Neurogenomic landscape associated with status-dependent cooperative behaviour.

The neurogenomic mechanisms mediating male-male reproductive cooperative behaviours remain unknown. We leveraged extensive transcriptomic and behavioural data on a neotropical bird species (Pipra filicauda) that performs cooperative courtship displays to understand these mechanisms. In this species, the cooperative display is modulated by testosterone, which promotes cooperation in non-territorial birds, but suppresses cooperation in territory holders. We sought to understand the neurogenomic underpinnings of three related traits: social status, cooperative display behaviour and testosterone phenotype. To do this, we profiled gene expression in 10 brain nuclei spanning the social decision-making network (SDMN), and two key endocrine tissues that regulate social behaviour. We associated gene expression with each bird's behavioural and endocrine profile derived from 3 years of repeated measures taken from free-living birds in the Ecuadorian Amazon. We found distinct landscapes of constitutive gene expression were associated with social status, testosterone phenotype and cooperation, reflecting the modular organization and engagement of neuroendocrine tissues. Sex-steroid and neuropeptide signalling appeared to be important in mediating status-specific relationships between testosterone and cooperation, suggesting shared regulatory mechanisms with male aggressive and sexual behaviours. We also identified differentially regulated genes involved in cellular activity and synaptic potentiation, suggesting multiple mechanisms underpin these genomic states. Finally, we identified SDMN-wide gene expression differences between territorial and floater males that could form the basis of 'status-specific' neurophysiological phenotypes, potentially mediated by testosterone and growth hormone. Overall, our findings provide new, systems-level insights into the mechanisms of cooperative behaviour and suggest that differences in neurogenomic state are the basis for individual differences in social behaviour.

Systems biology as a framework to understand the physiological and endocrine bases of behavior and its evolution-From concepts to a case study in birds.

Organismal behavior, with its tremendous complexity and diversity, is generated by numerous physiological systems acting in coordination. Understanding how these systems evolve to support differences in behavior within and among species is a longstanding goal in biology that has captured the imagination of researchers who work on a multitude of taxa, including humans. Of particular importance are the physiological determinants of behavioral evolution, which are sometimes overlooked because we lack a robust conceptual framework to study mechanisms underlying adaptation and diversification of behavior. Here, we discuss a framework for such an analysis that applies a "systems view" to our understanding of behavioral control. This approach involves linking separate models that consider behavior and physiology as their own networks into a singular vertically integrated behavioral control system. In doing so, hormones commonly stand out as the links, or edges, among nodes within this system. To ground our discussion, we focus on studies of manakins (Pipridae), a family of Neotropical birds. These species have numerous physiological and endocrine specializations that support their elaborate reproductive displays. As a result, manakins provide a useful example to help imagine and visualize the way systems concepts can inform our appreciation of behavioral evolution. In particular, manakins help clarify how connectedness among physiological systems-which is maintained through endocrine signaling-potentiate and/or constrain the evolution of complex behavior to yield behavioral differences across taxa. Ultimately, we hope this review will continue to stimulate thought, discussion, and the emergence of research focused on integrated phenotypes in behavioral ecology and endocrinology.

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.

Testosterone-mediated behaviour shapes the emergent properties of social networks.

Social networks can vary in their organization and dynamics, with implications for ecological and evolutionary processes. Understanding the mechanisms that drive social network dynamics requires integrating individual-level biology with comparisons across multiple social networks. Testosterone is a key mediator of vertebrate social behaviour and can influence how individuals interact with social partners. Although the effects of testosterone on individual behaviour are well established, no study has examined whether hormone-mediated behaviour can scale up to shape the emergent properties of social networks. We investigated the relationship between testosterone and social network dynamics in the wire-tailed manakin, a lekking bird species in which male-male social interactions form complex social networks. We used an automated proximity system to longitudinally monitor several leks and we quantified the social network structure at each lek. Our analysis examines three emergent properties of the networks-social specialization (the extent to which a network is partitioned into exclusive partnerships), network stability (the overall persistence of partnerships through time) and behavioural assortment (the tendency for like to associate with like). All three properties are expected to promote the evolution of cooperation. As the predictor, we analysed the collective testosterone of males within each social network. Social networks that were composed of high-testosterone dominant males were less specialized, less stable and had more negative behavioural assortment, after accounting for other factors. These results support our main hypothesis that individual-level hormone physiology can predict group-level network dynamics. We also observed that larger leks with more interacting individuals had more positive behavioural assortment, suggesting that small groups may constrain the processes of homophily and behaviour-matching. Overall, these results provide evidence that hormone-mediated behaviour can shape the broader architecture of social groups. Groups with high average testosterone exhibit social network properties that are predicted to impede the evolution of cooperation.

Common Field Data Limitations Can Substantially Bias Sexual Selection Metrics.

Sexual selection studies widely estimate several metrics, but values may be inaccurate because standard field methods for studying wild populations produce limited data (e.g., incomplete sampling, inability to observe copulations directly). We compared four selection metrics (Bateman gradient, opportunity for sexual selection, opportunity for selection, and smax') estimated with simulated complete and simulated limited data for 15 socially monogamous songbird species with extrapair paternity (4%-54% extrapair offspring). Inferring copulation success from offspring parentage creates nonindependence between these variables and systematically underestimates copulation success. We found that this introduces substantial bias for the Bateman gradient, opportunity for sexual selection, and smax'. Notably, 47.5% of detected Bateman gradients were significantly positive for females, suggesting selection on females to copulate with multiple males, although the true Bateman gradient was zero. Bias generally increased with the extent of other sources of data limitations tested (nest predation, male infertility, and unsampled floater males). Incomplete offspring sampling introduced bias for all of the metrics except the Bateman gradient, while incomplete sampling of extrapair sires did not introduce additional bias when sires were a random subset of breeding males. Overall, our findings demonstrate how biases due to field data limitations can strongly impact the study of sexual selection.

Reciprocity and behavioral heterogeneity govern the stability of social networks.

The dynamics of social networks can determine the transmission of information, the spread of diseases, and the evolution of behavior. Despite this broad importance, a general framework for predicting social network stability has not been proposed. Here we present longitudinal data on the social dynamics of a cooperative bird species, the wire-tailed manakin, to evaluate the potential causes of temporal network stability. We find that when partners interact less frequently and when social connectedness increases, the network is subsequently less stable. Social connectivity was also negatively associated with the temporal persistence of coalition partnerships on an annual timescale. This negative association between connectivity and stability was surprising, especially given that individual manakins who were more connected also had more stable partnerships. This apparent paradox arises from a within-individual behavioral trade-off between partnership quantity and quality. Crucially, this trade-off is easily masked by behavioral variation among individuals. Using a simulation, we show that these results are explained by a simple model that combines among-individual behavioral heterogeneity and reciprocity within the network. As social networks become more connected, individuals face a trade-off between partnership quantity and maintenance. This model also demonstrates how among-individual behavioral heterogeneity, a ubiquitous feature of natural societies, can improve social stability. Together, these findings provide unifying principles that are expected to govern diverse social systems.

Testosterone Modulates Status-Specific Patterns of Cooperation in a Social Network.

Stable cooperation requires plasticity whereby individuals are able to express competitive or cooperative behaviors depending on social context. To date, however, the physiological mechanisms that underlie behavioral variation in cooperative systems are poorly understood. We studied hormone-mediated behavior in the wire-tailed manakin (Pipra filicauda), a gregarious songbird whose cooperative partnerships and competition for status are both crucial for fitness. We used automated telemetry to monitor >36,000 cooperative interactions among male manakins over three field seasons, and we examined how circulating testosterone affects cooperation using >500 hormone samples. Observational data show that in nonterritorial floater males, high testosterone is associated with increased cooperative behaviors and subsequent ascension to territorial status. In territory-holding males, however, both observational and experimental evidence demonstrate that high testosterone antagonizes cooperation. Moreover, circulating testosterone explains significant variation (2%-8%) in social behavior within each status class. Collectively, our findings show that the hormonal control of cooperation depends on a male's social status. We propose that the status-dependent reorganization of hormone-regulatory pathways can facilitate stable cooperative partnerships and thus provide direct fitness benefits for males.

Sampling baseline androgens in free-living passerines: Methodological considerations and solutions.

Obtaining baseline hormone samples can be challenging because circulating hormone levels often change rapidly due to the acute stress of capture. Although field protocols are established for accurately sampling baseline glucocorticoid concentrations, fewer studies have examined how common sampling techniques affect androgens levels. Indeed, many studies focused on understanding the functional significance of baseline androgen levels use sampling methods known to activate the endocrine responses to stress. To understand how different field sampling protocols affect plasma androgen levels, we measured the androgen response to two types of capture stressors in a free-living tropical bird, the wire-tailed manakin (Pipra filicauda). First, we subjected males to a standardized capture and restraint protocol lasting either 15 or 30 min. Second, males were passively captured in nets that were filmed (to establish exact duration of time between capture and blood sampling) and checked every 30 min. The first study showed that circulating plasma androgen levels decreased significantly following both 15 and 30 min of restraint in a cloth bag, with a trend for the 30 min samples to be lower than the 15 min samples. Further, the change in androgen levels was dependent on an individual's initial androgen levels, with the individuals with the highest initial levels registering the largest declines. The results of the second study suggest that hanging in a mist net for extended periods of time also leads to a decrease in circulating androgen levels, but this effect was weaker than that of capture and restraint in a cloth bag. Our findings demonstrate that, overall, circulating androgen levels decrease in response to common sampling techniques; a finding that has important implications for studies measuring baseline androgen levels in free-living birds. Future studies should prioritize sampling individuals immediately upon removal from the mist net, as handling and restraint have a strong negative effect on circulating androgen levels. When constant monitoring of the mist net is not possible, investigators should use video cameras to record the amount of time an individual spends in the net prior to blood sampling and then statistically control for the effect of this variable in analyses.

Dynamic network partnerships and social contagion drive cooperation.

Both reciprocity and positive assortment (like with like) are predicted to promote the evolution of cooperation, yet how partners influence each other's behaviour within dynamic networks is not well understood. One way to test this question is to partition phenotypic variation into differences among individuals in the expression of cooperative behaviour (the 'direct effect'), and plasticity within individuals in response to the social environment (the 'indirect effect'). A positive correlation between these two sources of variation, such that more cooperative individuals elicit others to cooperate, is predicted to facilitate social contagion and selection on cooperative behaviour. Testing this hypothesis is challenging, however, because it requires repeated measures of behaviour across a dynamic social landscape. Here, we use an automated data-logging system to quantify the behaviour of 179 wire-tailed manakins, birds that form cooperative male-male coalitions, and we use multiple-membership models to test the hypothesis that dynamic network partnerships shape within-individual variation in cooperative behaviour. Our results show strong positive correlations between a bird's own sociality and his estimated effect on his partners, consistent with the hypothesis that cooperation begets cooperation. These findings support the hypothesis that social contagion can facilitate selection for cooperative behaviour within social networks.

Connectivity of wood thrush breeding, wintering, and migration sites based on range-wide tracking.

Many migratory animals are experiencing rapid population declines, but migration data with the geographic scope and resolution to quantify the complex network of movements between breeding and nonbreeding regions are often lacking. Determining the most frequently used migration routes and nonbreeding regions for a species is critical for understanding population dynamics and making effective conservation decisions. We tracked the migration of individual Wood Thrushes (Hylocichla mustelina) (n = 102) from across their range with light-level geolocators and, for the first time, quantified migration routes and wintering regions for distinct breeding populations. We identified regional and species-level migratory connectivity networks for this declining songbird by combining our tracking results with range-wide breeding abundance estimates and forest cover data. More than 50% of the species occupied the eastern wintering range (Honduras to Costa Rica), a region that includes only one-third of all wintering habitat and that is undergoing intensive deforestation. We estimated that half of all Wood Thrushes in North America migrate south through Florida in fall, whereas in spring approximately 73% funnel northward through a narrow span along the central U.S. Gulf Coast (88-93°W). Identifying migratory networks is a critical step for conservation of songbirds and we demonstrated with Wood Thrushes how it can highlight conservation hotspots for regional populations and species as a whole.

The Problem with Paradigms: Bateman's Worldview as a Case Study.

Bateman's principles, their corollaries and predictions constitute a paradigm for the study of sexual selection theory, evolution of mating systems, parental investment theory, and sexual dimorphism in male and female behavior. Some aspects of this paradigm have been challenged in recent years, while others have been supported by empirical and theoretical research. We re-examine Bateman's 1948 paper in detail, including some methodological problems. Additionally, we review three areas in which an over-reliance on Bateman's predictions about sexual dynamics hindered our ability to understand the potential importance of certain behaviors: 1) male mate choice and sperm allocation; 2) the role of females in initiating and soliciting extra-pair copulations and fertilizations; and 3) the role of females in lekking systems, in which recent evidence suggests that copulations with multiple males (polyandrous behavior) may be common. We conclude this introduction to the symposium by emphasizing the heuristic value of Bateman's contributions, as well as the problems that arise when Bateman's paradigm is viewed through the lens of modern behavioral ecology and evolutionary theory.