Net Rate Of Energy Gain Research Articles

Effects of sediment-protein concentration on feeding and growth rates of Abarenicola pacifica Healy et Wells (Polychaeta: Arenicolidae)

The relationships among concentration of sedimentary organic matter, deposit-feeding rate, and growth rate of Abarenicola pacifica were measured in laboratory experiments. When natural sediments were diluted with inorganic filler, feeding rate increased as the concentration of enzymatically available protein in the sediments decreased from ambient levels. Maximum feeding rates (2.4–7.0 g dry sediment · worm −1 · day −1) occurred at ≈ 0.05–0.1 mg protein · (g dry sediment) −1. Further decreases in sedimentprotein concentration resulted in decreased feeding rates. This type of functional response curve is similar to that reported for suspension-feeders and fits predictions of feeding models based on the energymaximization assumption. These models predict that peak feeding rates occur at the food concentration where the animal's energy-uptake mechanisms are saturated. We found, however, that growth rate decreased steadily as sediment-protein concentration decreased. Maximum net rate of energy gain did not correspond to the peak in sediment-processing rate.

Kleptoparasitism as a problem of prey choice: a study on mudflat-feeding curlews, Numenius arquata

Curlews feeding on intertidal flats attacked mainly conspecifics for prey, whereas they themselves also fell victim to kleptoparasitic attacks from gulls. There is no convincing evidence that robbers obtained other benefits, such as access to a good feeding site, besides the stolen prey itself. The data are consistent with the view that kleptoparasitism should be treated as a problem of prey choice, where attacks are initiated so as to maximize the net rate of energy gain. Attacks were directed mainly towards conspecifics handling large prey animals. There was no evidence of a simple producer-scrounger dichotomy. Instead, a dominance hierarchy seems more likely, where each individual could initiate attacks, but did so predominantly towards subdominant individuals, according to immediate profitability.

Bumblebee Foraging Behavior: Changes in Departure Decisions as a Function of Experimental Nectar Manipulations

The nectar-gathering foraging behavior of bumblebees (Bombus flavifrons and B. bifarius) was observed on control and nectar-enriched inflorescences of three Rocky Mountain herbs, Aconitum columbianum, Delphinium nelsonii and D. barbeyi, to determine whether bumblebees actually forage in a manner consistent with the learning mechanism proposed by Ollason. When gathering nectar from two of the three species, bumblebees encountering a series of enriched plants initially increased the number of flowers per inflorescence that they visited relative to control inflorescences. After the blossoms from a small number of enriched racemes were probed, the average number of flowers visited per plant returned to control levels. These results are consistent with the predictions made by Ollason and suggest that foragers' assessment of habitat quality is modified during a foraging bout. These results are used to estimate the extent of bumblebees' memories of previously encountered nectar rewards. The small estimated size of bumblebee memories may be adaptive under spatially heterogeneous reward schedules. INTRODUCTION Food resources for animals often occur in patches. Two problems shared by foraging organisms are, therefore, how to assess the quality of various food patches and how to decide when to leave one patch in search of another of higher quality. In an attempt to predict how an optimally foraging organism would deal with food patches of variable quality, Charnov (1976) formulated the marginal value theorem. In brief, the theorem states that a forager should leave its present patch when its rate of net energy intake drops below the average rate of net energy gain for the entire habitat. However, an animal's correct use of Charnov's rule requires a complete knowledge of the resource structure of the environment. It is unrealistic to expect that any organism would ever possess such knowledge, so how is it that observed foraging behavior is often consistent with the model's predictions (e.g., Krebs et al., 1974; Cowie, 1977; Pyke, 1978b, 1982; Heinrich, 1979; Hodges, 1981; Best and Bierzychudek, 1982; Cibula and Zimmerman, 1984)? Ollason (1980), recognizing that animals have incomplete knowledge of their environment, proposed a mechanism by which animals could make approximately optimal departure decisions. His rule states that foragers should continuously compare their current feeding rate with the average rate at which they remember feeding. When their current rate drops below their remembered average they should leave the patch in which they are feeding in search of another. A distinct assumption of Ollason's model is that animals, while gathering food, use the acquired information about reward schedules to continually update their estimate of overall habitat quality. Thus, Ollason's mechanism involves learning on the part of the foraging organism. The decisions actu'Address all correspondence to M. Zimmerman.

Optimal foraging and intraspecific diet differences in the lizard Cnemidophorus sexlineatus.

Previous studies have shown that adult and juvenile six-lined racerunners, Cnemidophorus sexlineatus, consume different sizes and taxa of arthropod prey. the purpose of this study was to determine if these differences could be explained in terms of energy cost and benefit parameters as related by the optimal diet model. Handling times and encounter rates with each of five categories of prey were determined by direct observation of lizard foraging behavior in the field. Energetic cost of search and energy content of prey were estimated from data in the literature. Mean values of all these parameters were used in the classic optimal diet model to determine which prey types yield the greatest rate of net energy gain for adult and juvenile racerunners. Grasshopper-like insects were the most valuable prey for adults, whereas plant and ground arthropods were the most valuable prey for juveniles. These findings correspond to the age-class specific diet differences.Each age-class adopts foraging tactics that increase the chance of finding the most valuable prey. Adult racerunners move hastily over a large area to find the relatively rare, but large and mobile grasshopper prey. Juveniles move much more slowly, and carefully investigate twigs and leaves to find smaller, cryptic plant and ground arthropods. However these foraging tactics do not preclude the taking of less valuable prey items, should they be encountered. This is because it is energetically better on average to eat the prey item rather than skipping it to search for better prey, except for the case of juvenile racerunners eating grasshoppers. That juvenile racerunners will attempt to capture and consume even very large grasshoppers is contrary to the expectations derived from the optimal diet model. This behavior may be the result of the foraging "rule of thumb" racerunners use to find their prey.

Foraging Strategies of Glaucous-Winged Gulls in a Rocky Intertidal Community

Foraging strategies of Glaucous-winged Gulls (Larus glaucescens) were studied in rocky intertidal habitats of the western Aleutian Islands, Alaska. Daily foraging activity was most intense at maximum low tide, and was concentrated in the lowest intertidal zones available to the birds. Barnacles (Balanus glandula) and mussels (Mytilus edulis) comprised most of the gull's diet during neap low tides, but these species were almost entirely abandoned during spring low tides in favor of sea urchins (Strongylocentrotus polyacanthus), chitons (Katharina tunicata), and limpets (Collisella pelta and Notoacmaea scutum). Sea urchins, chitons, and limpets, which had positive prey selection indices, were most abundant in the lower intertidal zones; barnacles and mussels, which had negative prey selection indices, were most abundant in the upper zones. Gulls also generally selected the larger individuals from each prey species, although sea urchins larger than the commissural bill width were avoided and limpets were selected in proportion to availability. Variation in prey availability also occurred among study areas with varying densities of sea otters (Enhydra lutris). With increasing depression of invertebrate prey by sea otter predation, gulls fed on a more diverse prey resource, and they switched to neritic fishes under intense sea otter predation. Preference experiments were conducted in the field, in which the common species and sizes of prey were made equally available to foraging gulls, thus eliminating search and capture times. In comparison with natural food choice, where sea urchins were most preferred, chitons became most preferred. We suggest that chitons are infrequent in natural diets because they adhere more strongly to the substratum than do sea urchins. Benefits of selective foraging were determined by comparing the net rate of energy gain of simulated random foragers with energy gained by selection of intertidal zones, prey species, and prey sizes. Observed selection patterns provided increased energy as gulls became more selective, and averaged 155% more than that obtained by the simulated random foragers. Results of the study support the two main predictions of optimal foraging theory in that (1) foraging patches (intertidal zones) and diets were selected such that net rates of energy gain were maximized, and (2) gulls became more selective foragers when energetically more profitable prey were more available.

Diet Selection and Optimization by Northwestern Crows Feeding on Japanese Littleneck Clams

Northwestern Crows {Corvus caurinus) feed extensively on littleneck clams {Venerupis japonica) despite the high cost in time and energy incurred in digging up the clams, opening them, and then extracting the edible parts. Casual observation also indicated that the crows fed preferentially on the larger clams. The purpose of this study was to quantify this apparent selectivity, to determine the energetics of the foraging process, and to compare the birds' net energy gains with the gains predicted by a random encounter model. The overall length of clams in the beach ranged from 10.0 to 43.0 mm. All sizes broke equally easily {X = 1.7 drops per clam), but the time taken to extract the meat from broken clams increased with size. Despite this, net energy gain per unit of handling time was an increasing function of clam size. Foraging crows ate clams between 24.0 and 43.0 mm long. They also dug up but rejected clams ranging from 13.0 to 35.0 mm long. The lower limit ofthe crows' diet, taken as the size which was eaten 50% ofthe times encountered, was 29.0 mm. We calculated that a diet consisting of clams >28.5 mm long yielded the maximum possible rate of net energy gain. These data clearly show that crows not only feed selectively on large clams but that their selectivity results in a net energy intake rate close to the maximum attainable.

Peripheral Foraging by Territorial Rufous Hummingbirds: Defense by Exploitation

Rufous Hummingbirds in the Sierra Nevada Mountains were observed to forage in the edge areas of their territories during the first few hours of the day, evenly over their territories during midday, and in the territory cores in late afternoon. This distribution of foraging effort appeared to be inconsistent with maximizing their rates of net energy gain during foraging. If Rufous Hummingbirds were to maximize their net energy gain during individual foraging bouts, foraging effort would have to be distributed evenly over the territory at all times of the day. We present a model which suggests that the observed feeding behavior of territorial Rufous Hummingbirds leads to higher daily net energy gains and lower food losses than if the birds maximized only net energy gain during each foraging bout during the day. Intrusion pressure accounts for the early—morning bias of foraging of edge areas. Intruders attempted to rob nectar from flowers in the edge areas of territories much more frequently than in the cores, and a higher proportion of these attempts were successful. Intruders frequently returned within 5—10 min to the sites that they had just robbed, and returned most frequently to those territories where they were able to rob for the longest time (on average) before being detected and chased. Nectar levels on territories were highest at dawn, and since edges were more prone to food losses than were cores, the territory owner minimized losses by feeding in the edge areas early in the morning. After chasing intruders from their territories, owners often returned and fed in the site just robbed by the intruder, which also reduced food losses by depleting nectar levels in areas prone to intrusion. According to the model, reduction of reintrusion rates by lowering the yield per intrusion and thereby discouraging robbers contributes more to the owner's benefit than does simple pre—emption of the resource by the owner. Thus, the peripheral foraging behavior of hummingbirds and their feeding response following intrusions are components of territory defense. Such defense by exploitation may be common when food resources are being defended, and should be considered along with aggressive exclusion of intruders (defense by interference) in the overall strategies of territorial animals. We close with a speculation that the late—afternoon foraging bias to the core may raise foraging efficiency during the time of day when the birds are heaviest and therefore have the highest foraging cost.

Optimal sugar concentrations of floral nectars ?dependence on sugar intake efficiency and foraging costs

A model is developed to elucidate the determinants of sugar concentrations in flower nectars. This model analyses the efficiency of sugar intake, or energy flux, which for nectarivores closely approximates the rate of net energy gain. For both steady state and some non-steady flows of nectars, this energy flux is shown to be maximal at particular sugar concentrations referred to here as the maximum flux concentration. Higher concentrations actually yield lower energy intake rates because the concomitant rapid increase in viscosity sharply reduces the rate of fluid intake. For pure sucrose solutions, the maximum flux concentration is 22%. For flower nectars, which are chemically more complex, the maximum flux concentration is predicted to be closer to 26%, using the first viscosity measures obtained for flower nectars. This concentration is shown to be essentially independent of the pollinator's feeding organ morphology and of the type of potential inducing nectar flow. It is proposed that this concentration applies for virtually all pollinators that select nectars with maximal energy flux.However not all pollinators are expected to select such nectars because this 26% concentration is not necessarily "optimal". The model predicts that optimal sugar concentrations vary for particular pollinators as a function of two primary factors: (1) the energy flux derived from the nectar, as discussed above, as well as (2) the relative contribution of transit costs to overall foraging costs. Relatively "dilute" nectars, with sugar concentrations close to the maximal flux value, are predicted for flowers pollinated by organisms that minimize feeding time to reduce high feeding costs, such as that of hovering or of exposure to enhanced predation while feeding. More concentrated nectars are predicted for flowers pollinated by nectarivores that incur high foraging transit costs relative to feeding costs.Flowers pollinated by hovering pollinators, including many hummingbirds, hawkmoths and bats, have nectars with mean sugar concentrations in close accord with the 26% maximum flux concentration predicted. Moreover, these nectars have relatively low concentrations of nonsugar constituents, which increase viscosity and thereby decrease sugar flux. Over 75% of the flowers examined in this study, which are pollinated primarily by territorial hummingbird species, provide nectars that allow sugar uptake with an efficiency of 90% or greater of the maximal value. According to the model, these data suggest that feeding costs of these pollinators far outweigh foraging transit costs. In contrast, the model suggests that flower nectars taken by traplining hummingbirds and by bees, with sugar concentrations significantly above the maximum flux value, reflect the higher costs of foraging flight relative to costs of feeding for these pollinators.Increasing temperature decreases nectar viscosity, and thereby increases absolute nectar uptake rates sharply. This leads to a number of predictions regarding foraging behavior as well as flower location, orientation, and color. However, the maximum flux concentration is shown to be practically invariable over a wide range of temperatures-increasing by only 2% sugar from 10°C to 30°C. Thus, contrary to previous expectations, little change in average sugar concentrations of flowers pollinated by particular groups of nectarivores is expected from cooler to warmer regions.

Models and Evidence for Feeding Control of Energy

The value of optimization theory is to provide falsifiable hypotheses and, when appropriate, alternative models of resource regulation. A consideration of alternatives suggests optimal energy regulation through feeding depends on time scale and supplies of energy relativeto demands. Maximization (on-off) control of energy reserves occurs over short intervals involved in consumption, while proportional control occurs over longer intervals (between meals) or as a consequence of trade-offs between energy regulation and nutrient or predation constraints. Maximization of rate of net energy gain occurs most frequently when energy supplies are low relative to demands and when energy and nutrients are not associated. This may be typical for small endothermic nectar feeders, while proportional controls are characteristic of feeding behaviors for many other animals. Controls for regulating energy content must also be affected by external environmental variation which may require use of “rules of thumb” that approximate the maximum rate of return.

Foraging tactics of a terrestrial salamander: Costs of territorial defence

Red-backed salamanders, Plethodon cinereus, established territories in laboratory chambers. Their foraging tactics, on two types of prey differing in caloric profitability and defence behaviour, were observed under a series of experimental conditions in which competitive threat was increased: no competitor present < familiar conspecific's pheromones present < unfamiliar conspecific's pheromones present < familiar conspecific intruder present < unfamiliar conspecific intruder present. As the degree of competitive threat increased, more time was devoted to territorial defence (displays and biting) at the expense of foraging. Simultaneously, the territorial residents gradually shifted from a specialized diet on the more profitable prey type to an indiscriminate diet, even though prey densities and the residents' encounter rates with each prey did not change. The presence of unfamiliar pheromones and both intruders led to approximately a 50% decrease in the residents' rates of net energy gain, about 80% of which was due to the time withdrawn from foraging and 20% due to change in diet. Changes in foraging time and diet both reflected the costs of territorial defence.

The diet of the New Holland honeyeater, Phylidonyris novaehollandiae

AbstractNew Holland honeyeaters collect nectar, manna or honeydew for energy and hawk small flying insects for protein. The insects taken were usually Diptera and Hymenoptera weighing 0.7 mg dry weight or less. Net rates of energy gain from hawking small flying insects were usually less than 20 J min−1 and sometimes negative and insufficient to meet the bird's daily energy requirements. Those from feeding on nectar, manna or honeydew were usually above 40J min−1 and often above 400J min−1 at dawn and the birds depended on these carbohydrates for energy. Nectar, manna and honeydew contained negligible amounts of protein, and the birds used small flying insects as sources of protein, and presumably other nutrients. Given that carbohydrate resources supply better rates of energy gain than insects. New Holland honeyeaters should collect their energy requirements from carbohydrates and only collect sufficient insects to satisfy their protein requirements. Estimates of the food intakes of both non‐breeding and breedig birds showed that they did this. Non‐breeding New Holland honeyeaters collected from 72 to 125 (mean 92) kJ of carbohydrates per day and 17 to 58 (mean 31) mg of protein per day. These meet the daily energy (75 kJ) and protein (20 mg) requirements of the birds. Breedig birds collected more carbohydrates and more insects, but in proportion to their increased energy and protein requirements respectively.New Holland honeyeaters are probably limited by their ability to meet their energy requirements from nectar, manna or honeydew and not by insects. Non‐breeding birds collected their protein requirements in about 10 min of insect‐feeding, but spent from 33 to 90% of the day collecting carbohydrates to meet their energy requirements. The maintenance requirement of 20 mg of protein per day for New Holland honeyeaters is about 25% of that estimated from standard equations for a bird of the same size. This low level may have evolved in response to low energy availability.

Foraging in bumblebees: rule of departure from an inflorescence

Most aspects of the bumblebees' rule of departure from an inflorescence of Aconitum columbianum are qualitatively what would be expected if the bumblebees maximize their net rate of energy gain. Flower revisitation increases with increases in the number of flowers already probed on an inflorescence and with decreases in the number of flowers available. Nectar volume per flower tends to decrease with increasing relative flower height and there is a positive correlation between nectar volumes of flowers from the same inflorescence. Consequently the bumblebees should be increasingly likely to leave an inflorescence with increases in either the number of flowers already probed or the relative height of the last flower and with decreases in either the number of flowers available or the nectar volume obtained at the last flower. The bumblebees showed all these trends.

Proportional Control for Daily Energy Regulation in Hummingbirds

Time budgets and food intake of captive hummingbirds (Eugenes fulgens and Lampornis clemenciae) were monitored for several days to estimate net energy exchange and to assess responses to depletion of energy reserves produced by food deprivation. Meal sizes averaged close to values predicted to maximize short-term rates of net energy gain if hummingbirds behaved optimally. Maximization of rate of net energy gain is analogous to an on-off model of control. However, longer-term, daily rates of energy storage were not maximized but generally were proportional to energy reserve depletion. High daily energy storage rates were associated with (1) high frequency feeding (one species, Eugenes), or (2) meal size (one species, Lampornis), and (3) low rates of energy expenditure between meals (both species). Proportional control may represent a compromise between characteristics of on-off controls, with high sensitivity to changes near set points, and integral controls, with low sensitivity to changes near set points.

Optimal Travel Speeds of Animals

Animal travel speeds have been hypothesized to satisfy several possible optimality criteria. These include (1) maximize net rate of energy gain (during foraging), (2) minimize energy cost per unit distance, and (3) minimize rate of energy expenditure. These criteria and others have, however, received little justification. General models of animal travel, coupled with the basic hypothesis that travel speeds are such that fitness is maximized, indicate that animals should sometimes behave in accordance with one of the above criteria. These models also indicate that animals should occasionally travel at their maximum sustainable speeds. In general, however, animals would be expected to travel at speeds that satisfy more complicated optimality criteria that include all the various counteracting factors.

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.

Beyond Selection: Optimal Ingestion Rate as a Function of Food Value

A simple model based on microphagous feeders (animals which process their food items in bulk with little chance of selecting food items on an individual basis) has been developed to predict how ingestion rate should vary with food quality if the net time rate of gain of some measure of food quality, say energy, is to be maximized. Three variants of the model were considered, in which absorption efficiency (1) was constant, (2) decreased linearly with increased ingestion rate, or (3) decreased exponentially. The optimal ingestion rate, which maximized the net rate of energy gain, depended on food quality and increased on higher quality foods. For certain parameter values, constantly maintaining the maximum ingestion rate resulted in maximal net energy gain. Experimental results from the literature on a variety of animals are not in consistent accord with these predictions. It seems likely that uncontrolled and confounding variables, such as food composition or palatability, may obscure the effects of food quality on ingestion rate.

Why hummingbirds hover and honeyeaters perch

Evidence is presented in support of the suggestion that a hovering bird is able to move between flowers more quickly than one that is perching. This advantage to hovering may be offset, however, by the higher energetic costs of hovering as compared with perching. This trade-off is evaluated in two field situations, one for perching honeyeaters and the other for hovering hummingbirds. In each case it is estimated that the birds employ the foraging mode (hovering versus perching) that results in the greatest net rate of energy gain.

On the mechanics and energetics of nectar feeding in butterflies

A mechanistic model describing the mechanics and energetics of nectar-feeding in butterflies is developed. The butterflies Collas eurytheme and Danaus plexippus are used to illustrate the model. Simulation results indicate that there are mechanical limitations upon the range of nectar sugar concentrations and nectar extraction times available to butterflies. There is a unique optimum for net rate of energy gain at 20–25% nectar sugar concentration which is independent of the metabolic rate and of proboscis shape and size over the ranges found in butterflies. The optimal nectar extraction rate depends upon the size and shape of the proboscis. These results are discussed in relation to the design of nectar feeding structures, optimal foraging strategy, and the evolution of insect pollination.

Optimal Foraging in Hummingbirds: Testing the Marginal Value Theorem

To a hummingbird, clusters of flowers on inflorescences represent patches and provide an ideal situation to test prediction of optimal patch-use. The basic question is what decision rule should a hummingbird use to decide whether or not to leave an inflorescence? The hypothesis is that hummingbirds will adopt the decision rule that maximizes their net rate of energy gain while foraging. This hypothesis leads to an analogue of Charnov's marginal value theorem which determines an optimal decision rule. The optimal decision ruleis then used to predict aspects of the hummingbirds' foraging, and these predictions are compared with field data The optimal decision rule is a function of how much information is used by the hummingbirds. Data indicate that a decision to leave an inflorescence is a function of the number of flowers visited, the number of flowers available on the inflorescence, and the amount of nectar obtained at the last flower. The optimal decision rule was calculated assuming no additional information is used.

Feeding: Models of Costs and Benefits in Energy Regulation

Models to predict feeding behavior at the level of consumption and use of energy involve either details of internal (physiological) controls or economic principles of regulation based on optimal (evolutionary) foraging theory. These two approaches will ultimately be related, but the former requires more information for specific predictions. The latter can provide predictions based on selected criteria for regulation. Meal sizes and feeding frequencies of hummingbirds are examined relative to two regulatory, criteria: maximizing rate of net energy gain and maximizing efficiency (intakes/expenditures) through a “crop emptying” model that incorporates energy intake from food and energy expenditures for short-term (meal to meal) maintenance and longer-term (overnight) energy storage. Experimental results suggest that the feeding behavior of hummingbirds is differentially sensitive to short-term and daily uses of energy. Changes in overnight energy storage requirements result primarily in changes in meal size, while changes in meal to meal maintenance requirements result primarily in feeding frequency changes. The economic models predict these responses. The feeding behavior of hummingbirds also appears to be sensitive to food quality, time spent flying to and from a food source, and costs associated with the weight of ingested food.