Female mate copying in the guppy {Poecilia reticulata): age-dependent effects

Virtually all studies of mate choice to date have assumed thatfemales choose mates independent of one another. Social cues,however, such as the mate choice of conspecifics, may also playan important role in such decisions. Previous work has shownthat female guppies of similar age copy each other's choiceof mates. Here we examine the effect of relative age on matechoice copying in the guppy, Poecilia reticulata, and examinewhether younger individuals are more likely to copy the matechoice of older conspecifics than vice versa. Results indicatethat younger females copy the mate choice of older females,but older individuals do not appear to be influenced by themate choice of younger individuals.     

Selfish long-term benefits of hoarding in the Siberian jay

We followed the consumption of cached food in Siberian jay flocksduring winter. Siberian jay flocks contain kin associationscomposed of parents in company with mature offspring. Further,flocks often contain immigrants not closely related to the groupmembers. We studied the extent to which Siberian jay offspringkept caches to themselves or whether they showed a kin biasand shared cached food mutualistically with relatives (parents/siblings)thus making an inclusive fitness gain. We recorded the retrievalof cached food using radio-ptilochronology, a technique thatdetects the consumption of a radioactively labeled food itemin the growth bar laid down in a growing feather the day ofconsumption. Food caching entailed a selfish benefit to thehoarder. The hoarding bird showed a substantial "recovery advantage"and retrieved its own cached food seven times as often as anyof its flock mates. There was no evidence for mutualistic sharingof caches among relatives. This selfish benefit persisted throughoutthe 7-week period for which we could follow the consumptionof labeled food.     

Spatial clumping of food increases its monopolization and defense by convict cichlids, Cichlasoma nigrofasciatum

The hypothesis that resource monopolization and defense increaseas the spatial clumping of resources increases was tested usinggroups of three convict cichlids competing for 120 Daphnia magnaprey. Spatial clumping was manipulated by varying the distance(3, 20, or 40 cm) between three tubes through which the preyappeared. As predicted, monopolization of prey (percentage eatenby the dominant fish) and frequency of aggression (chases perminute) by dominant fish increased significantly as the distancebetween the tubes decreased. However, there was no evidenceof individual flexibility in the aggressiveness (percentageof conspecifics chased) of dominant fish across treatments.Differences among dominant fish in aggressiveness were positivelycorrelated with their ability to monopolize prey, but the strengthof the correlation decreased as the distance between the tubesincreased. Aggression appears to be a more effective mechanismof interference competition when resources are clumped thanwhen resources are dispersed.     

Competition for landmark territories among male Polistes canadensis (L.) (Hymenoptera: Vespidae): large-size advantage and alternative mate-acquisition tactics

Alternative male mating tactics of insects at landmarks (leks)have only rarely been investigated. Some males of the paperwasp, Polistes canadensis (L.), were territorial at small treesalong the crests of dry ridges in Santa Rosa National Park,Costa Rica. Territories did not contain nests or resources forwhich females foraged. Contrary to other "hilltopping" species,male P. canadensis competed most intensely for territories insaddles along these ridges rather than at the highest points.Nonterritorial males patrolled small areas of the ridge line,following a path that took them to a number of territories.Many males switched between territoriality and patrolling, suggestingthat both size-related tactics belong to one conditional strategy.Males that were territorial on 2 or more days were larger thanthose that were territorial on only 1 day, and these in turnwere larger than permanent patrollers. Moreover, the mean sizeof territorial males was positively correlated with two measuresof territory attractiveness, suggesting that larger males monopolizepreferred sites. Mean age of territorial males was also relatedto territory attractiveness, but males of intermediate age claimedthe most attractive territories.     

Agonistic communication between males of a zaprochiline katydid (Orthoptera: Tettigoniidae)

The calls produced by male katydids serve to attract femalesand repel rivals. Males generally exhibit overt aggressive responsesto conspecifics that call within their territories, and intermalespacing is maintained by properties of the call. Here we examinethe acoustic behavior of a zaprochiline katydid in which maleaggressive behavior and even the ability to detect conspecificsappear greatly reduced. Despite reduced hearing sensitivity,males were shown to detect and respond to the calls of conspecificsusing substrate-borne vibrations. During playback experimentsmales increased their calling rates by decreasing the intervalbetween chirps. However, rather than exhibit overt aggressionmales would simply change their calling sites when the intensityof playback song was increased. These results were supportedby observations of natural encounters between males in the field.The interval between chirps decreased as the distance betweencalling males decreased, and encounters were always terminatedwhen one male moved away from the other. We suggest that theloss in auditory sensitivity and reduced aggressive behaviorof this species may be a consequence of the necessity to aggregatearound highly localized feeding sites.     

The influence of nest predation on mating strategies under polygyny

The mating status of an advertising male is one important factorfor female assessment. In species where already-mated malestry to attract additional females at some distance from theirmate, mating status may be difficult to assess. Limitationsin the time available to breed and a decrease in reproductivesuccess over the breeding season could make the costs outweighthe benefits of a time-consuming mate assessment process, whichautomatically postpones the onset of breeding. Time and energyconstraints will then limit the behavioral options for femalesin search of breeding opportunities, and females would suffera cost when assessing male mating status. Female choice maythen be better understood by analyzing the behavioral optionsfor females; the success of a strategy in which costly assessmentsare avoided and females choose mates indiscriminately, disregardingmating status ("fast" strategy), could be compared to the successof a strategy in which females carefully try to assess matingstatus ("coy" strategy). Nest predation should be included insuch an analysis because it influences the amount of male assistancefemales of polygynous males receive, as nest predation for onlyone of a polygynous male's two females leads to more male assistancefor the other female. In this study, I compare the outcome ofa "fast" strategy with that of a "coy" strategy under differentrates of nest predation. Although high probabilities of becominga mate of a polygynous male associated with a fitness reductiondecrease the relative success of a "fast" strategy, simulationsshow that high rates of nest predation considerably lower thedifference in the outcome of the two strategies. Therefore,the cost of assessing mating status must be relatively smallto be economical.     

Dominance, competition, and energetic reserves in the European starling, Sturnus vulgaris

We investigated the relationships between social dominance,competition for food, and strategies of body mass and fat regulationin the European starling (Sturnus vulgaris). In birds housedin groups of three, subdominant birds stored more fat than dominants.A removal experiment established a causal link between socialdominance and fat reserves; in groups that had the dominantindividual removed, the remaining birds reduced body mass andfat, relative to control groups that had the subordinate removed.In a second experiment, we investigated the influences of degreeof competition for food and dominance on body mass and fat reserves.Birds under high competition increased fat reserves and tendedto have higher body mass than birds under low competition. Theincrease in fat reserves was higher in the subdominants thanin the dominants. These results are consistent with hypothesesconcerning dominance-dependent access to food; subdominant birds,or birds under increased competition, may store more fat asan insurance against periods when food cannot be obtained. However,relations between dominance, body mass, and fat reserves mayalso arise through other proximate factors relating to dominance-dependentcosts and benefits of fat storage, such as predation risk andenergetic expenditure.     

Visual information, resource value, and sequential assessment in convict cichlid (Cichlasoma nigrofasciatum) contests

We staged contests between convict cichlids (Cichlasoma nigrofasciatum)that were matched for size and gender to test the influenceof prior information and resource value on the duration andstructure of fights. The contestants were separated before thecontest by either clear or opaque dividers to allow or preventvisual assessment, respectively. Contests were shorter in the"clear" than in the "opaque" treatment, suggesting that visualassessment occurred. The duration of lateral display, a noncontactdisplay, was shorter in the clear than in the opaque treatment,but the treatments did not differ significantly in the durationof three contact displays (biting, mouth wrestling, and circling).These results are consistent with the hypothesis that lateraldisplay provides primarily visual information, probably aboutbody size, whereas the other behavior patterns provide primarilynonvisual information, probably about strength. Second contestsbetween the same pair of fish were shorter than first contests,suggesting that the information acquired during the first contestmade it easier to resolve the second. After the subordinatefish from the second contest was given access to a mate, itfought more persistently so that third contests were longerthan second contests. Our results support the predictions ofthe sequential assessment model.     

Failure of simple optimal foraging models to predict residence time when patch quality is uncertain

Blue jays (Cyanocitta cristata) were presented with a foragingsituation in which half of the patches they encountered containedno prey and half contained a single prey item. Experimentallydetermined probability distributions controlled prey arrivaltimes in those patches that contained prey. Patch residencein empty patches was studied during four experiments. In thefirst, prey arrival was exponentially distributed. Residencetimes increased with travel time as predicted by a rate-maximizationmodel, but the bird stayed in empty patches much longer thanpredicted. During the second experiment, prey arrival was uniformlydistributed. The jays again stayed longer than optimal, andpatch residence times increased as travel time increased, althoughthe residence time that maximized rate of intake was independentof travel time under these conditions. In the third experiment,exponential and uniform patches were randomly intermixed. Thejays showed larger travel-time effects in the exponential thanin the uniform patch. However, the travel-time effect in theuniform patch was contrary to rate-maximization predictions,and the birds again overstayed in both patch types. In the fourthexperiment, prefeeding at the start of each foraging bout slightlyincreased overstaying rather than decreasing overstaying, aswould be expected if overstaying were due to underestimatingenvironmental quality. Consistent and dramatic overstaying anda travel-time effect under conditions where travel time hasno effect on optimal residence times suggest that the rate-maximizationapproach does not apply to foraging problems involving patchuncertainty.     

Area-restricted search by the plains pocket gopher (Geomys bursarius) in tallgrass prairie habitat

Because pocket gophers have the high energetic cost of excavatingburrows and an inability to detect distant food items throughthe soil, I hypothesized that individuals within establishedburrow systems would use area-restricted search as a foragingstrategy. To examine this hypothesis I compared gopher foragingeffort over a 10-month period between areas in which overallplant densities were experimentally varied. Gophers expendedapproximately 50% of their foraging effort in areas with thehighest plant density, even though these made up only 33% ofthe available area in experimental plots. In large, griddedareas sampled for an entire season as well as in small areasin which gophers foraged for less than 1 week, gopher foragingeffort was related to the density of a single leguminous plantspecies, Psoralea argophylla. In small plots where this plantspecies was at high density, gophers created more tunnel branches,thereby intensifying their search effort. Thus, area-restrictedsearch appears to increase the rate of encounter with the patchilydistributed Psoralea plants.     

The cost of producing a sexual signal: testosterone increases the susceptibility of male lizards to ectoparasitic infestation

According to current evolutionary theory, advertising traitsthat honestly indicate an organism's genetic quality might becostly to produce or maintain, though the kind of costs involvedin this process are controversial. Recently the immunocompetencehypothesis has proposed that testosterone (T) stimulates theexpression of male sexually selected traits while decreasingimmunocompetence. Even though some recent studies have shownan effect of T on ectoparasite load, the dual effect of thehormone has not been addressed in free-living populations. Herewe report results of an experiment in a free-living populationof the lizard Psammodromus algirus during the mating season.Males implanted with T had larger patches of breeding colorand behaved more aggressively than control males. In T-implantedmales, the increase in number of ticks during the mating seasonwas significantly higher than in control males and this negativelyaffected several hematological parameters. T-males sufferedsignificantly higher mortality than control males during theexperiment The results from the manipulation of T are consistentwith the dual effect of this hormone.     

Reproductive success of polygynous male corn buntings (Miliaria calandra) as confirmed by DNA fingerprinting

Despite a sex ratio approximating to unity, female corn buntingswere not equally distributed among males. In 1989 and 1990,41.2% of 50 males were monogamously paired, 29.4% were polygynous,and 23.5% were unpaired. Polygynous males usually paired withtwo females, although in 1990 three males were trigamous. Polygynousmales fledged more offspring from their territories than didmonogamous males, mainly because they had more mates. The fledgingsuccess per nesting female was slightly higher in territoriesof polygynous males, but not significantly so. DNA fingerprintingwas used to confirm the true paternity of 44 offspring from15 broods and the true maternity of 50 offspring from 16 broods.A further 12 offspring from three broods for which neither putativeparent was available were also fingerprinted. Actual reproductivesuccess of parents was close to that inferred from observationsof number of young raised. There was only one brood, containingtwo chicks (4.5% of offspring, or in 6.7% of broods), wherethe chicks were not fathered by the male defending the territory.However, this nest was close to the territory boundary, andthe defending male may have been assigned incorrectly. Therewere no cases of intraspecific brood parasitism (n = 16 broods).The copulation rate was low, and extrapair copulation attemptswere rare, probably because of the poor chances of sneakingonto a neighbor's territory undetected and the costs of leavinga territory unguarded.     

The location of landmark leks in the small heath butterfly, Coenonympha pamphilus: evidence against the hot-spot model

Two major theories have been developed to explain the locationand evolution of leks. According to the hot-spot model leksare male initiated and occur because males aggregate at locationsof maximum female home range overlap. The alternative femalepreference model, on the other hand, assumes that leks are femaleinitiated, and that females prefer to mate with males that aggregate.Small heath butterfly leks are situated close to landmarks suchas trees and bushes in open grassland. Censuses of wild populationsand experiments with artificial landmarks showed that theirattractiveness increased with height and width measured at themiddle of tree height. Landmark width at field layer heightwas negatively correlated with attractiveness, which suggeststhat a cornet shape is preferred to a pyramid shape. Togetherthese landmark size variables explained 60% of variation inlek size. Release experiments showed that landmarks were approachedby receptive virgin females, but mated females were indifferentto them. Landmark use was correlated to microclimatic conditions.Males preferred wider landmarks during cooler weather. Theyalso competed for the lee side of landmarks. There was no correlationbetween the location of landmarks and the dispersion of matedfemales or emergence sites of females. Hence, receptive femaledispersion was largely determined by their mate-locating behavior,which in turn appeared to result from male dispersion. Leksmay initially have developed around easily detected visual cuesthat offered a more suitable microclimate for male activity.Females visiting these locations would find a suitable matemore rapidly and save valuable time for oviposition. Althoughnonadaptive evolution cannot be excluded, taken together theresults support the female preference model for lek evolution.     

Conflict between sexes in the water strider, Gerris lacustris: a test of two hypotheses for male guarding behavior

We studied the effect of operational sex ratio on female reluctanceand male persistence to mate as well as on the length of copulationand postcopulatory guarding in Gerris lacustris by adding fivesurplus males or females to the basin with a pair in tandem.In the control treatment, a pair alone was tested. Accordingto the copulatory guarding hypothesis (CGH), males should prolongmating and guard females in the presence of surplus males. Accordingto the convenience polyandry hypothesis (CPH), females shouldshow lower levels of resistance to prolonged mating in the presenceof surplus males because the mating male protects the femaleagainst harassment from other males. As expected on the basisof both the CGH and CPH, mating (copulation + guarding) averagedlonger in the male-biased treatment. The behavior of males andfemales during mating suggested that both hypotheses hold true:females showed less resistance to prolonged mating (as predictedfrom CPH), and male behavior suggested stronger efforts to stayon the female when surplus males were present (as predictedfrom CGH). Comparisons of the treatment with surplus femaleswith the results from the mating pair without surplus individualssuggested that the capabilities of water striders in tandemto assess the sex of nearby nonmating striders are limited.     

Toward a theory of dominance hierarchies: effects of assessment, group size, and variation in fighting ability

We introduce assessment to the analysis of dominance hierarchiesby exploring the effect of an evolutionarily stable fightingrule when there is variation in resource holding potential (RHP)and RHP is not a perfectly reliable predictor of the outcomeof a fight. With assessment, the probability of a linear hierarchydecreases with group size but can remain appreciable for groupsof up to seven or eight individuals, whereas it decreases virtuallyto zero if there is no assessment. The probability of a hierarchythat correlates perfectly with RHP is low unless group sizeis small.     

Testosterone effects on the immune system and parasite infestations in the barn swallow (Hirundo rustica): an experimental test of the immunocompetence hypothesis

The immunocompetence hypothesis predicts that testosterone (T)enhances the expression of male secondary sexual characterswhile exerting a suppressive effect on the immune system therebyexposing hosts to higher intensities of parasite infestations.In a natural population of barn swallow (Hirundo rustica) males,the intensity of infestation by some ectoparasites was negativelycorrelated with tail length and was positively correlated withimmunoglobulin levels, but no clear relationship was observedbetween immune responses (leukocyte counts, immunoglobulins)and tail length. Males implanted with T had higher intensitiesof parasite infestations at the time of recapture than controlmales, and T-implanted males experienced an increase in countsof eosinophils. In T-implanted males, immunoglobulin levelsinitially decreased and then increased as time from implantationelapsed. Among T-implanted males, those with longer tails hada smaller increase in eosinophil counts, tended to experiencea smaller increase of parasite infestations, and were more likelyto survive until the following breeding season than those withshorter tails. The relationships between parasite burden, immunesystem, and exaggeration of tail length in the natural populationof males are consistent with some aspects of the immunocompetencehandicap hypothesis. The results from the manipulation of Tplasma levels are also partly consistent with the hypothesis,since T-implantation resulted in higher levels of parasite infestations,but contradict the assumption of an obligatory immunosuppressiveeffect of T. Higher activation of the immune system of T-implantedmales indicate that high T plasma levels imposed a two-foldcost because of the effects on parasites and the immune responseto parasites, and this suggests that the effect of T on parasitesmight not be mediated by the immune system of the host. Theresults of the manipulation of T plasma levels support the handicapversion of the immunocompetence hypothesis since high quality,long-tailed males paid less in terms of activation of the immunesystem, change in parasite infestations, and chances of survivalthan low-quality, short-tailed males.     

Environmental effects on prenatal growth rate in pronghorn and bighorn: further evidence for energy constraint on sex-biased maternal expenditure

Byers and Moodie (1990) proposed that high levels of maternalexpenditure in polygynous ungulates limit the ability of mothersto support elevated male fetal and neonatal growth rates. Thishypothesis assumes that females in high-expenditure speciesare at or near the maximum level possible and that females inlower-expenditure species are not. To test this assumption,we examined our long-term data on reproduction of pronghorn(Antilocapra americana) and bighorn (Ovis canadensis) at theNational Bison Range (western Montana, USA) and compared gestationlengths, birth weights, and prenatal growth rates of offspringborn following summers of low versus average or above averageprecipitation. In bighorn, these variables were unaffected bythe previous summer's rainfall, but in pronghorn, gestationlength and prenatal growth rate were significantly lower followingdry summers. Extended samples for both species confirmed earlierreports of sex-biased expenditure favoring males in bighornand the absence of sex-bias in pronghorn. Bighorn prenatal littergrowth rates and birth weights, corrected for maternal mass,are 63.5% and 38%, respectively, of pronghorn values. Thesedata support the Byers and Moodie (1990) contention that femalesof high-expenditure species do not show differential expenditureby offspring sex because they are at a reproductive expendituremaximum, whereas females of lower-expenditure species are ableto support excess expenditure in male offspring because optimalallocation to female offspring is farther from such a maximum.     

Patch assessment in foraging flocks of European starlings: evidence for the use of public information

A field experiment was carried out to determine whether group-foragingstarlings (Sturnus vulgaris) use public information to helpthem estimate the quality of an artificial resource patch anddepart accordingly. Three kinds of information are potentiallyavailable in a group: patch-sample information, pre-harvestinformation, and public information. These three types of informationcan be combined into four patch assessment strategies: (1) patch-samplealone; (2) patch-sample and pre-harvest; (3) patch-sample andpublic; and (4) patch-sample, pre-harvest, and public. Dependingon the foraging environment we presented to the starlings, eachassessment strategy made a unique set of predictions concerningthe patch departure decisions of pairs of birds based on differencesin their foraging success. The environment was manipulated intwo ways: by altering the variability in patch quality and bychanging compatibility, the ease with which individual birdscould simultaneously acquire both patch-sample and public information.Our observations on patch persistence and departure order demonstratethat the starlings used a combination of patch-sample and publicinformation, but not pre-harvest information, to estimate thequality of the experimental patch. Moreover, our results suggestthat starlings use public information only when it is easilyavailable and ignore it under incompatible conditions. Thisstudy provides the first evidence of public information usein a patch assessment problem.     

Sexual selection and extrapair fertilization in a socially monogamous passerine, the zebra finch (Taeniopygia gullata)

We investigated paternal exclusion rate (the percentage of youngreared by a male that were not his genetic offspring), and behavioraland reproductive variables influencing this rate, in a freelybreeding laboratory population of zebra finches (Tae-niopygiaguttala castanotis), a socially monogamous grassfinch. Priorto the experiment, each male founder was fitted with eithertwo red bands (creating a phenotype previously demonstratedto be attractive to females) or two green bands (unattractiveto females) as part of a unique combination of four leg bands.The overall paternal exclusion rate was 28%, as determined bymultilocus, minisatellite DNA fingerprinting of 278 offspringreared by 26 males and their mates. Mean exclusion rates were16% and 40% for red- and green-banded males, respectively. Exclusionrates were directly proportional to rates of female participationin unforced extrapair copulations (UEPCs) with red-banded malesthat occurred when females were fertile. Rates of fertile, forcedextrapair copulations (FEPCs) and fertile UEPCs involving green-bandedmales either failed to influence exclusion rate or varied inverselywith exclusion rate, indicating that extrapair fertilization(EPF) is under female control. Effort devoted by males to seekingEPFs increased exclusion rate. Results suggest that males placegreater effort into seeking fertile versus infertile EPCs andthat unattractive males accrue fitness gains through high parentalinvestment (PI), whereas attractive males benefit through decreasedPI and increased allocation to EPF.     

Parental provisioning and predation risk in rhinoceros auklets (Cerorhinca monocerata): effects on nestling growth and fledging

We examined the effects of predation risk on the behavior ofrhinoceros auklets (Cerorhinca monocerata) breeding at PineIsland, British Columbia, in 1990. Provisioning parents in someareas of the colony risked predation by bald eagles (Haliacetusleucocephalus). Chicks in high and low predation risk areasof the colony hatched on approximately the same date, receivedsimilar amounts of food to 46 days of age, grew at the samerate, reached similar peak masses, and fledged at similar masses.However, chicks in high predation areas fledged at a youngerage than did chicks in low predation areas. These data are consistentwith the hypothesis that parents in high risk areas terminatedprovisioning several days before those in lower risk areas.Mass at fledging was inversely related to age at fledging inboth high and low risk areas. The regression line for the highrisk habitats lies below that from the low risk habitats, aspredicted by a model that examines optimal time of fledgingfrom the perspective of the parents. We conclude that risk ofpredation represents a significant cost of reproduction to somerhinoceros auklets and that individual auklets within the colonyvary their behavior according to predation risk.