Testing the “ecologically noble savage” hypothesis: Interspecific prey choice by Piro hunters of Amazonian Peru

Native peoples have often been portrayed as natural conservationists, living in harmony with their environment. It is argued that this perspective is a result of an imprecise definition of conservation that emphasizes effects rather than actual behavior. Using foraging theory as a contrast, an operational definition of conservation is offered. Foraging theory assumes that foragers will behave to maximize their short-term harvesting rate. Hunting decisions that are costly in terms of short-term harvest rate maximization, yet increase the sustainability of the harvest are deemed conservation. Using this definition, alternative hypotheses are tested using data on the inter-specific prey choice decisions of a group of subsistence hunters, the Piro of Amazonian Peru. Results indicate that hunters do not show any restraint from harvesting species identified as vulnerable to over-hunting and local extinction. Decisions are made that are consistent with predictions of foraging theory. The phrase ecologically noble savage was coined by Kent Redford (1991) in a thought provoking article of the same name.     

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.     

Foraging efficiency of juvenile bluegill, Lepomis macrochirus, among different vegetated habitats

Optimal foraging theory is devoted to understanding how organisms maximize net energy gain. However, both the theory and empirical studies lack critical components, such as effects of environmental variables across habitats. We addressed the hypothesis that energetic returns of juvenile bluegill are affected by environmental variables characteristic of the vegetated habitats. Predicted optimal diet breadths were calculated and compared to prey items eaten by juvenile bluegill to determine if bluegill were foraging to maximize energetic gain. Differences in habitat profitability among vegetated sites were determined by comparing predictions of maximized energetic return rates (cals^-1) with prey contents of bluegill stomachs. Sizes of most prey items eaten by juvenile bluegill throughout the vegetated sites were smaller than the predicted optimal diet breadths. However, inclusion of smaller prey items in the diet did not seem to affect rate of energetic gain. Energetic return rates were maximized at the 1.5 and 2mm prey size classes and declined only slightly with inclusion of smaller prey sizes. Predicted energetic return rates and average mass in bluegill stomachs were related negatively. Average mass in bluegill stomachs also was associated negatively with Elodea canadensis stem densities and percent of light transfer, suggesting that foraging efficiency of bluegill decreased as plant density and percent of light increased. Results of our research indicate that maximization of energetic return rates is dependent upon availability of prey sizes that contribute to optimal foraging. Thus, determination of those habitats that provide the highest availability of benthic invertebrate prey with the least interference by stems is critical. Enhanced foraging capabilities can promote recruitment, faster growth, better body condition and survival.     

Two gerbils of the Negev: A long-term investigation of optimal habitat selection and its consequences

Optimal foraging theory has entered a new phase. It is not so much tested as used. It helps behavioural ecologists discover the nature of the information in an animals brain. It helps population ecologists reveal coefficients of interaction and their patterns of density-dependent variation. And it helps community ecologists examine niche relationships. In our studies on two species of Negev desert gerbil, we have taken advantage of the second and third of these functions. Both these gerbils prefer semi-stabilized dune habitat, and both altered their selective use of this habitat and stabilized sand according to experimental changes we made in their populations. Their changes in selectivity agree with a type of optimal foraging theory called isoleg theory. Isoleg theories provide examples of dipswitch theories – bundles of articulated qualitative predictions – that are easier to falsify than single qualitative predictions. By linking behaviour to population dynamics through isoleg theory, we were able to use the behaviour of the gerbils to reveal the shapes of their competitive isoclines. These have the peculiar non-linear shapes predicted by optimal foraging theory. Finally, when owl predation threatens, the behaviour of Gerbillus allenbyi reveals the shape of their victim isocline. As has long been predicted by predation theory and laboratory experiments, it is unimodal.     

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.     

Central place foraging in the Eastern chipmunk, Tamias striatus

Orians & Pearson (1979) considered the optimal foraging strategies of ‘central place foragers’, animals that repeatedly return with their food to a fixed location. We tested some of their predictions on eastern chipmunks, Tamias striatus. The rate of cheek pouch loading declines as the pouches fill; thus the optimal load size may vary, depending on the time required to travel between the feeding site and burrow. This ‘travel time’ may also affect the choice of feeding site. A method was developed to test this, and preliminary results confirm the hypothesis. Methodological and theoretical implications of these empirical results are discussed.     

Prey selection by thaidid gastropods: some observational and experimental field tests of foraging models

Summary Field observations and experiments revealed that predatory intertidal gastropods of the genus Thais (or Nucella) were able both to recognize the expected food value of encountered prey (expected energy or growth potential gained per unit handling time) and to monitor their average yield over time (average energy or growth potential gained per unit foraging time). They appeared to discriminate not only among prey species, but also among different sized individuals of the same prey species. The evidence supporting these interpretations included: 1) field observations of snails feeding preferentially on prey types of higher expected food value even though lower value prey types were available and abundant, 2) a very limited number of direct underwater observations of foraging snails rejecting encountered items that were either of lower expected value than the item finally eaten or not measurably different from it, and 3) field (=arena) experiments in which both average yield, and the distribution and abundance of potential prey were controlled: snails conditioned at a high average yield fed preferentially on high value items, while those animals conditioned at a low yield consumed prey in the proportions that they were encountered. These behaviors are all consistent with a prey-selection decision motivated by energetic considerations. Further, the field experiments indicated that these predatory gastropods could select items from a diverse array of prey so as to maximize growth in their natural environment. The behaviors were not consistent with three alternative foraging hypotheses: non-selective foraging, frequency-dependent foraging on prey types (here, sizes of particular prey species), and frequency-dependent foraging on prey species. Deviations from some of the quantitative predictions of optimal foraging theory appeared related to learning and risk.     

The ant’s estimation of distance travelled: experiments with desert ants,<Emphasis Type="Italic"> Cataglyphis fortis</Emphasis>

Foraging desert ants, Cataglyphis fortis, monitor their position relative to the nest by path integration. They continually update the direction and distance to the nest by employing a celestial compass and an odometer. In the present account we addressed the question of how the precision of the ants estimate of its homing distance depends on the distance travelled. We trained ants to forage at different distances in linear channels comprising a nest entrance and a feeder. For testing we caught ants at the feeder and released them in a parallel channel. The results show that ants tend to underestimate their distances travelled. This underestimation is the more pronounced, the larger the foraging distance gets. The quantitative relationship between training distance and the ants estimate of this distance can be described by a logarithmic and an exponential model. The ants odometric undershooting could be adaptive during natural foraging trips insofar as it leads the homing ant to concentrate the major part of its nest-search behaviour on the base of its individual foraging sector, i.e. on its familiar landmark corridor.     

Nectarivore foraging ecology: rewards differing in sugar types

Abstract.

• 1 Honey bees, visiting artificial flower patches, were used as a model system to study the effects of sugar type (sucrose, glucose, fructose, and mixed monosaccharide), caloric reward, and floral colour on nectarivore foraging behaviour. Observed behaviour was compared to the predictions of various (sometimes contradictory) foraging models.
• 2 Bees drank indiscriminately from flowers in patches with a blue-white flower dimorphism when caloric values of rewards were equal (e.g. 1M sucrose in both colours; 1 M sucrose versus 2 M monosaccharide of either type), but when nectar caloric rewards were unequal, they switched to the flower colour with the calorically greater reward.
• 3 In yellow-blue dimorphic flower patches, on the other hand, bees did not maximize caloric reward. Rather, bees were individually constant, some to blue, others to yellow, regardless of the sugar types or energy content of the rewards provided in the two flower morphs.
• 4 The results suggest that optimal foraging theory (maximization of net caloric gain per unit time) is a robust predictor of behaviour with regard to the sugar types common to nectars; such optimal foraging is, however, limited by a superstructure of individual constancy.

     

Does Foraging Behaviour Affect Female Mate Preferences and Pair Formation in Captive Zebra Finches?

Background

Successful foraging is essential for survival and reproductive success. In many bird species, foraging is a learned behaviour. To cope with environmental change and survive periods in which regular foods are scarce, the ability to solve novel foraging problems by learning new foraging techniques can be crucial. Although females have been shown to prefer more efficient foragers, the effect of males'' foraging techniques on female mate choice has never been studied. We tested whether females would prefer males showing the same learned foraging technique as they had been exposed to as juveniles, or whether females would prefer males that showed a complementary foraging technique.

Methodology/Principal Findings

We first trained juvenile male and female zebra finches (Taeniopygia guttata) to obtain a significant proportion of their food by one of two foraging techniques. We then tested whether females showed a preference for males with the same or the alternative technique. We found that neither a male''s foraging technique nor his foraging performance affected the time females spent in his proximity in the mate-choice apparatus. We then released flocks of these finches into an aviary to investigate whether assortative pairing would be facilitated by birds taught the same technique exploiting the same habitat. Zebra finches trained as juveniles in a specific foraging technique maintained their foraging specialisation in the aviary as adults. However, pair formation and nest location were random with regard to foraging technique.

Conclusions/Significance

Our findings show that zebra finches can be successfully trained to be foraging specialists. However, the robust negative results of the conditions tested here suggest that learned foraging specializations do not affect mate choice or pair formation in our experimental context.     

The patch distributed producer-scrounger game

Grouping in animals is ubiquitous and thought to provide group members antipredatory advantages and foraging efficiency. However, parasitic foraging strategy often emerges in a group. The optimal parasitic policy has given rise to the producer-scrounger (PS) game model, in which producers search for food patches, and scroungers parasitize the discovered patches. The N-persons PS game model constructed by Vickery et al. (1991. Producers, scroungers, and group foraging. American Naturalist 137, 847-863) predicts the evolutionarily stable strategy (ESS) of frequency of producers that depends on the advantage of producers and the number of foragers in a group. However, the model assumes that the number of discovered patches in one time unit never exceeds one. In reality, multiple patches could be found in one time unit. In the present study, we relax this assumption and assumed that the number of discovered patches depends on the producers’ variable encounter rate with patches (λ). We show that strongly depends on λ within a feasible range, although it still depends on the advantage of producer and the number of foragers in a group. The basic idea of PS game is the same as the information sharing (parasitism), because scroungers are also thought to parasitize informations of locations of food patches. Horn (1968) indicated the role of information-parasitism in animal aggregation (Horn, H.S., 1968. The adaptive significance of colonial nesting in the Brewer's blackbird (euphagus cyanocephalus). Ecology 49, 682-646). Our modified PS game model shows the same prediction as the Horn's graphical animal aggregation model; the proportion of scroungers will increase or animals should adopt colonial foraging when resource is spatiotemporally clumped, but scroungers will decrease or animals should adopt territorial foraging if the resource is evenly distributed.     

The influence of food supply on foraging behaviour in a desert spider

We tested the alternative hypotheses that foraging effort will increase (energy maximizer model) or decrease (due to increased costs or risks) when food supply increased, using a Namib desert burrowing spider, Seothyra henscheli (Eresidae), which feeds mainly on ants. The web of S. henscheli has a simple geometrical configuration, comprising a horizontal mat on the sand surface, with a variable number of lobes lined with sticky silk. The sticky silk is renewed daily after being covered by wind-blown sand. In a field experiment, we supplemented the spiders' natural prey with one ant on each day that spiders had active webs and determined the response to an increase in prey. We compared the foraging activity and web geometry of prey-supplemented spiders to non-supplemented controls. We compared the same parameters in fooddeprived and supplemented spiders in captivity. The results support the costs of foraging hypothesis. Supplemented spiders reduced their foraging activity and web dimensions. They moulted at least once and grew rapidly, more than doubling their mass in 6 weeks. By contrast, food-deprived spiders increased foraging effort by enlarging the diameter of the capture web. We suggest that digestive constraints prevented supplemented spiders from fully utilizing the available prey. By reducing foraging activities on the surface, spiders in a prey-rich habitat can reduce the risk of predation. However, early maturation resulting from a higher growth rate provides no advantage to S. henscheli owing to the fact that the timing of mating and dispersal are fixed by climatic factors (wind and temperature). Instead, large female body size will increase fitness by increasing the investiment in young during the period of extended maternal care.     

The strength of selection in the context of migration speed

I use a model of avian migration based on maximization of overall migration speed to compare the strength of selection acting on foraging performance and flight speed. Let the optimal foraging behaviour be u* and the optimal flight speed be v*. It is shown that at this optimum, the ratio of the strength of selection on foraging to the strength of selection on flight speed is theta = -(u*2Pgamma"/v*2gammaP"), where gamma is the rate of energy expenditure during flight and P is the rate at which energy is gained during foraging. The dimensionless ratio P/gamma is the ratio of time spent building up fuel to time spent flying which A. Hedenström and T. Alerstam showed was much greater than unity. Although theta depends on this ratio, it also depends on the curvatures of the functions, as represented by gamma" and P". I use this simple example to make some general points about the strength of selection.     

Predator foraging success and habitat complexity: quantitative test of the threshold hypothesis

Summary Numerous studies have demonstrated a negative relationship between increasing habitat complexity and predator foraging success. Results from many of these studies suggest a non-linear relationship, and it has been hypothesised that some threshold level of complexity is required before foraging success is reduced significantly. We examined this hypothesis using largemouth bass (Micropterus salmoides) foraging on juvenile bluegill sunfish (Lepomis macrochirus) in various densities of artificial vegetation. Largemouth foraging success differed significantly among the densities of vegetation tested. Regression analysis revealed a non-linear relationship between increasing plant stem density and predator foraging success. Logistic analysis demonstrated a significant fit of our data to a logistic model, from which was calculated the threshold level of plant stem desity necessary to reduce predator foraging success. Studies with various prey species have shown selection by prey for more complex habitats as a refuge from predation. In this stydy, we also examined the effects of increasing habitat complexity (i.e. plant stem density) on choice of habitat by juvenile bluegills while avoiding predation. Plant stem density significantly effected choice of habitat as a refuge. The relationship between increasing habitat complexity and prey choice of habitat was found to be positive and non-linear. As with predator foraging success, logistic analysis demonstrated a significant fit of our data to a logistic model. Using this model we calculated the threshold level of habitat complexity required before prey select a habitat as a refuge. This density of vegetation proved to be considerably higher than that necessary to significantly reduce predator foraging success, indicating that bluegill select habitats safe from predation.Implications of these results and various factors which may affect the relationships described are discussed.     

Maintaining Homeostasis by Decision-Making

Living organisms need to maintain energetic homeostasis. For many species, this implies taking actions with delayed consequences. For example, humans may have to decide between foraging for high-calorie but hard-to-get, and low-calorie but easy-to-get food, under threat of starvation. Homeostatic principles prescribe decisions that maximize the probability of sustaining appropriate energy levels across the entire foraging trajectory. Here, predictions from biological principles contrast with predictions from economic decision-making models based on maximizing the utility of the endpoint outcome of a choice. To empirically arbitrate between the predictions of biological and economic models for individual human decision-making, we devised a virtual foraging task in which players chose repeatedly between two foraging environments, lost energy by the passage of time, and gained energy probabilistically according to the statistics of the environment they chose. Reaching zero energy was framed as starvation. We used the mathematics of random walks to derive endpoint outcome distributions of the choices. This also furnished equivalent lotteries, presented in a purely economic, casino-like frame, in which starvation corresponded to winning nothing. Bayesian model comparison showed that—in both the foraging and the casino frames—participants’ choices depended jointly on the probability of starvation and the expected endpoint value of the outcome, but could not be explained by economic models based on combinations of statistical moments or on rank-dependent utility. This implies that under precisely defined constraints biological principles are better suited to explain human decision-making than economic models based on endpoint utility maximization.     

Optimization of short-term animal behaviour and the currency of time

Modern ageing theory is based on the observation that oxidative metabolism causes damage that results in a gradual loss of vitality, leading to senescence and death. If animals can oxidize only a given amount of substrate in a lifetime (i.e. the ‘metabolic time’ is allocated from a fixed budget), then behaviour may be aimed at maximizing benefits per unit of ‘metabolic time’ expended. We analyse the consequences of this view for two types of behaviour that are commonly expressed as rates with respect to clock time. Examples are given of locomotory behaviour in which the animals' preferred speeds are generally interpreted as a result of maximization of energetic efficiency. The same behaviour could be expected if animals were ‘speed maximizers’ with respect to metabolic time. Examples are given of foraging behaviour that is also best predicted on the basis of maximization of energetic efficiency. This makes sense only if energy is allocated from a fixed budget. However, foraging animals not only expend energy but also consume it so energy cannot be considered as being allocated from a fixed budget. The same behaviour could be expected if animals were ‘energy intake rate maximizers’ with respect to metabolic time. This makes sense if metabolic time is allocated from a fixed budget as suggested by ageing theory. The metabolic time concept can provide a crucial link between the optimum intensity of short-term behaviour and its long-term fitness consequences. We discuss the implications of this approach for the modelling of foraging behaviour. Copyright 2002 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.      

Flight Durations in Bumblebees Under Manipulation of Feeding Choices

Foraging bees spend less time flying between flowers of the same species than between individuals of different species. This time saving has been suggested as a possible advantage of flower-constant foraging. We hypothesized that the time required to switch flower type increases if (a) such switches are infrequent and (b) the bees need to decide whether to switch or not. Bumblebees were taught to forage on artificial feeders that were identical in morphology and reward schedule but differed in the color of their landing surface. In the first two experiments bees foraged alternatively between two feeders. In Experiment 1 the colors of the landing surfaces were switched every two or three visits, while in Experiment 2 they were switched every six or seven visits. In the third experiment, the bees were required to decide whether to make a color-constant or a color-shift flight. Intervisit time was defined as time elapsed between consecutive visits to feeders. When feeder colors were changed frequently (Experiment 1), we detected no difference between color-constant and color-shift intervisit times. When bees were repeatedly exposed to one color (Experiment 2), color shifts required a significantly longer time. When allowed to choose (Experiment 3), bees performed more color-constant flights than color-shift flights. Intervisit times were similar for color-constant and color-shift flights in this experiment. Intervisit times in Experiment 3 were significantly longer than in Experiment 2 and slightly but nonsignificantly longer than in Experiment 1. The results suggest that bees indeed save time though flower-constant foraging but that this time savings is a small (1 s/flower visit) under laboratory conditions, and appears only when switches between flower types are infrequent. The time saved may be more significant over long foraging trips, and when morphological differences between flower species are large, as often happens under natural conditions, providing a selective advantage to flower-constant foraging.     

Central place foraging in the white-fronted bee-eater

Orians & Pearson (1979) proposed a series of models of optimal central place foraging. I tested their model for single-prey loading species (those that carry items singly) with a natural population of white-fronted bee-eaters (Merops bullockoides) in Kenya. These insectivorous birds breed in colonies but defend nearby foraging territories from which they gather food for their nestlings. Qualitatively, two predictions of the model were upheld: as travel time increased, birds carried larger insects to their nests and spent longer periods of time foraging. Also, as distance increased, birds were increasingly selective of which insects they ate versus carried to the colony. Quantitative comparisons were also made using observed travel times and rates of insect capture. As distance increased, (1) the observed sizes of insects carried approached the values predicted by the Orians-Pearson model, (2) observed foraging times diverged from the predicted values, and (3) birds spent more time foraging between round trips in a relatively unprofitable area near the colony. These results are examined in light of the theory of central place foraging.     

Feeding behavior ofRhagoletis pomonella flies (Diptera: Tephritidae): Effect of initial food quantity and quality on food foraging,handling costs,and bubbling

Under seminatural conditions feeding and postfeeding behaviors of individual apple maggot flies, Rhagoletis pomonella(Diptera: Tephritidae), were recorded after flies were presented with yeast hydrolysate or sucrose droplets, varying in either concentration, amount of food solute, or total droplet volume. The objectives were (a) to establish, at a constant level of previous food deprivation, food ingestion thresholds in relation to food quality and quantity and (b) to study the effect of initial food quantity and quality on food handling time and subsequent food foraging behavior. For both carbohydrate and protein substrates, fly foraging time after feeding on a tree branchlet was positively related to total amount of food solute previously encountered on a leaf surface, though largely independent of food volume or concentration. The volume and concentration of food presented, however, significantly affected food handling and processing time and therefore foraging time. In fact, total branchlet residence time was more closely linked to food handling and processing time than to foraging time. Less time was needed for uptake of liquid than dry food, the latter requiring liquification by salivary secretion and eliciting considerable intermittent cleaning of mouthparts by feeding flies. Similar to the situation in other fluid feeders, uptake time in R. pomonelladecreased with increasing dilution, although below a threshold of a 30% concentration of solute, the rate of nutrient intake decreased rapidly. When the level of dilution and total volume of food ingested were great enough, engorged flies entered extended quiescent postfeeding periods during which they extrude orally droplets of liquid crop contents (bubbling). After this they reinitiated feeding, followed by more bubbling and feeding bouts. Multivariate logistic regression analysis suggested that bubbling behavior is determined by liquid food volume and degree of dilution, hunger, and temperature. Although thresholds triggering bubbling decreased with increasing temperature, higher temperature by itself did not result in bubbling behavior. This suggests that bubbling is not primarily a mechanism to achieve evaporative cooling as has been suggested but, rather, a behavior to eliminate excess water, thereby enabling engorged flies to continue feeding on diluted food sources.     

Direct queries for discovering network resource properties in a distributed environment

The development and performance of networkaware applications depends on the availability of accurate predictions of network resource properties. Obtaining this information directly from the network is a scalable solution that provides the accurate performance predictions and topology information needed for planning and adapting application behavior across a variety of networks. The performance predictions obtained directly from the network are as accurate as applicationlevel benchmarks, but the networkbased technique provides the added advantages of scalability and topology discovery. We describe how to determine network properties directly from the network using SNMP. We provide an overview of SNMP and describe the features it provides that make it possible to extract both available bandwidth and network topology information from network devices. The available bandwidth predictions based on network queries using SNMP are compared with traditional predictions based on application history to demonstrate that they are equally useful. To demonstrate the feasibility of topology discovery, we present results for a large Ethernet LAN.