Evolution of correlated complexity in the radically different courtship signals of birds-of-paradise
[['Adaptive radiations are driven by ecological differences that promote processes of diversification and speciation [1] .', 'In contrast , phenotypic radiations , which occur in the absence of clear ecological differentiation , are less well understood .', 'One commonly investigated mechanism for phenotypic diversification among ecologically similar taxa is variation in social and sexual selection pressures promoting signal or ornament diversification .', 'Ornamental radiations may come about as a consequence of variation in signaling environment [2 , 3] , sensory capabilities [4 , 5] , or pseudorandomly via mutation-order selection [6 , 7] or Fisher-Lande-Kirkpatrick processes [8–11] .', 'Most studies investigating patterns of ornamental diversification have focused on individual trait classes and simplified axes of variation; however , sexual selection does not act on single traits in isolation .', 'A more complete understanding of the processes driving ornamental diversification is possible only by investigating evolutionary relationships between the full suites of ornamental traits under selection .', 'Many animals rely on multiple ornamental traits to attract mates .', 'Advantages of multiple ornaments may include increased information transfer ( multiple messages ) , increased reliability ( redundancy ) , increased flexibility ( ensuring information transfer across contexts and environments ) , and increased memorability/discriminability [12–16] .', 'Multiple ornaments may be more common when costs associated with the display or evaluation of those ornaments are low [17] , as is likely the case in lekking species [12] .', 'Though we now have broad empirical support for many of the proposed adaptive benefits of multiple signals at the level of individual species , how these specific hypotheses map onto our understanding of phylogenetic patterns of ornament evolution is less clear .', 'Gaining insights into the macroevolutionary patterns of multiple ornament evolution is challenging , in part , owing to the difficulties of comparing highly divergent phenotypic traits across species .', 'For instance , even focusing on evolutionary patterns of a single trait ( e . g . , plumage color in birds ) across species can be difficult when traits possess different axes of variation ( e . g . , red versus blue ) .', 'Though ingenious new methods have been devised to compare highly divergent ornaments of a single signal type ( e . g . , plumage color [18] , electrical signals [19] , or song [20] ) , comparing ornamental complexity across signal types presents yet an additional layer of complication .', 'However , understanding the interrelationships of different classes of ornaments across phylogenetic scales can potentially provide valuable information about the evolutionary processes of communication , phenotypic radiation , and speciation that cannot be gathered from single-trait or single-species studies .', 'Following the evolution of multiple ornaments , selective pressures may favor different interrelationships among signal types .', 'If ornamental investment is governed by evolutionary tradeoffs , investment in one class of ornaments will come only at the expense of investment in another .', 'Evidence suggests that signal tradeoffs manifest as a negative correlation among ornament types across evolutionary time [21–27] , reflecting strong , consistent constraints imposed by ecology , physiology , and natural selection [28 , 29] .', 'Alternatively , instances in which ornamental traits show no evolutionary relationships [30–35] suggest long-term patterns of independent evolutionary trajectories .', 'In such cases , signals are functionally independent and may even have evolved for use in different contexts ( e . g . , territorial defense versus mate attraction ) .', 'When might we expect positive correlations among ornament classes across species ?', 'Theoretical [12 , 36] and empirical [37 , 38] work suggests that positive correlations among signals across species may reflect consistent selection acting similarly on separate axes of ornamental evolution .', 'Strong , consistent intersexual selection could generate these positive correlations ( sensu [37] ) , especially if the signals convey separate information [36] , resulting in functional integration among ornament elements [39 , 40] .', 'In such cases , positive correlations among signals across species would arise when selection favors an “integrated whole” of ornamental traits [41 , 42] , which we call the “courtship phenotype . ”', 'The courtship phenotype is the composite expression of all ornamental classes evaluated during courtship and may represent the composite target of selection .', 'Evolution may favor integrated , holistic mate evaluation strategies because of advantages that sensory overlap and redundancy offer ( e . g . , increased accuracy ) [12–16] .', 'Here , we examine broad evolutionary patterns of ornamental signal investment and complexity across the wildly diverse [43 , 44] , monophyletic [45] birds-of-paradise ( Paradisaeidae ) “…in which the process of sexual selection has gone to fantastic extremes” [46] ( Fig 1A ) .', 'We focus on the birds-of-paradise because this family exhibits extreme variation across species in multiple ornamental axes [43] ( e . g . , color [47–49] and behavior [50 , 51] ) while possessing broadly similar life histories and mating systems [43 , 44] .', 'Consequently , insights about the strength , direction , and diversification of ornamental phenotypes in this group may shed light on key processes of sexual selection and its power to generate phenotypic radiation when natural-selection–imposed constraints are minimized .', 'In this study , we use a unique natural history dataset to quantitatively evaluate behavioral , acoustic , and colorimetric ornamentation across 40 species of birds-of-paradise , as well as relationships between signals and display environment .'], ['Comparisons across signal types are inherently challenging for evolutionary biologists given that such signals are necessarily measured in different ways .', 'Additionally , comparisons within color , acoustic , and behavioral repertoires across taxa that vary widely ( e . g . , the birds-of-paradise ) present an additional methodological challenge: how does one compare phenotypes that may share no obvious overlapping characters ?', 'We addressed this obstacle with a two-pronged approach to quantify ornamental complexity for behavior , color , and sounds in the birds-of-paradise .', 'First , we broke down each ornament into a taxonomically unbounded character space that allowed classification of subunits across all species .', 'Second , we used the specific attributes of a given ornament for each individual for each species to categorize the ornament components before quantifying two conceptually aligned measures of complexity for each signal type .', 'Specifically , we evaluated richness ( the number of unique elements ) and diversity ( using an index dependent on the number and relative contribution of each element type ) using phylogenetic comparative approaches ( see Methods for additional details ) .', 'For behavioral analyses , we first broke down the courtship behaviors of all species into distinct subunits shared across species ( e . g . , S1 Video ) .', 'We then analyzed composite behavioral sequences across time using sliding-window analyses to compare maximally diverse behavioral repertoires for a set duration across species ( Fig 1B ) .', 'For colorimetric analyses , we relied on visual modeling of multispectral images to quantify the number and relative abundances of perceptually distinct color types across individuals and species .', 'Though different colors may have different underlying production mechanisms , our analyses simply focused on the number and distribution of distinguishable colors ( Fig 1C ) .', 'Similar to our behavioral analysis pipeline , we used acoustic properties and agglomerative clustering to classify distinct sound types used by birds-of-paradise in courtship contexts before employing a similar sliding-window analysis to identify maximally diverse acoustic sequences , facilitating comparisons across species ( Fig 1D ) .', 'In total , we analyzed 961 video clips , 176 audio clips , and 393 museum specimens .', 'From these analyses , we obtained quantitative diversity and richness metrics of ornamental complexity across the birds-of-paradise ( Fig 2 ) , which allowed us to rigorously evaluate patterns of correlated character evolution , as well as facilitating our investigation of the influence of breeding system and display environment on ornamental complexity .', 'Using multiple phylogenetic generalized least squares ( mPGLS ) analyses , which allowed us to control for the nonindependence of species due to their shared evolutionary history , as well as the potentially confounding influences of display environment ( both display height and proximity to courting conspecific males ) , we uncovered positive correlations between color and acoustic diversity ( Fig 3A ) , as well as between behavioral and acoustic diversity ( Fig 3B ) , consistent with the hypothesis that selection has acted similarly on these axes of ornamental complexity .', 'Interestingly , however , there was no significant relationship between color diversity and behavioral diversity , indicating independent evolutionary trajectories for these visually encoded aspects of courtship ornamentation ( S1 Table and S1 Fig ) .', 'Analyses of ornamental richness revealed the same pattern to those uncovered for ornamental diversity ( S2 Table ) .', 'Specifically , behavioral richness and acoustic richness were correlated , as were color richness and acoustic richness ( as was the case for both relationships involving ornamental diversity ) .', 'Behavior richness and acoustic richness , but not color richness , were influenced by stratum of the forest in which species display ( Fig 4A–4C ) .', 'Specifically , we found that behavioral richness exhibited a negative relationship with display height among birds-of-paradise , such that species that display on the forest floor had the largest behavioral repertoires ( S2 Table and Fig 4B ) .', 'Species that display on the forest floor are typically operating with lower-light environments , and consequently , these species appear to rely more heavily on complex dance sequences to attract mates .', 'Additionally , birds-of-paradise show increased acoustic ( Fig 4C ) richness as their display locations increase in height ( S2 Table ) , a result that partially corresponds to the predictions of sensory drive [52 , 53] whereby the openness of the upper canopy favors increasingly complex acoustic displays .', 'Similar to the patterns we uncovered for signal richness , we also found that behavioral diversity and acoustic diversity were influenced by display height ( S1 Table ) .', 'Species displaying in the forest understory exhibiting a marginally significant ( p = 0 . 051 ) trend for greater acoustic diversity relative to ground displaying species , and the behavioral diversity for ground-displaying species was higher than for both understory and canopy species ( S1 Table ) .', 'However , color diversity was not significantly influenced by display height .', 'Birds-of-paradise that display in classic leks have greater color richness ( Fig 4D and S2 Table ) , corresponding to the increased strength of sexual selection on males to “stand out” visually when being evaluated simultaneously in lekking contexts .', 'However , neither behavioral nor acoustic richness were significantly associated with the spatial distribution of displaying males .', 'Furthermore , none of the diversity metrics ( color , behavior , sound ) were significantly associated with the breeding system structure ( S1 Table ) .'], ['Our study provides evidence that selection has favored correlated levels of ornamental diversity across multiple signals among the birds-of-paradise .', 'This pattern of positive correlation among distinct ornament classes across evolutionary timescales and species suggests strong sexual selection on functionally integrated courtship phenotypes .', 'The degree to which phenotypic traits are coexpressed and functionally dependent upon one another can be referred to as functional integration [54] or interdependence [40] .', 'Courtship phenotypes with greater functional integration are therefore composed of ornaments that are typically expressed at similar levels and that are mutually interdependent in order to influence mate choice [39 , 41 , 42] .', 'Correlations among the signals that comprise the courtship phenotype also suggest a previously undescribed robustness in bird-of-paradise courtship phenotypes that may have played a key role in the extreme ornamental radiation exhibited by this taxon ( Fig 1 ) .', 'Evolutionary biologists dating back to Mayr [55] and even Darwin [56] have recognized the potential evolutionary implications of functional redundancy ( two or more structures performing the same function ) .', 'Functional redundancy , including “true” redundancy ( i . e . , structurally identical components with identical functions ) and degeneracy ( i . e . , structurally distinct components with similar functions ) [57] , facilitates evolutionary innovation ( i . e . , increases “evolvability” ) by increasing robustness .', 'Robust systems are those in which the overall structure and interconnectedness of parts provide protection from environmental or mutational instability [58] such that a given function is not lost if a single component fails .', 'Robustness increases evolvability by enabling elements to react to selection independently and diverge while maintaining original functions [57 , 59] .', 'All redundancy ( both “true” redundancy and degeneracy ) provides a measure of robustness , but robust systems are not necessarily redundant [60] .', 'Given the broad theoretical [59 , 61 , 62] and empirical [63–65] support for the idea that robustness can promote evolvability across a wide array of biological domains , we posit that the correlations among signal types within birds-of-paradise courtship phenotypes are at least partially responsible for the dramatic diversification and radiation of courtship signals displayed by birds-of-paradise .', 'If female birds-of-paradise make mate choice decisions based on sensory input from the multiple signals that comprise a composite courtship phenotype and information from those channels is correlated , then novel mutations changing the structure or form of a given ornament may occur without “necessary” information being lost [66] .', 'Consequently , over evolutionary time , we suggest it is the inherent functional overlap ( redundancy/degeneracy ) and structural interdependency ( robustness ) of courtship phenotypes that leads to increased phenotypic diversification ( evolvability ) in birds-of-paradise .', 'Phenotypic radiations in the absence of clear ecological differentiation may arise stochastically [67 , 68] and be heavily influenced by the specific intricacies of female choice [7 , 10 , 69 , 70] .', 'Birds-of-paradise clearly exhibit some ecological differentiation [43] , but broadly speaking , they tend to be heavily frugivorous and predominantly polygynous [71] .', 'They do not , however , all display to potential mates in the same contexts or microenvironments .', 'Some species display high in the canopy , some down on the forest floor , and others in the understory in between .', 'Likewise , some species display in large , cacophonous leks , some species in exploded leks ( exp leks ) in which males can hear but not see one another , and other species display solitarily .', 'Our results suggest that these differences have shaped the specific courtship and signaling strategies of each species ( Fig 4 and S1 and S2 Tables ) .', 'Birds with richer acoustic repertoires display high in the canopy , where there is less environmental interference ( e . g . , from cluttered branches ) , increasing the likelihood that females will be able to detect and discern numerous , elaborate sounds [53] .', 'Likewise , more behaviorally complex birds tend to display near the forest floor where there is less light ( and ability to perceive subtle variation in color ) but more area available for a courtship stage or “dance floor . ”', 'Birds that display in true leks have more colorful plumage , perhaps because females need to identify attractive individuals based on relatively unchanging traits , allowing them to compare among multiple displaying males simultaneously .', 'Display site and display context thus influence the specific forms of ornamentation possessed by individual species [72] , and taking them into account from an analytical perspective allows us to better understand patterns of signal coevolution and the potential importance of a functionally integrated courtship phenotype .', 'Signal efficacy and information content can exert strong influence on receiver preferences , and understanding both elements is integral when examining the evolution of complex , multicomponent courtship phenotypes [14 , 70 , 73 , 74] .', 'The influence of receiver preference is difficult to overstate , particularly in birds-of-paradise , for which recent work indicates that selection acting on female preferences controls the rate , extent , and phenotypic space available for ornamental radiations [70] .', 'Importantly , receiver preferences are influenced by the perceptual abilities [75 , 76] and psychology of signal receivers [77 , 78] , as well as the environments through which signals are transmitted [52]—all of which can markedly influence signal efficacy .', 'Additionally , the information content of multiple signals may increase the net amount of information transferred ( e . g . , multiple messages [16] ) or increase accuracy and reliability if multiple signals communicate the same message ( e . g . , redundant signals [12 , 16] ) .', 'The perceptual channels by which birds-of-paradise attract mates and those channels that are correlated at a phylogenetic scale provide tantalizing , though tentative , insights into the processes of efficient information transfer and receiver stimulation regulating mate choice in this group .', 'Specifically , the fact that significant positive correlations exist between acoustic and color signals ( auditory , visual ) , and between acoustic and behavioral signals ( auditory , visual ) , but not between color and behavioral signals ( visual , visual ) aligns with psychometric literature on information and sensory input .', 'When multiple sources of information are provided , information may be maximized if that information comes from separate channels ( e . g . , acoustic , visual ) and lost when arriving through a single sensory channel [79] ( but see [80] ) .', 'What exactly this “information” might be in birds-of-paradise ( quality [81] , attractiveness [69] , motivation [82] , etc . ) is not clear , but this result provides an interesting starting point for future investigations .', 'Phylogenetic comparative investigations of animal signals hold the potential to answer important questions about the evolutionary trajectories of communication over time [83 , 84] .', 'However , the data used to tackle key questions of signal evolution necessarily place upper and lower bounds on the confidence and interpretations one can make from such comparative studies .', 'It is our hope that the novel approaches we have developed to quantify color , sound , and behavior will be useful to other researchers interested in understanding signal variation at different scales .', 'Though our primary aim was to generate methodological pipelines that facilitated comparisons among the highly divergent birds-of-paradise , the basic framework we describe here may also be useful for comparisons of more similar taxa—including studies of intraspecific variation in signaling effort ( e . g . , through sliding-window analyses focused on bouts of maximal complexity ) or investment ( e . g . , by using receiver visual models to identify the number and perceptual similarity of color patches across individuals ) .', 'Consequently , we feel that our approaches complement recent suggestions for incorporating a systems biology approach to the study of animal communication [57] , wherein more comprehensive , higher-resolution data will only improve the validity and interpretability of analyses incorporating fitness surfaces and communication networks .', 'Evolutionary tradeoffs—increases in trait expression linked to reductions in another—are ubiquitous: “If there were no tradeoffs , then selection would drive all traits correlated with fitness to limits imposed by history and design” [85] .', 'Tradeoff thinking can inform our interpretations of both the marked interspecific variation in overall signal complexity ( Fig 2 ) and the finding that the ornaments of birds-of-paradise are positively correlated at phylogenetic scale ( Fig 3 ) .', 'Firstly , interspecific variation in overall signal complexity suggests tradeoffs between investment in courtship and some other , unmeasured , variable that differs across species ( e . g . , microenvironment , paternal care , resource competition , etc . ) .', 'Secondly , the absence of tradeoffs among signal types indicates an absence of differential costs on acoustic , behavioral , and chromatic signals .', 'Further , the correlation among ornamental classes suggests that selection is acting on functionally integrated courtship phenotypes for birds-of-paradise , a finding that indicates female birds-of-paradise make mate choice decisions incorporating holistic , multicomponent information sets comprised of the various ornaments possessed by males of their species .', 'Rather than being unique to birds-of-paradise , however , we suggest that this phenomenon is widespread among animals—though it is by varying degrees constrained , impeded , or obfuscated by conflicting and constraining processes and limitations imposed by ecology and natural selection .', 'The degree to which selection has facilitated the evolution of integrated , robust courtship phenotypes may in fact serve as a proxy for the overall strength and consistency of female-driven sexual selection in any taxa , for which the integration and correlation among ornaments comprising the courtship phenotype may shed important light on the history and strength of sexual selection in that particular group .'], ['The study was focused on vertebrates ( birds-of-paradise ) but used museum specimens ( physical and media ) , so no IACUC protocol was required .', 'We quantified the behavioral complexity of courtship display behaviors for the birds-of-paradise by scoring field-recorded video clips of 32 ( 80% ) paradisaeid species , primarily from the Macaulay Library at the Cornell Lab of Ornithology ( macaulaylibrary . org , S3 Table ) .', 'In total , we watched 961 clips from 122 individuals totaling 47 , 707 . 2 s ( approximately 795 . 12 min; mean clip duration = 49 . 64 s ) .', 'Courtship display behavior is highly variable among bird-of-paradise species , necessitating broad behavioral categories to facilitate investigations of behavioral evolution .', 'Specifically , one of us ( CDD ) blindly evaluated video clips of male birds-of-paradise displaying species-typical courtship behaviors [43] using a customized ethogram of behavioral units that enabled us to quantify all state and event behaviors exhibited by all species of Paradisaeidae ( S4 Table ) .', 'As with display behaviors , we quantified the acoustic complexity of courtship sounds produced by analyzing field-recorded audio/video clips of 32 ( 80% ) bird-of-paradise species .', 'In total , we analyzed sound from 176 clips from 59 individuals totaling 24 , 670 . 9 s ( approximately 411 min; mean clip duration = 140 . 18 s; S8 Table ) .', 'Though birds can generate sounds ( both vocally and mechanically ) in numerous contexts , we focused our analysis on recordings from known display sites or those matching written descriptions of courtship sound production [43] .']]
['Introduction', 'Results', 'Discussion', 'Methods']
['Ornaments used in courtship often vary wildly among species , reflecting the evolutionary interplay between mate preference functions and the constraints imposed by natural selection .', 'Consequently , understanding the evolutionary dynamics responsible for ornament diversification has been a longstanding challenge in evolutionary biology .', 'However , comparing radically different ornaments across species , as well as different classes of ornaments within species , is a profound challenge to understanding diversification of sexual signals .', 'Using novel methods and a unique natural history dataset , we explore evolutionary patterns of ornament evolution in a group—the birds-of-paradise—exhibiting dramatic phenotypic diversification widely assumed to be driven by sexual selection .', 'Rather than the tradeoff between ornament types originally envisioned by Darwin and Wallace , we found positive correlations among cross-modal ( visual/acoustic ) signals indicating functional integration of ornamental traits into a composite unit—the “courtship phenotype . ”', 'Furthermore , given the broad theoretical and empirical support for the idea that systemic robustness—functional overlap and interdependency—promotes evolutionary innovation , we posit that birds-of-paradise have radiated extensively through ornamental phenotype space as a consequence of the robustness in the courtship phenotype that we document at a phylogenetic scale .', 'We suggest that the degree of robustness in courtship phenotypes among taxa can provide new insights into the relative influence of sexual and natural selection on phenotypic radiations .']
['Animals frequently vary widely in ornamentation , even among closely related species .', 'Understanding the patterns that underlie this variation is a significant challenge , requiring comparisons among drastically different traits—like comparing apples to oranges .', 'Here , we use novel analytical approaches to quantify variation in ornamental diversity and richness across the wildly divergent birds-of-paradise , a textbook example of how sexual selection can profoundly shape organismal phenotypes .', 'We find that color and acoustic complexity , along with behavior and acoustic complexity , are positively correlated across evolutionary timescales .', 'Positive links among ornament classes suggests that selection is acting on correlated suites of traits—a composite courtship phenotype—and this integration may be partially responsible for the extreme variation in signal form that we see in birds-of-paradise .']
['taxonomy', 'organismal evolution', 'acoustics', 'ecology and environmental sciences', 'social sciences', 'vertebrates', 'animals', 'animal phylogenetics', 'phylogenetics', 'data management', 'animal behavior', 'zoology', 'ecological metrics', 'computer and information sciences', 'animal communication', 'birds', 'behavior', 'bioacoustics', 'evolutionary systematics', 'species diversity', 'physics', 'psychology', 'eukaryota', 'ecology', 'animal evolution', 'biology and life sciences', 'physical sciences', 'evolutionary biology', 'amniotes', 'organisms']