Abstract
Animal signals are inherently complex phenotypes with many interacting parts combining to elicit responses from receivers. The pattern of interrelationships between signal components reflects the extent to which each component is expressed, and responds to selection, either in concert with or independently of others. Furthermore, many species have complex repertoires consisting of multiple signal types used in different contexts, and common morphological and physiological constraints may result in interrelationships extending across the multiple signals in species’ repertoires. The evolutionary significance of interrelationships between signal traits can be explored within the framework of phenotypic integration, which offers a suite of quantitative techniques to characterize complex phenotypes. In particular, these techniques allow for the assessment of modularity and integration, which describe, respectively, the extent to which sets of traits covary either independently or jointly. Although signal and repertoire complexity are thought to be major drivers of diversification and social evolution, few studies have explicitly measured the phenotypic integration of signals to investigate the evolution of diverse communication systems. We applied methods from phenotypic integration studies to quantify integration in the two primary vocalization types (advertisement and aggressive calls) in the treefrogs Hyla versicolor, Hyla cinerea, and Dendropsophus ebraccatus. We recorded male calls and calculated standardized phenotypic variance–covariance (P) matrices for characteristics within and across call types. We found significant integration across call types, but the strength of integration varied by species and corresponded with the acoustic similarity of the call types within each species. H. versicolor had the most modular advertisement and aggressive calls and the least acoustically similar call types. Additionally, P was robust to changing social competition levels in H. versicolor. Our findings suggest new directions in animal communication research in which the complex relationships among the traits of multiple signals are a key consideration for understanding signal evolution.
Highlights
Animal signals are composed of multiple components, and the relationship between these components determines the effectiveness of signals in eliciting responses from receivers (Bradbury & Vehrencamp, 2011)
Phenotypic integration and modularity are key considerations for the study of the evolution of complex phenotypic traits, including signals, because the response to selection on one of a set of correlated characteristics depends on the strength and direction of selection acting on the other characteristics (Lande & Arnold, 1983; Phillips & Arnold, 1989; Schluter, 1996), potentially leading on the one hand to trade-offs or expression of suboptimal phenotypes (Blows & Hoffmann, 2005; Kirkpatrick, 2009; Roff & Fairbairn, 2007) and on the other hand to rapid diversification in form when trait covariance facilitates the response to selection (Agrawal & Stinchcombe, 2009)
Our findings must remain tentative because of the small sample of study species, we describe an analytical framework that we encourage other researchers to apply to additional species to gain a greater understanding of the evolutionary significance of signal integration
Summary
Animal signals are composed of multiple components, and the relationship between these components determines the effectiveness of signals in eliciting responses from receivers (Bradbury & Vehrencamp, 2011). Phenotypic integration and modularity are key considerations for the study of the evolution of complex phenotypic traits, including signals, because the response to selection on one of a set of correlated characteristics depends on the strength and direction of selection acting on the other characteristics (Lande & Arnold, 1983; Phillips & Arnold, 1989; Schluter, 1996), potentially leading on the one hand to trade-offs or expression of suboptimal phenotypes (Blows & Hoffmann, 2005; Kirkpatrick, 2009; Roff & Fairbairn, 2007) and on the other hand to rapid diversification in form when trait covariance facilitates the response to selection (Agrawal & Stinchcombe, 2009). Many recent studies have uncovered evidence for complex multivariate selection on signal form (Blows, Brooks, & Kraft, 2003; Brooks et al, 2005; Gerhardt & Brooks, 2009; Oh & Shaw, 2013; Tanner, Ward, Shaw, & Bee, 2017); it is important to quantify the corresponding pattern of interrelationships among multiple component signal traits in order to understand how selection has shaped, and continues to act on, complex animal signals
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