Context-dependent development of sexual ornamentation: implications for a trade-off between current and future breeding efforts

An age-related decline in the expression of a red carotenoid-based ornament in wild birds

The past decades have provided valuable information on how animals age in the wild. However, examples of male reproductive senescence are scarce. In particular, few studies have described an age-related decline in the expression of conspicuous traits influencing mating success. Red ornaments could be good candidates to detect this decline because their expression may depend on the availability of pigments (carotenoids) related to oxidative stress, the latter frequently linked to senescence. Furthermore, it has been argued that efficient mitochondrial metabolism is key to express red carotenoid-based ornaments, and mitochondrial dysfunction is usually associated with senescence. We studied the age-linked expression of a red carotenoid-based trait: the yellow-to-red plumage coloration of male common crossbills (Loxia curvirostra). This coloration has recently been experimentally related to mitochondrial function. Here, we analyzed longitudinal plumage coloration data obtained throughout 28 years in free-living birds. We detected an initial increase in redness during the first 2 years of life and a subsequent decline. The relationship between color and age was unrelated to within-individual body mass variability. As far as we know, this is the first demonstration of an age-related ketocarotenoid-based color decrease detected by simultaneously testing within- and between-individual variability in wild animals.

Lack of conspecific visual discrimination between second-year males and females in the Saffron Finch

Sexually dichromatic birds often show delayed plumage maturation, but second-year (SY) males may or may not be distinguishable from females. In competitive contexts, SY males receive a reduced amount of adult males’ aggression, either by mimicking females or through signaling their sex and inexperience as subordinate males. To the human eye, reproductive dull SY male Saffron Finches are indistinguishable from females, whereas after second-year (ASY) males are golden yellow. Our aim is to establish whether SY males are sexually dichromatic with females to the eye of conspecifics. We describe plumage variation in females, SY and ASY males and, in particular, analyze assortative mating by color by comparing a previously disregarded yellow feather patch shared by the three groups. We measured plumage reflectance of the forehead, breast, belly, and axillaries, and used a two-step avian visual model analysis to estimate the ability of Saffron Finches to distinguish between SY males and females. We find that those groups are indistinguishable to conspecifics by color. Furthermore, we find non-significant evidence of assortative mating directly related to the coloration of comparable feather patches between females and each type of male, though body condition of SY males is associated to that of their mates. Our results are compatible with both the female-mimicry and the status signaling hypotheses of evolution and maintenance of delayed plumage maturation. However, the singing behavior of males reveals their presence within the breeding site; the combined effect of song and dull coloration suggest that SY males are honestly revealing their sex and status to conspecifics.

Most Colorful Example of Genetic Assimilation? Exploring the Evolutionary Destiny of Recurrent Phenotypic Accommodation

Evolution of adaptation requires both generation of novel phenotypic variation and retention of a locally beneficial subset of this variation. Such retention can be facilitated by genetic assimilation, the accumulation of genetic and molecular mechanisms that stabilize induced phenotypes and assume progressively greater control over their reliable production. A particularly strong inference into genetic assimilation as an evolutionary process requires a system where it is possible to directly evaluate the extent to which an induced phenotype is progressively incorporated into preexisting developmental pathways. Evolution of diet-dependent pigmentation in birds-where external carotenoids are coopted into internal metabolism to a variable degree before being integrated with a feather's developmental processes-provides such an opportunity. Here we combine a metabolic network view of carotenoid evolution with detailed empirical study of feather modifications to show that the effect of physical properties of carotenoids on feather structure depends on their metabolic modification, their environmental recurrence, and biochemical redundancy, as predicted by the genetic assimilation hypothesis. Metabolized carotenoids caused less stochastic variation in feather structure and were more closely integrated with feather growth than were dietary carotenoids of the same molecular weight. These patterns were driven by the recurrence of organism-carotenoid associations: commonly used dietary carotenoids and biochemically redundant derived carotenoids caused less stochastic variation in feather structure than did rarely used or biochemically unique compounds. We discuss implications of genetic assimilation processes for the evolutionary diversification of diet-dependent animal coloration.

Evolution of long-term coloration trends with biochemically unstable ingredients

The evolutionarily persistent and widespread use of carotenoid pigments in animal coloration contrasts with their biochemical instability. Consequently, evolution of carotenoid-based displays should include mechanisms to accommodate or limit pigment degradation. In birds, this could involve two strategies: (i) evolution of a moult immediately prior to the mating season, enabling the use of particularly fast-degrading carotenoids and (ii) evolution of the ability to stabilize dietary carotenoids through metabolic modification or association with feather keratins. Here, we examine evolutionary lability and transitions between the two strategies across 126 species of birds. We report that species that express mostly unmodified, fast-degrading, carotenoids have pre-breeding moults, and a particularly short time between carotenoid deposition and the subsequent breeding season. Species that expressed mostly slow-degrading carotenoids in their plumage accomplished this through increased metabolic modification of dietary carotenoids, and the selective expression of these slow-degrading compounds. In these species, the timing of moult was not associated with carotenoid composition of plumage displays. Using repeated samples from individuals of one species, we found that metabolic modification of dietary carotenoids significantly slowed their degradation between moult and breeding season. Thus, the most complex and colourful ornamentation is likely the most biochemically stable in birds, and depends less on ecological factors, such as moult timing and migration tendency. We suggest that coevolution of metabolic modification, selective expression and biochemical stability of plumage carotenoids enables the use of unstable pigments in long-term evolutionary trends in plumage coloration.

A benign juvenile environment reduces the strength of antagonistic pleiotropy and genetic variation in the rate of senescence

The environment can play an important role in the evolution of senescence because the optimal allocation between somatic maintenance and reproduction depends on external factors influencing life expectancy.

The aims of this study were to experimentally test whether environmental conditions during early life can shape senescence schedules, and if so, to examine whether variation among individuals or genotypes with respect to the degree of ageing differs across environments.

We tested life‐history plasticity and quantified genetic effects on the pattern of senescence across different environments within a reaction norm framework by using an experiment on the three‐spined stickleback (Gasterosteus aculeatus, Linnaeus) in which F1 families originating from a wild annual population experienced different temperature regimes.

Male sticklebacks that had experienced a more benign environment earlier in life subsequently reduced their investment in carotenoid‐based sexual signals early in the breeding season, and consequently senesced at a slower rate later in the season, compared to those that had developed under harsher conditions. This plasticity of ageing was genetically determined. Both antagonistic pleiotropy and genetic variation in the rate of senescence were evident only in the individuals raised in the harsher environment.

The experimental demonstration of genotype‐by‐environment interactions influencing the rate of reproductive senescence provides interesting insights into the role of the environment in the evolution of life histories. The results suggest that benign conditions weaken the scope for senescence to evolve and that the dependence on the environment may maintain genetic variation under selection.

Ontogenetic stage-specific quantitative trait loci contribute to divergence in developmental trajectories of sexually dimorphic fins between medaka populations

Sexual dimorphism can evolve when males and females differ in phenotypic optima. Genetic constraints can, however, limit the evolution of sexual dimorphism. One possible constraint is derived from alleles expressed in both sexes. Because males and females share most of their genome, shared alleles with different fitness effects between sexes are faced with intralocus sexual conflict. Another potential constraint is derived from genetic correlations between developmental stages. Sexually dimorphic traits are often favoured at adult stages, but selected against as juvenile, so developmental decoupling of traits between ontogenetic stages may be necessary for the evolution of sexual dimorphism in adults. Resolving intralocus conflicts between sexes and ages is therefore a key to the evolution of age-specific expression of sexual dimorphism. We investigated the genetic architecture of divergence in the ontogeny of sexual dimorphism between two populations of the Japanese medaka (Oryzias latipes) that differ in the magnitude of dimorphism in anal and dorsal fin length. Quantitative trait loci (QTL) mapping revealed that few QTL had consistent effects throughout ontogenetic stages and the majority of QTL change the sizes and directions of effects on fin growth rates during ontogeny. We also found that most QTL were sex-specific, suggesting that intralocus sexual conflict is almost resolved. Our results indicate that sex- and age-specific QTL enable the populations to achieve optimal developmental trajectories of sexually dimorphic traits in response to complex natural and sexual selection.

Comparative ectoparasite loads of five seabird species in the Galapagos Islands

In this paper we describe the ectoparasitic lice (Insecta: Phthiraptera) found on 5 species of seabirds (magnificent frigatebird Fregata magnificens; great frigatebird Fregata minor ; Nazca booby Sula granti ; blue-footed booby Sula nebouxii ; and red-footed booby Sula sula ) on the Galapagos Archipelago. We found 9 species of ectoparasitic lice: 5 species of Pectinopygus ischnocerans, 1 infesting each host; 2 species of Colpocephalum amblyceran lice, 1 on each frigatebird species; and 2 shared amblycerans, Eidmanniella albescens (Piaget, 1880) found on Nazca and blue-footed boobies and Fregatiella aurifasciata (Kellogg, 1899) found on the 2 frigatebirds. We tested the relative importance and interactions of host sex, body size, host, island, host family, and breeding status and found that inter-island differences were the main predictors of prevalence and infestation intensity. These differences could be related to host density or weather, but further evidence is needed.

Developmental integration of feather growth and pigmentation and its implications for the evolution of diet-derived coloration

Variation in avian coloration is produced by coordinated pigmentation of thousands of growing feathers that vary in shape and size. Although the functional consequences of avian coloration are frequently studied, little is known about its developmental basis, and, specifically, the rules that link feather growth to pigment uptake and synthesis. Here, we combine biochemical, modeling, and morphometric techniques to examine the developmental basis of feather pigmentation in house finches (Carpodacus mexicanus)--a species with extensive variation in both growth dynamics of ornamental feathers and their carotenoid pigmentation. We found that the rate of carotenoid uptake was constant across a wide range of feather sizes and shapes, and the relative pigmented area of feathers was independent of the total amount of deposited carotenoids. Analysis of the developmental linkage of feather growth and pigment uptake showed that the mechanisms behind partitioning the feather into pigmented and nonpigmented parts and the mechanisms regulating carotenoid uptake into growing feathers are partially independent. Carotenoid uptake strongly covaried with early elements of feather differentiation (the barb addition rate and diameter), whereas the pigmented area was most closely associated with the rate of feather growth. We suggest that strong effects of carotenoid uptake on genetically integrated mechanisms of feather growth and differentiation provide a likely route for genetic assimilation of diet-dependent coloration.

Haste makes waste: accelerated molt adversely affects the expression of melanin-based and depigmented plumage ornaments in house sparrows

Background

Many animals display colorful signals in their integument which convey information about the quality of their bearer. Theoretically, these ornaments incur differential production and/or maintenance costs that enforce their honesty. However, the proximate mechanisms of production costs are poorly understood and contentious in cases of non-carotenoid-based plumage ornaments like the melanin-based badge and depigmented white wing-bar in house sparrows Passer domesticus. Costly life-history events are adaptively separated in time, thus, when reproduction is extended, the time available for molt is curtailed and, in turn, molt rate is accelerated.

Methodology/Principal Findings

We experimentally accelerated the molt rate by shortening the photoperiod in order to test whether this environmental constraint is mirrored in the expression of plumage ornaments. Sparrows which had undergone an accelerated molt developed smaller badges and less bright wing-bars compared to conspecifics that molted at a natural rate being held at natural-like photoperiod. There was no difference in the brightness of the badge or the size of the wing-bar.

Conclusions/Significance

These results indicate that the time available for molt and thus the rate at which molt occurs may constrain the expression of melanin-based and depigmented plumage advertisements. This mechanism may lead to the evolution of honest signaling if the onset of molt is condition-dependent through the timing of and/or trade-off between breeding and molt.

Structure of social networks in a passerine bird: consequences for sexual selection and the evolution of mating strategies

The social environment is a critical determinant of fitness and, in many taxa, is shaped by an individual's behavioral discrimination among social contexts, suggesting that animals can actively influence the selection they experience. In competition to attract females, males may modify sexual selection by choosing social environments in which they are more attractive relative to rivals. Across the population, such behaviors should influence sexual selection patterns by altering the relationship between male mating success and sexual ornament elaboration. Here we use network analysis to examine patterns of male social behavior in relation to plumage ornamentation and mating success in a free-living population of house finches. During the nonbreeding season, less elaborate males changed associations with distinct social groups more frequently, compared to more elaborate males that showed greater fidelity to a single social group. By the onset of pair formation, socially labile males effectively increased their attractiveness relative to other males in the same flocks. Consequently, males that frequently moved between social groups had greater pairing success than less social individuals with equivalent sexual ornamentation. We discuss these results in relation to conditional mating tactics and the role of social behavior in evolutionary change by sexual selection.

What factors drive prolactin and corticosterone responses to stress in a long-lived bird species (snow petrel Pagodroma nivea)?

Life-history theory predicts that individuals should adapt their parental investment to the costs and benefits of the current reproductive effort. This could be achieved by modulating the hormonal stress response, which may shift energy investment away from reproduction and redirect it toward survival. In birds, this stress response consists of a release of corticosterone that may be accompanied by a decrease in circulating prolactin, a hormone involved in the regulation of parental care. We lack data on the modulation of the prolactin stress response. In this study, we tested the hypothesis that individuals should modulate their prolactin stress response according to the fitness value of the current reproductive effort relative to the fitness value of future reproduction. Specifically, we examined the influence of breeding status (failed breeders vs. incubating birds) and body condition on prolactin and corticosterone stress responses in a long-lived species, the snow petrel Pagodroma nivea. When facing stressors, incubating birds had higher prolactin levels than failed breeders. However, we found no effect of body condition on the prolactin stress response. The corticosterone stress response was modulated according to body condition but was not affected by breeding status. We also performed an experiment using injections of adrenocorticotropic hormone (ACTH) and found that the modulation of the corticosterone stress response was probably associated with a reduction in ACTH release by the pituitary and a decrease in adrenal sensitivity to ACTH. In addition, we examined whether prolactin and corticosterone secretion were functionally linked. We found that these two hormonal stress responses were not correlated. Moreover, injection of ACTH did not affect prolactin levels, demonstrating that short-term variations in prolactin levels are not governed directly or indirectly by ACTH release. Thus, we suggest that the corticosterone and prolactin responses to short-term stressors are independent and may therefore mediate some specific components of parental investment in breeding birds. With mounting evidence, we suggest that examining both corticosterone and prolactin stress responses could be relevant to parental investment in vertebrates.