Modelling locust foraging: How and why food affects group formation

Locusts are short horned grasshoppers that exhibit two behaviour types depending on their local population density. These are: solitarious, where they will actively avoid other locusts, and gregarious where they will seek them out. It is in this gregarious state that locusts can form massive and destructive flying swarms or plagues. However, these swarms are usually preceded by the aggregation of juvenile wingless locust nymphs. In this paper we attempt to understand how the distribution of food resources affect the group formation process. We do this by introducing a multi-population partial differential equation model that includes non-local locust interactions, local locust and food interactions, and gregarisation. Our results suggest that, food acts to increase the maximum density of locust groups, lowers the percentage of the population that needs to be gregarious for group formation, and decreases both the required density of locusts and time for group formation around an optimal food width. Finally, by looking at foraging efficiency within the numerical experiments we find that there exists a foraging advantage to being gregarious.     

How anthropogenic noise affects foraging

The influence of human activity on the biosphere is increasing. While direct damage (e.g. habitat destruction) is relatively well understood, many activities affect wildlife in less apparent ways. Here, we investigate how anthropogenic noise impairs foraging, which has direct consequences for animal survival and reproductive success. Noise can disturb foraging via several mechanisms that may operate simultaneously, and thus, their effects could not be disentangled hitherto. We developed a diagnostic framework that can be applied to identify the potential mechanisms of disturbance in any species capable of detecting the noise. We tested this framework using Daubenton's bats, which find prey by echolocation. We found that traffic noise reduced foraging efficiency in most bats. Unexpectedly, this effect was present even if the playback noise did not overlap in frequency with the prey echoes. Neither overlapping noise nor nonoverlapping noise influenced the search effort required for a successful prey capture. Hence, noise did not mask prey echoes or reduce the attention of bats. Instead, noise acted as an aversive stimulus that caused avoidance response, thereby reducing foraging efficiency. We conclude that conservation policies may seriously underestimate numbers of species affected and the multilevel effects on animal fitness, if the mechanisms of disturbance are not considered.     

Social context affects risk taking by a satellite species in a mixed-species foraging group

Mixed-species flocks of birds form during winter in the easterndeciduous forests of North America. These flocks consist oftwo flock-leading nuclear species, tufted titmouse (Baeolophusbicolor) and Carolina chickadee (Poecile carolinensis), andseveral follower, or satellite, species, including downy woodpecker(Picoides pubescens) and white-breasted nuthatch (Sitta carolinensis).Hypotheses explaining the adaptiveness of participation in suchmixed-species foraging groups have focused on increased foragingsuccess and/or decreased predation risk. We tested the predictionthat if nuthatches join nuclear species to reduce predationrisk, they should be more reluctant to visit an exposed feederin the absence of titmice than in their presence. When the feederwas positioned 16 m from forest cover, latency to visit thefeeder was greater for both male and female nuthatches whentitmice were absent. Removal of titmice had no effect on latencyat 8 m. In the absence of titmice, nuthatches visited the feederless frequently at both distances. These results indicate thatreduced predation risk is a benefit that satellite species gainby flocking with nuclear species.     

Evolutionary drivers of group foraging: A new framework for investigating variance in food intake and reproduction

A proposed fundamental driver of group living is more reliable, predictable foraging and reproduction, i.e., reduced variance in food intake and reproductive output. However, existing theories on variance reduction in group foraging are simplistic, refer to variance at the level of individuals and groups without linking the two, and do not spell out crucial underlying assumptions. We provide a new, widely applicable framework for identifying when variance reduction conveys fitness benefits of group foraging in a wide range of organisms. We discuss critical limitations of established theories, the Central Limit Theorem and Risk‐Sensitive Foraging Theory applied to group foraging, and incorporate them into our framework while addressing the confusion over the levels of variance and identifying previously unaddressed assumptions. Through a field study on colonial spiders, Cyrtophora citricola, we demonstrate the importance of evaluating the level of food sharing as a critical first step, previously overlooked in the literature. We conclude that variance reduction provides selective advantages only under narrow conditions and does not provide a universal benefit to group foraging as previously proposed. Our framework provides an important tool for identifying evolutionary drivers of group foraging and understanding the role of fitness variance in the evolution of group living.     

How food type and brood influence foraging decisions of Lasius niger scouts

We investigated how the type of food (sucrose or protein) and the presence of brood influence foraging decisions of Lasius niger L. scouts. In particular, we studied whether and how these parameters alter the drinking behaviour of scouts and the allocation of workers to food retrieving and recruiting tasks. We analysed drinking and recruiting behaviour of single scouts from nests with or without brood that encountered a proteinaceous or sucrose droplet. A substantial fraction of scouts encountering a proteinaceous droplet did not ingest it and did not then return to the nest whereas nearly all drank at sugar droplets; brood presence did not influence this decision. Once an ant started drinking, it needed to drink a critical volume before returning to the nest; this critical volume did not depend on the type of food and the presence of brood. Scouts laid a trail only if they returned to the colony. Food type and brood presence altered the proportion of individuals that laid a trail but not the individual trail-laying intensity. We discuss the consequences of this decision system through simple individual assessments and decision rules, with regard to the self-organized foraging patterns of this species and the efficient collective exploitation of natural 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.     

How does food distance influence foraging in the ant Lasius niger: the importance of home-range marking

We study the influence of food distance on the individual foraging behaviour of Lasius niger scouts and we investigate which cue they use to assess their distance from the nest and accordingly tune their recruiting behaviour. Globally, the number of U-turns made by scouts increases with distance resulting in longer travel times and duration of the foraging cycle. However, over familiar areas, home-range marking reduces the frequency and thereby the impact of U-turns on foraging times leading to a quicker exploitation of food sources than over unmarked set-ups. Regarding information transfer, the intensity of the recruitment trail reaching the nest decreases with increasing food distance for all set-ups and is even more reduced in the absence of home-range marking. Hence, the probability of a scout continuing to lay a trail changes along the homeward journey but in a different way according to home-range marking. Over unexplored setups, at a given distance from the food source, the percentage of returning trail-laying ants remains unchanged for all tested nest-feeder distances. Hence, the tuning of the trail recruiting signal by scouts was not influenced by an odometric estimate of the distance already travelled by the ants during their outward journey to the food. By contrast, over previously explored set-ups, a distance-related factor – that is the intensity of home-range marking – strongly influences their recruiting behaviour. In fact, over a home-range marked bridge, the probability of returning ants maintaining their trail-laying behaviour increases with decreasing food distance while the gradient of home-range marks even induces ants which have stopped laying a trail to resume this behaviour in the nest vicinity. We suggest that home-range marking laid passively by walking ants is a relevant cue for scouts to indirectly assess distance from the nest but also local activity level or foraging risks in order to adaptively tune trail recruitment and colony foraging dynamics. Received 13 July 2004; revised 26 January and 20 May 2005; accepted 2 July 2005.     

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

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

How a simple adaptive foraging strategy can lead to emergent home ranges and increased food intake

Animals often alternate between searching for food locally and moving over larger distances depending on the amount of food they find. This ability to switch between movement modes can have large implications on the fate of individuals and populations, and a mechanism that allows animals to find the optimal balance between alternative movement strategies is therefore selectively advantageous. Recent theory suggests that animals are capable of switching movement mode depending on heterogeneities in the landscape, and that different modes may predominate at different temporal scales. Here we develop a conceptual model that enables animals to use either an area‐concentrated food search behavior or undirected random movements. The model builds on the animals’ ability to remember the profitability and location of previously visited areas. In contrast to classical optimal foraging models, our model does not assume food to be distributed in large, well‐defined patches, and our focus is on animal movement rather than on how animals choose between foraging patches with known locations and value. After parameterizing the fine‐scale movements to resemble those of the harbor porpoise Phocoena phocoena we investigate whether the model is capable of producing emergent home ranges and use pattern‐oriented modeling to evaluate whether it can reproduce the large‐scale movement patterns observed for porpoises in nature. Finally we investigate whether the model enables animals to forage optimally. We found that the model was indeed able to produce either stable home ranges or movement patterns that resembled those of real porpoises. It enabled animals to maximize their food intake when fine‐tuning the memory parameters that controlled the relative contribution of area concentrated and random movements.     

Modelling foraging ants in a dynamic and confined environment

In social insects, the superposition of simple individual behavioral rules leads to the emergence of complex collective patterns and helps solve difficult problems inherent to surviving in hostile habitats. Modelling ant colony foraging reveals strategies arising from the insects’ self-organization and helps develop of new computational strategies in order to solve complex problems. This paper presents advances in modelling ants’ behavior when foraging in a confined and dynamic environment, based on experiments with the Argentine ant Linepithema humile in a relatively complex artificial network. We propose a model which overcomes the problem of stagnation observed in earlier models by taking into account additional biological aspects, by using non-linear functions for the deposit, perception and evaporation of pheromone, and by introducing new mechanisms to represent randomness and the exploratory behavior of the ants.     

Optimal foraging: food patch depletion by ruddy ducks

Summary I studied the foraging behavior of ruddy ducks (Oxyura jamaicensis) feeding on patchily distributed prey in a large (5-m long, 2-m wide, and up to 2-m deep) aquarium. The substrate consisted of a 4x4 array of wooden trays (1.0-m long, 0.5-m wide, and 0.1-m deep) which contained 6 cm of sand. Any tray could be removed from the aquarium and loaded with a known number of prey. One bird foraged in the aquarium at a time; thus, by removing a food tray after a trial ended and counting the remaining prey, I calculated the number of prey consumed by the bird. I designed several experiments to determine if ruddy ducks abandoned a food patch in a manner consistent with the predictions of a simple, deterministic, patch depletion model. This model is based on the premise that a predator should maximize its rate of net energy intake while foraging. To accomplish this, a predator should only remain in a food patch as long as its rate of energy intake from that patch exceeds the average rate of intake from the environment. In the majority of comparisons, the number of food items consumed by the ruddy ducks in these experiments was consistent with the predictions of the foraging model. When the birds did not forage as predicted by the model, they stayed in the patch longer and consumed more prey than predicted by the model. An examination of the relation between rate of net energy intake and time spent foraging in the food patch indicated that by staying in a patch longer than predicted, the ruddy ducks experienced only a small deviation from maximum rate of net energy intake. These results provided quantitative support for the prediction that ruddy ducks maximize their rate of net energy intake while foraging.     

How group size affects vigilance dynamics and time allocation patterns: the key role of imitation and tempo

In the context of social foraging, predator detection has been the subject of numerous studies, which acknowledge the adaptive response of the individual to the trade-off between feeding and vigilance. Typically, animals gain energy by increasing their feeding time and decreasing their vigilance effort with increasing group size, without increasing their risk of predation ('group size effect'). Research on the biological utility of vigilance has prevailed over considerations of the mechanistic rules that link individual decisions to group behavior. With sheep as a model species, we identified how the behaviors of conspecifics affect the individual decisions to switch activity. We highlight a simple mechanism whereby the group size effect on collective vigilance dynamics is shaped by two key features: the magnitude of social amplification and intrinsic differences between foraging and scanning bout durations. Our results highlight a positive correlation between the duration of scanning and foraging bouts at the level of the group. This finding reveals the existence of groups with high and low rates of transition between activities, suggesting individual variations in the transition rate, or 'tempo'. We present a mathematical model based on behavioral rules derived from experiments. Our theoretical predictions show that the system is robust in respect to variations in the propensity to imitate scanning and foraging, yet flexible in respect to differences in the duration of activity bouts. The model shows how individual decisions contribute to collective behavior patterns and how the group, in turn, facilitates individual-level adaptive responses.     

The dilemma of foraging herbivores: dealing with food and fear

For foraging herbivores, both food quality and predation risk vary across the landscape. Animals should avoid low-quality food patches in favour of high-quality ones, and seek safe patches while avoiding risky ones. Herbivores often face the foraging dilemma, however, of choosing between high-quality food in risky places or low-quality food in safe places. Here, we explore how and why the interaction between food quality and predation risk affects foraging decisions of mammalian herbivores, focusing on browsers confronting plant toxins in a landscape of fear. We draw together themes of plant–herbivore and predator–prey interactions, and the roles of animal ecophysiology, behaviour and personality. The response of herbivores to the dual costs of food and fear depends on the interplay of physiology and behaviour. We discuss detoxification physiology in dealing with plant toxins, and stress physiology associated with perceived predation risk. We argue that behaviour is the interface enabling herbivores to stay or quit food patches in response to their physiological tolerance to these risks. We hypothesise that generalist and specialist herbivores perceive the relative costs of plant defence and predation risk differently and intra-specifically, individuals with different personalities and physiologies should do so too, creating individualised landscapes of food and fear. We explore the ecological significance and emergent impacts of these individual-based foraging outcomes on populations and communities, and offer predictions that can be clearly tested. In doing so, we provide an integrated platform advancing herbivore foraging theory with food quality and predation risk at its core.     

Predicting group size in primates: foraging costs and predation risks

We present a direct test of the long-standing hypothesis thatfood competition limits primate group size. Group size is acritical social variable because it constrains most other aspectsof social organization. We develop a simple population-specificindex of indirect feeding competition based on daily foragingcosts. This index explains nearly two-thirds of between-populationvariation in mean group sizes of mostly fruit-eating (but notof mostly leaf-eating) primates. Group size is also significantlyrelated to body size and terrestriality (or use of open country),which are suspected correlates of predation risk, although feedingcompetition remains an important predictor of group size evenwhen these correlates are controlled. Phylogeny also appearsto be important: the differences between observed mean populationgroup sizes and those predicted using ecological factors aremost positive for the Old World monkeys and most negative forthe lemuroids in our sample. The weak relationship between groupsize and feeding competition found for folivorous species maybe explained either by the energetic constraints of a leafydiet or by limits to group size imposed by infanticide as ahabitual male reproductive strategy.