Optimal Diet Model Research Articles

A Test of Foraging Models Using Dietary Diversity Indices for the Lomako Forest Bonobos

Optimal diet and functional response models are used to understand the evolution of primate foraging strategies. The predictions of these models can be tested by examining the geographic and seasonal variation in dietary diversity. Dietary diversity is a useful tool that allows dietary comparisons across differing sampling locations and time periods. Bonobos (Pan paniscus) are considered primarily frugivorous and consume fruits, leaves, insects, vertebrates, terrestrial herbaceous vegetation, and flowers. Frugivores, like bonobos, are valuable for examining dietary diversity and testing foraging models because they eat a variety of species and are subject to seasonal shifts in fruit availability. Frugivorous primate species thus allow for tests of how variation in dietary diversity is correlated with variation in ecological factors. We investigated measures of dietary diversity in bonobos at two research camps across field seasons within the same protected area (N’dele and Iyema) in Lomako Forest, Democratic Republic of the Congo. We compared the results of behavioral observation (1984/1985, 1991, 1995, 2014, and 2017) and fecal washing analysis (2007 and 2009) between seasons and study period using three diversity indices (Shannon’s, Simpson’s, and SW evenness). The average yearly dietary diversity indices at N’dele were Shannon’s Hʹ = 2.04, Simpson’s D = 0.82, and SW evenness = 0.88 while at Iyema, the indices were Shannon’s Hʹ = 2.02, Simpson’s D = 0.82, and SW evenness = 0.88. Behavioral observation data sets yielded significantly higher dietary diversity indices than fecal washing data sets. We found that food item (fruit, leaf, and flower) consumption was not associated with seasonal food availability for the 2017 behavioral observation data set. Shannon’s index was lower during periods when fewer bonobo dietary items were available to consume and higher when fruit was abundant. Finally, we found that optimal diet models best-explained patterns of seasonal food availability and dietary diversity. Dietary diversity is an essential factor to consider when understanding primate diets and can be a tool in understanding variation in primate diets, particularly among frugivores. Dietary diversity varies across populations of the same species and across time, and it is critical in establishing a complete understanding of how primate diets change over time.

An Optimal DASH Diet Model for People with Hypertension Using Linear Programming Approach

Selecting diets by quantitative techniques is becoming increasingly common. Linear programming is the most popular technique for the selection of least cost mixes of food to meet specific nutritional requirements for a particular group of persons for either general health or disease-related reason. Hypertension is a silent killer and its prevalence rate especially in the developing countries, which has been mostly associated to demographic, environmental and genetic factors, is becoming alarming. The DASH diet has been clinically proven to prevent and control hypertension. In this paper, a model that provides a Daily Optimal (minimum cost) DASH Diet plan for people with hypertension is formulated. The objective is to obtain daily minimum cost diet plans that satisfy the DASH Diets’ nutrients Tolerable Upper and Lower Intake for different daily Calorie Levels. The formulated DASH diet model was further illustrated using real data set with food samples gotten from the DASH eating plan chart. A DASH diet model for a hypertensive person with a 2000-daily-caloric need was formulated and its optimal diet plan for a day obtained with a total cost of 944.41 Naira. Optimal diet plans for other recommended daily calorie levels were also obtained.

Do oystercatchers (Haematopus ostralegus) select the most profitable limpets (Patella spp.)?

Eurasian oystercatchers (Haematopus ostralegus) feed on at least 15 different prey species in different habitats along the NW European coast. In each habitat, they specialise mostly on a single molluscan prey and within that prey type, they use various strategies to make foraging more profitable depending upon a number of internal and external factors. Oystercatchers resident on Lundy Island (SW England) feed mainly on limpets (Patella spp.). We found that the oystercatchers attacked both the common limpet, Patella vulgata and the black-footed limpet, Patella depressa, in any of four ways parallel striking, levering, hammering or pulling by weed. Oystercatchers took relatively deeper-shelled limpets in the 20–40mm length class, but this tendency reversed in limpets >40mm long. The oystercatchers showed a selective preference for smaller, deeper, irregular-bottomed limpets, and attacked them on the anterior region. The oystercatchers showed a strong preference for 20–30mm long limpets (Jacob's preference index relative to other length classes D=0.85) and increasing avoidance to larger length classes. But the calculated profitability for successfully opened limpets increased monotonically with the length, and the largest limpets had the highest profitability. Hence, the oystercatchers apparently did not select the most profitable limpets according to the simplest rate maximising optimal diet model; however we argue that this is because handling time varies as a function of substrate condition, so that a more complex model is needed.

Moving on with foraging theory: incorporating movement decisions into the functional response of a gregarious shorebird.

Models relating intake rate to food abundance and competitor density (generalized functional response models) can predict forager distributions and movements between patches, but we lack understanding of how distributions and small-scale movements by the foragers themselves affect intake rates. Using a state-of-the-art approach based on continuous-time Markov chain dynamics, we add realism to classic functional response models by acknowledging that the chances to encounter food and competitors are influenced by movement decisions, and, vice versa, that movement decisions are influenced by these encounters. We used a multi-state modelling framework to construct a stochastic functional response model in which foragers alternate between three behavioural states: searching, handling and moving. Using behavioural observations on a molluscivore migrant shorebird (red knot, Calidris canutus canutus), at its main wintering area (Banc d'Arguin, Mauritania), we estimated transition rates between foraging states as a function of conspecific densities and densities of the two main bivalve prey. Intake rate decreased with conspecific density. This interference effect was not due to decreased searching efficiency, but resulted from time lost to avoidance movements. Red knots showed a strong functional response to one prey (Dosinia isocardia), but a weak response to the other prey (Loripes lucinalis). This corroborates predictions from a recently developed optimal diet model that accounts for the mildly toxic effects due to consuming Loripes. Using model averaging across the most plausible multi-state models, the fully parameterized functional response model was then used to predict intake rate for an independent data set on habitat choice by red knot. Comparison of the sites selected by red knots with random sampling sites showed that the birds fed at sites with higher than average Loripes and Dosinia densities, that is sites for which we predicted higher than average intake rates. We discuss the limitations of Holling's classic functional response model which ignores movement and the limitations of contemporary movement ecological theory that ignores consumer-resource interactions. With the rapid advancement of technologies to track movements of individual foragers at fine spatial scales, the time is ripe to integrate descriptive tracking studies with stochastic movement-based functional response models.

Recent experience-driven behaviour optimizes foraging

Owing to the trade-off between time for searching for and exploiting resources, choosing a profitable food resource is key for an animal's survival and reproductive success. Although the optimal diet model (ODM) from optimal foraging theory (OFT) has demonstrated this point successfully by modelling the global maximization of energy intake rate for foraging, the behavioural mechanisms by which animals achieve optimization, especially when facing a novel environment, have yet to be understood. Here we propose a recent experience-driven (RED) decision-making rule of foraging that relies only on short-term memory of recent feeding experience and hunger aversion. This contrasts with the ODM, which assumes both global information access to the forager and its apparent ability to reach optimal choices. Despite relying only on limited past experience and hunger aversion, this behavioural rule not only accounts for nearly the same energy intake rate as predicted by the ODM but also can flexibly respond to changing environments by rapidly reaching new optimization. The RED behaviour also predicts a more realistic partial preference for diet choice, contrasting with the zero-one rule of OFT which allows for no partial preference within the forager's diet. The partial preference simply emerges from the RED strategy, as opposed to decision-making errors imposed from behavioural and physiological constraints. The RED rule further elucidates three potential traits that natural selection can act upon for optimizing foraging: (1) short-term memory of recent feeding experience, (2) ability to recognize encountered resources, and (3) basic neural set-up for alternative responses to environmental cues. This simple and flexible RED rule thus provides a behavioural mechanism for optimal foraging, and can be further applied to decision-making theory in psychology and economics.

Seasonal changes in mollusc abundance in a tropical intertidal ecosystem, Banc d'Arguin (Mauritania): Testing the ‘depletion by shorebirds’ hypothesis

At temperate latitudes densities and biomass of intertidal molluscs tend to be strongly seasonal. Here we provide a comparative study on seasonality of bivalves and gastropods in the tropical intertidal seagrass-covered soft sediment environment of Banc d'Arguin, Mauritania (20°N, 16°W). In this system, benthivorous shorebirds exert considerable predation pressure with strong seasonal variations. It has been proposed that during the period when (adult) shorebirds are absent (May–August) benthic biomass would be able to recover, but a first test was inconclusive. Over a full year (March 2011–February 2012), each month we sampled benthic invertebrates at sixteen permanent sites. The total of 3763 specimens comprised 20 species, representing eight orders and 19 families. Bivalves were much more common than gastropods. The bivalve Loripes lucinalis dominated the assemblage throughout the year (58% of total number), followed by Dosinia isocardia (10%), Senilia senilis (8%) and the gastropod Gibbula umbilicalis (6%). Average biomass amounted to 32 g AFDM/m2, of which the large West-African bloody cockle Senilia made up three-quarter, Loripes 16%, Gibbula 2% and Dosinia 1%. Across the 20 species, lowest densities were reached in late spring (May) and summer (Aug.), whereas highest densities occurred in autumn (Oct.). The lowest overall density of 676 specimens/m² in August more than doubled to a peak density of 1538 specimens/m² in October, most of the increase being due to strong recruitment in both Loripes (densities increasing from 322 specimens/m² in Sept. to 785 specimens/m² in Oct.) and Dosinia (densities increasing from 18 specimens/m² in Aug. to 265 specimens/m² in Sept.). Our results suggest that by the time the feathered molluscivore predators returned in high numbers to Banc d'Arguin (after their summer breeding season in the Arctic), benthic animals were at a peak. In order to quantitatively understand the seasonal changes in mollusc abundance, we build upon a recently published optimal diet model in which the most abundant molluscivore shorebird, the red knot (Calidris canutus), could choose between Loripes and Dosinia. Observed changes in densities of these two bivalves closely match depletion trajectories predicted by the model. We conclude that molluscivore shorebirds are able to deplete their food stocks in the course of their ‘winter’ in a tropical intertidal area.

The application of temporal difference learning in optimal diet models

An experience-based aversive learning model of foraging behaviour in uncertain environments is presented. We use Q-learning as a model-free implementation of Temporal difference learning motivated by growing evidence for neural correlates in natural reinforcement settings. The predator has the choice of including an aposematic prey in its diet or to forage on alternative food sources. We show how the predator's foraging behaviour and energy intake depend on toxicity of the defended prey and the presence of Batesian mimics. We introduce the precondition of exploration of the action space for successful aversion formation and show how it predicts foraging behaviour in the presence of conflicting rewards which is conditionally suboptimal in a fixed environment but allows better adaptation in changing environments.

Toxin constraint explains diet choice, survival and population dynamics in a molluscivore shorebird

Recent insights suggest that predators should include (mildly) toxic prey when non-toxic food is scarce. However, the assumption that toxic prey is energetically as profitable as non-toxic prey misses the possibility that non-toxic prey have other ways to avoid being eaten, such as the formation of an indigestible armature. In that case, predators face a trade-off between avoiding toxins and minimizing indigestible ballast intake. Here, we report on the trophic interactions between a shorebird (red knot, Calidris canutus canutus) and its two main bivalve prey, one being mildly toxic but easily digestible, and the other being non-toxic but harder to digest. A novel toxin-based optimal diet model is developed and tested against an existing one that ignores toxin constraints on the basis of data on prey abundance, diet choice, local survival and numbers of red knots at Banc d'Arguin (Mauritania) over 8 years. Observed diet and annual survival rates closely fit the predictions of the toxin-based model, with survival and population size being highest in years when the non-toxic prey is abundant. In the 6 of 8 years when the non-toxic prey is not abundant enough to satisfy the energy requirements, red knots must rely on the toxic alternative.

Short timescale rate maximization by gulls and implications for predation on size-structured prey

The timescale over which a predator estimates changes in prey encounter rates will play an important role in maximization of energetic returns from foraging in habitats where prey availability is highly variable through time. However, studies that explicitly test the temporal scale over which foragers track changes in prey availability are surprisingly rare. The increasingly well-recognized impact of terrestrial predators (e.g., birds) on intertidal food webs is likely to depend on their ability to track prey fluctuations in these highly variable environments. Here, we compare the predictions of 2 optimal diet models: a “classic” model in which prey encounter rate estimates are based on long-term, site-level abundance averages, and a model in which encounter rate estimates change at regular intervals throughout the foraging period. We parameterized these models using data from a field study on glaucous-winged gulls (Larus glaucescens) foraging on various sizes of the sea star Pisaster ochraceus. Predictions from the classic model, which assumes constant diet breadth throughout the tide cycle, did not match field obser vations of diet breadth. The “tide-sensitive” model, which assumes that gulls track tide-related changes in prey abundance, provided a better fit to observational data, explaining the full range of Pisaster sizes consumed by gulls. We conclude that gulls track short-term changes in prey encounter rates within a single low tide period to maximize foraging returns. We also present data for high rates of Pisaster removal by gulls, challenging the view of this sea star as a top predator in its intertidal communities. Key wor ds: diet breadth, glaucous-winged gull, intertidal ecology , Pisaster ochraceus, prey encounter rate. [Behav Ecol]

Optimal and Non-optimal Behavior Across Species

We take a behavioral approach to decision-making and, apply it across species. First we review quantitative theories that provide good accounts of both non-human and human choice, as, for example, in operant analogues to foraging (including the optimal diet model and delay-reduction theory). Second we show that for all species studied, organisms will acquire observing responses, whose only function is to produce stimuli correlated with the schedule of reinforcement in effect. Observing responses are maintained only by “good news”: “no news” is preferred to “bad news”. We then review two areas of decision-making in which human participants (but not necessarily non-humans) tend to make robust errors of judgment or to approach decisions non-optimally. The first area is the sunk-cost effect in which participants persist in a losing course of action, ignoring the currently operative marginal utilities. The second area is base-rate neglect in which participants overweight case cues (such as witness testimony or medical diagnostic tests) and underweight information about the base rates or probabilities of the events in question. In both cases we argue that the poor decisions we make are affected by the misapplication of previously learned rules and strategies that have utility in other situations. These conclusions are strengthened both by the behavioral approach taken and by the data revealed in cross-species comparisons.

Why Afro-Siberian Red KnotsCalidris Canutus Canutushave Stopped Staging in the Western Dutch Wadden Sea During Southward Migration

Afro-Siberian Red Knots Calidris canutus canutus use the western Dutch Wadden Sea as a refuelling area during southward migration from Taimyr to West Africa. Here we document the decline of their food stocks in this area, based on a yearly large-scale benthic mapping effort, from 1996 to 2005. For each benthic sampling position, intake rate (mg/s, ash-free dry mass) was predicted by an optimal diet model based on digestive rate maximization. Over the ten years, when accounting for a threshold value to meet energetic fuelling demands, subspecies canutus lost 86% of its suitable foraging area. Over this period, the proportion of probable canutus in mist-net catches in July—August declined relative to overwintering islandica Knots. This suggests that canutus dropped even more in numbers than islandica, for which we showed earlier a food-explained decline in numbers. We discuss the possible causality between a decline in the quality of intertidal mudflats in the Dutch Wadden Sea and population declines of Knots in the West-African wintering quarters.

Do wildlife laws work? Species protection and the application of a prey choice model to poaching decisions

Legislation for the protection of species is a global conservation tool. However, in many developing countries lack of resources means that effectiveness relies on voluntary compliance, leading to contradictory assumptions. On one hand, laws introduced without effective enforcement mechanisms carry an implicit assumption that voluntary compliance will occur. On the other hand, it is often openly assumed that, without enforcement, there will in fact be no compliance. Which assumption holds has rarely been rigorously tested. Here we show that laws for the protection of some species of large mammal have no effect on the prey choice patterns of primarily commercial hunters in the Democratic Republic of Congo, confirming the second assumption. We established this result by using an optimal diet model to predict the pattern of prey choice in the absence of regulation. Prey choice patterns predicted by the model were accurate across a range of conditions defined by time, space and type of hunting weapon. Given that hunters will not comply voluntarily, the protection of vulnerable species can only take place through effective enforcement, for example by wildlife authorities restricting access to protected areas, or by traditional authorities restricting the sale of protected species in local markets.

Oystercatchers use colour preference to achieve longer-term optimality.

The optimal diet model entails that foragers look beyond the individual prey encounter, to at least the level of intake rate across a bout of foraging, but optimization over a longer time remains controversial. In this paper, we show how oystercatchers increase their intake over the longer term using mussel colour as a cue. Wintering oystercatchers Haematopus ostralegus feed extensively on mussels Mytilus edulis in the estuaries of southern Britain. They show a marked preference for brown-shelled mussels over the commoner black-shelled morph, and we show that this enables them to maximize their rate of energy gain over a longer period than a single foraging bout. The brown and black mussels did not differ in ventral thickness and energy content, which are the main criteria for mussel selection and most important for short-term optimization. The brown mussels contained significantly less moisture, so by selecting them, oystercatchers could pack more mussel flesh into their limited oesophageal storage capacity. This enables them to increase their overall consumption during a feeding bout and increases their long-run energy gain rate, to an extent that is large enough to be significant for survival, especially during the short exposure of the mussel beds in winter.

Interference competition and the functional response of oystercatchers searching for cockles by touch

We examined interference competition for food in oystercatchers,Haematopus ostralegusL., feeding on cockles,Cerastoderma eduleL., when kleptoparasitism was infrequent. These birds opened cockles by hammering a hole in the shell, searched for them by touch and experienced densities of feeding conspecifics that ranged from 0 to 2362.5 birds/ha. Handling times were not significantly correlated with competitor density, but the probability of successfully opening a cockle declined significantly as competitor density increased because birds were more likely to abandon cockles they had found. Birds were also significantly more likely to carry cockles away from where they were found prior to attempting to open them as competitor density increased. We used an optimal diet model to predict maximum energy intake rates achievable for birds feeding on a given prey population, and experiencing a range of competitor densities. Despite affecting foraging behaviour, the model showed that competitor density had a negligible impact on overall intake rates. Although kleptoparasitism was rare in our study population, only 1.5% (9/586) of cockles being lost to parasites, a recent model suggests that it was likely to be profitable, under the conditions experienced by our birds. We suggest that kleptoparasitism might be infrequent because birds could reduce its likelihood by adjusting their behaviour, with only a minimal cost in terms of a reduced intake rate. Behaviour-based models of interference competition, therefore, need to consider a range of potentially complex avoidance behaviours when attempting to describe the dynamics of this process.

The Relative Importance of Habitat Structure and of Prey Characteristics for the Foraging Success of Water Pipits (Anthus spinoletta)*

AbstractWhile many studies on foraging have related energy gain to the density and the size of prey, only few have investigated whether and how habitat structure modifies the gain through affecting foraging success. In this study, the influences of habitat structure and prey characteristics on the foraging success of water pipits, Anthus spinoletta, were investigated experimentally. The birds take longer to find prey in tall than in short vegetation. The effects of vegetation on searching times differ between prey types. These differences are probably caused by variation in prey behaviour and in cryptic colouration, but not by prey size. Searching times increase with decreasing density for mealworms and tipulids, but not for caterpillars. Handling large prey items requires more time than handling smaller prey. Tipulids and caterpillars, which were offered alive, are handled for a longer time than dead mealworms of corresponding size. The success of attacks on flying insects is probably influenced by the prey's flight speed: fast houseflies are missed more often than slow tipulids. Overall, the results show that the time costs of foraging water pipits are influenced to a comparable degree by vegetation structure, by prey density and by other specific prey characteristics such as camouflage, hiding behaviour or agility. The amount of food gathered per unit time is determined primarily by factors that affect searching times, and less by handling and travelling times. Insertion of our data into an optimal diet model leads to the prediction that water pipits should be generalist foragers, which agrees with the observed behaviour.

Behavioral flexibility in prey selection by bacterivorous nanoflagellates

We used laboratory experiments to study the effects of changing food abundance on the selective feeding of two heterotrophic nanoflagellates on particles of similar size but differing nutritional quality (latex beads and fluorescently stained bacteria). Both Bodo saltans and Spumella sp. exhibited similar shifts in selectivity with changes in food concentration. Flagellates that were cultured under food‐limiting conditions showed a modest but significant preference for beads when both particles were offered simultaneously. However, both flagellates exhibited strong discrimination against the inert beads within 30–60 min after the addition of a satiating concentration of live bacteria. As bacterial abundance declined over 24 h, discrimination against the inert beads gradually relaxed. The observed pattern of concentration‐dependent selectivity is in agreement with the predictions of optimal diet models.

Food selection by calanoid copepods in response to between‐lake variation in food abundance

SUMMARY1. Food selection experiments were conducted by acclimating calanoid copepods (Eudiaptomus spp.) in suspensions of natural seston and then adding pairs of dual‐labelled (14C/32P) algae. Each feeding trial measured selectivity between a small, high‐quality reference alga, Chlamydomonas reinhardii, and a test alga that differed in size and/or food quality. The influence of food concentration on food selection was tested by using seston from two lakes with contrasting food abundance and by including treatments with filtered lake water ('starved’) and seston diluted with filtered water or enriched with cultured algae.2. Copepods that had been starved or acclimated to natural seston with low food abundance preferred the larger of two labelled algae, regardless of the nutritional quality of the algae. In agreement with the predictions of an optimal diet model, however, copepods that were acclimated to high food conditions discriminated against low‐quality foods, including digestion‐resistant algae and dead algae.3. Selectivity coefficients showed excellent agreement with a previous study involving the same taxa of copepods and labelled algae but in which the copepods had been acclimated to pairs of cultured algae rather than natural seston. Thus, these comparisons emphasize the importance of food availability in modulating copepod selectivity for foods that differ in nutritional quality and suggest that such behaviour occurs in nature.

Foraging Responses of Kudus to Seasonal Changes in Food Resources: Elasticity in Constraints

Foraging behavior provides the mechanistic link between species performance and food resources in the environment. I document the foraging responses of a browsing ruminant, the kudu (Tragelaphus strepsiceros), to changes in food abundance and quality over the seasonal cycle and interpret the results in the context of optimal foraging theory. The study was carried out on hand—reared but free—ranging animals in a savanna region with summer wet season and winter dry season in Transvaal, South Africa. During the dry season, kudus expanded their diet to include evergreen and unpalatable deciduous woody species neglected during the wet season. The acceptability rating of woody plants was correlated with food value expressed in terms of a protein—condensed tannin index, relative to handling time during ingestion or digestion. However, dietary expansion alone was inadequate to satisfy the daily energy requirement during this period. The animals also increased the fraction of trees of palatable species that was accepted for feeding, extended feeding duration at feeding stations, and increased encounter rate with evergreen trees still retaining leaves. Both total time active and proportion of active time spent foraging increased over the dry season. Digestive capacity was apparently increased to accommodate a higher daily food intake, which compensated for reduced diet quality. Only in September at the end of the dry season, when little foliage remained, were compensatory adjustments inadequate to meet energy requirements. The functional response relating consumption rate to seasonal changes in food abundance showed little variation, despite a decline in foliage biomass by more than an order of magnitude. The kudus were neither energy maximizers, nor time minimizers, but rather targeted on their energy requirements with least overall cost. Findings demonstrate that the foraging time and digestive capacity constraints assumed in optimal diet models are somewhat elastic. Dietary predictions obtained using average parameter settings may be misleading, because constraints may become effective only under extreme conditions. Further research is needed to establish the costs associated with stretching physiological constraints towards their upper tolerance limits.

Sex ratios in natural populations ofAphelinus mali (Hym.: Aphelinidae) in relation to host size and host density

In an apple orchard at Armidale, the Northern Tablelands of NSW, population sex ratios ofAphelinus mali (Haldeman), an endoparasitoid of the woolly apple aphid,Eriosoma lanigerum (Hausmann) varied from 0.51 (proportion of males) at low host densities to female-biased at high host densities (proportion of males ranged from 0.35–0.39). This shift in sex ratio seems to be caused by the differences in allocation of sons and daughters to hosts of different sizes. In the fieldA. mali parasitizes all life stages (four nymphal instars and adult) of the woolly aphid upon encountering. According to Hughes'(1979) optimal diet model, such general host acceptance seems to be the best strategy. However, it allows the host nymphs or adults to continue to develop or reproduce until about to mummify (pupate). No mortality was observed when first or second-instar hosts were parasitized in the laboratory. Field collected small mummified hosts yielded male-biased sex ratios whereas large mummified hosts produced mainly females. In the laboratory, progeny from smaller hosts (first to third-instars) produced sex ratios which were not significantly different from 0.5 whereas progeny from larger hosts (third and fourth-instars) produced female-biased sex ratio. During winter (June–August) and early spring (September–October) when the host populations in the orchard were predominantly nymphs, the parasitoid tended to allocate equal resources to male and female offspring. In contrast, at peak population densities in summer and autumn (December–May) when larger hosts were available, the sex ratios were female-biased. The host size ofE. lanigerum andA. mali is, therefore, an important component in the dynamics of host-parasitoid interactions.

Evaluating optimal diet models for an African browsing ruminant, the kudu: How constraining are the assumed constraints?

The linear programming model (LPM) of Belovsky (1978, 1986) and modifications of the classical or contingency model incorporating a digestive constraint (CM) were tested using foraging data recorded for kudus (Tragelaphus strepsiceros) browsing savanna vegetation over the late wet season. Food choice was between the herbaceous and woody plant components for LPM and among plant species or categories for CM. The constraints considered were consumption (cropping) rate, foraging time and digestive capacity. Woody communities dominated byBurkea andAcacia represented alternative habitat types. Following a minor adjustment, LPM represented the overall average diet and predicted the dietary differences between habitat types. However, the kudus failed to respond dietarily to variations among days and foraging sessions (meals) in the parameters constraining intake. The kudus accepted a wider dietary range than predicted to be optimal by CM. Evidence suggested that neither foraging time, nor digestive capacity, formed an effective constraint under the study conditions. Thermal tolerance and gut space may become limiting only towards the extremes of environmental variability that animals experience. LPM is vulnerable to circularity if average parameter values are used to estimate constraint settings. The energy maximizer—time minimizer dichotomy fails to take into account the fitness consequences of alternative foraging responses. CM is less cryptic in its application than LPM and so has greater heuristic value, despite its predictive failures. However, there may be no consistent ranking of food types where multiple constraints that are variable in their effectiveness apply. Dynamic programming models offer a solution, but pose a formidable challenge in complex natural environments.