Brain surface morphology and ecological and macroevolutionary inferences of avian New World suboscines (Aves, Passeriformes, Tyrannides)

The New World suboscines (Passeriformes and Tyrannides) are one of the biggest endemic vertebrate radiations in South America, including the families Furnariidae and Tyrannidae. Avian brain morphology is a reliable proxy to study their evolution. The aim of this work is to elucidate whether the brains of these families reflect the ecological differences (e.g., feeding behavior) and to clarify macroevolutionary aspects of their neuroanatomy. Our hypotheses are as follows: Brain size is similar between both families and with other Passeriformes; brain morphology in Tyrannides is the result of the pressure of ecological factors; and brain disparity is low since they share ecological traits. Skulls of Furnariidae and Tyrannidae were micro-computed tomography-scanned, and three-dimensional models of the endocast were generated. Regression analyses were performed between brain volume and body mass. Linear and surface measurements were used to build phylomorphospaces and to calculate the amount of phylogenetic signal. Tyrannidae showed a larger brain disparity than Furnariidae, although it is not shaped by phylogeny in the Tyrannides. Furnariidae present enlarged Wulsts (eminentiae sagittales) but smaller optic lobes, while in Tyrannidae, it is the opposite. This could indicate that in Tyrannides there is a trade-off between the size of these two visual-related brain structures.

Evolution of Avian Eye Size Is Associated with Habitat Openness, Food Type and Brain Size

The eye is the primary sensory organ that obtains information from the ecological environments and specifically bridges the brain with the extra environment. However, the coevolutionary relationships between eye size and ecological factors, behaviours and brain size in birds remain poorly understood. Here, we investigate whether eye size evolution is associated with ecological factors (e.g., habitat openness, food type and foraging habitat), behaviours (e.g., migration and activity pattern) and brain size among 1274 avian species using phylogenetically controlled comparative analyses. Our results indicate that avian eye size is significantly associated with habitat openness, food type and brain size. Species living in dense habitats and consuming animals exhibit larger eye sizes compared to species living in open habitats and consuming plants, respectively. Large-brained birds tend to possess larger eyes. However, migration, foraging habitat and activity pattern were not found to be significantly associated with eye size in birds, except for nocturnal birds having longer axial lengths than diurnal ones. Collectively, our results suggest that avian eye size is primarily influenced by light availability, food need and cognitive ability.

Draft genome assemblies of four manakins

Manakins are a family of small suboscine passerine birds characterized by their elaborate courtship displays, non-monogamous mating system, and sexual dimorphism. This family has served as a good model for the study of sexual selection. Here we present genome assemblies of four manakin species, including Cryptopipo holochlora, Dixiphia pipra (also known as Pseudopipra pipra), Machaeropterus deliciosus and Masius chrysopterus, generated by Single-tube Long Fragment Read (stLFR) technology. The assembled genome sizes ranged from 1.10 Gb to 1.19 Gb, with average scaffold N50 of 29 Mb and contig N50 of 169 Kb. On average, 12,055 protein-coding genes were annotated in the genomes, and 9.79% of the genomes were annotated as repetitive elements. We further identified 75 Mb of Z-linked sequences in manakins, containing 585 to 751 genes and an ~600 Kb pseudoautosomal region (PAR). One notable finding from these Z-linked sequences is that a possible Z-to-autosome/PAR reversal could have occurred in M. chrysopterus. These de novo genomes will contribute to a deeper understanding of evolutionary history and sexual selection in manakins.

Measurement(s)	whole genome sequencing
Technology Type(s)	BGISEQ-500 Sequencing
Sample Characteristic - Organism	Pipridae

Dancing drives evolution of sexual size dimorphism in manakins

Body size mediates life history, physiology and inter- and intra-specific interactions. Within species, sexes frequently differ in size, reflecting divergent selective pressures and/or constraints. Both sexual selection and differences in environmentally mediated reproductive constraints can drive sexual size dimorphism, but empirically testing causes of dimorphism is challenging. Manakins (Pipridae), a family of Neotropical birds comprising approximately 50 species, exhibit a broad range of size dimorphism from male- to female-biased and are distributed across gradients of precipitation and elevation. Males perform courtship displays ranging from simple hops to complex aerobatic manoeuvres. We tested associations between sexual size dimorphism and (a) agility and (b) environment, analysing morphological, behavioural and environmental data for 22 manakin species in a phylogenetic framework. Sexual dimorphism in mass was most strongly related to agility, with males being lighter than females in species performing more aerial display behaviours. However, wing and tarsus length dimorphism were more strongly associated with environmental variables, suggesting that different sources of selection act on different aspects of body size. These results highlight the strength of sexual selection in shaping morphology-even atypical patterns of dimorphism-while demonstrating the importance of constraints and ecological consequences of body size evolution.

Layered evolution of gene expression in "superfast" muscles for courtship

The process by which new complex traits evolve has been a persistent conundrum throughout the history of evolutionary inquiry. How multiple physiological changes at the organism level and genetic changes at the molecular level combine is still unclear for many traits. Here, we studied the displays of manakins, who beat their wings together at nearly twice the speed of other songbirds to produce a loud “snap” that attracts mates. We simultaneously analyzed evolution of gene expression levels and gene sequences to identify key genes related to muscle contractions and tissue regeneration after stress. Our results show how innovative behavioral traits evolve as a layered process where recent molecular shifts build on ancestral genetic evolutionary changes.

Identifying the molecular process of complex trait evolution is a core goal of biology. However, pinpointing the specific context and timing of trait-associated changes within the molecular evolutionary history of an organism remains an elusive goal. We study this topic by exploring the molecular basis of elaborate courtship evolution, which represents an extraordinary example of trait innovation. Within the behaviorally diverse radiation of Central and South American manakin birds, species from two separate lineages beat their wings together using specialized “superfast” muscles to generate a “snap” that helps attract mates. Here, we develop an empirical approach to analyze phylogenetic lineage-specific shifts in gene expression in the key snap-performing muscle and then integrate these findings with comparative transcriptomic sequence analysis. We find that rapid wing displays are associated with changes to a wide range of molecular processes that underlie extreme muscle performance, including changes to calcium trafficking, myocyte homeostasis and metabolism, and hormone action. We furthermore show that these changes occur gradually in a layered manner across the species history, wherein which ancestral genetic changes to many of these molecular systems are built upon by later species-specific shifts that likely finalized the process of display performance adaptation. Our study demonstrates the potential for combining phylogenetic modeling of tissue-specific gene expression shifts with phylogenetic analysis of lineage-specific sequence changes to reveal holistic evolutionary histories of complex traits.

A novel exome probe set captures phototransduction genes across birds (Aves) enabling efficient analysis of vision evolution

The diversity of avian visual phenotypes provides a framework for studying mechanisms of trait diversification generally, and the evolution of vertebrate vision, specifically. Previous research has focused on opsins, but to fully understand visual adaptation, we must study the complete phototransduction cascade (PTC). Here, we developed a probe set that captures exonic regions of 46 genes representing the PTC and other light responses. For a subset of species, we directly compared gene capture between our probe set and low-coverage whole genome sequencing (WGS), and we discuss considerations for choosing between these methods. Finally, we developed a unique strategy to avoid chimeric assembly by using "decoy" reference sequences. We successfully captured an average of 64% of our targeted exome in 46 species across 14 orders using the probe set and had similar recovery using the WGS data. Compared to WGS or transcriptomes, our probe set: (1) reduces sequencing requirements by efficiently capturing vision genes, (2) employs a simpler bioinformatic pipeline by limiting required assembly and negating annotation, and (3) eliminates the need for fresh tissues, enabling researchers to leverage existing museum collections. We then utilized our vision exome data to identify positively selected genes in two evolutionary scenarios-evolution of night vision in nocturnal birds and evolution of high-speed vision specific to manakins (Pipridae). We found parallel positive selection of SLC24A1 in both scenarios, implicating the alteration of rod response kinetics, which could improve color discrimination in dim light conditions and/or facilitate higher temporal resolution.

Integrative Studies of Sexual Selection in Manakins, a Clade of Charismatic Tropical Birds

The neotropical manakins (family Pipridae) provide a great opportunity for integrative studies of sexual selection as nearly all of the 51 species are lek-breeding, an extreme form of polygyny, and highly sexually dimorphic both in appearance and behavior. Male courtship displays are often elaborate and include auditory cues, both vocal and mechanical, as well as visual elements. In addition, the displays are often extremely rapid, highly acrobatic, and, in some species, multiple males perform coordinated displays that form the basis of long-term coalitions. Male manakins also exhibit unique neuroendocrine, physiological, and anatomical adaptations to support the performance of these complex displays and the maintenance of their intricate social systems. The Manakin Genomics Research Coordination Network (Manakin RCN, https://www.manakinsrcn.org) has brought together researchers (many in this symposium and this issue) from across disciplines to address the implications of sexual selection on evolution, ecology, behavior, and physiology in manakins. The objective of this paper is to present some of the most pertinent and integrative manakin research as well as introducing the papers presented in this issue. The results discussed at the manakin symposium, part of the 2021 Society for Integrative and Comparative Biology Conference, highlight the remarkable genomic, behavioral, and physiological adaptations as well as the evolutionary causes and consequences of strong sexual selection pressures that are evident in manakins.

Correlated evolution of acrobatic display and both neural and somatic phenotypic traits in manakins (Pipridae)

Brightly colored manakin (Aves: Pipridae) males are known for performing acrobatic displays punctuated by non-vocal sounds (sonations) in order to attract dull colored females. The complexity of the display sequence and assortment of display elements involved (e.g., sonations, acrobatic maneuvers, and cooperative performances) varies considerably across manakin species. Species-specific display elements coevolve with display-distinct specializations of the neuroanatomical, muscular, endocrine, cardiovascular, and skeletal systems in the handful of species studied. Conducting a broader comparative study, we previously found positive associations between display complexity and both brain mass and body mass across 8 manakin genera, indicating selection for neural and somatic expansion to accommodate display elaboration. Whether this gross morphological variation is due to overall brain and body mass expansion (concerted evolution) versus size increases in only functionally relevant brain regions and growth of particular body ("somatic") features (mosaic evolution) remains to be explored. Here we test the hypothesis that cross-species variation in male brain mass and body mass is driven by mosaic evolution. We predicted positive associations between display complexity and variation in the volume of the cerebellum and sensorimotor arcopallium, brain regions which have roles in sensorimotor processes, and learning and performance of precisely timed and sequenced thoughts and movements, respectively. In contrast, we predicted no associations between the volume of a limbic arcopallial nucleus or a visual thalamic nucleus and display complexity as these regions have no-specific functional relationship to display behavior. For somatic features, we predicted that the relationship between body mass and complexity would not include contributions of tarsus length based on a recent study suggesting selection on tarsus length is less labile than body mass. We tested our hypotheses in males from 12 manakin species and a closely related flycatcher. Our analyses support mosaic evolution of neural and somatic features functionally relevant to display and indicate sexual selection for acrobatic complexity may increase the capacity for procedural learning via cerebellar enlargement and maneuverability via a reduction in tarsus length in species with lower overall complexity scores.

Sexual selection and introgression in avian hybrid zones: Spotlight on Manacus

Hybrid zones offer a window into the processes and outcomes of evolution, from species formation or fusion to genomic underpinnings of specific traits and isolating mechanisms. Sexual selection is believed to be an important factor in speciation processes, and hybrid zones present special opportunities to probe its impact. The manakins (Aves, Pipridae) are a promising group in which to study the interplay of sexual selection and natural hybridization: they show substantial variation across the family in the strength of sexual selection they experience, they readily hybridize within and between genera, and they appear to have formed hybrid species, a rare event in birds. A hybrid zone between two manakins in the genus Manacus is unusual in that plumage and behavioral traits of one species have introgressed asymmetrically into populations of the second species through positive sexual selection, then apparently stalled at a river barrier. This is one of a handful of documented examples of asymmetric sexual trait introgression with a known selective mechanism. It offers opportunities to examine reproductive isolation, introgression, plumage color evolution, and natural factors enhancing or constraining the effects of sexual selection in real time. Here, we review previous work in this system, propose new hypotheses for observed patterns, and recommend approaches to test them.

Positive allometry of sexually selected traits: Do metabolic maintenance costs play an important role?

Sexual selection drives the evolution of some of the most exaggerated traits in nature. Studies on sexual selection often focus on the size of these traits relative to body size, but few focus on energetic maintenance costs of the tissues that compose them, and the ways in which these costs vary with body size. The relationships between energy use and body size have consequences that may allow large individuals to invest disproportionally more in sexually selected structures, or lead to the reduced per-gram maintenance cost of enlarged structures. Although sexually selected traits can incur energetic maintenance costs, these costs are not universally high; they are dependent on the relative mass and metabolic activity of tissues associated with them. Energetic costs of maintenance may play a pervasive yet little-explored role in shaping the relative scaling of sexually selected traits across diverse taxa. Also see the video abstract here: https://youtu.be/JyuoQIeA33Q.

Breeding season length predicts duet coordination and consistency in Neotropical wrens (Troglodytidae)

Many animals produce coordinated signals, but few are more striking than the elaborate male-female vocal duets produced by some tropical songbirds. Yet, little is known about the factors driving the extreme levels of vocal coordination between mated pairs in these taxa. We examined evolutionary patterns of duet coordination and their potential evolutionary drivers in Neotropical wrens (Troglodytidae), a songbird family well known for highly coordinated duets. Across 23 wren species, we show that the degree of coordination and precision with which pairs combine their songs into duets varies by species. This includes some species that alternate their song phrases with exceptional coordination to produce rapidly alternating duets that are highly consistent across renditions. These highly coordinated, consistent duets evolved independently in multiple wren species. Duet coordination and consistency are greatest in species with especially long breeding seasons, but neither duet coordination nor consistency are correlated with clutch size, conspecific abundance or vegetation density. These results suggest that tightly coordinated duets play an important role in mediating breeding behaviour, possibly by signalling commitment or coalition of the pair to mates and other conspecifics.

Do Different Methods Yield Equivalent Estimations of Brain Size in Birds?

The ratio of brain size to body size (relative brain size) is often used as a measure of relative investment in the brain in ecological and evolutionary studies on a wide range of animal groups. In birds, a variety of methods have been used to measure the brain size part of this ratio, including endocranial volume, fixed brain mass, and fresh brain mass. It is still unclear, however, whether these methods yield the same results. Using data obtained from fresh corpses and from published sources, this study shows that endocranial volume, mass of fixed brain tissue, and fresh mass provide equivalent estimations of brain size for 48 bird families, in 19 orders. We found, however, that the various methods yield significantly different brain size estimates for hummingbirds (Trochilidae). For hummingbirds, fixed brain mass tends to underestimate brain size due to reduced tissue density, whereas endocranial volume overestimates brain size because it includes a larger volume than that occupied by the brain.

Egg recognition and brain size in a cuckoo host

The evolution of animal brain size and cognitive ability is a topic of central significance in evolutionary ecology. Interspecific brood parasitism imposes severe selection pressures on hosts favoring the evolution of cuckoo egg recognition and rejection. However, recognizing and rejecting foreign parasitic eggs are enormous cognitive challenges for cuckoo hosts, which might select for an increase in brain size in birds with this capacity. To explore the association between cuckoo parasitism and the evolution of brain size in cinereous tits (Parus cinereus), we used two types of experimental parasitic eggs, real mimetic white-rumped munia (Lonchura striata) eggs and non-mimetic blue model eggs, to test the egg recognition ability of female cinereous tits, thereby comparing brain size variation among individuals that were able to recognize foreign eggs and those that lacked this ability. Interestingly, our results however did not support the prediction that cuckoo parasitism selects for an increase in brain size of host birds, since brain size of egg rejecters was not significantly larger than that of accepters. Hence, this study suggested that the evolution of cognitive ability did not allow recognition of foreign eggs by female cinereous tits. That was the case despite the evolution of a larger brain may have allowed for a reduction in the cost of brood parasitism by cuckoos.

Sex Steroids as Regulators of Gestural Communication

Gestural communication is ubiquitous throughout the animal kingdom, occurring in species that range from humans to arthropods. Individuals produce gestural signals when their nervous system triggers the production of limb and body movement, which in turn functions to help mediate communication between or among individuals. Like many stereotyped motor patterns, the probability of a gestural display in a given social context can be modulated by sex steroid hormones. Here, we review how steroid hormones mediate the neural mechanisms that underly gestural communication in humans and nonhumans alike. This is a growing area of research, and thus we explore how sex steroids mediate brain areas involved in language production, social behavior, and motor performance. We also examine the way that sex steroids can regulate behavioral output by acting in the periphery via skeletal muscle. Altogether, we outline a new avenue of behavioral endocrinology research that aims to uncover the hormonal basis for one of the most common modes of communication among animals on Earth.

Variation in sexual brain size dimorphism over the breeding cycle in the three-spined stickleback

Snapshot analyses have demonstrated dramatic intraspecific variation in the degree of brain sexual size dimorphism (SSD). Although brain SSD is believed to be generated by the sex-specific cognitive demands of reproduction, the relative roles of developmental and population-specific contributions to variation in brain SSD remain little studied. Using a common garden experiment, we tested for sex-specific changes in brain anatomy over the breeding cycle in three-spined stickleback (Gasterosteus aculeatus) sampled from four locations in northern Europe. We found that the male brain increased in size (ca. 24%) significantly more than the female brain towards breeding, and that the resulting brain SSD was similar (ca. 20%) for all populations over the breeding cycle. Our findings support the notion that the stickleback brain is highly plastic and changes over the breeding cycle, especially in males, likely as an adaptive response to the cognitive demands of reproduction (e.g. nest construction and parental care). The results also provide evidence to suggest that breeding-related changes in brain size may be the reason for the widely varying estimates of brain SSD across studies of this species, cautioning against interpreting brain size measurements from a single time point as fixed/static.

Synergistic selection regimens drive the evolution of display complexity in birds of paradise

Integrated visual displays that combine gesture with colour are nearly ubiquitous in the animal world, where they are shaped by sexual selection for their role in courtship and competition. However, few studies assess how multiple selection regimens operate on different components of these complex phenotypes on a macroevolutionary scale. Here, we study this issue by assessing how both sexual and ecological selection work together to influence visual display complexity in the birds of paradise. We first find that sexual dichromatism is highest in lekking species, which undergo more intense sexual selection by female choice, than non-lekking species. At the same time, species in which males directly compete with one another at communal display courts have more carotenoid-based ornaments and fewer melanin ornaments. Meanwhile, display habitat influences gestural complexity. Species that dance in the cluttered understorey have more complex dances than canopy-displaying species. Taken together, our results illustrate how distinct selection regimens each operate on individual elements comprising a complex display. This supports a modular model of display evolution, wherein the ultimate integrated display is the product of synergy between multiple factors that select for different types of phenotypic complexity.

Neuromuscular mechanisms of an elaborate wing display in the golden-collared manakin (Manacus vitellinus)

Many species perform elaborate physical displays to court mates and compete with rivals, but the biomechanical mechanisms underlying such behavior are poorly understood. We address this issue by studying the neuromuscular origins of display behavior in a small tropical passerine bird, the golden-collared manakin (Manacus vitellinus). Males of this species court females by dancing around the forest floor and rapidly snapping their wings together above their back. Using radio-telemetry, we collected electromyographic (EMG) recordings from the three main muscles that control avian forelimb movement, and found how these different muscles are activated to generate various aspects of display behavior. The muscle that raises the wing (supracoracoideus, SC) and the primary muscle that retracts the wing (scapulohumeralis caudalis, SH) were activated during the wing-snap, whereas the pectoralis (PEC), the main wing depressor, was not. SC activation began before wing elevation commenced, with further activation occurring gradually. By contrast, SH activation was swift, starting soon after wing elevation and peaking shortly after the snap. The intensity of this SH activation was comparable to that which occurs during flapping, whereas the SC activation was much lower. Thus, light activation of the SC likely helps position the wings above the back, so that quick, robust SH activation can drive these appendages together to generate the firecracker-like snap sonation. This is one of the first looks at the neuromuscular mechanisms that underlie the actuation of a dynamic courtship display, and it demonstrates that even complex, whole-body display movements can be studied with transmitter-aided EMG techniques.

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.

Evolutionary patterns of adaptive acrobatics and physical performance predict expression profiles of androgen receptor - but not oestrogen receptor - in the forelimb musculature

1. Superior physical competence is vital to the adaptive behavioral routines of many animals, particularly those that engage in elaborate socio-sexual displays. How such traits evolve across species remains unclear. 2. Recent work suggests that activation of sex steroid receptors in neuromuscular systems is necessary for the fine motor skills needed to execute physically elaborate displays. Thus, using passerine birds as models, we test whether interspecific variation in display complexity predicts species differences in the abundance of androgen and estrogen receptors (AR and ERα) expressed in the forelimb musculature and spinal cord. 3. We find that small-scale evolutionary patterns in physical display complexity positively predict expression of the AR in the main muscles that lift and retract the wings. No such relationship is detected in the spinal cord, and we do not find a correlation between display behavior and neuromuscular expression of ERα. Also, we find that AR expression levels in different androgen targets throughout the body - namely the wing muscles, spinal cord, and testes - are not necessarily correlated, providing evidence that evolutionary forces may drive AR expression in a tissue-specific manner. 4. These results suggest co-evolution between the physical prowess necessary for display performance and levels of AR expression in avian forelimb muscles. Moreover, this relationship appears to be specific to muscle and AR-mediated, but not ERα-mediated, signaling. 5. Given that prior work suggests that activation of muscular AR is a necessary component of physical display performance, our current data support the hypothesis that sexual selection shapes levels of AR expressed in the forelimb skeletal muscles to help drive the evolution of adaptive motor abilities.