Sexual selection for large male size in a polyandrous butterfly: the effect of body size on male versus female reproductive success in Pieris napi

Sexual selection theory predicts that the larger sex shouldbe that for which fitness increases at the faster rate withsize. In butterflies, as in most invertebrates, females areusually the larger sex, but previous comparative analysis hasshown that relative male size increases with female polyandryamong butterflies. In agreement with this pattern, males arelarger than females in the strongly polyandrous green-veinedwhite butterfly, Pieris napi L., and in this article we assessthe size dependence of reproductive success in both sexes. Inan experiment where virgin males and females were released inthe field, we found no strong association between size and malemating success. However, laboratory experiments showed thatthere was a strong correlation between size and the ejaculatethat the male delivered to the female at mating and that largeejaculates delayed female remating for a longer time comparedto small ejaculates. Moreover, female P. napi utilize male-derivednutrients received at mating to increase their fecundity. Hence,large males sire more offspring both by way of donating morenutrients to female egg production and by way of delaying femaleremating (given that the last male to mate with the female willfather most of the offspring). Laboratory experiments showedthat the association between size and fecundity was low, ornonexistent, among P. napi females allowed to mate only once.However, weak size dependence was found for polyandrous females.We hypothesize that size dependence of female fecundity maybe especially weak among polyandrous butterflies because a fundamentalsource of variation in fecundity relates to their ability tofind nutrient giving males, an ability which may be unrelatedto female size. According to this hypothesis there is a causalassociation between weak size dependence of female fecundityand polyandry, and a strong size dependence of male reproductivesuccess that may underlie the comparative pattern of positivecorrelation between relative male size and polyandry.     

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.     

Dynamic discarding decisions: foraging theory for high-grading in a trawl fishery

The decision by fishermen to discard or retain fish of low valueto make room for more valuable fish in the hold of a boat (high-grading)is similar to diet choice problems faced by natural foragers.In our study, we apply the rationale of diet choice theory tohigh-grading behavior in the Oregon trawl fishery by treatingfishermen as foragers who must decide how much of each net'shaul to "ingest" before searching for more prey. We derive astate-dependent, temporal model of discarding behavior withina fishing trip. This optimization considers the availabilityof differently valued fish, trip quotas set by the regulatoryagency, and the risk of premature trip termination due to lossof gear or injury. The results indicate that those parametersaffect discarding behavior through their effect on the probabilityof exceeding the allowable catch, which we consider analogousto gut capacity. High-grading (partial prey consumption) occurredthroughout many simulated trips. The predictions were consistentwith the trends in discarding observed in the Oregon trawl fleet.Behavioral models such as ours can be useful to fishery managersby providing a means to explore the potential responses of fishermento new regulations before they are implemented.     

Trade-offs between postcopulatory riding and mate location in the blue milkweed beetle

In the blue milkweed beetle, Chrysochus cobaltinus (Coleoptera:Chrysomelidae), males remain stationary on females' backs forprolonged periods after a single, brief copulation. Lone malesoften attack pairs, and takeovers, in which rival males replacethe resident on the female's back, are common. I used removalexperiments to measure the effect of postcopulatory riding onlatency to remating. Females whose males were removed were morelikely to remate in the next 2 h than females that were allowedto remain paired. When females were removed, males were alsomore likely to remate in the 2-h period following removal thanwere males allowed to continue riding. This indicates that malesdelay remating by their mates at the expense of mating opportunitieswith additional females. The predicted reproductive successof guarding and nonguarding males was calculated using a modelbased upon experimentally derived latencies to remating andsimulated levels of last male paternity. Guarders outperformednonguarders when the proportion of offspring sired by the lastmale (P₂) was 0.4 or greater. Data on survival and lifetimemating success were used to evaluate survival trade-offs ofpostcopulatory riding. Although males sacrificed feeding timeto ride on females' backs, there was a positive relationshipbetween the proportion of time males spent riding and theirlongevity in the patch. These results indicate that postcopulatoryriding has no survival costs and indicate that postcopulatoryriding can be an effective paternity assurance mechanism evenwhen takeovers are common.     

The definition of eusociality

We describe more precise definitions for the term "eusociality"and other social systems. Our criterion for eusociality is thepresence of castes, which are groups of individuals that becomeirreversibly behaviorally distinct at some point prior to reproductivematurity. Eusocial societies are characterized by two traits:(1) helping by individuals of the less-reproductive caste, and(2) either behavioral totipotency of only the more reproductivecaste (facultative eusociality) or totipotency of neither caste(obligate eusociality). We define "cooperative breeding" asalloparental care without castes. Cooperatively breeding societiesmay comprise two types, semisocial (distribution of lifetimereproductive success bimodal), and quasisocial (distributionof lifetime reproductive success unimodal), but this hypothesisrequires empirical analysis. Our definitions conceptually unifystudies of arthropod and vertebrate sociality.     

Correlates of male quality in the field cricket, Gryllus campestris L.: age, size, and symmetry determine pairing success in field populations

Sexual selection in field crickets (Gryllidae) is well documented.Several studies have identified male traits, such as age andbody size, that influence pairing success in the field. HereI show how these traits covary with male quality by examiningthe degree of fluctuating asymmetry in male Gryllus campestris.Older males were both larger and more symmetrical. Principalcomponents analysis suggests that these three variables wereessentially measures of the same trait, male quality. A comparisonof paired and calling males showed that males of high qualitywere more successful in obtaining mates; paired males were older,larger, and more symmetrical. The area of the harp covariedwith morphological traits and determined the carrier frequencyof the male's call. Information related to male quality wastherefore available for female discrimination.     

On the economics of sit-and-wait foraging: site selection and assessment

We present two models of optimal resource exploitation for sit-and-waitforagers. The first model assumes immediate recognition of sitequality and that site quality does not change over time. Thismodel predicts a forager's minimum acceptable site quality.We present a graphical analysis to show how (1) the distributionof site qualities, (2) the travel time between sites, (3) costof search, and (4) expected duration of the foraging processinfluence the minimum acceptable rate. Our second model allowssite qualities to change and relaxes the assumption of immediaterecognition. This model defines conditions of (1) state duration,(2) recognition time, (3) site abundance, and (4) cost of searchwhere the optimal policy is to stay put in a site regardlessof experience. We discuss the implications of these models forthe design and interpretation of field experiments of site useand habitat selection.     

Predation, sensitivity, and sex: why female black rhinoceroses outlive males

Among sexually dimorphic, polygynous mammals, adult femalestend to outlive males and respond more strongly to predatorsthan males. We asked whether a monomorphic, polygynous speciesvirtually immune to predation due to large size (black rhinoceros,Diceros bicornis) conforms to this pattern. Data on 193 interactionswith lions (Panthera leo) and spotted hyenas (Crocuta crocuta)in two nonhabituated populations in Namibia studied from 1991to 1993 revealed that: (1) females were more vigilant or aggressivethan males to either of the potential predators; and (2) whethersolitary or with calves, females attacked more often than males.Although solitary females tended to be more aggressive to lionsthan to hyenas, neither females with calves or males seemedto discriminate between the two carnivores. We also simulatedthe behavior of human predators (poachers) during 69 encounterswith rhinoceroses. While both sexes abandoned local sites becauseof our presence, females ran farther than males, covering upto 40 km in a day. These findings implicate a behavioral mechanismto explain why secondary sex ratios favor females - males aremore prone to human predation, a prediction consistent withdata from 12 populations throughout Africa. Black rhinocerosesappear to be an unanticipated exception to the well-establishedpattern of male-biased mortality in polygynous mammals; in theabsence of intense human predation (a recent event), male mortalityfails to exceed that of females, suggesting that intrasexualcompetition in a polygynous mammal: may not be the primary causeof unbalanced secondary sex ratios. Our results on the causesof sex differences in mortality and in responsiveness to differentpredators reinforce the relevance of behavioral ecology to conservation;such information is necessary for planning how best to minimizenegative human influences on the few remaining wild Africanrhinos.     

Energetic constraints and foraging efficiency

Previous research considers foraging options that differ interms of their gross rate of gain b and rate of energy expenditurec. This research argues that maximizing efficiency b/c willmaximize net energetic gain when there is an upper limit onthe amount of energy that can be assimilated. This analysisdoes not include the expenditure during the time for which theanimal is unable to forage because of this constraint. Whenthis expenditure is included, maximizing efficiency is no longeroptimal. Instead the best feeding option is the one with thehighest value of b/(c – c₁), where c, is the metabolicrate when the animal is not foraging.     

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.     

Dynamics of female choice for parental care in a fish species where care is facultative

In the peacock wrasse (Symphodus tinca), females either placetheir eggs in a nest under the care of a male or disperse theireggs widely so that they receive no protection. The same femalecan spawn in both modes. Females appear to prefer males withnests early in the nesting cycle, and they spawn less oftenthan expected in late-cycle nests and with non-nesting males.Survival and hatching success are consistently higher for eggsplaced in early nests, particularly in mid-season when egg predationis intense and hatching times are relatively long. Nevertheless,30%-80% of females place their eggs outside nests, even whenhatching success is more than four times greater with care.A model incorporating search time for nests correctly predictedthe qualitative changes in the tendency of females to choosecare or no care over the course of the mating season. Extensionsof the model suggest that in the early part of the season, whennests are rare and the relative survival advantage of parentalcare is small, females should sample no more than one nest beforeopting for no care. In mid-season, when the advantages of careare highest and between-nest travel times are low, females areestimated to visit at least 8 nests before abandoning the effort,corresponding to a giving-up time of about 43 min of search.Later in the season, when short hatching times reduce the relativebenefit of care, females are estimated to visit between 4 and5 nests before giving up, corresponding to about 31 min of search.We suggest that the variability in parental care in this speciesarises from seasonal changes in relative costs and benefitsof care for the two sexes.     

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.     

Frequency-dependent selection on information-transfer strategies at breeding colonies: a simulation study

Through computer simulations, we model three different foodfinding strategies: searcher, no information transfer, watcher,limited information transfer; follower, full information transfer.The aim of this article was to study how frequency-dependentselection affects the proportion of these strategies at a simulatedcolony under different patterns of food distribution. Furthermore,we determined how information transfer in a population witha mixed evolutionarily stable strategy (ESS) modified the averageforaging efficiency of an individual compared to that of anindividual in a population with mutual information exchange.We found that the proportion of information gaining strategiesincreases as the food resources become more clumped. The improvementin foraging efficiency through the operation of an informationcenter need not require mutuality in information exchange. Onthe basis of the presented study, at the ESS only a small percentageof colony members need discover food patches, yet the foragingefficiency may be high because of the operation of an informationcenter.     

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.     

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.     

Predicting group size in primates: foraging costs and predation risks

We present a direct test of the long-standing hypothesis thatfood competition limits primate group size. Group size is acritical social variable because it constrains most other aspectsof social organization. We develop a simple population-specificindex of indirect feeding competition based on daily foragingcosts. This index explains nearly two-thirds of between-populationvariation in mean group sizes of mostly fruit-eating (but notof mostly leaf-eating) primates. Group size is also significantlyrelated to body size and terrestriality (or use of open country),which are suspected correlates of predation risk, although feedingcompetition remains an important predictor of group size evenwhen these correlates are controlled. Phylogeny also appearsto be important: the differences between observed mean populationgroup sizes and those predicted using ecological factors aremost positive for the Old World monkeys and most negative forthe lemuroids in our sample. The weak relationship between groupsize and feeding competition found for folivorous species maybe explained either by the energetic constraints of a leafydiet or by limits to group size imposed by infanticide as ahabitual male reproductive strategy.     

Egg presence, egg loss, and female mate preferences in the sand goby (Pomatoschistus minutus)

We studied the effect of egg presence on female mate choicein a fish with paternal care. Females who were allowed a freechoice between two males mated within a shorter time than femaleswho were randomly assigned to a particular male. When a secondfemale was allowed to choose among the males, she preferredthe same male as the previous female. This result shows thatfemales are concordant in their mate choice. When the initialfemale was randomly assigned to mate with one of two males (forcedchoice), the second female mated randomly with respect to thefirst one. Thus females do not prefer males with eggs. If theinitial female was given a free choice, but the eggs were removedfrom the chosen male, the test female mated randomly. When boththe males initially had mated but one randomly determined male'seggs were removed, the test female preferred the male who wasstill guarding eggs. These experiments show that females avoidspawning in unsuccessful nests. When the females in the freechoice/egg removal experiment mated with the unsuccessful malethere was a considerably bigger size difference in favor ofthis male than when the females mated with the other male. Weconclude that female sand gobies show clear mate preferences,but that they do not prefer males with eggs over males withouteggs. They do, however, avoid mating with males guarding unsuccessfulnests. We therefore suggest that egg loss could be an importantfactor selecting for egg preference.     

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.     

Investigating structure and temporal scale in social organizations using identified individuals

Studies of individually identified animals can produce substantialdata sets containing information on the structure and temporalscale of social organizations. However, methods of analyzingsuch data are not well established. Important features of asocial organization are revealed by plotting the rate of persistenceof the associations between pairs of individuals over a rangeof time lags (lagged association rate). The consistency of long-termrelationships can be characterized using the rate of associationof pairs of individuals between their first and last observedassociations (intermediate association rate). A hierarchicalseries of models featuring exponentially decaying lagged associationrates may be fitted to these data. This technique retrievedthe essential parameters of five simulated social organizationsand, when used on real data, portrayed the essential featuresof the patterns of temporal change in relationships betweenanimals. The method should be especially useful for analyzingfissionfusion societies containing 10–10, 000 individuallyidentifiable animals.