Optimal foraging and ontogeny; food selection by Haplochromis piceatus

Summary We examined the influence of satiation level, prey density and light intensity on food uptake rate through the ontogeny of Haplochromis piceatus. Prey handling in the buccal cavity was found to be the main factor limiting prey uptake rate under light circumstances and at a sufficiently high prey density. Food uptake rate per unit body weight of different sizes of H. piceatus was equal when feeding on Chaoborus but decreased with increasing fish size when feeding on Daphnia magna. In choice experiments with Chaoborus and D. magna, prey selection by H. piceatus of all sizes was according to the predictions based on Charnov's 1976 model.     

Odour-mediated long-range avoidance of interspecific competition by a solitary endoparasitoid: a time-saving foraging strategy

1. In studies on optimal foraging strategies, long-range decisions in the pursuit of resource are rarely considered. This is also the case for sympatric parasitoids, which may be confronted with the decision to accept or reject host larvae that are already parasitized by a competing species. They can be expected to reject already parasitized hosts if it is likely that they will lose the resulting intrinsic competition. However, examples of such interspecific host discrimination are rare. 2. We propose that parasitoids that are not egg-limited should reject inferior hosts only if it saves them time, and that this will be achieved mainly when the parasitoids are able to detect competitors from a distance. We tested this hypothesis using the sympatric parasitoids Cotesia marginiventris (Cresson) and Campoletis sonorensis (Cameron). 3. C. sonorensis was found to be the superior intrinsic competitor but, upon contact with a host larva, both wasps readily accepted hosts that had already been parasitized by the other species. However, in an olfactometer experiment, C. marginiventris females were found to strongly avoid the odour of their superior competitor. 4. These results are in accordance with a time optimization scenario, whereby the inferior competitor accepts competition if it costs only an egg, but avoids competition if it may save time that can be allocated to the search for more profitable hosts. 5. Models on host discrimination strategies in parasitoids had not yet considered discrimination from a distance. Long-range foraging decisions can also be expected for other organisms that have to choose between resources of varying suitability and profitability.     

The effect of prey density on foraging mode selection in juvenile lumpfish: balancing food intake with the metabolic cost of foraging

1. In many species, individuals will alter their foraging strategy in response to changes in prey density. However, previous work has shown that prey density has differing effects on the foraging mode decisions of ectotherms as compared with endotherms. This is likely due to differences in metabolic demand; however, the relationship between metabolism and foraging mode choice in ectotherms has not been thoroughly studied. 2. Juvenile lumpfish Cyclopterus lumpus forage using one of two modes: they can actively search for prey while swimming, or they can 'sit-and-wait' for prey while clinging to the substrate using a ventral adhesive disk. The presence of these easily distinguishable foraging modes makes juvenile lumpfish ideal for the study of foraging mode choice in ectotherms. 3. Behavioural observations conducted during laboratory experiments showed that juvenile lumpfish predominantly use the 'cling' foraging mode when prey is abundant, but resort to the more costly 'swim' mode to seek out food when prey is scarce. The metabolic cost of active foraging was also quantified for juvenile lumpfish using swim-tunnel respirometry, and a model was devised to predict the prey density at which lumpfish should switch between the swim and cling foraging modes to maximize energy intake. 4. The results of this model do not agree with previous observations of lumpfish behaviour, and thus it appears that juvenile lumpfish do not try to maximize their net energetic gain. Instead, our data suggest that juvenile lumpfish forage in a manner that reduces activity and conserves space in their limited aerobic scope. This behavioural flexibility is of great benefit to this species, as it allows young individuals to divert energy towards growth as opposed to activity. In a broader context, our results support previous speculation that ectotherms often forage in a manner that maintains a minimum prey encounter rate, but does not necessarily maximize net energy gain.     

Optimal foraging and community structure: implications for a guild of generalist grassland herbivores

Summary A particular linear programming model is constructed to predict the diets of each of 14 species of generalist herbivores at the National Bison Range, Montana. The herbivores have body masses ranging over seven orders of magnitude and belonging to two major taxa: insects and mammals. The linear programming model has three feeding constraints: digestive capacity, feeding time and energy requirements. A foraging strategy that maximizes daily energy intake agrees very well with the observed diets. Body size appears to be an underlying determinant of the foraging parameters leading to diet selection. Species that possess digestive capacity and feeding time constraints which approach each other in magnitude have the most generalized diets. The degree that the linear programming models change their diet predictions with a given percent change in parameter values (sensitivity) may reflect the observed ability of the species to vary their diets. In particular, the species which show the most diet variability are those whose diets tend to be balanced between monocots and dicots. The community-ecological parameters of herbivore body-size ranges and species number can possibly be related to foraging behavior.     

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.     

The foraging behavior of granivorous rodents and short-term apparent competition among seeds

The foraging behavior of a predator species is thought to bethe cause of short-term apparent competition among those preyspecies that share the predator. Short-term apparent competitionis the negative indirect effect that one prey species has onanother prey species via its effects on predator foraging behavior.In theory, the density-dependent foraging behavior of granivorousrodents and their preference for certain seeds are capable ofinducing short-term apparent competition among seed species.In this study, I examined the foraging behavior of two heteromyidrodent species (family Heteromyidae), Merriam's kangaroo rats(Dipodomys merriami) and little pocket mice (Perognathus longimembris).In one experiment I tested the preferences of both rodent speciesfor the seeds of eight plant species. Both rodent species exhibiteddistinct but variable preferences for some seeds and avoidanceof others. However, the differences in preference appearedto have only an occasional effect on the strength of the short-termapparent competition detected in a field experiment. In anotherexperiment, I found that captive individuals of both rodentspecies had approximately equal foraging effort (i.e., timespent foraging) in patches that contained a highly preferredseed type (Oryzopsis hymenoides) regardless of seed densityand the presence of a less preferred seed type (Astragalus cicer)in the patches. The rodents also harvested a large proportionof O. hymenoides seeds regardless of initial seed density;this precluded a negative indirect effect of A. cicer on O.hymenoides. But there was a negative indirect effect of O.hymenoides on A. cicer caused by rodents having a lower foragingeffort in patches that only contained A. cicer seeds than inpatches that contained A. cicer and O. hymenoides seeds. Theindirect interaction between O. hymenoides and A. cicer thusrepresented a case of short-term apparent competition thatwas non-reciprocal. Most importantly, it was caused by theforaging behavior of the rodents.     

Optimal diet selection by white-tailed deer: Balancing reproduction with starvation risk

Summary Energy intake rates of wintering deer vary over time because of variation in the abundance and quality of their natural foods. Accordingly, there is a chance that energy requirements will not be satisfied in a feeding period. This is especially critical because deer are reproductive during winter; hence selecting diets to minimize the risk of starvation may not maximize fitnss. I examined diet selection by white-tailed deer (Odocoileus virginianus) using a risk-sensitive foraging model which predicts the optimal diet when foragers face starvation risks during a reproductive period. Optimal diets were estimated by quantifying the mean and variance in energy intake rate deer could obtain when selecting different potential diets and substituting these values into functions for estimating offspring production and starvation risk. I conducted a field experiment to ask whether deer selected deciduous and coniferous twigs according to model predictions. Starvation risk was manipulated by providing deer supplemental feed. When faced with starvation risks, deer appeared to select diets that balanced offspring production with starvation risk. When starvation risk was climinated, deer tended to select diets that simply maximized their mean energy intake rates.     

斑痣悬茧蜂对不同寄主密度斑块的选择和最优搜寻行为

在圆柱形透明有机玻璃罩内,观察了斑痣悬茧蜂对不同寄主密度斑块的选择和滞留时间,并用Cox比例风险模型分析了寄主密度、产卵次数和搜寻特征对斑痣悬茧蜂在斑块上的驻留时间及其影响因素.结果表明:斑痣悬茧蜂对高密度寄主斑块的初次选择次数、访问次数均高于低密度寄主斑块,导致在高密度斑块上具有更高的寄生率;斑痣悬茧蜂在各斑块的驻留时间随寄主密度的增加而增加.Cox模型分析结果显示,斑块内寄主密度和产卵数均对其离开斑块的倾向起消极作用,即随寄主密度和产卵数的增加而倾向于留在斑块上;而寄主密度与产卵数的互作反而对寄生蜂离开斑块倾向具有促进作用,这两种作用可使寄生蜂利用寄主斑块获得最大适合度.     

Predators induce interspecific herbivore competition for food in refuge space

Resource competition among herbivorous arthropods has long been viewed as unimportant because herbivore populations are controlled by predators. Although recently resurrected as an organizing force in arthropod communities on plants, there is still general agreement that resource competition among herbivores is reduced by predators. Here we show the reverse: predators induce interspecific resource competi-tion among herbivores. We found that thrips larvae ( Frankliniella occidentalis ) use the web produced and inhabited by the spider mite Tetranychus urticae as a refuge from predation by the phytoseiid mite Neoseiulus cucumeris . Thrips larvae prefer clean plant parts, but move into the web upon perceiving volatile cues associated with thrips and predators. This behaviour leads to lower predation risk, but also to reduced developmental rate and lower production of thrips larvae due to competition with spider mites. In addition, thrips larvae consume spider-mite eggs. Thus, predators induce interspecific competition and intraguild predation among herbivores within refuge space, even when host plants have an overall green appearance.     

Optimal traits when there are several costs: the interaction of mortality and energy costs in determining foraging behavior

I analyzed the interaction of different types of costs in determiningoptimal behavior using mathematical models. The analysis concentrateson foraging behavior and asks (1) whether the cost factor thathas the greatest effect on fitness generally has the greatesteffect on optimal trait values and (2) whether increasing thesize of one type of cost makes the optimal behavior absolutelyor relatively more sensitive to that cost. The foraging costsconsidered are energy expenditure, predation risk, and othermortality factors. It is shown that increasing the magnitudeof one cost often decreases the relative and absolute sensitivityof the optimal foraging strategy to that cost. The relativefitness effects of different costs generally differ from therelative sensitivities of the optimal strategies to the costfactors. Researchers should therefore measure the shapes ofcost curves rather than their average magnitudes to determinewhich of several costs can be ignored in cost-benefit analyses.     

Sexual selection and the allometry of earwig forceps

Summary Positive intraspecific allometry, the tendency for large individuals to have relatively larger morphological traits, is thought to be more likely for secondary sexual traits than naturally selected traits. This is because secondary sexual traits are often used to signal individual quality and positive allometry should arise where the costs and/or benefits of signalling are size dependent. Here we examine the allometric relationships between forceps length, a sexually selected trait and elytra length, a naturally selected trait, in 42 species of earwig. Both forceps and elytra showed positive allometry. However, the degree of allometry was greater for forceps as predicted. If allometry arises due to sexual selection we would predict a greater degree of allometry in species with more exaggerated secondary sexual traits. Across species, the degree of forcep allometry did increase with forcep exaggeration. The relevance of positive allometry to reliable signalling is discussed.     

Optimal diving behaviour for foraging in relation to body size

Overall, large animals dive longer and deeper than small animals; however, after the difference in body size is taken into account, smaller divers often tend to make relatively longer dives. Neither physiological nor theoretical explanations have been provided for this paradox. This paper develops an optimal foraging diving model to demonstrate the effect of body size on diving behaviour, and discusses optimal diving behaviour in relation to body size. The general features of the results are: (1) smaller divers should rely more heavily on anaerobic respiration, (2) larger divers should not always make longer dives than smaller divers, and (3) an optimal body size exists for each diving depth. These results explain the relatively greater diving ability observed in smaller divers, and suggest that if the vertical distribution of prey in the water column is patchy, there is opportunity for a population of diving animals to occupy habitat niches related to body size.     

Ritualized behavior during competition for food between two formicinae

Summary Polyrhachis laboriosa andCamponotus brutus are two syntopic ants of the African equatorial forest. Although they occupy two different ecological niches (nesting area, diet, rhythms of activity), they are in competition for the exploitation of large permanent food sources. C. brutus, which is nocturnal, changed its rhythm of activity in the presence of large permanent food sources to exploit it day and night, whileP. laboriosa, diurnal, did not change its rhythm of activity. Encounters between workers of the two species at the food source always resulted in duels, even though several other workers were present. When attacked byC. brutus, P. laboriosa workers showed a novel flee-return strategy (fleeing over a 20-cm distance and returning to the source) instead of escaping definitively from the source and displayed ritualized behavior (i.e., fleeing, raising the gaster, flexing the gaster). These types of behavior appeasedC. brutus workers and enabledP. laboriosa to avoid overt aggression and to exploit the source in spite of the presence of a competitor. C. brutus also exhibited ritualized behavior during the duels (i.e., back-and-forth jerking of the body, series of light bites on a leg).C. brutus intimidatedP. laboriosa rather than really attacking it. This ritualization, used at an interspecific level, may be the result of a coevolutionary process or the effect of learning that certain types of behavior are beneficial. In any case, both species benefit from the possibility of exploiting large permanent food sources through confrontations that never lead to overt aggression.     

Optimal foraging by a suspension-feeding copepod: responses to short-term and seasonal variation in food resources

Laboratory radioisotope experiments were used to investigate the effects of phytoplankton seasonal succession on the selectivity and clearance rates of a suspension-feeding copepod in two Indiana lakes. Responses to particle size and quality were tested by allowing adult female Diaptomus birgei feeding in natural seston to select between a small (6×7 m) flagellate (Chlamydomonas reinhardii) and a large, poor quality food (heat-killed Carteria olivieri, 22×25 m). Short-term responses were tested in one lake by additional treatments in which copepods were acclimated for 5–6 h in filtered lake water (starved) or natural seston with added Chlamydomonas (enriched). Copepods from both lakes fed selectively on the small live flagellate during the spring bloom of edible phytoplankton but fed selectively on the larger, poor quality particle during the clear water phase when food was scarce. These results are interpreted as an interaction between the concentration-dependent selectivity for high quality foods predicted by optimal diet theory and a perceptual bias for large-sized particles. Selectivity for high-quality food was intermediate and clearance rates were depressed when total phytoplankton abundance was high but dominated by filamentous cyanobacteria. In each experiment copepods also responded to the short-term manipulations by exhibiting weaker discrimination against the poor quality particle in the starvation treatment and stronger discrimination in the enriched treatment. A two-way mixed model ANOVA revealed substantial short term (37%) and seasonal (53%) components to the total variance in selectivity. Clearance rates were also influenced by both phytoplankton succession and the short-term resource manipulations. As expected, clearance rates on the poor quality food were more sensitive to the abundance of alternative foods. These results show how the feeding behavior of a freshwater copepod is modulated by both seasonal and short-term variation in natural food.     

Optimal central-place foraging by beavers: Tree-size selection in relation to defensive chemicals of quaking aspen

Summary At a newly occupied pond, beavers preferentially felled aspen smaller than 7.5 cm in diameter and selected against larger size classes. After one year of cutting, 10% of the aspen had been cut and 14% of the living aspen exhibited the juvenile growth form. A phenolic compound which may act as a deterrent to beavers was found in low concentrations in aspen bark, and there was no significant regression of relative concentration of this compound on tree diameter. At a pond which had been intermittently occupied by beavers for over 20 years, beavers selected against aspen smaller than 4.5 cm in diameter, and selected in favor of aspen larger than 19.5 cm in diameter. After more than 28 years of cutting at this site, 51% of the aspen had been cut and 49% of the living aspen were juvenileform. The phenolic compound was found in significantly higher concentrations in aspen bark than at the newly occupied site, and there was a significant negative regression of relative concentration on tree diameter. The results of this study show that responses to browsing by trees place constraints on the predictive value of standard energy-based optimal foraging models, and limitations on the use of such models. Future models should attempt to account for inducible responses of plants to damage and increases in concentrations of secondary metabolites through time.