% lower growth, than walleye pollock receiving dispersed food. Sablefish demonstrated only a weak swim speed response, with no difference in growth between food distributions. Our results demonstrate that although social foraging may increase encounter rates with food, in some specles there may also be an energetic cost for this behavior, which will have a n influence upon energet~c efficiency, potentially affecting growth and survival.

Resource Consumption Variance Within and Among Individuals: On Coloniality in Spiders

Several models for the economics of sociality demonstrate that the probability distribution of an individual's resource consumption could depend on the size of its social group. When groups share food clumps discovered by any member, expected resource consumption may increase or decline with group size, but the individual's resource consumption variance will ordinarily decline as group size increases. Hence, the formation and dissolution of social groups may represent risk—sensitive responses to foraging success. A series of field studies report greater mean prey consumption and decreased prey consumption variance as group size in colonial spiders increases. Several hypotheses attempt to explain this effect. Coloniality may promote acquisition of information concerning temporal variation in prey availability. Colony membership may permit individuals to steal food when the number of captured prey varies spatially. Finally, colonality can enhance food acquisition because prey (that might otherwise escape) ricochet from one spider's web to another group member's web. Our purpose was to quantify each hypothesis and ask if the resulting models predict reduced resource consumption variance as group size increases. We model each mechanism and conclude that stealing prey is the simplest explanation for variance reduction in spider colonies. We emphasize that variance among individuals need not provide a good estimate of the within—individual resource consumption variance, and it is the latter variance to which risk sensitivity responds.

Spatial clumping of food increases its monopolization and defense by convict cichlids, Cichlasoma nigrofasciatum

The hypothesis that resource monopolization and defense increase as the spatial clumping of resources increases was tested using groups of three convict cichlids competing for 120 Daphnia magna prey. Spatial clumping was manipulated by varying the distance (3, 20, or 40 cm) between three tubes through which the prey appeared. As predicted, monopolization of prey (percentage eaten by the dominant fish) and frequency of aggression (chases per minute) by dominant fish increased significantly as the distance between the tubes decreased. However, there was no evidence of individual flexibility in the aggressiveness (percentage of conspecifics chased) of dominant fish across treatments. Differences among dominant fish in aggressiveness were positively correlated with their ability to monopolize prey, but the strength of the correlation decreased as the distance between the tubes increased. Aggression appears to be a more effective mechanism of interference competition when resources are clumped than when resources are dispersed.

Co-operation and defection: Playing the field and the ESS

If food is patchily dispersed, food clumps being very rich, but rare and hard to find, each individual in a foraging flock then faces an evident dilemma: whether to co-operate and participate in the search, thus enhancing the rate by which rich patches are discovered, or to defect and let others do the searching, thus avoiding any possible expenditures and risks involved in the search (but enjoying the abundant resources once a rich patch is discovered). This conflict (and its possible solution) is treated as an example in the analysis of the synergistic n-player game presented in this paper. After deriving conditions for the existence of a mixed ESS in such games, the evolutionary stability of the mixed strategy against invasions by pure strategists, in particular against invasions by recognizable defectors, is analyzed. Whereas in any "degenerating" mixed-strategy model a recognizable defector can invade and spread, a "non-degenerating" model can sometimes yield a mixed ESS which is immune to such invasions.

Black-tailed deer feeding bouts: dynamic events

Foraging bouts of captive black-tailed deer (Odocoileus hemionus columbianus Richardson) were investigated to examine how searching for food affects diet selection. We determined food preference for three types of food under ad libitum conditions and then studied the foraging of two deer in a 0.5-ha, vegetation-free pen in which we controlled food availability and distribution of the same three types of food. Our hypotheses included the following: (i) clumping of preferred food into patches would enable animals to better exploit food distributions; (ii) the switch from preferred to lower-ranked food would be gradual as preferred food was less frequently encountered; and (iii) deer would respond to a lower abundance of preferred foods by eating more of lower-ranked food items at each feeding location. Searching for food alone did not alter diet selection from ad libitum conditions. Deer nearly exhausted their highly preferred food item before switching to lower-ranked ones. Amount of preferred food already eaten during a trial was positively correlated with the time that animals continued searching before switching to lower-ranked food items. Switching was related to amount and type of food encountered and not to amount of food in the pen. Clumping of the preferred food had no significant effect on the amount of food eaten, but did significantly influence types of food encountered by one deer. When preferred food was abundant, it was not always completely eaten the first time a feeding platform was visited. Increases in the intake rates of nonpreferred food items resulted from deer visiting more feeding stations containing nonpreferred food items and not from deer eating more food at each feeding station.