Honeyeater foraging: A test of optimal foraging theory

Honeyeaters (Meliphagidae) were observed foraging for nectar from Lambertia formosa inflorescences, each of which has seven flowers. The frequency distribution of numbers of flowers probed per visit to an inflorescence was found to be bimodal, with one peak at two and the other at seven. It is hypothesized that this frequency distribution results from a rule of departure from inflorescences that maximizes the net rate of energy gain. Patterns of nectar distribution were determined for a large sample of inflorescences. In addition the extent to which the honeyeaters re-probe flowers during a visit to an inflorescence was estimated. From these data and from field measurements of the times required by the honeyeaters to perform the various foraging behaviours, computer simulations of honeyeater foraging were constructed. These simulations led in turn to optimal frequency distributions of numbers of flowers probed per inflorescence that were bimodal but had peaks at 1 and 7 instead of 2 and 7. Although the observed and predicted behaviour were consequently similar, the difference between them was nevertheless significant. This difference could have been due to the birds' transient occupancy of the study area.     

Behavioural consequences of hunting by expectation: A simulation study of foraging tactics

Foraging behaviour has been simulated using a model that predicts the path taken by an animal foraging for particulate food, the path being defined by the animal's remembrance of its previous foraging success. This is represented in the model by a two-dimensional vector with its modulus encoding an exponentially smoothed average of the animal's feeding rate and its orientation encoding the average direction of travel. As food is ingested the amount ingested and its position are used to update the remembrance, and the animal turns in the adjusted direction travelling at a speed inversely proportional to the average feeding rate. Foraging paths simulated in a patchy environment are shown to have the following properties: (i) They tend to avoid crossing the boundaries of patches from the inside. (ii) They tend to avoid intersecting themselves. (iii) When they do intersect themselves they usually do so more or less at right angles. (iv) They ascend gradients of density of food.     

Agent-based model approach to optimal foraging in heterogeneous landscapes: effects of patch clumpiness

Optimal foraging theory concerns animal behavior in landscapes where food is concentrated in patches. The efficiency of foraging is an effect of both the animal behavior and the geometry of the landscape; furthermore, the landscape is itself affected by the foraging of animals. We investigated the effect of landscape heterogeneity on the efficiency of an optimal forager. The particular aspect of heterogeneity we considered was "clumpiness"– the degree to which food resource patches are clustered together. The starting point for our study was the framework of the Mean Value Theorem (MVT) by Charnov. Since MVT is not spatially explicit, and thus not apt to investigate effects of clumpiness, we built an agent-based (or individual-based) model for animal movement in discrete landscapes extending the MVT. We also constructed a model for generating landscapes where the clumpiness of patches can be easily controlled, or "tuned", by an input parameter. We evaluated the agent based model by comparing the results with what the MTV would give, i.e. if the spatial effects were removed. The MVT matched the simulations best on landscapes with random patch configuration and high food recovery rates. As for our main question about the effects of clumpiness, we found that, when landscapes were highly productive (rapid food replenishment), foraging efficiency was greatest in clumped landscapes. In less productive landscapes, however, foraging efficiency was lowest in landscapes with a clumped patch distribution.     

A generalized model of optimal diets

The scope of current optimal diet theory is greatly restricted by certain rather stringent assumptions upon which it rests. One of these is that the type of prey a predator encounters next is not influenced by the last type encountered. The purpose of this paper is to relax this and certain other assumptions and, in so doing, arrive at a set of rules for determining the structure of the optimal diet which are analogous to, but more general than, those of current theory. Once obtained, these rules are contrasted with their earlier analogues. The major findings are that (1) prey types are not necessarily added to the optimal diet in order of decreasing energy to handling time ratio, and (2) the abundance of a type initially excluded from the diet is not necessarily irrelevant in determining whether or not that type will be included in the future. These findings show that, in the more general case considered, the structure of the optimal diet may be quite different than predicted by current theory.     

A mathematical framework for optimal foraging of herbivores

The aim of this paper is to study a model of optimal foraging of herbivores (with special reference to ungulates) assuming that food distribution is arbitrary. Usually the analysis of foraging of herbivores in the framework of optimal foraging theory is based on the assumption of a patchy food distribution. We relax this assumption and we construct more realistic models. The main constraint of our model is the total amount of food which the animal may eat and the currency is the total foraging time. We represent total foraging time as a variational expression depending on food eaten and the length of the path. We prove that there exists a threshold for food acquisition. More explicitly, it exists a positive real number such that, at any point x of the path, the animal either eats till the density of food is decreased to the value or, if the density of food at x is less than , there it does not eat. We discuss the results and emphasize some biologically important relationships among model parameters and variables. Finally, we try to give a sound biological interpretation of our results.     

General activity and foraging tactics in a water bug

Diplonychus indicus is a highly versatile predator that forages both actively and from ambush. However, no correlations between predatory mode changes and predatory performance have yet been evidenced. The hypotheses that time spent foraging actively was proportional to time spent locomotory active and that time spent ambushing was proportional to time spent quiescent were tested during animal development. Locomotory activity increases during development due to increases in both frequency and duration of swimming bouts. The frequency of position changes increases as well. Eggbearing males were less active than other adults. However, the proportion of active foraging did not vary significantly with developmental stage and no correlations between activity level and predatory mode were found. Changes in predatory tactics inDiplonychus indicus differ from those reported in other predators as they are not related to any of the usual biotic or abiotic factors.     

Food sources and foraging tactics in tropical rain pools

Pools on exposed rocks are common over much of Africa. Based on dimensions and position, those examined are of three types. Each type is inhabited by larvae of virtually a single dipteran species at high densities (over two million larvae m^-2).
Location of the pools suggests that food might be a limiting factor. However, events, including defecation in pools by civets and genets, fruit fall and wind-borne pollen, apparently ensure that this is not the case. In this environment of superabundance animals are presumably free to choose favoured items of food.
Each animal species does, indeed, take a characteristic assemblage of food items. However, each species is shown to eat whatever it can swallow, differences in gut contents being due to differences in the characteristic food items available in each type of pool. Most algae are excluded because they are too large or inaccessible, which means that the pool food chains are based largely on allochthonous detritus. There is no reason to believe that food type, perse , has any influence in determining which of the three dipteran species is present.     

Age-specific forced polymorphism: implications of ontogenetic changes in morphology for male mating tactics

Age-specific forced polymorphism is the presence of two or more distinct phenotypes (here we consider only males) that occur in separate sexually mature age groups (e.g., horns in older males but not younger males). The life-stage morph maturation hypothesis posits that all younger males that possess a particular structure can transform into older males with a different structure, most likely via the influence of hormones. The life-stage morph selection hypothesis posits that polymorphism is due to intense selection resulting in a highly nonrandom sample of younger males surviving to become older males, thus leading to different mean phenotypes in different age groups. We conducted an extensive review of literature from the past 20 years (1983-2003) for cases of age-specific forced polymorphism. Overall, we found only a few cases that fit our criteria of age-specific forced polymorphism, and we argue that most (e.g., orangutans, elephant seals) have likely arisen via the life-stage morph maturation mechanism, but we also present several examples (e.g., green anole lizards) that appear to be candidates for life-stage morph selection. However, none of the reviewed studies provided enough information (e.g., age of morphs, growth patterns of the morphological structure) to definitively invoke either of the two mechanisms. We suggest that age-specific forced polymorphism is more common than reflected in this review and that future studies should gather demographic and laboratory data that will directly compare the life-stage morph maturation and life-stage morph selection hypotheses.     

Age-specific differences in the winter foraging strategies of rooks Corvus frugilegus

Summary Foraging efficiency and intraspecific competition were compared between wild adult and immature rooks Corvus frugilegus with respect to flock size. Behavioural time budgets, and observations of prey selection and prey energetic values revealed that adult rooks in large flocks (> 50 individuals) consumed smaller, less profitable prey, but allocated more time to feeding and fed at a faster rate and with greater success than adults in small flocks. By contrast, immature rooks in flocks of more than 30 individuals allocated proportionally less time to feeding, fed at a lower rate and fed with no increase in success rate than when foraging in smaller flocks. Agonistic encounters and the avoidance of adults by immature rooks appeared responsible for such inefficient foraging. Hence immature rooks showed a preference for smaller flocks (< 50 individuals) with low adult: immature ratios while adults preferred larger flocks (> 50 individuals). We discuss the possible influence of competitive disadvantages on immature rook distribution, flock composition and post-natal dispersal.     

Flexible search tactics and efficient foraging in saltatory searching animals

Summary Foraging is one of the most important endeavors undertaken by animals, and it has been studied intensively from both mechanistic-empirical and optimal foraging perspectives. Planktivorous fish make excellent study organisms for foraging studies because they feed frequently and in a relatively simple environment. Most optimal foraging studies of planktivorous fish have focused, either on diet choice or habitat selection and have assumed that these animals used a cruise search foraging strategy. We have recently recognized that white crappie do not use a cruise search strategy (swimming continuously and searching constantly) while foraging on zooplankton but move in a stop and go pattern, searching only while paused. We have termed thissaltatory search. Many other animals move in a stop and go pattern while foraging, but none have been shown to search only while paused. Not only do white crappie search in a saltatory manner but the components of the search cycle change when feeding on prey of different size. When feeding on large prey these fish move further and faster after an unsuccessful search than when feeding on small prey. The fish also pause for a shorter period to search when feeding on large prey. To evaluate the efficiency of these alterations in the search cycle, a net energy gain simulation model was developed. The model computes the likelihood of locating 1 or 2 different size classes of zooplankton prey as a function of the volume of water scanned. The volume of new water searched is dependent upon the dimensions of the search volume and the length of the run. Energy costs for each component of the search cycle, and energy gained from the different sized prey, were assessed. The model predicts that short runs produce maximum net energy gains when crappie feed on small prey but predicts net energy gains will be maximized with longer runs when crappie feed on large prey or a mixed assemblage of large and small prey. There is an optimal run length due to high energy costs of unsuccessful search when runs are short and reveal little new water, and high energy costs of long runs when runs are lengthy. The model predicts that if the greater search times observed when crappie feed on small prey are assessed when they feed on a mixed diet of small and large prey, net energy gained is less than if small prey are deleted from the diet. We believe the model has considerable generality. Many animals are observed to move in a saltatory manner while foraging and some are thought to search only while stationary. Some birds and lizards are, known to modify the search cycle in a manner similar to white crappie.Components of the search cycle and dimensions of the location space SST (sec) Successful search time — the average time stationary prior to a pursuit - USST (sec) Unsuccessful search time — the average time stationary prior to a run - PT (sec) Pursuit time-PL/SS — the time to pursue prey at a given distance away. It is calculated by dividing the pursuit distance by swim speed - RT (sec) Run time-RL/SS — the time to complete a run of a given length. It is calculated by dividing the run length, by swim speed - PL (cm) Pursuit length-distance moved to attack prey - RL (cm) Run length-distance moved between consecutive searches - SS (cm/sec) Swim speed — the speed of movement during a pursuit or run - LS (l) Location space — the area or volume within which prey are located. In the case of white crappie the search space is shaped like a pie wedge with the fish positioned at the apex of the wedge - LA (^o) Location angle—the angle of the wedge-shaped search space - LH (cm) Location height—the height of the wedge-shaped search space - LD (cm) Location distance—the length of long axis of the wedge-shaped search space. Components of the location probability model RND Random number-random number generated through BASICA - SV (l) Search volume—the volume of water actually searched after one run of given length - SV_MAX (l) Maximum search volume—the greatest search volume that can be based upon LA, LH, LD and unaffected by the previous search - SV_R (l) Search volume researched—that volume of SV_MAX that is researched where RLo Search volume unsearched—that volume of SV_MAX not previously searched - AD (#/1) Absolute density—the density of zooplankton prey in numbers per liter - VD (#) Visual density—the number of zooplankton prey in the search volume - LP (%) Location probability—the probability that one or more prey are in the search volume Components of the net energy gain model NEG (cal/sec) Net energy gain-total calories ingested, less total calories used, divided by total time. - E _e (cal) Energy expended on the search cycle - E _i (cal) Energy intake - e _p (cal) Energy content of a given individual prey - P _i Total number of prey ingested - e _r (cal) Energy expended while searching - e _s (cal) Energy expended while swimming - T _t (sec) Total time-time expended to eat a given number of prey     

Time's crooked arrow: optimal foraging and rate-biased time perception

Time perception is critical to animal behaviours requiring anticipation of future events based on present information about the environment. Most models of animal foraging assume that animals are capable of measuring absolute time despite evidence that animals measure time with predictable biases in mean and variance. We incorporate the evidence for a rate-biased subjective time perception into a classic model of optimal foraging, the marginal value theorem. If acceleration of the clock rate is proportional to food intake rate and time is perceived similarly when in transit between patches as it is while waiting in a patch following eating, organisms are predicted to follow the predictions of the marginal value theorem exactly. However, a nonlinear relationship between clock rate and food intake rate, unequal wait and transit time perception, or any lag in the clock predicts characteristic suboptimal behaviour. We discuss how this mechanism for suboptimal behaviour compares with others in the literature and how it can be recognized in experiments. Copyright 2002 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.     

Modeling hunter-gatherer decision making: complementing optimal foraging theory

While optimal foraging theory has been of considerable value for understanding hunter-gatherer subsistence patterns, there is a need for a complementary approach to human foraging behavior which focuses on decision-making processes. Having made this argument, the paper proposes the type of modeling approach that should be developed, using decision making during encounter foraging as an example. This model concerns the individual decision maker attempting to improve his foraging efficiency, rather than maximize it, under the constraint of limited information and with conflicting goals. This is illustrated by applying it to the Valley Bisa hunters using computer simulation.     

Strategy maps: Generalised giving-up densities for optimal foraging

Finding a common currency for benefits and hazards is a major challenge in optimal foraging theory, often requiring complex computational methods. We present a new analytic approach that builds on the Marginal Value Theorem and giving-up densities while incorporating the nonlinear effect of predation risk. We map the space of all possible environments into strategy regions, each corresponding to a discrete optimal strategy. This provides a generalised quantitative measure of the trade-off between foraging rewards and hazards. This extends a classic optimal diet choice rule-of-thumb to incorporate the hazard of waiting for better resources to appear. We compare the dynamics of optimal decision-making for three foraging life-history strategies: One in which fitness accrues instantly, and two with delays before fitness benefit is accrued. Foragers with delayed-benefit strategies are more sensitive to predation risk than resource quality, as they stand to lose more fitness from a predation event than instant-accrual foragers.     

Macronutrient modifications of optimal foraging theory: An approach using indifference curves applied to some modern foragers

The use of energy (calories) as the currency to be maximized per unit time in Optimal Foraging Models is considered in light of data on several foraging groups. Observations on the Ache, Cuiva, and Yora foragers suggest men do notattempt to maximize energetic return rates, but instead often concentrate on acquiring meat resources which provide lower energetic returns. The possibility that this preference is due to the macronutrient composition of hunted and gathered foods is explored. Indifference curves are introduced as a means of modeling the tradeoff between two desirable commodities, meat (protein-lipid) and carbohydrate, and a specific indifference curve is derived using observed choices in five foraging situatiuons. This curve is used to predict the amount o meat that Mbuti foragers will trade for carbohydrate, in an attempt to test the utility of the approach.     

A general version of optimal foraging theory: The effect of simultaneous encounters

Optimal foraging theory is extended so as to treat cases where a choice between several options is required. A new version of optimal foraging theory is derived under this assumption of simultaneous encounters of prey species and proved by using a set-theoretic approach. On the basis of this new version, it is demonstrated that, in general, no unique ranking of food types can be specified only from knowledge of the intrinsic properties of the food types. It is demonstrated that a food type may become less frequent in the diet as a result of becoming more abundant in the environment; that an increase in the abundance of a food type represented in the diet may have the effect that new food types enter the diet; and that an increase in the overall food abundance may imply that new types are included and/or old ones are excluded.     

Fluctuations in prey density: effects on the foraging tactics of scolopendrid centipedes

Foraging animals are faced with the problem of acquiring information about prey populations and utilizing that information in making foraging decisions. In this paper the effect of variation in prey density on the search tactics and mechanisms of prey density assessment in the centipede Scolopendra polymorpha are examined. Centipedes exhibited a prey density-dependent repertoire of search tactics. After 50 min of exposure to high prey density, centipedes switched from active search to ambush-like tactics, while maintaining a high rate of search at the lowest prey density. An initial period of sampling of prey density was involved in the switch in search behaviour and it is suggested that the encounter rate with prey was the key element in density assessment. When the prey density was changed from low to high, centipedes switched from active search to ambush tactics and when prey density was changed from high to low centipedes switched from ambush to active search within 40 min. Such behaviour may decrease the unreliability of sampling information and the risk involved in foraging decisions in variable environments.