Signal detection shapes ornament allometry in functionally convergent Caribbean Anolis and Southeast Asian Draco lizards

Abstract

Visual ornaments have long been assumed to evolve hyper‐allometry as an outcome of sexual selection. Yet growing evidence suggests many sexually selected morphologies can exhibit other scaling patterns with body size, including hypo‐allometry. The large conspicuous throat fan, or dewlap, of arboreal Caribbean Anolis lizards was one ornament previously thought to conform to the classical expectation of hyper‐allometry. We re‐evaluated this classic example alongside a second arboreal group of lizards that has also independently evolved a functionally equivalent dewlap, the Southeast Asian Draco lizards. Across multiple closely related species in both genera, the Anolis and Draco dewlaps were either isometric or had hypo‐allometric scaling patterns. In the case of the Anolis dewlap, variation in dewlap allometry was predicted by the distance of conspecifics and the light environment in which the dewlap was typically viewed. Signal efficacy, therefore, appears to have driven the evolution of hypo‐allometry in what was originally thought to be a sexually selected ornament with hyper‐allometry. Our findings suggest that other elaborate morphological structures used in social communication might similarly exhibit isometric or hypo‐allometric scaling patterns because of environmental constraints on signal detection.

Signal size allometry inAnolislizard dewlaps

“…A recent study of dewlap size allometry in Caribbean anoles reported surprising evidence for hypo -allometry, i.e. dewlap size was proportionately smaller with increasing body size [14]. This result was especially surprising as it was counter to all other studies (e.g.…”

“…[11][12][13]), including our results here. The paper introduced a new method based on measurements taken from videos of lizards displaying in nature [14]. Though an innovative and potentially important method, when we examined the underlying data associated with the paper, we found that the dewlap size measurements are vastly larger than our measurements of individuals of the same species (see electronic supplementary material).…”

“…Specifically, allometric slopes ( b ) can be described using the equation Y = aX b , where Y in our study represents dewlap size and X represents body size. Dewlap size (area) was first linearized with respect to body size (SVL) by square-root transformation and then both variables were natural log transformed [5,14]. Ordinary least squares regressions were used to determine allometric scaling relationships [5,14].…”

“…Dewlap size (area) was first linearized with respect to body size (SVL) by square-root transformation and then both variables were natural log transformed [5,14]. Ordinary least squares regressions were used to determine allometric scaling relationships [5,14]. Isometry ( b = 1) describes a relationship where relative trait size is constant with increasing body size.…”

“…Co-occurring species of different ecomorph classes rarely have dewlaps comprised of similar colours or patterns [14], which suggests a role for dewlaps as signals for species recognition ( [17][18][19], but see [20]). However, members of the same ecomorph class, though convergent in ecology, morphology and behaviour, rarely are convergent in dewlap phenotype [14,20]. Comparative studies of dewlap size allometry, among either convergent species of the same ecomorph class or co-occurring species of different ecomorph classes, have been surprisingly overlooked.…”