Condition-dependent sexual traits and social dominance in the house finch

Elaboration of costly sexual traits can reduce investment inother aspects of reproduction, such as parental care or intrasexualcompetition, which may lead to the evolution of alternativemating tactics. In house finches (Carpodacus mexicanus), lesselaborately ornamented (dull) males tend to dominate more elaborated(redder) males, but redder males pair earlier and invest morein parental care. This suggests that males may pursue alternativeparental or competitive tactics, depending on the elaborationof their sexual trait. Elevation of testosterone, a hormonethat is closely associated with condition in male house finches,influences dominance and sexual behaviors but is antagonisticto parental behaviors. We tested the hypothesis that the higherdominance status of dull males reflects an alternative testosterone-dependentmating tactic. First, we experimentally manipulated the testosteronelevels of captive males and measured the effect on dominancerank, and second, we measured the association of testosteroneelevation and plumage hue in free-living males. We found that,as predicted, testosterone elevation increased dominance rankin captive males. However, in free-living males, testosteronelevels were higher in redder males, suggesting that testosteroneis dissociated from dominance status under natural circumstances.This may be because the context of social interactions and thehigher motivation of dull males to access food resources havea stronger influence on the outcome of dominance interactionsthan does the physiological effects of testosterone elevation.In turn, the strong positive correlation between testosteronelevels and plumage elaboration likely reflects the common conditiondependence of these traits.     

No evidence for differential maternal allocation to offspring in the house sparrow (Passer domesticus)

We tested the differential maternal allocation hypothesis ina population of house sparrows. We experimentally altered theattractiveness of males by treating them with implants filledwith crystalline testosterone (T) or left empty (C). We subsequentlymonitored maternal investment as a function of male hormonaltreatment and the size of the black patch of feathers on thethroat (i.e., the badge), a sexually selected trait. The differentialallocation hypothesis predicts that females should adjust theirinvestment with respect to the benefits they receive by matingwith an attractive male. Given that both circulating levelsof T and badge size are condition-dependent traits, we expectedthat females mated with T males and/or with large-badged malesshould invest more into current reproduction. Contrary to thisprediction, we found no evidence that suggested differentialmaternal allocation in this population of house sparrows. Femaleinvestment in yolk T, yolk mass, clutch size, chick brooding,and feeding was not affected by male hormonal treatment or bymale badge size. As expected, T males invested less into chickbrooding and feeding. More surprisingly, females did not compensatethe reduced paternal contribution to chick feeding. As a consequence,the breeding success of T pairs was largely reduced comparedwith that of C pairs. The absence of differential allocationin a system in which it could have an adaptive role raises thequestion about the possible constraints or overriding factorsoperating on patterns of reproductive investment in this species.     

Mating success in lekking males: a meta-analysis

Traits that are correlated with mating success are likely tobe subject to sexual selection. In lekking species, a male'smating success can be estimated as the number of females thathe copulates with. Earlier reviews of sexual selection in lekkingspecies have been inconclusive, suggesting that different traitsmay be important in different species. To obtain a more completeunderstanding of the outcome of sexual selection in this matingsystem, we performed a meta-analysis in which we combined theresults from different studies across a wide variety of taxa.Our aim was to synthesize available information about correlatesof male mating success in lekking species. We found that behavioraltraits such as male display activit aggression rate, and lekattendance were positively correlated with male mating success.Further, territory position was negatively correlated with malemating success, such that males with territories close to thegeometric center of the leks had higher mating success thanother males. The size of "extravagant" traits, such as birdstails and ungulate antlers, and age were positively correlatedwith male mating success. Male morphology (measure of body size)and territory size showed small effects on male mating success.Our results confirm some of the suggestions put forward by earlierreviews but add more rigor to the condusions drawn. Part ofthe variation across studies still remain unaccounted for. Furtherstudies are needed to perform proper meta-analyses that cantake factors like phylogeny and sexual dimorphism into account.     

Assortative mating by size: A meta-analysis of mating patterns in water striders

Summary Assortative mating by size is a common mating pattern that can be generated by several different behavioural mechanisms, with different evolutionary implications. Assortative mating is typically associated with sexual selection and has been regarded as an attribute of populations, species, mating systems or even higher order taxa. In most animal groups, however, appropriate analyses of assortative mating at these different levels are lacking and the causes and forms of assortative mating are poorly understood. Here, we analyse 45 different population level estimates of assortative mating and non-random mating by size in seven confamiliar species of water striders that share a common mating system. A hierarchical comparative analysis shows that virtually all the variance within the clade occurs among samples within species. We then employ meta-analysis to estimate the overall strength of assortative mating, to determine the form of assortative mating and to further assess potential differences among species as well as the probable causes of assortative mating in this group of insects. We found overall weak but highly significant positive assortative mating. We show that analyses of the degree of heteroscedasticity in plots of male versus female size are critical, since the evolutionary implications of true and apparent assortative mating differ widely and conclude that the positive assortative mating observed in water striders was of the true rather than the apparent form. Further, within samples, mating individuals were significantly larger than non-mating individuals in both males and females. All of these non-random mating patterns were consistent among species and we conclude that weak positive assortative mating by size is a general characteristic of those water strider species that share this mating system. We use our results to illustrate the importance of distinguishing between different forms of assortative mating, to discriminate between various behavioural causes of assortative mating and to assess potential sources of interpopulational variance in estimates of assortative mating. Finally, we discuss the value of using meta-analytic techniques for detecting overall patterns in multiple studies of non-random mating.     

Female-specific color is a signal of quality in the striped plateau lizard (Sceloporus virgatus)

Recent theoretical and empirical studies confirm that male matechoice and/or female–female mate competition can be expressedin the absence of sex-role reversal. Such reproductive patternsmay select for the evolution of female traits that indicatefemale phenotypic or genotypic quality among non–role-reversedspecies. Although attention to the evolution and function offemale ornaments is increasing, additional focus is needed onfemale-specific ornaments (those not expressed in conspecificmales) and on nonavian systems in order to gain a broad understandingof how selection acts directly on ornamentation of female animals.In the striped plateau lizard, Sceloporus virgatus, only femalesdevelop orange throat patches during the reproductive season.The color peaks in expression near the time of ovulation andappears to stimulate male courtship. Here, I examine whetherthis female-specific ornament can be used by males to reliablyassess female phenotypic quality. Using multivariate regressionanalyses, I show that the area of the orange color patch predictsbody condition and mite load, the chroma (i.e., saturation)of the color patch predicts body size, and both patch area andchroma reliably predict average egg mass. Thus, female reproductivecolor may function as a condition-dependent signal, indicatingphenotypic quality to potential mates.     

Resource value and the context dependence of receiver behaviour

Many animals use signals of fighting ability to minimize the costs of competition. Theory predicts that signals must be costly to remain reliable indicators of their bearer's abilities, but many signals of fighting ability lack obvious developmental costs. Instead, receivers are thought to maintain signal accuracy by behaving aggressively towards individuals with inaccurate signals (i.e. social costs). Models predict that the evolutionary stability of social cost signals depends on receivers trusting signals in certain contexts and testing signal accuracy in other contexts. Here, I use the signals of agonistic ability in Polistes dominulus wasps to provide the first experimental evidence that receiver responses to social cost signals are context dependent. During contests over low-value resources, wasps trust signals; they avoid patches of food guarded by rivals with elaborate signals. As the value of the resource increases, wasps become more likely to test signal accuracy. In fact, receivers challenge guards regardless of their signal phenotype when the resource is sufficiently valuable. Context-dependent receiver responses are likely to be an important behavioural mechanism underlying the evolution of social costs, as context-dependent responses allow receivers to minimize the costs of conflict while also ensuring signal accuracy.     

Addition of arthropod cocoons to house wren nests is correlated with delayed pairing

Males in the cavity-nesting house wren (Troglodytes aedon) frequentlyadd arthropod cocoons to their nests during building, possiblyas an ornamental cue for female choice. We tested this hypothesisby comparing the time to pairing for males that did and didnot add cocoons to their nests and for males in whose nestswe manipulated the number of cocoons prior to pairing. We alsotested the hypothesis that females acquire fitness-related benefitsby selecting mates based on their use of cocoons. The use ofcocoons by males was not consistently related to habitat, butthe number of cocoons added per nest increased during the courseof the breeding season. Contrary to prediction, the time topairing for males adding cocoons was significantly longer thanthat for males without cocoons in their nests at both unmanipulatedand experimental nests. There was also no consistent fitness-relatedbenefit for females related to the use of cocoons by their mates.Therefore, we conclude that females did not prefer males thatadded cocoons to their nests, and that the increased time topairing for males that add cocoons likely results in fitness-relatedcosts brought about by delayed breeding. Nonetheless, male housewrens routinely use cocoons, and why they do so remains unknown.     

Quantitative genetics of immune function and body size in the house cricket,Acheta domesticus

Female house crickets are attracted to male calling song containing a relatively high number of syllables per ‘chirp’, which tends to be produced by large males. In a previous study, we showed that this song characteristic is also positively and independently correlated with haemocyte load, an important determinant of the ability to produce an encapsulation response in insects. Females will therefore tend to select males with high encapsulation ability (and large body size) as mates. The present study demonstrates that variation in haemocyte load and body size, together with a second parameter of immune function (the ability to encapsulate a synthetic substrate), is heritable in the same population. Moreover, all three traits are shown to be positively genetically correlated. In favouring males that produce calling song with the preferred characteristics, females should therefore also tend to produce larger offspring with a greater ability to produce an encapsulation response.     

Clutch size and the costs of incubation in the house wren

Trade-offs in the allocation of finite resources among differentstages of a breeding attempt as well as between different reproductiveevents should shape the evolution of life-history traits. Toinvestigate the effects of incubation effort on within-broodand between-brood trade-offs in house wrens (Troglodytes aedon),we manipulated the clutch size that females incubated. We isolatedeffects of incubation by reversing the manipulation at hatchingto allow all parents to provision their natural brood sizes.Females that incubated enlarged clutches had longer incubationperiods than control females, both early and late in the season,suggesting that the experimental treatment increased incubationeffort. Contrary to predictions, however, increased incubationeffort did not adversely affect the allocation of effort tonestling provisioning. Rather, in the early season, but notin the late season, females that incubated enlarged clutchesappeared to allocate more effort to nestling provisioning, producingheavier and larger fledglings than control females. Althoughfemales with enlarged early-season clutches consequently lostmore mass than control females, this was likely an adaptiveresponse to reduce wing loading in anticipation of high provisioningdemands. There were no treatment-related differences in fledglingmass or size, or in female mass loss, in the late season. Thus,elevated incubation demands negatively affected a fitness-relatedtrait (duration of incubation) that may constrain clutch sizebut not the allocation of resources to subsequent stages ofthe same breeding event or to subsequent breeding events. Wesuggest that environmental conditions may mediate clutch-sizeeffects on trade-offs in allocation of resources between incubationand nestling provisioning.     

Effects of extra-pair and within-pair reproductive success on the opportunity for selection in birds

The number of mates, their fecundity, and the number of extra-pairfertilizations can all affect male reproductive success in biparentalspecies. Extra-pair mating in birds has been of particular interest,because it could generate strong levels of sexual selectioneven when a species is socially monogamous. We examined howextra-pair fertilizations affect the opportunity for selectionin the sexually dimorphic common yellowthroat (Geothlypis trichas)and the sexually monomorphic house wren (Troglodytes aedon).We were able to identify sires for at least 95% of all nestlings,and, thus, we were able to make a nearly complete accountingof male reproductive success. Although extra-pair fertilizationswere common in yellowthroats (26% of young) it contributed little(21%) to the total variance in male reproductive success. Mostof the variance in reproductive success (58%) was attributableto the male's within-pair success, which was influenced primarilyby the number of young produced by each mate and the proportionof within-pair young sired. Despite a moderate level of extra-pairfertilizations (10% of young) in house wrens, almost all ofthe variance in male reproductive success (97%) was attributableto within-pair success, particularly the number of social mates.Although extra-pair fertilizations generally increase the variancein male reproductive success, within-pair reproductive successmay be the major source of variation in male reproductive success.Thus, sexual dimorphism in monogamous birds may be influencedmore by the number of mates and their fecundity than by extra-pairmatings.     

Paternal care as a conditional strategy: distinct reproductive tactics associated with elaboration of plumage ornamentation in the house finch

When individuals in a population differ in physiological conditionand residual reproductive value, selection should favor phenotypicplasticity in reproductive investment such that individualsare able to adopt the reproductive tactic that results in thehighest fitness under given conditions. Here we examined reproductivetactics in relation to the elaboration of condition-dependentsexual ornamentation (carotenoid breast coloration) in a Montanapopulation of the house finches (Carpodacus mexicanus). Malesused distinct reproductive tactics depending on elaborationof their sexual ornamentation. Males with red pigmentation (maximum ornament elaboration) paired with females that nestedearlier, but these males did little provisioning of incubatingfemales and nestlings. In contrast, males with yellow colorationpaired with females that nested later, but these males fedfemale and nestlings more. Consequently, for red males offspringrecruitment was primarily affected by earlier nest initiation, whereas in yellow males it was affected most by male provisioning.In males with intermediate plumage coloration, all measuredcomponents, nest initiation, provisioning of incubating female,and nestling feeding, strongly contributed to offspring recruitment.The fitness consequences of alternative reproductive tacticsof males were influenced by breeding experience and fidelityof their mates. Among first-time breeders, red males achievedthe highest fecundity because of the advantage gained throughearly nesting and pairing with more experienced females andbecause of compensation by their mates for low male provisioningof nestlings. Among experienced breeders, males with intermediateplumage coloration achieved the highest fecundity because ofthe combined benefits of relatively early pairing and high parental care. High variation in sexual ornamentation in a Montana populationof house finches may favor distinct associations of sexualdisplays with a particular set of reproductive behaviors.     

Nest size predicts the effect of food supplementation to magpie nestlings on their immunocompetence: an experimental test of nest size indicating parental ability

Post-mating sexually selected signals are expected to indicateparental quality. The good parent model assumes that expressionof the sexual character positively reflects parental ability,resulting in a potential link between the exaggeration of thecharacter and nestling-fitness traits. We tested this predictionin a population of a monogamous passerine, the magpie (Picapica), for which nest size is known to act as a post-matingsexually selected signal. We provided a food supplement to halfof the magpie nestlings in each nest, keeping the other halfas control nestlings. We found that food-supplemented nestlingsexperienced a significantly higher T-cell-mediated immune responseand a tendency to an increased condition index. In accordancewith the good parent model, we found that nest size was positivelyrelated to T-cell mediated immune response for control magpie,whereas this relationship was nonexistent in food-supplementednestlings. In addition, the difference in T-cell mediated immuneresponse between food-supplemented and control nestlings ofthe same nest was principally explained by nest size. Basedon our results, we discuss that magpie pairs with large nestsprovided their nestlings with higher quality food as comparedto pairs with smaller nests, nest size thereby being an indicatorof parental ability. To our knowledge, this is the first studyshowing a link between a post-mating sexually selected signaland nestling immunocompetence, a trait closely related to fitnessin birds.     

Aerodynamic costs of long tails in male barn swallows Hirundo rustica and the evolution of sexual size dimorphism

Exaggerated tail feathers of birds constitute a standard exampleof evolution of extravagant characters due to sexual selection.Such secondary sexual traits are assumed to be costly to produceand maintain, and they usually are accompanied by morphologicaladaptations that tend to reduce their costs. The aerodynamiccosts for male barn swallows Hirundo rustica of having longtails were quantified using aerodynamics theory applied to morphologicaldata from seven European populations. Latitudinal differencesin tail length were positively correlated with differences inflight costs predicted by aerodynamics theory. A positive relationshipbetween aerodynamic costs of long tails and the degree of sexualsize dimorphism was found among populations. Latitudinal differencesin foraging costs may result in tail length being relativelysimilar in males and females in southern populations, whereasthe low foraging costs for males in northern populations mayallow them to cope with higher aerodynamic costs, giving riseto large sexual size dimorphism. Enlargement of wingspan inmales can alleviate but not eliminate the costs of tail exaggeration,and therefore differences in aerodynamic costs of male ornamentswere maintained among populations. Sexual size dimorphism in thebarn swallow arises as a consequence of latitudinal differencesin the advantages of sexual selection for males and the costsof long tails for males and females.