Foraging Cost of a Long Tail Ornament: An Experiment with Sand Martin Females

The handicap hypothesis assumes that sexual ornaments impose a viability cost upon the bearers. There have been few empirical tests of this assumption. Previous studies show evidence for the cost of a tail ornament in male birds: a negative relationship between an experimentally increased tail ornament (long tail streamers) and efficiency at foraging for nestlings. However, it must be admitted, that the apparent impairing effect of an elongated tail could be a result of a decrease in male parental effort in response to an increase of female parental effort, which might have occurred in response to increased male attractiveness (differential allocation of female parental effort). In this study, the effect of differential parental expenditure was eliminated by lengthening the tail in female, rather than male, sand martins ( Riparia riparia ). Tail-elongated females decreased the rate at which they fed nestlings, and captured more but smaller insects. There was no simultaneous increase of feeding rate in the males that could explain the decrease of feeding rate in the females. These results confirm the existence of a cost of a tail ornament in birds feeding in flight, as is expressed in terms of impaired flight and foraging capacity.     

Examining the Cost of Experimentally Imitated Ornaments

Long forked tail ornament in male barn swallows (Hirundo rustica) were suggested to impose a condition‐dependent viability cost ( Møller 1989 ; reviewed in Møller 1994 ; Møller & de Lope 1994 ; Møller et al. 1995 ): long tails impair male flight and foraging ability in terms of mean size of prey captured. In a recent study, we ( Matyjasiak et al. 1999 ) showed such a foraging cost of an experimentally imitated tail ornament in the sand martin (Riparia riparia), which have no tail ornament. We lengthened the tail in females, instead of in males, and thus controlled experimentally for the possible effect of differential allocation of female parental expenditure (differential‐allocation hypothesis, Burley 1986 ). Cuervo (2000) raises important issues in his comments about our paper. He questions the method used in our study ( Matyjasiak et al. 1999 ). He points out that ‘in order to test the cost of flight of an ornament, the trait to be experimentally manipulated has to be an ornament’, and that we should have not only elongated but also shortened tail feathers. Secondly, he suggests that we did not provide, in our article, the exclusive evidence for the handicap model of sexual selection. Thirdly, he disagrees with our suggestion that the results from the barn swallow experiments could be confounded by the differential allocation of female parental expenditure. Here, we outline the areas of agreement and disagreement between Cuervo's critique and our paper. Firstly, we agree with Cuervo regarding the importance of tail shortening in studies of fully developed tail ornaments, which has already been pointed out by other authors ( Thomas & Rowe 1997 ; Evans & Thomas 1997 ). However, shortening of an ornament that is at equilibrium would always produce a decrease in costs. Therefore it will only confirm that an ornament is costly. However, if shortening is done to individuals of various, known condition, it may result in crucial information for distinguishing between hypotheses of sexual selection. We agree that for a character that is not an ornament, as in our experiments, shortening would not give new insights apart from another way of confirming the measures used in the barn swallow studies. However, we disagree with Cuervo's claims of irrelevance of our experiment to the issues of the evolution of signals. We have chosen sand martins as a model species because the shape of its tail resembles that in ancestors of modern tail‐ornamented swallows, from which tail feather elongation under sexual selection may have started ( Matyjasiak et al. 2000 ). Possible scenarios of the early evolution of a tail ornament may be examined by experimentally adding such an ornament, which requires manipulating original, non‐ornamental traits (e.g. Goötmark 1994, 1996 ). We disagree with Cuervo that the existence of traits that may reduce the costs of ornaments (e.g. Møller 1996 ) eliminates the validity of tail manipulation studies in species without ornaments. We believe that such cost‐reducing traits may not have existed during the early stages of evolution of tail ornaments (as well as other sexual traits) but may have developed later, and our experiment may represent such a situation very well. Hence, by imitating the initial development of a forked tail ornament in sand martins, we performed a biologically relevant manipulation. Secondly, we do not state in our paper that we have tested or proved the handicap principle. We only mention in the last paragraph of the Discussion, that our results are consistent with this principle. However, we do admit that mentioning only the handicap hypothesis in the Introduction and Discussion only, and omitting other processes by which costly tail ornaments might arise, was unwarranted, as Cuervo properly argued; this might have left a false impression that our goal was to test the handicap hypothesis. We do believe that we have properly measured the costs of tail elongation, and this was our goal, as stated in the Abstract and in the last paragraph of the Introduction. Nowhere in the paper did we state that we actually aimed to test the differential costs of ornaments that are crucial to the handicap hypothesis. This issue was the objective of a different paper ( Matyjasiak et al. 2000 ) in which we consider conditions important for the early evolution of tail ornaments, with special emphasis on the handicap principle. Thirdly, we are more cautious at interpreting the barn swallow studies and we do not exclude a possibility that differential allocation of parental expenditure can affect the size of prey brought by males. Differential allocation does exist in this species ( Møller 1992 ; de Lope & Møller 1993 ), as shown by the higher feeding rate by females responding to male higher attractiveness (longer tails). We believe that even though simple logical, intuitive reasoning may suggest that prey size should be unaffected by a differential allocation mechanism, it is not just reasoning but empirical proof that is needed here. Because there was no proof for a lack of the effect of differential allocation on the size of prey, we thought of an independent way of illustrating the costs of tail elongation in swallows, using a species without possible differential allocation effects. We thought that if we obtained results confirming the barn swallow studies by Møller and collaborators, we could help in validating the measures of tail elongation costs used in those studies. This was possible using the sand martin females for reasons discussed in the paper, all of which point to the lack of any differential allocation effects in this species. From this point of view, experimentally coupling tail elongation with tail shortening in our study appears unnecessary. Even though, as argued by Cuervo (2000) , it seems likely that the prey size in male swallows is unaffected by a change in feeding rate of females in response to male attractiveness (an assumption inherent in the barn swallow studies), another scenario is also possible. Imagine a female that has increased the amount of food for nestlings, in response to increased sexual attractiveness of her male partner due to experimental elongation of his tail. In such a situation the male has been relieved from a considerable duty of providing the young with a large amount of food. Hence, it is possible that such a male shifts from maximising the energy brought to nestlings per unit time (i.e. maximising foraging rate) to the criterion of obtaining the daily energetic needs at the least expense (i.e. minimising foraging costs). Such shifts may lead to including some non‐preferred, small insects that can be captured quickly and inexpensively in terms of energy, because it does not require the use of expensive flapping flight, which is required to capture larger, preferred insects ( Waugh 1978 ; Bryant & Turner 1982 ; Turner 1980, 1982 ). By including more small insects in their catch at the expense of a reduced foraging rate, attractive males could save energy for future use. Barn swallows appear to compromise between maximising their foraging efficiency (maximising foraging gains per costs) and maximising energy intake per unit time (see fig. 5.7 in Turner 1980 and table IV in Turner 1982 ). Hence, if sexually attractive males aim at minimising foraging costs rather than maximising foraging rate for nestlings, then we cannot exclude the possibility that this may result in smaller insects being caught, on average, by males with experimentally longer tails. By a similar reasoning, it is theoretically possible that differential allocation effects could lead to larger mean prey size in males with experimentally shortened tails ‐ an effect actually shown in the barn swallow studies ( de Lope & Møller 1993 ; Møller & de Lope 1994 ; Møller et al. 1995 ). Hence, whether shortening, elongating or performing both experimental manipulations, in our view we cannot be entirely sure whether the results are affected by differential allocation. This was sufficient motivation for us to investigate, independently, the usefulness of the change in prey size as an indicator of foraging costs due to elongated tails. In contrast to Cuervo's opinion, we believe that our results are relevant to the issues important for the evolution of forked tail ornaments. We have measured the costs of a character that imitates the hypothetical early stages of the evolution of sexual ornaments, and we may use these results to discuss the early evolution of sexual ornaments (see Matyjasiak et al. 2000 ). We also confirmed the validity of measures of tail elongation costs used in the barn swallow studies.     

DIFFERENTIAL COSTS OF A SECONDARY SEXUAL CHARACTER: AN EXPERIMENTAL TEST OF THE HANDICAP PRINCIPLE

The evolution of reliable signaling can be explained by the handicap principle, which assumes that (1) the cost of a signal guarantees its reliability, and (2) cheating is prevented because the cost of a unit of display is greater for low-quality than for high-quality individuals. A test of these two assumptions was performed using manipulations of the length of the outermost tail feathers of male barn swallows Hirundo rustica, a trait currently subject to a directional female mate preference. We found that survival decreased with tail elongation and increased with tail shortening of males, supporting the assumption that the secondary sexual character is costly. Naturally long-tailed males were better able to survive with an elongated tail, whereas naturally short-tailed males improved their survival following tail shortening. This observation supports the second assumption of a differential cost of a signal. One mechanism imposing differential costs on sexually signaling barn swallows is foraging. Males with elongated tails captured smaller, less profitable Diptera, whereas males with shortened tails captured large, profitable prey items. The conditions for reliable sexual signaling by the tail ornament of male barn swallows are thus fulfilled.     

Vane emargination of outer tail feathers improves flight manoeuvrability in streamerless hirundines, Hirundinidae

Recent studies have suggested that the proximal part of the swallow (Hirundo rustica) tail streamer appears to aid turning flight, as expected if streamers evolved initially purely through natural selection for enhanced manoeuvrability. However, the evolution of slender aerodynamically advantageous streamers is also predicted by an alternative hypothesis, which suggests that such a trait could develop primarily to ameliorate the aerodynamic cost of a long size-dimorphic tail. To distinguish between these hypotheses, we have investigated for the effect on manoeuvrability of trimming the tips of the outer tail feathers into short streamers, without lengthening these feathers, in two streamerless hirundine species--the house martin (Delichon urbica) and the sand martin (Riparia riparia). This allowed us to examine the aerodynamic costs and benefits of streamers at an early evolutionary stage that predates elongation of the outermost tail feathers through female choice. We showed that such initial streamers enhance manoeuvrability in streamerless hirundines, confirming the findings of recent studies. However, in contrast to these studies, we showed that improved manoeuvrability resulting from streamers could arise before the outermost tail feathers have become elongated (e.g. owing to female choice). The occurrence of such an aerodynamic advantage depends on the ancestral shape of a forked tail. This provides support for the hypothesis that streamers, like those in the barn swallow, might evolve initially purely through natural selection for enhanced manoeuvrability.     

Central place foraging by swallows (Hirundinidae): The question of load size

Some predictions of Orians & Pearson's (1979) models for central place foragers (CPF) were tested with three species of swellows (Hirundinidae). House martins (Delichon urbica) and sand martins (Riparia riparia) brought larger food loads to the nest mainly when foraging distances were great, whereas swallows (Hirundo rustica) gathered large loads when food was plentiful. For all three species the outcome conformed qualitatively with the predictions of the CPF models. Overall, house martins were the most sensitive to travel time effects, but in a quantitative test the predicted load size was 20–40% less than the observed size for a range of realistic travel times. Additional models are presented which emphasize the significance of foraging techniques and foraging costs for optimal load size in multiple prey loaders.     

Sexual selection in the barn swallow Hirundo rustica. VI. Aerodynamic adaptations

Secondary sexual characters are assumed to be costly to produce and maintain, and this will select for morphological modifications that reduce the magnitude of such costs. Here we test whether a feather ornament, the sexually exaggerated outermost tail feathers of male barn swallows Hirundo rustica, a trait currently subject to a directional female mate preference, and other aspects of the morphology used for flight have been modified to increase aerodynamic performance. This was done by making comparisons among sexes within populations, among individuals varying in tail length within populations, and among populations from different parts of Europe. Male barn swallows experienced reduced drag from their elongated tail feathers by morphological modifications of the ornamental feathers as compared to females. Morphological features of the outermost tail feathers were unrelated to tail length in both males and females within populations. Wing and tail morphology (length of central tail feathers and wings, wing span, wing area, wing loading, and aspect ratio) was modified in males compared to females. Barn swallows with long tails had morphological tail and wing modifications that reduced the cost of a large ornament, and similar modifications were seen among populations. The costs of the exaggerated secondary sexual character were therefore reduced by the presence of cost-reducing morphological modifications. The assumptions of reliable signalling theory, that signals should be costly, but more so to low than to high quality individuals, were not violated because long-tailed male barn swallows had the largest cost-reducing morphological characters.     

Adaptive fault bar distribution in a long‐distance migratory,aerial forager passerine?

Fault bars are translucent bands produced by stressful events during feather formation. They weaken feathers and increase their probability of breakage, and thus could compromise bird fitness by lowering flight performance. It has been recently suggested ('fault bar allocation hypothesis') that birds could have evolved adaptive mechanisms for reducing fault bar load on the feathers with the highest function during flight. We tested this hypothesis by studying first-year individuals of the long-distance migratory, aerial forager barn swallow Hirundo rustica . We predicted that fault bars should be less abundant on the outermost wing and tail feathers, but more frequent on the tail than on the outermost wing feathers. Accordingly, we found that fault bars occurred more often in tertials than in primaries or secondaries. Tail feathers had fewer fault bars than tertials, but more than primaries. Within the tail, the distribution pattern of fault bars was W-shaped, with the highest fault bar load occurring on the streamers and on the two central feathers. Because streamers are the most important tail feathers for flight performance, this finding seems to contradict the 'fault bar allocation hypothesis'. However, flight performance is much less sensitive to changes in the shape of the tail than of the wings, which could explain why evolutionary forces have not counteracted the increase of fault bars associated with feather elongation during the recent evolution of streamers in the tail of hirundines.  © 2005 The Linnean Society of London, Biological Journal of the Linnean Society , 2005, 85 , 455–461.     

Sex-limited expression of ornamental feathers in birds

Extravagant secondary sexual characters show sexual size dimorphismin some species but are completely sex limited in others. Sexualornamentation has been hypothesized to benefit mainly malesthrough sexual selection, but the costs of secondary sexualcharacters initially would be experienced by both sexes. Theevolution of sexual size dimorphism of ornaments and, eventually, the complete sex-limited expression of these characters, willdepend on the effects of sexual and natural selection on thetwo sexes. A phylogenetic analysis controlling for similaritiesdue to common ancestry of 60 independent evolutionary originsof feather ornamentation in birds was used to investigate ecologicalfactors correlated with sexual size dimorphism and sex-limited expression of secondary sexual characters. When the size ofan ornament is large relative to body size, the trait willbe particularly costly for females, resulting in selectionfor increased sexual size dimorphism of the ornament. Indeed,sexual size dimorphism of ornaments was positively related to the relative size of male ornaments but was unrelated torelative size of female ornaments. Species with polygynousand lekking mating systems with little or no male parentalcare (in particular nest building and incubation) demonstratedsex-limited expression of ornaments as compared to monogamous species. Species with no food provisioning of offspring by themale showed a trend for increased sexual size dimorphism ofornaments. Therefore, large natural selection costs duringreproduction imposed by the expression of secondary sexualcharacters are related to the evolution of sexual size dimorphismof ornaments and eventually their complete loss from females.     

THE HANDICAP PROCESS FAVORS EXAGGERATED,RATHER THAN REDUCED,SEXUAL ORNAMENTS

Why are traits that function as secondary sexual ornaments generally exaggerated in size compared to the naturally selected optimum, and not reduced? Because they deviate from the naturally selected optimum, traits that are reduced in size will handicap their bearer, and could thus provide an honest signal of quality to a potential mate. Thus if secondary sexual ornaments evolve via the handicap process, current theory suggests that reduced ornamentation should be as frequent as exaggerated ornamentation, but this is not the case. To try to explain this discrepancy, we analyze a simple model of the handicap process. Our analysis shows that asymmetries in costs of preference or ornament with regard to exaggeration and reduction cannot fully explain the imbalance. Rather, the bias toward exaggeration can be best explained if either the signaling efficacy or the condition dependence of a trait increases with size. Under these circumstances, evolution always leads to more extreme exaggeration than reduction: although the two should occur just as frequently, exaggerated secondary sexual ornaments are likely to be further removed from the naturally selected optimum than reduced ornaments.     

Uropygial gland volume and malaria infection are related to survival in migratory house martins

Pathogens such as bacteria, fungi and malaria and related haemosporidians provoke negative effects on the fitness of their hosts. Animals have developed a range of defensive mechanisms to resist or eliminate these parasitic infections and their negative fitness costs. The uropygial gland secretion has been proposed to act as defensive barrier of skin and plumage in the fight against bacteria and fungi, and may prevent birds from acquiring haemosporidian infections. Thus, the secretion of uropygial glands of birds may favour survival of individuals. However, whether uropygial gland secretion influence survival remains unknown. Here we explore if the size of the uropygial gland and malaria infection influence survival of house martins Delichon urbica. We showed, for the first time, that the volume of the uropygial gland positively predicted survival prospects of malaria infected house martins. Malaria infected birds had the lowest probability of survival, with the effect of gland size on survival prospects depending on infection: infected house martins with larger uropygial glands were better able to survive to the next breeding season, while infected birds with small uropygial glands were not. These results highlight the importance of uropygial gland secretion in the life history of wild birds.     

Convergent evolution of the tradeoff between egg size and tail fork depth in swallows and swifts

Convergent evolution provides strong evidence of the power of natural selection, particularly for distantly related taxa. Swallows and swifts are such distantly related taxa; both are specialised to feed on flying insects and have similar morphological features, such as long wings. These birds also exhibit deeply forked tails in some species, but their function remains unclear; some have argued that fork tails have evolved due to sexual selection to attract mates, while others claim that viability selection for efficient foraging favours fork tails. A recent phylogenetic analysis found the negative relationship between female tail fork depth and egg size in swallows perhaps due to foraging costs of fork tails during egg formation, but its generality remains unclear. Here, using swifts, which differ from swallows by foraging on weak‐flying insects, we found that egg size significantly decreased with increasing female fork depth. Because female fork depth was not significantly related to clutch size, clutch size would not compensate for the relationship between egg size and fork depth. The current finding using swifts, together with the previous finding in swallows, provide strong support for an evolutionary tradeoff between the female plumage ornament and reproductive investment, as predicted by sexual selection theory.     

The dynamics of honesty: modelling the growth of costly, sexually-selected ornaments

The handicap principle suggests that individuals of superior quality can more easily bear the cost of developing extravagant ornaments. Consequently, ornament size should provide reliable information about quality or condition. Previous models have largely ignored the process of ornament growth, focusing only on final ornament size. We model ornament growth schedules for individuals of different qualities, where higher quality individuals experience lower costs of carrying energy reserves of a given size, but where all individuals pay a net cost of carrying ornaments of a given size. If the costs of ornament production ensure that final ornament size reliably signals quality, the information conveyed by the signal can change dramatically during growth. Higher quality individuals should delay growth until closer to breeding. Taking a snapshot of partially developed ornaments prior to breeding would show them to be larger in poorer quality individuals. The claim that costly ornaments honestly signal quality thus needs to be understood in a dynamic context, and may only hold during some phases of growth.     

HOW TO COMPENSATE FOR COSTLY SEXUALLY SELECTED TAILS: THE ORIGIN OF SEXUALLY DIMORPHIC WINGS IN LONG-TAILED BIRDS

Recent work on birds suggests that certain morphological differences between the sexes may have evolved as an indirect consequence of sexual selection because they offset the cost of bearing extravagant ornaments used for fighting or mate attraction. For example, long-tailed male sunbirds and widowbirds also have longer wings than females, perhaps to compensate for the aerodynamic costs of tail elaboration. We used comparative data from 57 species to investigate whether this link between sexual dimorphism in wing and tail length is widespread among long-tailed birds. We found that within long-tailed families, variation in the extent of tail dimorphism was associated with corresponding variation in wing dimorphism. One nonfunctional explanation of this result is simply that the growth of wings and tails is controlled by a common developmental mechanism, such that long-tailed individuals inevitably grow long wings as well. However, this hypothesis cannot account for a second pattern in our data set: as predicted by aerodynamic theory, we found that, comparing across long-tailed families, sexual dimorphism in wing length varied with tail shape as well as with sex differences in tail length. Thus, wing dimorphism was generally greater in species with aerodynamically costly graduated tails than in birds with cheaper, streamer-shaped tails. This result was not caused by confounding phylogenetic effects, because it persisted when phylogeny was controlled for, using an independent comparisons method. Our findings therefore confirm that certain aspects of sexual dimorphism may sometimes have evolved through selection for traits that reduce the costs of elaborate sexually selected characters. We suggest that future work aimed at understanding sexual selection by investigating patterns of sexual dimorphism should attempt to differentiate between the direct and indirect consequences of sexual selection.     

Experimental evidence that female ornamentation increases the acquisition of sperm and signals fecundity

Mate choice can lead to the evolution of sexual ornamentation. This idea rests on the assumption that individuals with more elaborate ornaments than competitors have higher reproductive success due to gaining greater control over mating decisions and resources provided by partners. Nevertheless, how the resources and quality of sexual partners that individuals gain access to are influenced by the ornamentation of rival individuals remains unclear. By experimentally concealing and subsequently revealing female ornaments to males, we confirm in the fowl, Gallus gallus, that female ornamentation influences male mating decisions. We further show, by manipulating the relative ornament size of females, that when females had larger ornaments than competitors they were more often preferred by males and obtained more sperm, especially from higher quality males, as measured by social status. Males may benefit by investing more sperm in females with larger ornaments as they were in better condition and produced heavier eggs. Female ornament size also decreased during incubation, providing a cue for males to avoid sexually unreceptive females. This study reveals how inter-sexual selection can lead to the evolution of female ornaments and highlights how the reproductive benefits gained from mate choice and bearing ornaments can be dependent upon social context.     

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.     

Use of trace elements in feathers of sand martin Riparia riparia for identifying moulting areas

We investigated whether trace elements in tail feathers of an insectivorous and long-distance migratory bird species could be used to identify moulting areas and hence migratory pathways. We analysed tail feathers from birds of different age and sex collected from a range of different breeding sites across Europe. The site of moult had a large effect on elemental composition of feathers of birds, both at the European and African moulting sites. Analysis of feathers of nestlings with known origin suggested that the elemental composition of feathers depended largely upon the micro-geographical location of the colony. The distance between moulting areas could not explain the level of differences in trace elements. Analysis of feathers grown by the same individuals on the African wintering grounds and in the following breeding season in Europe showed a large difference in composition indicating that moulting site affects elemental composition. Tail feathers moulted in winter in Africa by adults breeding in different European regions differed markedly in elemental composition, indicating that they used different moulting areas. Analysis of tail feathers of the same adult individuals in two consecutive years showed that sand martins in their first and second wintering season grew feathers with largely similar elemental composition, although the amounts of several elements in tail feathers of the older birds was lower. There was no difference between the sexes in the elemental composition of their feathers grown in Africa. Investigation of the trace element composition of feathers could be a useful method for studying similarity among groups of individuals in their use of moulting areas.     

Mutual ornamentation, sexual selection, and social dominance in the black swan

We investigated the adaptive significance of a sexually monomorphicornament in the black swan Cygnus atratus. Both sexes grow curledfeathers on their wings (range 7–22 curled feathers perwing), which are displayed prominently in a range of socialinteractions. The number of curled feathers increased untilthe birds reached sexual maturity (at 2 years of age) but didnot vary with age thereafter. We found evidence for both sexualand social functions of the ornament. Paired, mature individualsof both sexes had higher numbers of curled feathers than unpaired,mature birds, and individuals paired assortatively with respectto curled feather number, suggesting the feathers may be involvedin mutual sexual selection. More ornamented individuals weredominant in agonistic interactions with birds of the same sexand pairing status. Highly ornamented pairs were also more likelyto maintain extended tenancy of preferred cygnet feeding areas,which resulted in improved offspring survival. The curled feathersthus appear to function as a signal of social dominance, whichis highly correlated with reproductive success and is thereforea reliable signal of parental quality in mate choice.     

灰椋鸟身体大小和内脏器官形态的两性差异

鸟类性二态现象广泛存在,比如身体大小、羽色等,性二态很可能是自然选择和性选择共同作用的结果.为了探索和更好地了解雀形目鸟类身体大小性二态的进化,在2019年繁殖季节早期研究了灰椋鸟(Sturnus cineraceus)野外种群身体大小和内脏器官形态的两性差异.结果表明,除嘴宽外,其他身体特征参数均雄性显著大于雌性,表...     

Sexual selection of multiple handicaps in the red-collared widowbird: female choice of tail length but not carotenoid display

Although sexual selection through female choice explains exaggerated male ornaments in many species, the evolution of the multicomponent nature of most sexual displays remains poorly understood. Theoretical models suggest that handicap signaling should converge on a single most informative quality indicator, whereas additional signals are more likely to be arbitrary Fisherian traits, amplifiers, or exploitations of receiver psychology. Male nuptial plumage in the highly polygynous red-collared widowbird (Euplectes ardens) comprises two of the commonly advocated quality advertisements (handicaps) in birds: a long graduated tail and red carotenoid coloration. Here we use multivariate selection analysis to investigate female choice in relation to male tail length, color (reflectance) of the collar, other aspects of morphology, ectoparasite load, display rate, and territory quality. The order and total number of active nests obtained are used as measures of male reproductive success. We demonstrate a strong female preference and net sexual selection for long tails, but marginal or no effects of color, morphology, or territory quality. Tail length explained 47% of male reproductive success, an unusually strong fitness effect of natural ornament variation. Fluctuating tail asymmetry was unrelated to tail length, and had no impact on mating success. For the red collar, there was negative net selection on collar area, presumably via its negative relationship with tail length. None of the color variables (hue, chroma, and brightness) had significant selection differentials, but a partial effect (selection gradient) of chroma might represent a color preference when tail length is controlled for. We suggest that the red collar functions in male agonistic interactions, which has been strongly supported by subsequent work. Thus, female choice targets only one handicap, extreme tail elongation, disregarding or even selecting against the carotenoid display. We discuss whether long tails might be better indicators of genetic quality than carotenoid pigmentation. As regards the evolution of multiple ornaments, we propose that multiple handicap signaling is stable not because of multiple messages but because of multiple receivers, in this case females and males.     

Evolutionary dynamics of a sexual ornament in the house sparrow (Passer domesticus): the role of indirect selection within and between sexes

The relative contribution of sexual and natural selection to evolution of sexual ornaments has rarely been quantified under natural conditions. In this study we used a long-term dataset of house sparrows in which parents and offspring were matched genetically to estimate the within- and across-sex genetic basis for variation and covariation among morphological traits. By applying two-sex multivariate "animal models" to estimate genetic parameters, we estimated evolutionary changes in a male sexual ornament, badge size, from the contribution of direct and indirect selection on correlated traits within males and females, after accounting for overlapping generations and age-structure. Indirect natural selection on genetically correlated traits in males and females was the major force causing evolutionary change in the male ornament. Thus, natural selection on female morphology may cause indirect evolutionary changes in male ornaments. We observed however no directional phenotypic change in the ornament size of one-year-old males during the study period. On the other hand, changes were recorded in other morphological characters of both sexes. Our analyses of evolutionary dynamics in sexual characters require application of appropriate two-sex models to account for how selection on correlated traits in both sexes affects the evolutionary outcome of sexual selection.