Alejandro Kacelnik

Some effects of energy costs on foraging strategies

Abstract We consider the effect of including energy costs on the optimal strategy for animals exploiting a depleting food resource. In the context of central place foraging this leads to the problem of what load size should be brought back to the central place. Two strategies are discussed: (i) maximize gross rate of energy delivery and (ii) maximize net rate of energy delivery. The optimal load size (or optimal patch time) for net maximizers is not always larger than for gross maximizers, as has been claimed. Instead, the difference in optimal load size has the same sign as the difference between metabolic rates of travelling and foraging. We point out that the influence of costs has not always been correctly incorporated in experimental tests of the theory.

Starlings exploiting patches: the effect of recent experience on foraging decisions

Laboratory and field experiments have shown that, as predicted by the marginal value model, starlings, Sturnus vulgaris, stay longer in a food patch when the average travel time between patches is long. A laboratory analogue of a patchy environment was used to investigate how starlings respond to rapidly fluctuating changes in travel time in order to find out the length of experience over which information is integrated. When there was a progressive increase in the amount of work required to obtain successive food items in a patch (experiment 1), birds consistently took more prey after long than after short travel times; travel experience before the most recent had no effect on the number of prey taken. Such behaviour does not maximize the rate of energy intake in this environment. The possibility that this is the result of a simple constraint on crop capacity is rejected as, when successive prey were equally easy to obtain up until a stepwise depletion of the patch (experiment 2), birds took equal numbers of prey per visit after long and short travel times: the rate-maximizing behaviour. A series of models are developed to suggest the possible constraints on optimal behaviour that affect starlings in the type of environment mimicked by experiment 1.

FORAGING STRATEGY, WORKER MORTALITY, AND THE GROWTH OF THE COLONY IN SOCIAL INSECTS

Models of optimal foraging often assume that maximizing the long-term net rate of gain is equivalent to maximizing fitness. We present a model that challenges this view on theoretical grounds for foragers from colonies of social insects. To maximize the number of reproductive forms produced by a colony, the production of new workers should be at a maximum during the ergonomic stage of a colonys development. However, when the mortality rate of workers depends on foraging performance, then maximization of the net rate no longer maximizes fitness. Under such conditions, the model predicts deviations from net-rate maximization (gain per unit of time) that may resemble the maximization of efficiency (gain per metabolic expenditure), as found in our earlier studies.

Effects of the knowledge of partners on learning rates in zebra finches Taeniopygia guttata

Abstract Many interpretations of the adaptive value of group living involve tranfer of knowledge. However, according to learning theory, being in a pair with a knowledgeable partner can have paradoxical consequences. Obtaining food by following a skilled companion may reduce the ability of naive individuals to learn about clues that signal the occurrence of food. This study examined the relation between learning and following in paris of zebra finches. Knowledgeable partners were trained to obtain food from a computer-controlled dispenser by using the information provided by a signal. For non-knowledgeable partners, the signal was irrelevant and could not be used to predict foraging opportunities. The rate of learning about the signal by naive birds that shared the experience of either knowledgeable or nonknowledgeable tutors was then examined. Naive birds learned more slowly as a result of being in a pair with a knowledgeable than a non-knowledgeable partner. Well-informed mates acted as a reliable cue to predict foraging opportunities, and thus overshadowed the independent signal. The knowledge of a partner influences learning rates in naive individuals, but in the opposite direction to that predicted by earlier accounts of learning in social contexts.

Psychological mechanisms and the Marginal Value Theorem: effect of variability in travel time on patch exploitation

Abstract The Marginal Value Theorem (MVT) describes the behaviour that maximizes the ratio of expected gain over expected foraging time in a patchy environment. When travel time is variable, the MVT rationale and its predictions are sensitive only to the mean travel time and not to the spread or skew of the distribution. Two mechanistic arguments contradict these predictions. First, tests of the MVT have previously shown that there is a disproportionate influence of the last travel time, and second, psychological models of information processing suggest that memory for time intervals is strongly dependent on the scatter of the distribution experienced. These mechanistic concepts, combined with Jensens inequality, suggest that patch exploitation should decrease as the scatter of the travel distribution increases. In a Skinner box experiment with pigeons, Columba livia , the problem was examined by simulating three environments with identical patches and the same mean travel time, but different travel time variability. Patch exploitation decreased with increasing variance in travel time. The results are used to argue in favour of the inclusion of realistic psychological properties as constraints in functional models of behaviour. Although both the MVT and the mechanistic models account for some features of the results, none of them can explain all the findings.

Central place foraging: a reappraisal of the ‘loading effect’

Abstract Animals that provision a central place usually bring back larger loads when foraging far from home. This positive correlation between average load size and distance is typically explained as rate-maximizing behaviour in the face of a trade-off between travel costs and a decelerating rate of prey gain in food patches (the ‘loading effect’). By using feeders to provide wild parent starlings, Sturnus vulgaris , with constant rates of prey loading, a positive load-distance correlation was shown to exist in the absence of a loading effect (experiment I). However, in a laboratory simulation where no load was transported (experiment II). the average number of prey eaten in patch visits by self-feeding starlings was invariant with travel distance, so the explanation of the load-distance correlation in experiment I must lie in featues peculiar to central place foraging. Bottlenecks in ingestion by chicks and interruption by visual detection of nest disturbance (experiment III) were rejected as causes of the correlation. Risks of dropping prey in flight appeared low, but the risk of kleptoparasitism received weak support. The travel-load size correlation may be an adaptive response to load transport costs, as return travel times increased with the load size being carried (experiment IV).

The tale of the screaming hairy armadillo, the guinea pig and the marginal value theorem

Abstract Foraging by screaming hairy armadillos, Chaetophractus vellerosus, and guinea pigs, Cavia porcellus, was studied in the laboratory. The main question was whether patch exploitation varies with overall capture rate as predicted by the marginal value theorem (MVT). Armadillos in experiment I and guinea pigs in experiment II experienced a single travel time between depleting patches of two kinds: good and poor. There were two treatments, which differed in the quality of poor patches. MVT predicts that within a treatment, more prey should be taken from good than from poor patches and between treatments, good patches should be exploited in inverse relation to the quality of poor patches and poor patches should be exploited in direct relation to their own quality. In experiment III, guinea pigs experienced three treatments which differed in the travel requirement, while the two patch types remained the same. MVT predicts that within a treatment more prey should be taken from good than from poor patches and that between treatments more prey should be taken from both patch types as travel requirement increases. The qualitative predictions were supported in the three experiments. The quantitative fit was good but there was a bias towards more severe patch exploitation than predicted. The results indicate that in these species patch exploitation depends on overall food availability as predicted by the MVT when overall food availability differs either because of patch type composition or because of differences in travel requirement between patches.

Timing Mechanisms in Optimal Foraging: Some Applications of Scalar Expectancy Theory

Uncertainty and its effects on optimal foraging predictions have been extensively discussed (for a review see Stephens & Krebs 1986). Uncertainty has usually been assumed to result from stochastic properties of the environment (Caraco 1980; McNamara & Houston 1980) or from transient incomplete knowledge while learning proceeds (Krebs, Kacelnik & Taylor 1978; McNamara & Houston 1985; Shettleworth, Krebs, Stephens & Gibbon 1988). Nevertheless, uncertainty in foraging may in addition originate from intrinsic properties of the predator’s information-processing capabilities. Here we consider foraging problems of ambiguity in the psychological system responsible for the perception, storage and retrieval of information about the duration of time intervals.

Short-term rate maximization when rewards and delays covary

Abstract In nature foragers must exploit resources that vary randomly in both the energy acquired per item (reward) and the time required to pursue, capture and process an item (delay). Furthermore, rewards and delays associated with particular resources may often covary significantly. An analytical model asks how variance-covariance levels for rewards and delays could influence choice of resources when lack of information or cognitive limitation implies that a consumer attempts to maximize its short-term rate of energy gain. Both greater expected reward and reduced expected delay clearly should enhance preference for a resource. The model predicts that increased delay variance and reduced reward-delay covariance should increase a foragers preference for a resource. A forager should be risk-averse towards reward variance when the reward-delay covariance is positive, but should become risk-prone towards reward variance when the reward-delay covariance is negative.

On the fitness functions relating parental care to reproductive value

Models of maximization of reproductive value are based on functions relating the intensity of parental care to two main fitness components: current offspring viability and future offspring production. A trade-off between the two components may be highly dependent on the shape of the underlying functions. Most theoretical models of parental care, however, have only presented an intuitive analysis of their properties. We present an analytical study of some properties of the offspring viability curve in birds. Reproductive value may be maximized in a region of the offspring viability curve that is decelerating. However, when future offspring production decreases at an increasing rate with the intensity of parental care, reproductive value may also be maximized in an accelerating portion of the curve. It is shown that the quantitative and qualitative details of parental care models depend to a large extent on the shape of the underlying fitness functions.