Polygamy slows down population divergence in shorebirds

Long-distance seasonal migration to the tropics promotes genetic diversity but not gene flow in boreal birds

Differences in life history can cause co-distributed species to evolve contrasting population genetic patterns, even as they occupy the same landscape. In high-latitude animals, evolutionary processes may be especially influenced by long-distance seasonal migration, a widespread adaptation to seasonality. Although migratory movements are intuitively linked to dispersal and therefore promotion of gene flow, their evolutionary genetic consequences remain poorly understood. Using ~1,700 genomes from 35 co-distributed boreal-breeding bird species that differ in non-breeding latitude and thus migration distance, we find that most long-distance migrants unexpectedly exhibit spatial genetic structure, despite their strong movement propensity. This result suggests evolutionary effects of philopatry-the tendency of many migrants to return to the same breeding site year after year, resulting in restricted dispersal. We further demonstrate that migration distance and genetic diversity are strongly positively correlated in our study species. This striking relationship suggests that the adaptive seasonal shifts in biogeography inherent to long-distance migration may enhance population stability, preserving genetic diversity in long-distance migrants relative to shorter-distance migrants that winter in harsher conditions at higher latitudes. Our results suggest that the major impact of long-distance seasonal migration on population genetic evolution occurs through promotion of demographic stability, rather than facilitation of dispersal.

Large-scale sampling of potential breeding sites in male ruffs

The traditional narrative of the life cycle of migratory birds is that individuals perform long-distance movements between a breeding and a wintering site, but are largely resident at those sites. Although this pattern may apply to socially monogamous species with biparental care, in polygamous systems, the sex that only provides gametes may benefit from continuing to move and sample several potential breeding sites during a single breeding season. Such behaviour would blur the distinction between migration and breeding. We used satellite telemetry to study movements during the breeding season of males of the ruff Calidris pugnax, a lekking wader with a polygynous mating system and female-only parental care. Ruffs have a unique life-history, with three distinct genetically determined male mating strategies: aggressive 'independents', submissive 'satellites', and female-mimicking 'faeders'. Within the breeding season, ruff males visited up to 23 sites (median: 11) and travelled up to 9029 km (median: 4435 km) covering a considerable part of their known breeding range. All three male morphs displayed breeding site sampling, indicating that they might gain similar benefits from this behaviour. Our findings suggest that large-scale breeding range sampling may be a common feature of migratory species with female-only care and strong male-male competition.

Social mating systems in birds: resource-defense polygamy-but not lekking-is a macroevolutionarily unstable trait

Our understanding of the evolution of social mating systems is largely based on an atemporal ecological framework, whereas macroevolutionary and phylogenetic perspectives looking at the causes of mating systems variation are less developed. Here, we present analyses of the evolution of social mating systems in birds at an unprecedented scale, including 66% of the world's birds and using trait-dependent speciation and extinction models. We found that lekking (no social bond between the sexes) is very rarely lost, in accordance with the hypothesis that a male shifting to investing in one rather than multiple mates would suffer a severe fitness cost. In contrast, resource-defense polygamous lineages (with a weak, transient socio-sexual bond) frequently revert back to monogamy (strong, durable socio-sexual bond) and have an elevated extinction fraction. We tentatively attribute this to the impossibility of females settling on an optimal parental care strategy under this system. Finally, we found that most gains of lekking have been directly from monogamy rather than through an intermediate stage of resource-defense polygamy.

Evolution of reproductive strategies: sex roles, sex ratios and phylogenies

Behavioural variations associated with breeding-termed reproductive strategies-are some of the striking behaviours that have occupied naturalists for 1000s of years. How an animal seeks, competes for and/or chooses a mate? Do they breed with a single partner, or do they change partners between breeding events? How and when do they look after their young? Behavioural biologists, ecologists and evolutionary biologists have investigated these questions using quantitative methods since 1970s. In Debrecen, with the support and mentoring of Prof Zoltán Varga, we are investigating the causes and implications of reproductive strategies since 1988. This article reviews some of the core ideas in reproductive strategies research and explains the influence of Prof Varga on the development of these ideas. My main thesis here is that both integrative thinking and adopting a multi-pronged research approach using an explicit phylogenetic framework-both of these have been spearheaded by Prof Varga throughout his lifetime-can reveal novel aspects of reproductive strategies. Importantly, some of these academic insights have direct implications for preserving species and their habitats in the wild, and thus benefit biodiversity conservation.

Egg characteristics vary longitudinally in Arctic shorebirds

Arctic environments are changing rapidly and if we are to understand the resilience of species to future changes, we need to investigate alterations in their life histories. Egg size and egg shape are key life-history traits, reflecting parental investment as well as influencing future reproductive success. Here we focus on egg characteristics in two Arctic shorebirds, the Dunlin (Calidris alpina) and the Temminck's stint (Calidris temminckii). Using egg photos that encompass their full breeding ranges, we show that egg characteristics exhibit significant longitudinal variations, and the variation in the monogamous species (Dunlin) is significantly greater than the polygamous species (Temminck's stint). Our finding is consistent with the recent "disperse-to-mate" hypothesis which asserts that polygamous species disperse further to find mates than monogamous species, and by doing so they create panmictic populations. Taken together, Arctic shorebirds offer excellent opportunities to understand evolutionary patterns in life history traits.

Country-wide genetic monitoring over 21 years reveals lag in genetic recovery despite spatial connectivity in an expanding carnivore (Eurasian otter, Lutra lutra) population

Numerous terrestrial mammal species have experienced extensive population declines during past centuries, due largely to anthropogenic pressures. For some species, including the Eurasian otter (Lutra lutra), environmental and legal protection has more recently led to population growth and recolonization of parts of their historic ranges. While heralded as conservation success, only few such recoveries have been examined from a genetic perspective, i.e. whether genetic variability and connectivity have been restored. We here use large-scale and long-term genetic monitoring data from UK otters, whose population underwent a well-documented population decline between the 1950s and 1970s, to explore the dynamics of a population re-expansion over a 21-year period. We genotyped otters from across Wales and England at five time points between 1994 and 2014 using 15 microsatellite loci. We used this combination of long-term temporal and large-scale spatial sampling to evaluate 3 hypotheses relating to genetic recovery that (i) gene flow between subpopulations would increase over time, (ii) genetic diversity of previously isolated populations would increase and that (iii) genetic structuring would weaken over time. Although we found an increase in inter-regional gene flow and admixture levels among subpopulations, there was no significant temporal change in either heterozygosity or allelic richness. Genetic structuring among the main subpopulations hence remained strong and showed a clear historical continuity. These findings highlight an underappreciated aspect of population recovery of endangered species: that genetic recovery may often lag behind the processes of spatial and demographic recovery. In other words, the restoration of the physical connectivity of populations does not necessarily lead to genetic connectivity. Our findings emphasize the need for genetic data as an integral part of conservation monitoring, to enable the potential vulnerability of populations to be evaluated.

Shorebirds and the Dispersal of Bipolar Plant Species to South America

Among the most distantly separated plant species are those that are found on the polar regions of the northern and southern hemispheres, the so-called bipolar species. Two routes of introduction have been proposed—long-distance dispersal (LDD) and mountain hopping (MH). Shorebirds have been implicated in the distribution of the bipolar species by several authors, but the most likely participants and the most probable routes of introduction have been little investigated. The Global Biodiversity Information Facility database was accessed to determine the geographic range of those angiosperm species that have been reported to have bipolar distributions. A bipolar plant species was considered most likely to have been dispersed by LDD if it has a distinct disjunct distribution between North and South America, and through MH if it is found in intermediate latitudes. The Atlas of Bird Migrations and the Cornell Birds of the World database were searched to discover which birds make long-distance migrations from Arctic North America to the tip of South America, and their mode of travel. Twenty-three plant species have been identified as bipolar. LDD appears to have been more important than MH in their dispersal, as seventeen (75%) have disjunct distributions and six (25%) are found in intermediate latitudes. The most likely players in the LDD dispersal of the bipolar plant species are the Eskimo Curlew, Hudsonian Godwit, Red Knot, Ruddy Turnstone and Whimbrel. Of these five long flyers, the Hudsonian Godwit may have delivered the most seeds as its breeding and migration ranges overlap with the most bipolar species, 12 in all.

No evidence for female kin association, indications for extragroup paternity, and sex-biased dispersal patterns in wild western gorillas

Characterizing animal dispersal patterns and the rational behind individuals' transfer choices is a long-standing question of interest in evolutionary biology. In wild western gorillas (Gorilla gorilla), a one-male polygynous species, previous genetic findings suggested that, when dispersing, females might favor groups with female kin to promote cooperation, resulting in higher-than-expected within-group female relatedness. The extent of male dispersal remains unclear with studies showing conflicting results. To investigate male and female dispersal patterns and extragroup paternity, we analyzed long-term field observations, including female spatial proximity data, together with genetic data (10 autosomal microsatellites) on individuals from a unique set of four habituated western gorilla groups, and four additional extragroup males (49 individuals in total). The majority of offspring (25 of 27) were sired by the group male. For two offspring, evidence for extragroup paternity was found. Contrarily to previous findings, adult females were not significantly more related within groups than across groups. Consistently, adult female relatedness within groups did not correlate with their spatial proximity inferred from behavioral data. Adult females were similarly related to adult males from their group than from other groups. Using R ST statistics, we found significant genetic structure and a pattern of isolation by distance, indicating limited dispersal in this species. Comparing relatedness among females and among males revealed that males disperse farer than females, as expected in a polygamous species. Our study on habituated western gorillas shed light on the dispersal dynamics and reproductive behavior of this polygynous species and challenge some of the previous results based on unhabituated groups.

Long-distance dispersal of the beach strawberry, Fragaria chiloensis, from North America to Chile and Hawaii

BACKGROUND AND AIMS

The beach strawberry, Fragaria chiloensis, is found in a narrow coastal band from the Aleutian Islands to central California and then jumps thousands of kilometres all the way to Hawaii and Chile. As it probably had a North American origin, it must have been introduced to the other locations by long-distance dispersal. The aim of this study was to determine which agent carried the beach strawberry to its Pacific and South American locations.

METHODS

A deductive framework was constructed to separate between the possible modes of long-distance dispersal involving animals, wind and ocean currents. Bird migration was subsequently identified as the most likely scenario, and then the routes, habitats, feeding preferences and flight distances of all the shorebird species were evaluated to determine the most likely carrier.

KEY RESULTS

Six species migrate between North America and Chile and feed on the beaches and rocky shores where F. chiloensis grows naturally: Black-bellied Plovers, Greater Yellowlegs, Ruddy Turnstones, Sanderlings, Whimbrels and Willets. Of these, only two eat fruit and migrate in long continuous flight: Ruddy Turnstones and Whimbrels. Two species travel between North America and Hawaii, eat fruit and forage on the beaches and rocky shores where F. chiloensis grows naturally: Pacific Golden-plovers and Ruddy Turnstones. Ruddy Turnstones eat far less fruit than Pacific Golden-plovers and Whimbrels, making them less likely to have introduced the beach strawberry to either location.

CONCLUSIONS

We provide evidence that F. chiloesis seeds were probably dispersed to Hawaii by Pacific Golden-plovers and to Chile by Whimbrels.

Successful breeding predicts divorce in plovers

When individuals breed more than once, parents are faced with the choice of whether to re-mate with their old partner or divorce and select a new mate. Evolutionary theory predicts that, following successful reproduction with a given partner, that partner should be retained for future reproduction. However, recent work in a polygamous bird, has instead indicated that successful parents divorced more often than failed breeders (Halimubieke et al. in Ecol Evol 9:10734-10745, 2019), because one parent can benefit by mating with a new partner and reproducing shortly after divorce. Here we investigate whether successful breeding predicts divorce using data from 14 well-monitored populations of plovers (Charadrius spp.). We show that successful nesting leads to divorce, whereas nest failure leads to retention of the mate for follow-up breeding. Plovers that divorced their partners and simultaneously deserted their broods produced more offspring within a season than parents that retained their mate. Our work provides a counterpoint to theoretical expectations that divorce is triggered by low reproductive success, and supports adaptive explanations of divorce as a strategy to improve individual reproductive success. In addition, we show that temperature may modulate these costs and benefits, and contribute to dynamic variation in patterns of divorce across plover breeding systems.

Allelic diversity and patterns of selection at the major histocompatibility complex class I and II loci in a threatened shorebird, the Snowy Plover (Charadrius nivosus)

Background

Understanding the structure and variability of adaptive loci such as the major histocompatibility complex (MHC) genes is a primary research goal for evolutionary and conservation genetics. Typically, classical MHC genes show high polymorphism and are under strong balancing selection, as their products trigger the adaptive immune response in vertebrates. Here, we assess the allelic diversity and patterns of selection for MHC class I and class II loci in a threatened shorebird with highly flexible mating and parental care behaviour, the Snowy Plover (Charadrius nivosus) across its broad geographic range.

Results

We determined the allelic and nucleotide diversity for MHC class I and class II genes using samples of 250 individuals from eight breeding population of Snowy Plovers. We found 40 alleles at MHC class I and six alleles at MHC class II, with individuals carrying two to seven different alleles (mean 3.70) at MHC class I and up to two alleles (mean 1.45) at MHC class II. Diversity was higher in the peptide-binding region, which suggests balancing selection. The MHC class I locus showed stronger signatures of both positive and negative selection than the MHC class II locus. Most alleles were present in more than one population. If present, private alleles generally occurred at very low frequencies in each population, except for the private alleles of MHC class I in one island population (Puerto Rico, lineage tenuirostris).

Conclusion

Snowy Plovers exhibited an intermediate level of diversity at the MHC, similar to that reported in other Charadriiformes. The differences found in the patterns of selection between the class I and II loci are consistent with the hypothesis that different mechanisms shape the sequence evolution of MHC class I and class II genes. The rarity of private alleles across populations is consistent with high natal and breeding dispersal and the low genetic structure previously observed at neutral genetic markers in this species.

Phylogenetic relationships between different raccoon dog (Nyctereutes procyonoides) populations based on four nuclear and Y genes

BACKGROUND

The raccoon dog (Nyctereutes procyonoides), endemic to East Asia, is classified as six subspecies according to their geographical distribution including a population introduced to Europe. Studies on phylogenetic relationship or population genetics in both native and introduced areas have been carried out recently. Lately, opinions that Japanese raccoon dogs should be classified as a different species were asserted based on several studies using karyotypes, morphometric characters, mtDNA, and microsatellites analysis. However, no data pertaining to the nuclear DNA (nDNA) or Y chromosome are available.

OBJECTIVE

To estimate the relationship among the species using different genes is necessary in understanding of the history of this species.

METHOD

Therefore, we investigated nDNA and Y chromosomes in our study to define relationships: (1) between continental raccoon dog populations, (2) between original and introduced groups, and (3) between continental and Japanese groups.

RESULTS

The analysis of four nuclear (CHRNA1, VTN, TRSP, WT1) and ZFY genes indicated that there had been no genetic differentiation among the continental populations. However, significant differences were observed between continental and Japanese raccoon dogs in VTN and ZFY genes implying genetic differentiation has been going between them.

CONCLUSION

To better understand the phylogenetic relationship among raccoon dog populations, further study will be necessary.

Novel insights into chromosome evolution of Charadriiformes: extensive genomic reshuffling in the wattled jacana (Jacana jacana, Charadriiformes, Jacanidae)

The order Charadriiformes comprises three major clades: Lari and Scolopaci as sister group to Charadrii. Until now, only three Charadriiformes species have been studied by chromosome painting: Larus argentatus (Lari), Burhinus oedicnemus and Vanellus chilensis (Charadrii). Hence, there is a lack of information concerning the third clade, Scolapaci. Based on this, and to gain a better understanding of karyotype evolution in the order Charadriiformes, we applied conventional and molecular cytogenetic approaches in a species belonging to clade Scolopaci - the wattled jacana (Jacana jacana) - using Gallus gallus and Zenaida auriculata chromosome-specific probes. Cross-species evaluation of J. jacana chromosomes shows extensive genomic reshuffling within macrochromosomes during evolution, with multiple fission and fusion events, although the diploid number remains at high level (2n=82). Interestingly, this species does not have the GGA7-8 fusion, which was found in two representatives of Charadrii clade, reinforcing the idea that this fusion may be exclusive to the Charadrii clade. In addition, it is shown that the chromosome evolution in Charadriiformes is complex and resulted in species with typical and atypical karyotypes. The karyotypic features of Scolopaci are very different from those of Charadrii and Lari, indicating that after divergence, each suborder has undergone different chromosome rearrangements.

Population genomics of two congeneric Palaearctic shorebirds reveals differential impacts of Quaternary climate oscillations across habitats types

Intracontinental biotic divisions across the vast Palaearctic region are not well-characterized. Past research has revealed patterns ranging from a lack of population structure to deep divergences along varied lines of separation. Here we compared biogeographic patterns of two Palaearctic shorebirds with different habitat preferences, Whimbrel (Numenius phaeopus) and Eurasian curlew (N. arquata). Using genome-wide markers from populations across the Palaearctic, we applied a multitude of population genomic and phylogenomic approaches to elucidate population structure. Most importantly, we tested for isolation by distance and visualized barriers and corridors to gene flow. We found shallow Palaearctic population structure in subpolar bog and tundra-breeding whimbrels, consistent with other species breeding at a similarly high latitude, indicating connectivity across the tundra belt, both presently and during southward shifts in periods of global cooling. In contrast, the temperate grassland-breeding Eurasian curlew emerged in three distinct clades corresponding to glacial refugia. Barriers to gene flow coincided with areas of topographic relief in the central Palaearctic for whimbrels and further east for Eurasian curlews. Our findings highlight the interplay of historic and ecological factors in influencing present-day population structure of Palaearctic biota.

Demographic Histories and Genome-Wide Patterns of Divergence in Incipient Species of Shorebirds

Understanding how incipient species are maintained with gene flow is a fundamental question in evolutionary biology. Whole genome sequencing of multiple individuals holds great potential to illustrate patterns of genomic differentiation as well as the associated evolutionary histories. Kentish (Charadrius alexandrinus) and the white-faced (C. dealbatus) plovers, which differ in their phenotype, ecology and behavior, are two incipient species and parapatrically distributed in East Asia. Previous studies show evidence of genetic diversification with gene flow between the two plovers. Under this scenario, it is of great importance to explore the patterns of divergence at the genomic level and to determine whether specific regions are involved in reproductive isolation and local adaptation. Here we present the first population genomic analysis of the two incipient species based on the de novo Kentish plover reference genome and resequenced populations. We show that the two plover lineages are distinct in both nuclear and mitochondrial genomes. Using model-based coalescence analysis, we found that population sizes of Kentish plover increased whereas white-faced plovers declined during the Last Glaciation Period. Moreover, the two plovers diverged allopatrically, with gene flow occurring after secondary contact. This has resulted in low levels of genome-wide differentiation, although we found evidence of a few highly differentiated genomic regions in both the autosomes and the Z-chromosome. This study illustrates that incipient shorebird species with gene flow after secondary contact can exhibit discrete divergence at specific genomic regions and provides basis to further exploration on the genetic basis of relevant phenotypic traits.

Endless forms of sexual selection

In recent years, the field of sexual selection has exploded, with advances in theoretical and empirical research complementing each other in exciting ways. This perspective piece is the product of a "stock-taking" workshop on sexual selection and sexual conflict. Our aim is to identify and deliberate on outstanding questions and to stimulate discussion rather than provide a comprehensive overview of the entire field. These questions are organized into four thematic sections we deem essential to the field. First we focus on the evolution of mate choice and mating systems. Variation in mate quality can generate both competition and choice in the opposite sex, with implications for the evolution of mating systems. Limitations on mate choice may dictate the importance of direct vs. indirect benefits in mating decisions and consequently, mating systems, especially with regard to polyandry. Second, we focus on how sender and receiver mechanisms shape signal design. Mediation of honest signal content likely depends on integration of temporally variable social and physiological costs that are challenging to measure. We view the neuroethology of sensory and cognitive receiver biases as the main key to signal form and the 'aesthetic sense' proposed by Darwin. Since a receiver bias is sufficient to both initiate and drive ornament or armament exaggeration, without a genetically correlated or even coevolving receiver, this may be the appropriate 'null model' of sexual selection. Thirdly, we focus on the genetic architecture of sexually selected traits. Despite advances in modern molecular techniques, the number and identity of genes underlying performance, display and secondary sexual traits remains largely unknown. In-depth investigations into the genetic basis of sexual dimorphism in the context of long-term field studies will reveal constraints and trajectories of sexually selected trait evolution. Finally, we focus on sexual selection and conflict as drivers of speciation. Population divergence and speciation are often influenced by an interplay between sexual and natural selection. The extent to which sexual selection promotes or counteracts population divergence may vary depending on the genetic architecture of traits as well as the covariance between mating competition and local adaptation. Additionally, post-copulatory processes, such as selection against heterospecific sperm, may influence the importance of sexual selection in speciation. We propose that efforts to resolve these four themes can catalyze conceptual progress in the field of sexual selection, and we offer potential avenues of research to advance this progress.

Mate fidelity in a polygamous shorebird, the snowy plover (Charadrius nivosus)

Social monogamy has evolved multiple times and is particularly common in birds. However, it is not well understood why some species live in long-lasting monogamous partnerships while others change mates between breeding attempts. Here, we investigate mate fidelity in a sequential polygamous shorebird, the snowy plover (Charadrius nivosus), a species in which both males and females may have several breeding attempts within a breeding season with the same or different mates. Using 6 years of data from a well-monitored population in Bahía de Ceuta, Mexico, we investigated predictors and fitness implications of mate fidelity both within and between years. We show that in order to maximize reproductive success within a season, individuals divorce after successful nesting and re-mate with the same partner after nest failure. Therefore, divorced plovers, counterintuitively, achieve higher reproductive success than individuals that retain their mate. We also show that different mating decisions between sexes predict different breeding dispersal patterns. Taken together, our findings imply that divorce is an adaptive strategy to improve reproductive success in a stochastic environment. Understanding mate fidelity is important for the evolution of monogamy and polygamy, and these mating behaviors have implications for reproductive success and population productivity.

Genetic, phenotypic and ecological differentiation suggests incipient speciation in two Charadrius plovers along the Chinese coast

Background

Speciation with gene flow is an alternative to the nascence of new taxa in strict allopatric separation. Indeed, many taxa have parapatric distributions at present. It is often unclear if these are secondary contacts, e.g. caused by past glaciation cycles or the manifestation of speciation with gene flow, which hampers our understanding of how different forces drive diversification. Here we studied genetic, phenotypic and ecological aspects of divergence in a pair of incipient shorebird species, the Kentish (Charadrius alexandrinus) and the White-faced Plovers (C. dealbatus), shorebirds with parapatric breeding ranges along the Chinese coast. We assessed divergence based on molecular markers with different modes of inheritance and quantified phenotypic and ecological divergence in aspects of morphometric, dietary and climatic niches.

Results

Our integrative analyses revealed small to moderate levels of genetic and phenotypic distinctiveness with symmetric gene flow across the contact area at the Chinese coast. The two species diverged approximately half a million years ago in dynamic isolation with secondary contact occurring due to cycling sea level changes between the Eastern and Southern China Sea in the mid-late Pleistocene. We found evidence of character displacement and ecological niche differentiation between the two species, invoking the role of selection in facilitating divergence despite gene flow.

Conclusion

These findings imply that ecology can indeed counter gene flow through divergent selection and thus contributes to incipient speciation in these plovers. Furthermore, our study highlights the importance of using integrative datasets to reveal the evolutionary history and assist the inference of mechanisms of speciation.

Electronic supplementary material

The online version of this article (10.1186/s12862-019-1449-5) contains supplementary material, which is available to authorized users.

Population divergence and gene flow in two East Asian shorebirds on the verge of speciation

Genetic isolation of populations over evolutionary time leads to the formation of independent species. We examined a pair of shorebirds – the Kentish Plover Charadrius alexandrinus and the enigmatic White-faced Plover C. dealbatus – which display strong plumage differentiation, yet show minimal genetic divergence based on previous mitochondrial and microsatellite work. Two scenarios may lead to this situation: (1) they represent clinal or poorly diverged populations with limited genomic differentiation despite substantial plumage variation, or (2) they are diverging taxa at the cusp of speciation, with ongoing limited gene flow obliterating signals of differentiation in traditional genetic markers. We compared the genotypes of 98 plovers (59 Kentish Plovers, 35 White-faced Plovers and 4 genomic hybrids) sampled in eastern Asia and Europe using ddRADSeq to harvest over 8000 genome-wide SNPs. In contrast to previous studies, our analyses revealed two well defined genomic clusters, with limited hybridization and a narrow contact zone. We also uncovered significant differences in bill length and further sex-specific differences in size, which may signal differences in mate choice between Kentish and White-faced Plovers. Our results support the hypothesis that this shorebird duo is on the verge of speciation.

Latitudinal variation in biophysical characteristics of avian eggshells to cope with differential effects of solar radiation

Solar radiation is an important driver of animal coloration, not only because of the effects of coloration on body temperature but also because coloration may protect from the deleterious effects of UV radiation. Indeed, dark coloration may protect from UV, but may increase the risk of overheating. In addition, the effect of coloration on thermoregulation should change with egg size, as smaller eggs have higher surface-volume ratios and greater convective coefficients than larger eggs, so that small eggs can dissipate heat quickly. We tested whether the reflectance of eggshells, egg spottiness, and egg size of the ground-nesting Kentish plover Charadrius alexandrinus is affected by maximum ambient temperature and solar radiation at breeding sites. We measured reflectance, both in the UV and human visible spectrum, spottiness, and egg size in photographs from a museum collection of plover eggshells. Eggshells of lower reflectance (darker) were found at higher latitudes. However, in southern localities where solar radiation is very high, eggshells are also of dark coloration. Eggshell coloration had no significant relationship with ambient temperature. Spotiness was site-specific. Small eggs tended to be light-colored. Thermal constraints may drive the observed spatial variation in eggshell coloration, which may be lighter in lower latitudes to diminish the risk of overheating as a result of higher levels of solar radiation. However, in southern localities with very high levels of UV radiation, eggshells are of dark coloration likely to protect embryos from more intense UV radiation. Egg size exhibited variation in relation to coloration, likely through the effect of surface area-to-volume ratios on overheating and cooling rates of eggs. Therefore, differential effects of solar radiation on functions of coloration and size of eggshells may shape latitudinal variations in egg appearance in the Kentish plover.

Sex-biased breeding dispersal is predicted by social environment in birds

Sex-biased dispersal is common in vertebrates, although the ecological and evolutionary causes of sex differences in dispersal are debated. Here, we investigate sex differences in both natal and breeding dispersal distances using a large dataset on birds including 86 species from 41 families. Using phylogenetic comparative analyses, we investigate whether sex-biased natal and breeding dispersal are associated with sexual selection, parental sex roles, adult sex ratio (ASR), or adult mortality. We show that neither the intensity of sexual selection, nor the extent of sex bias in parental care was associated with sex-biased natal or breeding dispersal. However, breeding dispersal was related to the social environment since male-biased ASRs were associated with female-biased breeding dispersal. Male-biased ASRs were associated with female-biased breeding dispersal. Sex bias in adult mortality was not consistently related to sex-biased breeding dispersal. These results may indicate that the rare sex has a stronger tendency to disperse in order to find new mating opportunities. Alternatively, higher mortality of the more dispersive sex could account for biased ASRs, although our results do not give a strong support to this explanation. Whichever is the case, our findings improve our understanding of the causes and consequences of sex-biased dispersal. Since the direction of causality is not yet known, we call for future studies to identify the causal relationships linking mortality, dispersal, and ASR.

Social interactions predict genetic diversification: an experimental manipulation in shorebirds

Mating strategy and social behavior influence gene flow and hence affect levels of genetic differentiation and potentially speciation. Previous genetic analyses of closely related plovers Charadrius spp. found strikingly different population genetic structure in Madagascar: Kittlitz’s plovers are spatially homogenous whereas white-fronted plovers have well segregated and geographically distinct populations. Here, we test the hypotheses that Kittlitz’s plovers are spatially interconnected and have extensive social interactions that facilitate gene flow, whereas white-fronted plovers are spatially discrete and have limited social interactions. By experimentally removing mates from breeding pairs and observing the movements of mate-searching plovers in both species, we compare the spatial behavior of Kittlitz’s and white-fronted plovers within a breeding season. The behavior of experimental birds was largely consistent with expectations: Kittlitz’s plovers travelled further, sought new mates in larger areas, and interacted with more individuals than white-fronted plovers, however there was no difference in breeding dispersal. These results suggest that mating strategies, through spatial behavior and social interactions, are predictors of gene flow and thus genetic differentiation and speciation. Our study highlights the importance of using social behavior to understand gene flow. However, further work is needed to investigate the relative importance of social structure, as well as intra- and inter-season dispersal, in influencing the genetic structures of populations.

Genetic isolation in an endemic African habitat specialist

The Chestnut-banded Plover Charadrius pallidus is a Near-Threatened shorebird species endemic to mainland Africa. We examined levels of genetic differentiation between its two morphologically and geographically distinct subspecies, C. p. pallidus in southern Africa (population size 11 000-16 000) and C. p. venustus in eastern Africa (population size 6500). In contrast to other plover species that maintain genetic connectivity over thousands of kilometres across continental Africa, we found profound genetic differences between remote sampling sites. Phylogenetic network analysis based on four nuclear and two mitochondrial gene regions, and population genetic structure analyses based on 11 microsatellite loci, indicated strong genetic divergence, with 2.36% mitochondrial sequence divergence between individuals sampled in Namibia (southern Africa) and those of Kenya and Tanzania (eastern Africa). This distinction between southern and eastern African populations was also supported by highly distinct genetic clusters based on microsatellite markers (global F_ST  = 0.309, GST' = 0.510, D = 0.182). Behavioural factors that may promote genetic differentiation in this species include habitat specialization, monogamous mating behaviour and sedentariness. Reliance on an extremely small number of saline lakes for breeding and limited dispersal between populations are likely to promote reproductive and genetic isolation between eastern and southern Africa. We suggest that the two Chestnut-banded Plover subspecies may warrant elevation to full species status. To assess this distinction fully, additional sample collection will be needed, with analysis of genetic and phenotypic traits from across the species' entire breeding range.