Social Mating System Research Articles

No evidence for an adaptive role of early molt into breeding plumage in a female fairy wren

Abstract The evolution of ornaments as sexually selected signals is well understood in males, but female ornamentation remains understudied. Fairy wrens offer an excellent model system, given their complex social structure and mating systems, and the diversity of female ornamentation. We investigated whether early molt into ornamental breeding plumage plays an adaptive role in females of the monogamous purple-crowned fairy wren Malurus coronatus, the only fairy wren known to have female seasonal plumage. Using 6 years of monitoring, we found that the timing of female molt was similar to males, but there was no evidence for assortative mating. Like males (previous study), older and dominant individuals acquired their breeding plumage earlier; however, in contrast to males, early molt did not seem to be costly since unfavorable environmental conditions or previous reproductive effort did not delay molt. Early female molt was not associated with any indicator of reproductive quality nor did it attract additional offspring care by their partners. We also found no association between early molt and the likelihood of acquiring a dominant (breeding) position or with the presence or proximity to same-sex rivals. Our study results, which are similar to previous findings in conspecific males, suggest that directional selection for early molt might be relaxed in this species, in contrast to other genetically polygamous fairy wrens in which early molt predicts extrapair mating success in males. However, the finding that molt timing is status dependent raises the possibility that other attributes of the ornament may fulfill an adaptive function in females.

Testing predictions of inclusive fitness theory in inbreeding relatives with biparental care.

Inclusive fitness theory predicts that parental care will vary with relatedness between potentially caring parents and offspring, potentially shaping mating system evolution. Systems with extra-pair paternity (EPP), and hence variable parent–brood relatedness, provide valuable opportunities to test this prediction. However, existing theoretical and empirical studies assume that a focal male is either an offspring's father with no inbreeding, or is completely unrelated. We highlight that this simple dichotomy does not hold given reproductive interactions among relatives, complicating the effect of EPP on parent–brood relatedness yet providing new opportunities to test inclusive fitness theory. Accordingly, we tested hierarchical hypotheses relating parental feeding rate to parent–brood relatedness, parent kinship and inbreeding, using song sparrows (Melospiza melodia) experiencing natural variation in relatedness. As predicted, male and female feeding rates increased with relatedness to a dependent brood, even controlling for brood size. Male feeding rate tended to decrease as paternity loss increased, and increased with increasing kinship and hence inbreeding between socially paired mates. We thereby demonstrate that variation in a key component of parental care concurs with subtle predictions from inclusive fitness theory. We additionally highlight that such effects can depend on the underlying social mating system, potentially generating status-specific costs of extra-pair reproduction.

Rapid decreases in relative testes mass among monogamous birds but not in other vertebrates.

Larger testes produce more sperm and therefore improve reproductive success in the face of sperm competition. Adaptation to social mating systems with relatively high and low sperm competition are therefore likely to have driven changes in relative testes size in opposing directions. Here, we combine the largest vertebrate testes mass dataset ever collected with phylogenetic approaches for measuring rates of morphological evolution to provide the first quantitative evidence for how relative testes mass has changed over time. We detect explosive radiations of testes mass diversity distributed throughout the vertebrate tree of life: bursts of rapid change have been frequent during vertebrate evolutionary history. In socially monogamous birds, there have been repeated rapid reductions in relative testes mass. We see no such pattern in other monogamous vertebrates; the prevalence of monogamy in birds may have increased opportunities for investment in alternative behaviours and physiologies allowing reduced investment in expensive testes.

Molecular evidence of extra-pair paternity and intraspecific brood parasitism by the Magellanic Penguin (Spheniscus magellanicus)

Molecular studies have shown that, although most avian species are socially monogamous, they can exhibit different reproductive strategies, such as extra-pair paternity (EPP) and intraspecific brood parasitism (IBP). We investigated the genetic mating system of the Magellanic Penguin, Spheniscus magellanicus, a species that is reported to be monogamous, with little or no evidence of extra-pair paternity, high partner faithfulness over the years and intense biparental care. Thus, we proposed that if EPP does occur, the rate will be low and could be related to increased genetic diversity among offspring or the body condition of parents. As a consequence, we hypothesize that offspring from EPP will exhibit faster growth than non-EPP offspring, thus increasing their survival probability. In contrast to expectations, EPP was detected in 31% of the offspring and 48% of the nests had at least one extra-pair young. In addition, 6% of chicks were not the offspring of the putative mother or either member of the social pair, indicating evidence of IBP. However, we did not find a difference in genetic diversity between nests with or without EPP, nor did we find differences in parent body condition. Offspring from EPP showed slightly faster growth than non-EPP offspring, although this difference was not statistically significant. These results reveal a high rate of EPP and a low rate of IBP, both of which were previously unknown for this species. Our findings demonstrate that the Magellanic penguin has a more complex social mating system than previously thought.

Correlated evolution between repertoire size and song plasticity predicts that sexual selection on song promotes open-ended learning.

Some oscine songbird species modify their songs throughout their lives ('adult song plasticity' or 'open-ended learning'), while others crystallize their songs around sexual maturity. It remains unknown whether the strength of sexual selection on song characteristics, such as repertoire size, affects adult song plasticity, or whether adult song plasticity affects song evolution. Here, we compiled data about song plasticity, song characteristics, and mating system and then examined evolutionary interactions between these traits. Across 67 species, we found that lineages with adult song plasticity show directional evolution toward increased syllable and song repertoires, while several other song characteristics evolved faster, but in a non-directional manner. Song plasticity appears to drive bi-directional transitions between monogamous and polygynous social mating systems. Notably, our analysis of correlated evolution suggests that extreme syllable and song repertoire sizes drive the evolution of adult song plasticity or stability, providing novel evidence that sexual selection may indirectly influence open- versus closed-ended learning.

Equitable Chick Survival in Three Species of the Non-Migratory Shorebird Despite Species-Specific Sexual Dimorphism of the Young.

Simple SummaryWe aimed to determine whether seasonal brood sex-ratio, sex-biased chick survival, and sex specific dimorphism at hatching or during growth occurs among three species of resident Australian shorebird. Our results describe no sex-bias in chick production, survival or growth rates between sexes for any of the three species studied. Sex-biases in populations can have important implications for species’ social biology, population demography and mating systems. It has recently been suggested that in some shorebirds, sex-specific bias in survival of precocial young may occur. This may be driven by variation in the brood sex-ratio and/or the sexual size dimorphism of young birds, which may influence predator escape capacity. Understanding the survival of young birds remains a significant knowledge gap for many taxa, especially when young birds are mobile and cryptic. Our aims were to estimate the sex-ratio variation in three species of Australian resident shorebird, specifically to determine: (1) whether seasonal brood sex-ratio variation at hatching is occurring, (2) the extent of any sex-biased chick survival, (3) if sex specific dimorphism at hatching or during growth occurs; and, (4) whether escape capacity differs between the sexes. We radio-tracked 50 Masked Lapwing Vanellus miles, 42 Red-capped Plover Charadrius ruficapillus and 27 Hooded Plover Thinornis cucullatus chicks from individual broods, examined the likelihood of hatchlings being male or female based on the hatching date within the breeding season, and compared size at hatching, growth and mortality of chicks of different sexes. There was no sex-bias with the hatching date across the breeding season, nor were there differences in survival or growth rates between sexes for any of the three species studied. In one species, male hatchlings had longer tarsi than females, but this did not result in differential escape propensity or improved survival. In conclusion, the hatching date, survival and growth of chicks from three species of resident shorebird was not influenced by their sex.

Confounding social and mating systems predictably lead to biased results when examining the evolution of cooperative breeding in cichlids: A response to Tanaka et al.

AbstractIn 2017, we demonstrated that transitions to cooperative breeding in Lamprologine cichlid fishes were not related to a species’ social mating system (Dey et al. 2017. Nature Ecology & Evolution, 1, 137). This contrasted previous evidence that monogamy (and a low degree of promiscuity) promoted transitions to cooperative breeding in other taxa. Recently, Tanaka et al. (2018. Ethology, 124, 777–789) critiqued our study and argued that a re‐analysis of the data shows transitions to cooperative breeding are promoted by non‐monogamous mating systems. Here, we show that Tanaka et al.'s critique contains numerous inaccuracies. In addition, we show that the results put forth by Tanaka et al. emerge only under the extreme scenario in which all cooperative breeding species are classified as non‐monogamous, which we argue arises because Tanaka et al. confound social systems and mating systems. While we agree that there is uncertainty regarding the mating system of some Lamprologine species, we argue this uncertainty was sufficiently addressed through the extensive sensitivity analyses conducted in our original study.

Population distribution and group structure of the invasive shrimp Palaemon macrodactylus (Caridea: Palaemonidae) living in a fouling community

ABSTRACTBehaviour traits like sociality have been identified as factors promoting invasiveness. Studying the manner in which animals are distributed in their natural environment relative to conspecifics and other species provides basic information about behavioural aspects, including social tendencies, aggressiveness, nesting and mating system, among others. The invasive shrimp Palaemon macrodactylus has shown aggressive behaviour in laboratory conditions, whereas in natural conditions it seems to live aggregated, sharing patchy habitats with other species such as the small goby fish Gobiosoma parri. In this study we investigated the abundance, density, group structure (i.e. size, sex, reproductive condition), and the possible coexistence of P. macrodactylus with G. parri in the fouling community of Mar del Plata harbour (Argentina), where this shrimp has invaded and established a stable population since 2000. Samples taken included all individuals present in each fouling patch. The results show that P. macrodactylus, inhabiting in the fouling community, are in groups of up to 20 individuals, without sex or size structure and that a positive correlation exists with the goby fish. This suggests a gregarious behaviour, and tolerance to other species using the same area, at least in an environment with high availability of refuges, traits that may be crucial for a successful invader. The persistence of such behaviour in different contexts and the effect of refuge should be investigated in order to fully understand and make predictions of new invasions.

Monogamy without parental care? Social and genetic mating systems of avian brood parasites.

Classic evolutionary theory predicts that monogamy should be intimately linked with parental care. It has long been assumed, therefore, that avian brood parasites-which lay their eggs in the nests of 'host' species and provide little, if any, parental care-should be overwhelmingly promiscuous. However, recent studies have revealed that the social mating systems of brood parasites are surprisingly diverse, encompassing lek polygyny, monogamy, polygamy and promiscuity. What ecological or phylogenetic factors explain this variation, and why are some brood parasites apparently monogamous? Here we review the social and genetic mating systems of all 75 brood parasitic species for which data are available and evaluate several hypotheses that may help explain these patterns. We find that social monogamy is widespread, often co-occurring with territoriality and cooperative behaviour by the mated pair. Comparative studies, though preliminary, suggest that in some species, monogamy is associated with low host density and polygamy with higher host density. Interestingly, molecular data show that genetic and social mating systems can be entirely decoupled: genetic monogamy can occur in parasitic species that lack behavioural pair-bonds, possibly as a by-product of territoriality; conversely, social monogamy has been reported in parasites that are genetically polygamous. This synthesis suggests that social and genetic monogamy may result from very different selective pressures, and that male-female cooperative behaviours, population density and territoriality may all interact to favour the evolution of monogamous mating in brood parasites. Given that detailed descriptive data of social, and especially genetic, mating systems are still lacking for the majority of brood parasitic species, definitive tests of these hypotheses await future work. This article is part of the theme issue 'The coevolutionary biology of brood parasitism: from mechanism to pattern'.

Thermoregulatory Requirements Shape Mating Opportunities of Male Proboscis Bats

The spatiotemporal distribution of females is a major factor affecting animal social systems. Predation risk and the distribution of feeding resources often determine where females are found, but abiotic factors (e.g. temperature) can also shape the distribution of females and therefore variation in social organization and mating systems. Given the predicted future changes in climatic variation, it is vital to understand how animal mating systems and the sexual selection process may be altered by temperature. In bats, female distribution is tightly linked to roosting ecology and particularly to the microclimatic conditions at the roost. Proboscis bats (Rhynchonycteris naso) form cohesive and stable multi-male-multi-female groups and inhabit exposed day roosts (e.g. tree trunks, vines, buildings). Strong selection to remain inconspicuous to visually oriented predators in the exposed day roosts has been suggested to promote a rather rare male mating strategy termed site-specific dominance where males defend females directly but are successful in doing so only in their own territory. The choice of open-roost structures can result in the bats roosting under direct sunlight, making individuals susceptible to overheating. Here we investigate whether regular relocations of R. naso social groups among male territories are a mechanism of behavioral thermoregulation. Our results suggest that in general R. naso choose the warmest suitable roost sites within a roost, possibly to minimize the energetic costs of thermoregulation. However, on days with high midday temperatures at the primary roost site, bats commonly relocate to alternative, cooler sites within their roosts. These thermoregulatory relocations entail that a social group regularly switches among the territories of several males. Thus, the need for behavioral thermoregulation determines the spatial distribution of females and shapes the mating opportunities of males during the day. This is supported by our result that territorial males defending primary roost sites are reproductively more successful than territorial males of alternate roost sites. In line with other studies, our findings suggest that the increase in ambient temperatures associated with climate change has the potential to affect the intensity of sexual selection in bat species and may have far-reaching behavioral, demographic and evolutionary consequences for their populations.

Evolutionary diversity as a catalyst for biological discovery.

The tremendous diversity of animal behaviors has inspired generations of scientists from an array of biological disciplines. To complement investigations of ecological and evolutionary factors contributing to behavioral evolution, modern sequencing, gene editing, computational and neuroscience tools now provide a means to discover the proximate mechanisms upon which natural selection acts to generate behavioral diversity. Social behaviors are motivated behaviors that can differ tremendously between closely related species, suggesting phylogenetic plasticity in their underlying biological mechanisms. In addition, convergent evolution has repeatedly given rise to similar forms of social behavior and mating systems in distantly related species. Social behavioral divergence and convergence provides an entry point for understanding the neurogenetic mechanisms contributing to behavioral diversity. We argue that the greatest strides in discovering mechanisms contributing to social behavioral diversity will be achieved through integration of interdisciplinary comparative approaches with modern tools in diverse species systems. We review recent advances and future potential for discovering mechanisms underlying social behavioral variation; highlighting patterns of social behavioral evolution, oxytocin and vasopressin neuropeptide systems, genetic/transcriptional "toolkits," modern experimental tools, and alternative species systems, with particular emphasis on Microtine rodents and Lake Malawi cichlid fishes.

Male Scent Gland Signals Mating Status in Greater Spear-Nosed Bats, Phyllostomus hastatus.

Chemical signals are ubiquitous, but often overlooked as potentially important for conveying information relevant for sexual selection. The male greater spear-nosed bat, Phyllostomus hastatus, possesses a sexually dimorphic gland on the chest that produces an odoriferous secretion. Here, we investigate the potential for this glandular secretion to act as a sexually selected olfactory signal by examining gland activity in and out of the mating season and determining if variation in its chemical composition reflects variation in male mating status or attributes of the individual. Based on gas chromatography-mass spectrometry (GC-MS) measurements of samples collected from wild bats roosting in caves in Trinidad, West Indies, we find that males that defend and roost with groups of females (harem holders) have significantly different chemical profiles from males found roosting in all male groups (bachelors). Additionally, profiles differed significantly among individuals. Taken together, these results suggest that this chemical signal has the potential to communicate both mating status and individual identity and thus could be used to mediate interactions among individuals within this harem-based social mating system.

Asymmetric assortative mating and queen polyandry are linked to a supergene controlling ant social organization.

Nonrecombining genomic variants underlie spectacular social polymorphisms, from bird mating systems to ant social organization. Because these "social supergenes" affect multiple phenotypic traits linked to survival and reproduction, explaining their persistence remains a substantial challenge. Here, we investigate how large nonrecombining genomic variants relate to colony social organization, mating system and dispersal in the Alpine silver ant, Formica selysi. The species has colonies headed by a single queen (monogynous) and colonies headed by multiple queens (polygynous). We confirmed that a supergene with alternate haplotypes-Sm and Sp-underlies this polymorphism in social structure: Females from mature monogynous colonies had the Sm/Sm genotype, while those from polygynous colonies were Sm/Sp and Sp/Sp. Queens heading monogynous colonies were exclusively mated with Sm males. In contrast, queens heading polygynous colonies were mated with Sp males and Sm males. Sm males, which are only produced by monogynous colonies, accounted for 22.9% of the matings with queens from mature polygynous colonies. This asymmetry between social forms in the degree of assortative mating generates unidirectional male-mediated gene flow from the monogynous to the polygynous social form. Biased gene flow was confirmed by a significantly higher number of private alleles in the polygynous social form. Moreover, heterozygous queens were three times as likely as homozygous queens to be multiply mated. This study reveals that the supergene variants jointly affect social organization and multiple components of the mating system that alter the transmission of the variants and thus influence the dynamics of the system.

First draft genome of an iconic clownfish species (Amphiprion frenatus).

Clownfishes (or anemonefishes) form an iconic group of coral reef fishes, principally known for their mutualistic interaction with sea anemones. They are characterized by particular life history traits, such as a complex social structure and mating system involving sequential hermaphroditism, coupled with an exceptionally long lifespan. Additionally, clownfishes are considered to be one of the rare groups to have experienced an adaptive radiation in the marine environment. Here, we assembled and annotated the first genome of a clownfish species, the tomato clownfish (Amphiprion frenatus). We obtained 17,801 assembled scaffolds, containing a total of 26,917 genes. The completeness of the assembly and annotation was satisfying, with 96.5% of the Actinopterygii Benchmarking Universal Single-Copy Orthologs (BUSCOs) being retrieved in A.frenatus assembly. The quality of the resulting assembly is comparable to other bony fish assemblies. This resource is valuable for advancing studies of the particular life history traits of clownfishes, as well as being useful for population genetic studies and the development of new phylogenetic markers. It will also open the way to comparative genomics. Indeed, future genomic comparison among closely related fishes may provide means to identify genes related to the unique adaptations to different sea anemone hosts, as well as better characterize the genomic signatures of an adaptive radiation.

Behavioral Ecology of Elliot’s Laughingthrush (Trochalopteron (Garrulax) elliotii; Timaliidae): I. Breeding Biology and Social Behavior

Babblers (Timaliidae s. l.) are a diverse group of passerine birds distributed mainly in Asia. The behavior and vocalization of most babbler species are poorly known. We provide the first description of the social organization and behavior of Elliot’s laughingthrush, a member of Timaliidae s. l. Data on its breeding biology are given as well. The study was conducted in the Hunan Hupingshan National Nature Reserve, south-central China. Elliot’s laughingthrushes are common breeders in secondary mountain forests up to 1900 m above sea level. Nests can be found in bamboo and scrub thickets. Clutch size is 2–3 eggs. The social mating system is monogamy. Each pair defends a small territory (mean size, 1710 m2). The borders between adjacent territories are established during territorial interactions at the onset of the nesting period, with participation of both males and females. We discuss our data in the context of differences in the behavioral ecology between the tropical and temperate bird species.

Do extra-group fertilizations increase the potential for sexual selection in male mammals?

Fertilizations by males outside the social breeding group (extra-group paternity, EGP) are widespread in birds and mammals. EGP is generally proposed to increase male reproductive skew and thereby increase the potential for sexual selection, but the generality of this relationship is unclear. We extracted data from 27 mammals in seven orders and used phylogenetic comparative methods to investigate the influence of EGP and social mating system on measures of inequality in male fertilization success, which are indices of the potential for sexual selection. We find that EGP and social mating system can predict the potential for sexual selection in mammalian populations, but only when considered jointly and not individually. EGP appears to increase the potential for sexual selection but only when the degree of social polygyny is relatively low. When social polygyny is high, EGP appears to result in a more uniform distribution of reproduction and a decrease in the potential for sexual selection. A possible explanation to be investigated is that the phenotype of extra-group fathers differs systematically across social mating systems. Our findings have implications for the use of EGP and social mating system as indices of sexual selection in comparative analyses of trait evolution under sexual selection.

Social mating system, male parental care contribution and life history traits of a southern Sedge Wren (Cistothorus platensis platensis) population: a comparison with northern Sedge Wrens (Cistothorus platensis stellaris)

The study of geographic variation of social mating systems can shed new light on our understanding of how ecological variables shape extant mating associations. We report data on the social mating system, parental care and life history traits of a temperate population of southern Sedge Wrens (Cistothorus platensis platensis) in South America. We compared our results with published records of two temperate populations of northern Sedge Wren (Cistothorus platensis stellaris) in North America. The southern temperate population had a lower social polygyny rate, greater male contribution to feeding nestlings and smaller clutch sizes than northern temperate populations. A similar pattern of low rates of social polygyny and smaller clutch sizes in the south versus moderate rates of social polygyny and bigger clutch sizes in the north has been reported for the House Wren (Troglodytes aedon). This suggests that different selective forces may be operating in northern and southern wren populations. Future work in additional study populations is essential to establishing the generality of our results.

High fidelity: extra‐pair fertilisations in eight Charadrius plover species are not associated with parental relatedness or social mating system

Extra‐pair paternity is a common reproductive strategy in many bird species. However, it remains unclear why extra‐pair paternity occurs and why it varies among species and populations. Plovers (Charadrius spp.) exhibit considerable variation in reproductive behaviour and ecology, making them excellent models to investigate the evolution of social and genetic mating systems. We investigated inter‐ and intra‐specific patterns of extra‐pair parentage and evaluated three major hypotheses explaining extra‐pair paternity using a comparative approach based on the microsatellite genotypes of 2049 individuals from 510 plover families sampled from twelve populations that constituted eight species. Extra‐pair paternity rates were very low (0 to 4.1% of chicks per population). No evidence was found in support of the sexual conflict or genetic compatibility hypotheses, and there was no seasonal pattern of extra‐pair paternity (EPP). The low prevalence of EPP is consistent with a number of alternative hypotheses, including the parental investment hypothesis, which suggests that high contribution to care by males restricts female plovers from engaging in extra‐pair copulations. Further studies are needed to critically test the importance of this hypothesis for mate choice in plovers.

Insight into the Evolution of the Avian Vocal Organ, or Syrinx, from Enhanced‐Contrast X‐ray Computed Tomography and Fossil Data

From complex songs to simple honks, squawks, or caws, birds produce sounds using a unique vocal organ, the syrinx. Located close to the heart at the tracheobronchial junction, vocal folds or membranes attached to modified mineralized rings vibrate to produce sound. Not thought to commonly enter the fossil record, the few reported fossilized parts of the syrinx are geologically young (i.e., Pleistocene and Holocene). The only known older syrinx is an Eocene specimen, which was not described or figured. Data on the relationship between soft tissue structures and syringeal three‐dimensional geometry has also been limited. Our team of physiologists and paleontologists recently described the first remains of a fossil syrinx from the Mesozoic, which are preserved in three dimensions in a specimen from the Late Cretaceous of Antarctica. With both cranial and postcranial remains, the new specimen is the most complete to be recovered from a part of the radiation of living birds, Aves. Enhanced‐contrast x‐ray computed tomography (CT) of syrinx structure in twelve extant non‐passerine birds, as well as CT imaging of the Cretaceous and the Eocene syrinxes, informs both the reconstruction of ancestral states in birds and properties of the vocal organ in the extinct species. Fused rings in the Cretaceous fossil form a well‐mineralized pessulus, a derived neognath bird feature, proposed to anchor enlarged vocal folds, or labia. Left‐right bronchial asymmetry, only known in extant birds with two sound sources is similarly present.The new data show the fossilization potential of the avian vocal organ and beg the question why these remains have not been found in other dinosaurs. The lack of other Mesozoic tracheal and bronchial ring remains, and the poorly‐mineralized condition in archosaurian taxa without a syrinx, may indicate a complex syrinx was a late arising feature in the evolution of birds, well after the origin of flight and respiratory innovations. While a role for complexity in social structure and mating system has been extensively investigated with reference to increases in brain size in other vertebrates including humans, in the early evolution of birds these hypotheses have not been explored. We propose that first more complex visual signals and then vocal communication and song may have played important roles in the evolution of the larger extant avian brain.Support or Funding InformationGordon and Betty Moore Foundation Grant GBMF4498, US National Science Foundation (OPP ANT‐1141820, OPP 0927341 and EAR 1355292 to J.C.); Argentine Agencia Nacional de Promoción Científica y Técnica (PICT 2010‐066; F.E.N).

Biogeography predicts macro-evolutionary patterning of gestural display complexity in a passerine family.

Gestural displays are incorporated into the signaling repertoire of numerous animal species. These displays range from complex signals that involve impressive and challenging maneuvers, to simpler displays or no gesture at all. The factors that drive this evolution remain largely unclear, and we therefore investigate this issue in New World blackbirds by testing how factors related to a species’ geographical distribution and social mating system predict macro‐evolutionary patterns of display elaboration. We report that species inhabiting temperate regions produce more complex displays than species living in tropical regions, and we attribute this to (i) ecological factors that increase the competitiveness of the social environment in temperate regions, and (ii) different evolutionary and geological contexts under which species in temperate and tropical regions evolved. Meanwhile, we find no evidence that social mating system predicts species differences in display complexity, which is consistent with the idea that gestural displays evolve independently of social mating system. Together, these results offer some of the first insight into the role played by geographic factors and evolutionary context in the evolution of the remarkable physical displays of birds and other vertebrates.