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.     

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.     

Black-and-white plumage in male pied flycatchers (Ficedula hypoleuca) reduces the risk of predation from sparrowhawks (Accipiter nisus) during the breeding season

Bright colors in birds might signal that they are undesirableas prey (aposematic), an idea that has been difficult to test.When stuffed pied flycatchers Ficedula hypoleuca are exposedto migrating sparrowhawks Accipiter nisus in spring or in autumn,the hawks attack cryptic females more often than bright males.To achieve better statistical control and to assess whethermale plumage also reduces predation risk in the breeding seasonand in the nesting habitat of the pied flycatcher, I placedpairs of male and female flycatcher mounts in similar positionsnear 22 nests of sparrowhawks. The hawks attacked mainly femalemounts, verifying that the preference is real. The sparrowhawkscaught at least 19 live pied flycatchers; 12 young, 5 adultmales, 1 adult female, and 1 female or young. Hawks that caughtan adult male seemed to prefer attacking female mounts. I discussthree interpretations of these results, suggesting that black-and-whitemale flycatchers may benefit from being a novel and aberrantprey, at least early in the breeding season.     

Divergence in male mating tactics between two populations of the soapberry bug: II. Genetic change and the evolution of a plastic reaction norm in a variable social environment

Many social behaviors are conditional, but behavioral comparisonsbetween populations do not normally distinguish genetic andenvironmental causation. As a result, the opportunity to testpredictions about the evolution of strategic conditionality(genotype x environment interaction) is lost. We apply theseconcepts in an examination of how interpopulation differencesin mean and variance of sex ratio have led to genetic differencesin the allocation of male effort to mate guarding versus nonguardingbetween genetically isolated populations of the soapberry bugin Oklahoma and Florida. We observed the mating behavior ofmales from the two populations at a series of experimental sexratios, and modeled their mating decisions as first-order Markovchains of independent mating states. Likelihood ratio testsof these behavioral sequences showed that the populations differedsignificantly in their response to sex ratio, and that onlymales from the variable environment (Oklahoma) altered theirbehavior in response to differences in female availability amongthe treatments. The flexible strategy of this population maybe adaptive and probably has evolved in response to sex ratiovariability.     

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.     

Temperature affects male and female potential reproductive rates differently in the sex-role reversed pipefish, Syngnathus typhle

The differences in potential reproductive rate between the sexescan be used to predict the operational sex ratio and the patternsand intensity of mating competition and hence sexual selectionin a population. This article describes how one environmentalcomponent, temperature, affects potential reproductive ratesof the two sexes in the paternally brooding, sex-role reversedpipefish (Syngnathus typhle). Males brooded embryos much longer(on average 58 days) in cold water (about 10°C) than inwarmer water (35 days at about 15°C). As a consequence,the potential reproductive rate (number of eggs brooded perday) of males was significantly higher in warm water. In females,however, potential reproductive rate, i.e., number of eggs producedper day given an unlimited access to mates, was not significantlydifferent between temperatures. In both sexes, potential reproductiverate was positively related to body size. At both temperatures,females had the potential to reproduce faster than males. Asa result, the operational sex ratio will become female biasedand sex-roles reversed, as is the case in this species. Sincetemperature differently influenced the potential reproductiverates of males and females, with the sexual difference largerat lower temperatures, more intense female-female competitionis predicted at low temperatures.     

Differential grooming rate and tick load of territorial male and female impala, Aepyceros melampus

During the breeding season in Zimbabwe territorial male impalawere found to engage in much less self oral grooming and allogroomingthan females, presumably as a reflection of the need to remainvigilant in herding females and repulsing challenging bachelormales. Territorial males spent an average of 11 min engagedin all types of grooming during a 12-h day, compared with 40min grooming by females. Rutting activity and time spent scanningpeaked in May, while self oral grooming and feeding by territorialmales was lowest at this time. The decrement in grooming bymales (relative to females) represented half of all time devotedto rutting behaviors. Territorial males appeared to sacrificefeeding and grooming time in exchange for more time devotedto vigilant activities essential to mating success. In a comparableregion of Zimbabwe, territorial males were also found to harborabout six times as many adult ticks as the females on the sameterritories during the breeding season. Because grooming isunderstood to be effective in removing ticks, the higher tickload of territorial males was attributed to their reduced groomingbehavior. However, testosterone and adrenal cortical steroids,which are elevated in territorial male impala, are known todepress the immune system, and so may be important in controllingparasite infections. Sexually active males of many species aregenerally found to harbor more parasites than females. The differencein tick load between territorial male and female impala mayreflect both behavioral and hormonal parameters.     

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.     

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.     

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.     

The effect of group size manipulations on the foraging behavior of black-tailed prairie dogs

Individuals foraging in large groups are thought to benefitbecause they are better able to detect and avoid predators.As a consequence, individuals in groups can adopt more risky,but rewarding, foraging behaviors without exposing themselvesto excessive danger. I experimentally manipulated the size ofblack-tailed prairie dog (Cynomys ludovicianus) groups to determineif individuals in large groups do forage in a more risky manner.I found that prairie dogs foraged more alertly and in less riskylocations (nearer to burrows, nearer to the center of the group,and in shorter vegetation) when group size was reduced. Effectsof group-size reductions were reversed when removed individualswere replaced, and persisted for at least three weeks in experimentswhere group size was permanently reduced. My results provideevidence that the relationships between group size and bothalertness and risk-place foraging are causal.     

Early arrival, initiation of nesting, and social status: an experimental study of breeding female red-winged blackbirds

Red-winged blackbirds (Agelaius phoeniceus) arrive at breedingsites several months before nests are constructed. Males inthis highly polygynous species presumably return early to defenddesirable territories. Females, however, also begin to arrivealmost two months before nesting is attempted. Early returnto breeding sites could enhance reproductive success by increasinga female's social status and thereby allowing earlier nesting.I measured the effect of experimentally delayed arrival on thetiming of nesting, reproductive success, and social status offemales. Birds were captured as they arrived in early springand detained in an aviary. These experimental females were laterreleased at their capture sites before control females beganconstruction of nests; controls had arrived during the sameperiod as experimentals but were not detained. Experimentalfemales nested, on average, more than a week later than controls,although I could detect no effect of timing on reproductivesuccess during this study. By manipulating arrival date, butnot covariates of arrival time such as age or experience, thisresult indicates that timing of arrival directly influencedlaying date. Delayed females were subordinate to control femaleswith the same mate. This decline in social status may have beenresponsible for the delay in nesting and could have fitnessconsequences for females in some years, because the offspringof earlier nesting, dominant birds are more likely to receivemale parental care in this population.     

Parental effort of kestrels (Falco tinnunculus) in nest defense: effects of laying time, brood size, and varying survival prospects of offspring

We studied the nest defense behavior of Eurasian kestrels (Falcotinnunculus) towards a stuffed pine marten (Martes martes) througha 3-year vole cycle (1990–92) in western Finland. Survivalprobability of offspring decreases with a later start of breeding,and, therefore, early breeders should protect their offspringmore than late ones. We found this true for males during theincubation period, but not for females. In addition, we expectedthe nest defense intensity to increase with offspring number.During the incubation period, this was true for females, butnot for males. During the nestling phase, parents did not adjusttheir defense effort to natural or manipulated (by one to twoyoung) brood size. Survival prospects of kestrel offspring werehighest in the increasing vole year 1991 and lowest in the decreasingvole year 1992, and, therefore, we expected the defense activityof kestrels to follow the same trend. However, the oppositeresult appeared true for females with a similar tendency formales. Most hypotheses predicting avian nest defense behaviorwere not supported by our data. Temporally heterogeneous environmentand low degree of nest-site tenacity of migratory kestrels maymake them unfamiliar with environmental variation and survivalprospects of their offspring. Therefore, fitness benefits ofparental care are not predictable, and kestrels may thus adjusttheir parental effort to their own future reproductive potential(i.e., number of future breeding attempts), rather than to somecurrent investment indicator, like offspring age and number.