Individual Intake Rates Research Articles

Cooperative foraging expands dietary niche but does not offset intra-group competition for resources in social spiders

Group living animals invariably risk resource competition. Cooperation in foraging, however, may benefit individuals in groups by facilitating an increase in dietary niche. To test this, we performed a comparative study of social and solitary spider species. Three independently derived social species of Stegodyphus (Eresidae) occupy semi-arid savannas and overlap with three solitary congeners. We estimated potential prey availability in the environment and prey acquisition by spiders in their capture webs. We calculated dietary niche width (prey size) and breadth (taxonomic range) to compare resource use for these six species, and investigated the relationships between group size and average individual capture web production, prey biomass intake rate and variance in biomass intake. Cooperative foraging increased dietary niche width and breadth by foraging opportunistically, including both larger prey and a wider taxonomic range of prey in the diet. Individual capture web production decreased with increasing group size, indicating energetic benefits of cooperation, and variance in individual intake rate was reduced. However, individual biomass intake also decreased with increasing group size. While cooperative foraging did not completely offset resource competition among group members, it may contribute to sustaining larger groups by reducing costs of web production, increasing the dietary niche and reducing the variance in prey capture.

Latitudinal trends in habitat quality of shallow‑water flatfish nurseries

The habitat quality of European shallow-water nurseries was studied for 3 common flatfish species based on juvenile growth conditions. Field growth of 0-group plaice Pleuronectes platessa, flounder Platichthys flesus and sole Solea solea, from both published and unpublished studies, was compared with maximum growth predicted by a bioenergetics model based on the dynamic energy budget theory. In plaice and flounder, realized growth ratio decreased consistently during the growing season in most of the nurseries analyzed, indicating a widespread pattern of declining conditions. In sole, growth performance was not maximal, but as opposed to the other species, no clear temporal trend in realized growth ratio was observed. A latitudinal comparison of realized growth ratio over the various nurseries indicated clear positive trends for plaice and flounder, with better growth conditions at northern latitudes. In sole, despite some variability, the same trend was found during part of the summer. In the absence of clear gradients in benthic prey biomass, we hypothesize that increased food limitation in southern locations is most likely caused by interspecific competition reducing maximum individual intake rates. These results suggest that, in the context of global warming, habitat quality of southern European nurseries for juvenile growth may be particularly affected by the combined interaction of food and thermal constraints.

A unifying approach to allometric scaling of resource ingestion rates under limiting conditions

Individual resource ingestion rates depend on both individual body size and resource supply. A component of the latter, namely resource availability, is also body‐size dependent. This raises the question of the adequacy of simple scaling laws to describe the body‐size dependency of resource ingestion. Here we propose a model which integrates resource ingestion drivers by merging a scaling law for feeding metabolism and Holling's functional responses into a single mathematical framework. At any fixed level of resource supply, the model predicts a log‐log concave‐down relationship between resource ingestion rates and body size, rather than a simple scaling law. Deviations from the latter are accounted for by the body size dependency of resource limitations. Experimental and literature data describing patterns of perceived resource availability and individual intake rates under limiting conditions with increasing individual body size are used to validate the model's assumptions and predictions. The model incorporates and extends well‐established theoretical approaches and is intended as a step towards the integration of metabolic theory with behavioral ecology and population dynamics.

Prey choice affects the trade-off balance between predation and starvation in an avian herbivore

Animals make economic decisions on where to feed based on balancing intake rates with the risk of starvation. When foraging, they must also manage time allocation to feeding, antipredator behaviour and resource guarding. These behaviours are usually believed to be mutually exclusive. The most intense foraging effort is predicted to occur in low-risk situations, while more antipredator behaviour is seen in high-risk situations. We studied the time allocation of individual light-bellied brent geese, Branta bernicla hrota, and the proportion of the flock engaged in foraging and vigilance behaviour in high- and low-risk habitats. Geese faced with a depleted resource in a low-risk intertidal situation responded by foraging in a higher-risk terrestrial habitat. Individual intake rates and levels of agonistic interactions were higher during terrestrial feeding, but group vigilance was maintained, with certain flock positions showing disproportionate levels of vigilance. In contrast to previous studies, agonistic interactions were more frequent when the geese were feeding on a nonpatchy resource, suggesting that different mechanisms drive agonistic encounters in different habitats. We discuss these results in the context of a growing population with limited resources. By remaining behaviourally plastic, the geese are able to adapt novel trade-off strategies to use alternative prey types.

GROUP FORAGING BY A KLEPTOPARASITIC FISH: A STRONG INFERENCE TEST OF SOCIAL FORAGING MODELS

Animals that obtain food by using the investment of other foragers (klep- toparasites) often do so in groups. We tested whether group formation by a kleptoparasitic fish, the western buffalo bream (Kyphosus cornelii), fit the predictions of five social foraging models. Two aggregation economy models assumed that there was some benefit to grouping shared by group members, such as reduced predation risk or increased ability to gain access to resources. These models and a third, the dispersion (ideal free) economy model, assumed that kleptoparasites had perfect information regarding the quality of opportunities for klep- toparasitism. The other two models did not make this assumption. These producer-scrounger models assumed that some kleptoparasites (producers) discovered opportunities, while oth- ers used producers to reduce the costs of foraging. These last two models differed in whether foragers could estimate the state of current opportunities for kleptoparasitism. We compared typical group size, and the influence of group size on intake rate and the success of klep- toparasitic attempts, with the predictions of these models. We found that typical group size was larger during periods when opportunities for kleptoparasitism were poor than when good and that there was no influence of group size on the likelihood that the group was successful at kleptoparasitizing. Individual intake rate declined with group size for small group sizes, but increased with group size in large groups. However, large groups were rare. For small groups, only a producer-scrounger model allowing foragers to update their information could not be rejected. For large groups, neither that model nor the aggregation economy with foraging benefits model could be rejected. We compare these results with those of other studies of kleptoparasitic groups.

DOMINANCE AND PATCH DISTRIBUTION OF GREAT TITS PARUS MAJOR: A PRELIMINARY TEST OF THEORETICAL PREDICTIONS

1. Predictions from existing models on 'ideal free distribution of unequal competitors' were tested by comparing the intake rates of the birds in relation to their bowl preferences. 2. Three types of distributions across two patches (1 and 2, where 1 provides higher food production rate than 2) were tested: truncated (H 1 > L 1 H 2 > L 2 ), semi-truncated (H 1 > H 2 > L 1 L 2 ), and mixed (H 1 H 2 > L 1 L 2 ). H i indicates highest individual intake rate in patch i, L i indicates lowest individual intake rate in patch i. 3. The birds were studied in groups of six. They were fed by presenting two bowls containing mealworms, one bowl had twice the food-input rate of the other. 4. Four birds visited the high-density bowl more often than the low-density bowl, whereas the other two birds showed no or opposite preferences. 5. The semi-truncated distribution was the only distribution that could not be rejected by any criteria. 6. When members of the experimental group were exchanged, the changes in individual intake rates suggest that the intake rate is related to each individual's quantitative competitive ability rather than to the individual's rank.

The value of titration experiments: a reply to Moody et al. (1996)

Behavioural ecology has been remarkably successful as a discipline, in part because much of the theory that has been developed is readily testable through manipulative experiments in the laboratory and the ®eld. Although the preferred currency for most theories is ®tness, it has usually been possible to incorporate modi®cations or simplifying assumptions that allow the use of more readily observed, surrogate measures of ®tness. Admittedly this does require some compromise, but in the grand tradition of behavioural ecology this trade-o€ is easy to solve: the bene®ts outweigh the costs. Moody et al. (1996) (henceforth referred to as MHM), explored the role that risk dilution will have on the results of experiments that investigate predation risk ± foraging trade-o€s. Assuming that the probability of being killed by a predator is inversely related to the number of animals feeding at a site, they developed a model that incorporates risk dilution into ideal free distributions (IFDs) under predation risk. Due to risk dilution, their model suggests that patch choice decisions will be a€ected not only by the presence of predators and relative food availability, but also by the total number of animals. Their suggestions are very useful in advancing our theoretical understanding of this problem. However, we disagree with their critical assessment of the use of the IFDs as an experimental tool to quantify decisions involving con icting demands. In 1989, Abrahams and Dill (henceforth referred to as AD) published a paper that described an experimental technique that could be used to quantify the energetic equivalence of the risk of predation and, more importantly, presented the results of experiments that tested its validity. The approach is based upon a continuous-input IFD. With this type of IFD, when food is the only parameter that describes patch quality, the spatial distribution of the foragers will match the spatial distribution of their food. A large number of authors have demonstrated that under experimental conditions, animals closely conform to an IFD (for a review see Milinski and Parker 1991). Incorporation of an additional patch parameter (i.e. the risk of predation) should result in a change in the spatial distribution of animals that re ects the relative change in patch quality. Di€erences in individual intake rates will then provide a measure of the assessed value of this parameter from the animals' perspective, and can be determined empirically. This approach describes our ®rst experiment. Our second experiment then tested a prediction (derived from the results of the ®rst experiment) concerning how much additional food must be added to the dangerous patch in order for the foragers to consider it equivalent to the safe patch. To make this prediction, we assumed a linear relationship between energy and ®tness. This was not intended to describe the actual relationship between these two parameters, but to make it possible to interpret deviations in the spatial distribution with respect to the relationship between energy and ®tness. For example, if more animals than we predicted used the dangerous location after the addition of extra food, then we concluded that the ®tness bene®ts of the patch increased by more than the manipulated energy bene®ts. Conversely, if the addition of extra food resulted in fewer animals than predicted using the dangerous location, then we concluded that the ®tness bene®ts of the patch increased by less than the manipulated energy bene®ts. These experiments demonstrated that even with the linearity assumption, we could fairly accurately predict the amount of additional food that must be added to a dangerous patch to o€set the risk of predation. MHM were critical of this approach for three reasons: (1) our calculations did not incorporate risk dilution; (2) the relationship between ®tness and energy should have no in uence on the calculations necessary to Behav Ecol Sociobiol (1998) 44: 147±148 O Springer-Verlag 1998

Risk sensitivity in starlings: variability in food amount and food delay

Starlings' preferences for constant versus variable food sources were studied in the laboratory. The constant alternative gave a fixed amount of food after a fixed delay. The variable alternative offered either a varying amount of food after a fixed delay (treatment A) or a fixed amount of food after a variable delay (treatment B). In both treatments the ratio of amount of food over trial length (the sum of intertrial interval plus delay and handling times) of the constant alternative equaled the average of the two ratios of the variable alternative. The variable ratios were 30% higher and 30% smaller than the fixed ratio. In free-choice trials (both options available in each trial), the subjects were risk-averse or indifferent in treatment A and indifferent or riskprone in treatment B. In no-choice trials (only one source available per trial), the latency to respond was longer in the variable than in the constant source in treatment A and the opposite in treatment B. The greater preference for variability in time than for variability in reward amount is not consistent with either maximizing the ratio of expected energy over expected time or the expected ratio of energy over time for individual trials. There was a negative correlation between individual intake rate and degree of risk proneness for both kinds of variability. We present a model of choice based on an information-processing theory for temporal memory that accounts for the different effects of variability in delay and in amount but cannot explain the effects of intake rate. [Behav Ecol 1991;2:301–308]