Absence of protandry in the spring migration of a population of Song Sparrows Melospiza melodia

Early male arrival at breeding sites, or protandry, is thought to have evolved from intrasexual competition among males for access to mates or breeding resources. Males of polygynous species tend to be larger than females and have exaggerated secondary sexual traits. Additionally, such species show a high degree of protandry, suggesting that timing of arrival is sexually selected. Species showing limited sexual dimorphism and showing sexual monochromatism may be expected to show limited early male arrival. However, there are very few studies of migration timing of the sexes in such species because individuals cannot be readily identified to sex in the hand. In this study, we genetically sexed birds and found no evidence for early male arrival, for a population of migratory Song Sparrows Melospiza melodia . For our study population, males and females display limited sexual size dimorphism and are sexually monochromatic which is characteristic of the species. Fat scores for males were inversely associated with timing of arrival, whereas for females, larger-winged birds arrived sooner – suggesting that early migration timing may be selected for in both sexes.     

Sex‐related variation in migration phenology in relation to sexual dimorphism: a test of competing hypotheses for the evolution of protandry

Timing of arrival/emergence to the breeding grounds is under contrasting natural and sexual selection pressures. Because of differences in sex roles and physiology, the balance between these pressures on either sex may differ, leading to earlier male (protandry) or female (protogyny) arrival. We test several competing hypotheses for the evolution of protandry using migration data for 22 bird species, including for the first time several monochromatic ones where sexual selection is supposedly less intense. Across species, protandry positively covaried with sexual size dimorphism but not with dichromatism. Within species, there was weak evidence that males migrate earlier because, being larger, they are less susceptible to adverse conditions. Our results do not support the ‘rank advantage’ and the ‘differential susceptibility’ hypotheses, nor the ‘mate opportunity’ hypothesis, which predicts covariation of protandry with dichromatism. Conversely, they are compatible with ‘mate choice’ arguments, whereby females use condition‐dependent arrival date to assess mate quality.     

Correlates of timing of spring migration in birds: a comparative study of trans-Saharan migrants

The evolution of migratory strategies in birds is likely to have been influenced by ecological as well as socio-sexual factors in both wintering and breeding areas. In this comparative study, we analysed timing of spring passage of 38 long-distance migratory bird species that winter south of the Sahara desert and breed in Europe, in relation to wintering and breeding latitudes, moult strategy, nesting site (open vs. cavity), and sexual dimorphism in size and coloration, which may reflect intensity of sexual selection. We employed a large data set consisting of more than 190 000 individuals ringed during spring migration in the Mediterranean Sea. We found that the species that migrated earlier were those wintering farther north, nesting in cavities and showing the largest degree of sexual size dimorphism (SSD). However, sexual dichromatism was not related to migration date. Among passerine species, moulting wing-feathers in Africa delayed migration. We found no support for the energetic constraint hypothesis, which proposes that early arrival selects for large male size, since early arriving species were not larger than late arriving ones. Thus, the observed associations suggest that variation in migration schedules at the interspecific level may have evolved in relation to ecological factors and SSD, possibly reflecting the intensity of mating competition.  © 2005 The Linnean Society of London, Biological Journal of the Linnean Society , 2005, 85 , 199–210.     

Degree of protandry reflects level of extrapair paternity in migratory songbirds

Males of most migratory organisms, including many birds, precede female conspecifics on their journey to the breeding areas. Several hypotheses have been proposed to explain the evolution of protandrous migration, yet they have rarely been tested at the interspecific level. Here, we provide correlational support for the “mate opportunity” hypothesis, which assumes that selection favours protandry in polygynous species where males gain significant fitness benefits from arriving earlier than females. Drawing on phenological data collected at two northern European stopover sites, we show that the time-lag in spring passage between males and females of five Palearctic migratory songbird species is positively associated with levels of extrapair paternity available from the literature. This suggests that males arrive relatively more in advance of females in species with high sperm competition where sexual selection through female choice is intense. Thus, protandry may arise from selection on the relative arrival timing of males and females rather than from selection within one of the sexes.     

Sexually selected dichromatism in the hihi Notiomystis cincta: multiple colours for multiple receivers

Why do some bird species show dramatic sexual dichromatism in their plumage? Sexual selection is the most common answer to this question. However, other competing explanations mean it is unwise to assume that all sexual dichromatism has evolved by this mechanism. Even if sexual selection is involved, further work is necessary to determine whether dichromatism results from competition amongst rival males, or by female choice for attractive traits, or both. Here, we test whether sexually dichromatic hihi (Notiomystis cincta) plumage is currently under sexual selection, with detailed behavioural and genetic analyses of a free‐living island population. Bateman gradients measured for males and females reveal the potential for sexual selection, whilst selection gradients, relating reproductive success to specific colourful traits, show that there is stabilizing selection on white ear tuft length in males. By correlating colourful male plumage with different components of reproductive success, we show that properties of yellow plumage are most likely a product of male–male competition, whilst properties of the black and white plumage are an outcome of both male–male competition and female choice. Male plumage therefore potentially signals to multiple receivers (rival males and potential mates), and this may explain the multicoloured appearance of one of the most strikingly dichromatic species in New Zealand.     

Why do female migratory birds arrive later than males?

1. In migratory birds males tend to arrive first on breeding grounds, except in sex-role reversed species. The two most common explanations are the rank advantage hypothesis, in which male-male competition for breeding sites drives stronger selection for early arrival in males than females, and the mate opportunity hypothesis, which relies on sexual selection, as early arrival improves prospects of mate acquisition more for males than for females. 2. To date, theoretical work has focused on selection for early arrival within a single sex, usually male. However, if fitness depends on territory quality, selection for early arrival should operate on both sexes. Here we use two independent modelling approaches to explore the evolution of protandry (male-first arrival) and protogyny (female-first arrival) under the rank advantage and mate opportunity hypotheses. 3. The rank advantage hypothesis, when operating alone, fails to produce consistent patterns of protandry, despite our assumption that males must occupy territories before females. This is because an individual of either sex benefits if it out-competes same-sex competitors. Rather than promoting protandry, the rank advantage mechanism can sometimes result in protogyny. Female-female competition is stronger than male-male competition early in the season, if females compete for a resource (territories occupied by males) that is initially less common than the resource of interest to males (unoccupied territories). 4. Our results support the mate opportunity hypothesis as an explanation of why protandry is the norm in migratory systems. Male-biased adult sex ratios and high levels of sperm competition (modelled as extra-pair young: EPY) both produce protandry as a result of sexual selection. Protogyny is only observed in our models with female-biased sex ratios and low EPY production. 5. We also show that the effects of sex ratio biases are much stronger than those of EPY production, explore the evidence for sex ratio biases and extra-pair paternity in migratory species and suggest future research directions.     

Sexual dichromatism in frogs: natural selection, sexual selection and unexpected diversity

Sexual dichromatism, a form of sexual dimorphism in which males and females differ in colour, is widespread in animals but has been predominantly studied in birds, fishes and butterflies. Moreover, although there are several proposed evolutionary mechanisms for sexual dichromatism in vertebrates, few studies have examined this phenomenon outside the context of sexual selection. Here, we describe unexpectedly high diversity of sexual dichromatism in frogs and create a comparative framework to guide future analyses of the evolution of these sexual colour differences. We review what is known about evolution of colour dimorphism in frogs, highlight alternative mechanisms that may contribute to the evolution of sexual colour differences, and compare them to mechanisms active in other major groups of vertebrates. In frogs, sexual dichromatism can be dynamic (temporary colour change in males) or ontogenetic (permanent colour change in males or females). The degree and the duration of sexual colour differences vary greatly across lineages, and we do not detect phylogenetic signal in the distribution of this trait, therefore frogs provide an opportunity to investigate the roles of natural and sexual selection across multiple independent derivations of sexual dichromatism.     

Sexual imprinting misguides species recognition in a facultative interspecific brood parasite

Sexual reproduction relies on the recognition of conspecifics for breeding. Most experiments in birds have implicated a critical role for early social learning in directing subsequent courtship behaviours and mating decisions. This classical view of avian sexual imprinting is challenged, however, by studies of megapodes and obligate brood parasites, species in which reliable recognition is achieved despite the lack of early experience with conspecifics. By rearing males with either conspecific or heterospecific brood mates, we experimentally tested the effect of early social experience on the association preferences and courtship behaviours of two sympatrically breeding ducks. We predicted that redheads (Aythya americana), which are facultative interspecific brood parasites, would show a diminished effect of early social environment on subsequent courtship preferences when compared with their host and congener, the canvasback (Aythya valisineria). Contrary to expectations, cross-fostered males of both species courted heterospecific females and preferred them in spatial association tests, whereas control males courted and associated with conspecific females. These results imply that ontogenetic constraints on species recognition may be a general impediment to the initial evolution of interspecific brood parasitism in birds. Under more natural conditions, a variety of mechanisms may mitigate or counteract the effects of early imprinting for redheads reared in canvasback broods.     

The evolution of male infanticide in relation to sexual selection in mammalian carnivores

The evolution of infanticide by males has often been explained by the sexual selection hypothesis, which posits that infanticide improves male reproductive success by shortening the interbirth intervals of the mothers of the killed offspring. In Carnivora, however, the fitness advantages assumed in this hypothesis have been shown in only a few species, and it has been argued that male infanticide may be nonadaptive in pinniped carnivores. According to the sexual selection hypothesis, male infanticide is expected to be more prevalent in species in which males are subjected to stronger sexual selection through intrasexual competition over mates. We examined a phylogenetically corrected relationship between male infanticide and sexual size dimorphism (SSD) as a measure of the intensity of sexual selection in carnivores. Our analyses failed to detect a significant association between the occurrence of male infanticide and SSD across carnivores, although they showed that, among fissipeds (typically terrestrial carnivores), males in species with stronger male-biased SSD are significantly more likely to commit infanticide. This suggests that the evolution of male infanticide is correlated with intense sexual selection in fissipeds. In pinnipeds (Odobenidae, Otariidae, and Phocidae), there was no significant association between male infanticide and SSD. Assuming that SSD represents the intensity of sexual selection on males, this result is consistent with the argument that infanticide by male pinnipeds is not a sexually selected behaviour.     

Field evidence challenges the often‐presumed relationship between early male maturation and female‐biased sexual size dimorphism

Female‐biased sexual size dimorphism (SSD) is often considered an epiphenomenon of selection for the increased mating opportunities provided by early male maturation (i.e., protandry). Empirical evidence of the adaptive significance of protandry remains nonetheless fairly scarce. We use field data collected throughout the reproductive season of an SSD crab spider, Mecaphesa celer, to test two hypotheses: Protandry provides fitness benefits to males, leading to female‐biased SSD, or protandry is an indirect consequence of selection for small male size/large female size. Using field‐collected data, we modeled the probability of mating success for females and males according to their timing of maturation. We found that males matured earlier than females and the proportion of virgin females decreased abruptly early in the season, but unexpectedly increased afterward. Timing of female maturation was not related to clutch size, but large females tended to have more offspring than small females. Timing of female and male maturation was inversely related to size at adulthood, as early‐maturing individuals were larger than late‐maturing ones, suggesting that both sexes exhibit some plasticity in their developmental trajectories. Such plasticity indicates that protandry could co‐occur with any degree and direction of SSD. Our calculation of the probability of mating success along the season shows multiple male maturation time points with similar predicted mating success. This suggests that males follow multiple strategies with equal success, trading‐off access to virgin females with intensity of male–male competition. Our results challenge classic hypotheses linking protandry and female‐biased SSD, and emphasize the importance of directly testing the often‐assumed relationships between co‐occurring animal traits.