Spatial clumping of food and social dominance affect interference competition among ruddy turnstones

In studying the success of foraging animals, studies of interferencecompetition have put emphasis on effects of competitor density,whereas studies of resource defense have focused on the effectsof the spatial distribution of food within patches. Very fewstudies have looked at both factors simultaneously, that is,determined whether the effects of competitor density on foragingsuccess depend on the spatial distribution of food. We studiedthe behavior and the foraging success of ruddy turnstones (Arenariainterpres) using an experiment in which we varied both the presenceof a competitor and the food distribution. Because turnstonesmay differ strongly in their relative dominance status, we alsoexperimentally varied the foragers' relative dominance status.We found that the presence of a competitor only reduced theforaging success of subordinate birds foraging at the clumpedfood distribution. At this condition, dominant and subordinatebirds differed markedly in their foraging success. Contraryto our expectations, we did not observe more agonistic behaviorat the clumped food distribution. This indicates that the amountof agonistic behavior observed may be a bad indicator of interferenceeffects. These findings have specific implications for modelsof interference competition. Most notably they show that theeffects of competitor density on agonistic behavior and foragingsuccess may well depend on the spatial distribution of foodand the foragers' relative dominance status. Additionally, ourresults suggest that social dominance will not be fully understoodwithout considering long-term processes such as the formationand maintenance of social dominance hierarchies.     

Evolution of dominance under frequency-dependent intraspecific competition

A population-genetic analysis is performed of a two-locus two-allele model, in which the primary locus has a major effect on a quantitative trait that is under frequency-dependent disruptive selection caused by intraspecific competition for a continuum of resources. The modifier locus determines the degree of dominance at the trait level. We establish the conditions when a modifier allele can invade and when it becomes fixed if sufficiently frequent. In general, these are not equivalent because an unstable internal equilibrium may exist and the condition for successful invasion of the modifier is more restrictive than that for eventual fixation from already high frequency. However, successful invasion implies global fixation, i.e., fixation from any initial condition. Modifiers of large effect can become fixed, and also invade, in a wider parameter range than modifiers of small effect. We also study modifiers with a direct, frequency-independent deleterious fitness effect. We show that they can invade if they induce a sufficiently high level of dominance and if disruptive selection on the ecological trait is strong enough. For deleterious modifiers, successful invasion no longer implies global fixation because they can become stuck at an intermediate frequency due to a stable internal equilibrium. Although the conditions for invasion and for fixation if sufficiently frequent are independent of the linkage relation between the two loci, the rate of spread depends strongly on it. The present study provides further support to the view that evolution of dominance may be an efficient mechanism to remove unfit heterozygotes that are maintained by balancing selection. It also demonstrates that an invasion analysis of mutants of very small effect is insufficient to obtain a full understanding of the evolutionary dynamics under frequency-dependent selection.     

Patch use, apprehension, and vigilance behavior of Nubian Ibex under perceived risk of predation

Foraging theory predicts that animals will sacrifice feedingeffort in order to reduce predation risk. Once a forager choosesa habitat, it must decide how to allocate its foraging effort.Nubian Ibex are diurnal, social, cliff-dwelling herbivores.Many of their characteristics seem to have evolved as responsesto predation risk. In order to assess the effects that perceivedrisk of predation might have on foraging behavior of free-rangingNubian Ibex in the Negev Desert, Israel, we measured giving-updensities (GUDs) in artificial food patches and used them togauge apprehension level. (Apprehension can be defined as areduction in attention devoted to performing an activity asa consequence of reallocating attention to detecting or respondingto predation risk. A forager can also be vigilant. Vigilanceis often defined as time spent scanning the surroundings withthe head up.) We also quantified time budgeting using focalobservation of individual Nubian Ibex. Habitat preferences andpatch selectivity as a measure of apprehension were considered.In particular, we tested the effect of distance from refugeon GUDs, the effect of micropatch structure on selectivity,and the effect of distance from the refuge and group size onNubian Ibex vigilance level and apprehension. Nubian Ibex allocatetheir foraging effort more toward patches closer to the escapeterrain. At the same time, Nubian Ibex are more apprehensiveat intermediate distances from the cliff edge than nearer thecliff, and their use of vigilance increases with distance fromthe cliff edge. These results suggest that Nubian Ibex may switchfrom apprehension to a more extreme behavior of vigilance atgreater distances from the refuge. This study demonstrated theuse of antipredatory behaviors, apprehension, and vigilanceby a forager. Estimating apprehension and vigilance levels ofa forager simultaneously gives a more complete and accuratepicture of how the habitat is perceived by them and combinedwith measurements of GUD allow a more accurate assessment ofhabitat quality.     

Environmental and genetic determinants of ovicide in parasitic wasp Bracon hebetor

Superparasitism occurs when a parasitoid lays a second clutchof eggs on a host previously parasitised by herself or a conspecific.Ovicide refers to a parasitoid destroying an existing clutchof eggs on a parasitized host before laying a second clutch.We investigated environmental and genetic determinants of ovicidein the parasitic wasp Bracon hebstor. Characterization of egglayingbehavior revealed that B. hebetor commits ovicide during thehost examination phase of oviposition. The temporal costs ofovicide were found to be relatively small for females that experiencedlow rates of host encounter, whereas the costs of ovicide increasedfor females that experienced a high rate of host encounter.Individual wasps committed ovicide on conspecifically parasitizedhosts more frequently than on self-parasitized hosts. Manipulationexperiments suggested that B. hebetor females learn about theirenvironment while foraging and commit ovicide on the basis ofthe travel time between successive hosts. Significant differenceswere also found in ovicidal behavior among laboratory and fieldpopulations of B. hebetor. The implications of our results forclutch size theory and the evolution of ovicide are discussed.     

Influence of environmental conditions on patch exploitation strategies of parasitoids

In patchy environments, patch-leaving decision rules are a keycomponent of the foraging behavior of parasitoids that haveto share their searching time between the different host patchesto optimize the number of ovipositions. It has been experimentallyshown that one of the proximate cues involved in patch-leavingdecision is the number of ovipositions performed by the parasitoidwhile in the patch. Ovipositions can have either a decrementalor an incremental effect on the patch residence time dependingon parasitoid species. Previous theoretical studies have suggestedthat environmental conditions and the ability of parasitoidsto reliably estimate the number of available hosts on a patchwhen entering it should influence how patch departure dependson ovipositions. We developed an individual-based model to testa large variety of patch-leaving decision rules in differentenvironmental conditions. This model includes a more realisticrepresentation of individual acquisition of information thanprevious theoretical work. In accordance with previous results,we show that the best use of information depends on environmentalconditions. Moreover, we identify the environmental factorsthat have a decisive influence on the most appropriate typeof rule (incremental or decremental). Decremental mechanismsare most efficient only in restricted conditions characterizedby a large number of patches and few parasitoids. The sensitivityof decision rules to frequency-dependent processes was testedby numerical invasibility experiments. Rare mutants using anyalternative rule never outperform populations using a high-performancerule. Incremental rules seem to be more sensitive than decrementalones to frequency-dependent processes.     

Sex allocation of parasitic wasps: local mate competition,dispersal before mating and host quality variation

We constructed a sex allocation model for parasitic wasps to explain the wide variation in their sex ratio, considering the effects of local mate competition, partial dispersal of progeny before mating, and heterogeneity in host quality among patches. We conducted an experiment to compare with the predictions of our model. We considered the following situations. First, the hosts are distributed in discrete patches: a number of female wasps visit and oviposit in each patch. Second, all the progeny do not mate within the natal patch; some of them disperse to take part in population-wide random mating. We calculated ES sex ratios in cases where there are two kinds of patches: good ones and poor ones. We examined the dependency of ES sex ratios on several parameters, i.e., 1) the probability that a daughter mates in her natal patch, 2) the ratio of the female fitness of the good patch to that of the poor patch, 3) the proportion of poor patches, and 4) the number of foundresses in a patch. The result of our experiment showed the same tendency as the calculation in case where the LMC effect is high in each patch. We briefly discuss a possible selection pressure for dispersal of progeny, with special reference to the mating structure of parasitic wasps.     

Patch exploitation in a producer-scrounger system: test of a hypothesis using flocks of spice finches (Lonchura punctulata)

Group foraging allows individuals (scroungers) to obtain fooddiscovered by others (producers). Producer-scrounger game modelstypically overlook the costs and benefits of patch exploitation,assuming instead that producers and scroungers abandon patchessimultaneously once depletion occurs. Here we develop an extensionof the marginal value theorem of patch exploitation that includesthe producer-scrounger dynamics and examine the propensity ofa producer to abandon its patch before depletion once scroungersarrive. We predict that early departures should occur more oftenwhen expected searching time decreases and when competitionintensity in the patch increases. Competition intensity is expectedto increase when more scroungers are present or when patchesare smaller. We tested these predictions using a within-subjectexperimental design with six captive flocks of spice finches(Lonchura punctulata), each composed of one producer and threescroungers. As predicted, producers abandoned their food discoveriesmore frequently once scroungers arrived when searching timewas short, when more scroungers were present, and when patcheswere small. The results show that the producers of a patch oftenleave as scroungers join their food discoveries because thepayoffs from leaving exceed those from staying     

Alternative forms of competition and predation dramatically affect habitat selection under foraging--predation-risk trade-offs

Habitat selection under foraging—predation-risk trade-offshas been a frequent topic of interest to theoretical behavioraland evolutionary ecologists. However, most habitat selectionmodels assume that individuals compete exploitatively for resourcesand that predation is either density independent or dilutedcompletely by competitor number, despite empirical evidencethat other forms of competition and predation also occur innature. I developed an individual-based model for studyingthe effects of alternative forms of competition and predationon the process of habitat selection under foraging—predation-risktrade-offs. To make the model more relevant to natural populations,I assumed that individuals vary continuously in traits relatedto competitive ability and vulnerability to predation and allowedresources and predators to be distributed across more than twohabitats. The results of my investigation demonstrate thatthe predicted pattern of habitat selection can be affecteddramatically by the form predation is assumed to take. Whenpredation is density dependent or frequency dependent, individualswill tend to be distributed across habitats according to theirabsolute vulnerability to predation. In contrast, when predationis density dependent or vulnerability dependent, individualswill tend to segregate by competitive ability. Whether oneassumes that individuals compete for resources via exploitationor interference also influences the predicted pattern of habitatselection. In general, interference competition results in amore even distribution of competitors across habitats.     

The influence of developmental mortality on optimal sex allocation under local mate competition

In panmictic populations, optimal sex allocation is, under theassumptions of Fisher's model, not influenced by the probabilityof offspring developmental mortality, or by differences in mortalitybetween the sexes. In contrast, when mating opportunities areconfined to siblings, developmental mortality can influenceoptimal sex allocation. Many animal species have both localmating and developmental mortality. We show that when developmentalmortality is random for individual offspring, optimal sex allocationis influenced by mortality among males but not among females.Male mortality increases the allocation to males, but this shouldnever be male biased, even under extreme male mortality. Thisresult applies both when mothers are able to control the sexof individual offspring precisely, and when sex is allocatedwith binomial probability. The influence of mortality becomesprogressively larger when the variance of the distribution ofmortality over clutches diminishes. The reduction in fitnessis greater than the proportion of mortality, especially at smallclutch sizes, and mortality reduces the advantage of producingprecise sex ratios, and of local mate competition in general.     

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.     

Evolution of fighting behavior under asymmetric competition: an experimental test with juvenile salmonids

Large-dominant and small-subordinate species engaging in asymmetricinterference competition may optimize behavior under differenttrade-offs between the chance of winning and the cost of fighting.If fighting behavior is heritable and under selection, theorysuggests that large-dominant and small-subordinate species shouldevolve aggressive and passive fighting behaviors, respectively.To test this prediction, I manipulated the size and competitiveasymmetry of juveniles from sympatric populations of large-dominantcoho salmon (Oncorhynchus kisutch) and small-subordinate steelheadtrout (O. mykiss) and asked whether differences in fightingbehavior persisted independently of competitive ability. I observedfighting behavior during dyadic contests in two habitats, mutuallypreferred pools and energetically demanding riffles, under eachof three size treatments: natural size asymmetry, asymmetryremoved, and reversed size asymmetry. The results supportedthe prediction. Competitive ability depended primarily on size;large individuals of both species dominated smaller heterospecifics,and neither species dominated when size matched. Fighting behaviordepended primarily on species identity; coho salmon used a higherproportion of aggressive chases, whereas steelhead trout useda higher proportion of passive displays. Large individuals weremore likely to chase, and small individuals were more likelyto display. As evidence that asymmetric competition is associatedwith behavioral divergence, these results complement previouswork on morphological divergence under asymmetric competitionand provide a richer context for other features of the coho–steelheadsystem.     

Effects of Foraging Experiences on Residence Time of the Predatory Mite <Emphasis Type="BoldItalic">Neoseiulus womersleyi</Emphasis> in a Prey Patch

We investigated the effects of foraging experiences on the residence time of Neoseiulus womersleyi in a currently inhabited prey (Tetranychus urticae) patch. Satiated predators that had experienced starvation stayed longer in a current patch than those that had not experienced starvation. Satiated predators that had experienced a prey-rich patch showed approximately the same residence time in the current patch irrespective of the number of prey therein. By contrast, satiated predators that had experienced a prey-poor patch stayed longer in a current patch of high prey density than in one of low prey density. N. womersleyi appears to determine residence time in the current patch based on foraging experiences together with the quantity of prey in the current patch.     

The use and misuse of public information by foraging red crossbills

Group foragers may assess patch quality more efficiently bypaying attention to the sampling behavior of group members foragingin the same patch (i.e., using "public information"). To determinewhether red crossbills (Loxia curvirostra) use public informationto aid their patch departure decisions, we conducted experimentsthat compared the sampling behavior of crossbills foraging ona two-patch system (one patch was always empty, one patch containingseeds) when alone, in pairs, and in flocks of three. When foragingalone, crossbills departed from empty patches in a way thatwas qualitatively consistent with energy maximization. We foundevidence for the use of public information when crossbills werepaired with two flock mates, but not when paired with one flockmate. When foraging with two flock mates, crossbills sampledapproximately half the number of cones on the empty patch beforedeparting as compared to when solitary. Furthermore, as expected ifpublic information is used, the variance in both the numberof cones and time spent on the empty patch decreased when crossbillsforaged with two flock mates as compared to when alone. Althoughhigh frequencies of scrounging reduce the availability of publicinformation, scrounging is usually uncommon in crossbills, apparentlybecause they exploit divisible patches. Consequently, publicinformation is likely to be important to crossbills in the wild.We also show that feeding performance is greatly diminishedwhen the feeding performances of flock mates differ. This providesa mechanism that will favor assortative grouping by phenotypewhen phenotypes affect feeding performance, which may in turnpromote speciation in some groups of animals.