Phylogenetic signal in the skull of cowbirds (Icteridae) assessed by multivariate and cladistic approaches

Abstract

Closely related species are expected to resemble each other because of inheritance of features from their common ancestors, which are therefore said to carry phylogenetic signal. Cowbirds (Molothrus) are icterids well known from being interspecific brood parasites, but their taxonomy and evolutionary relationships have varied considerably based on morphology alone. In turn, these relationships became unsupported by molecular phylogenies, lending lower value to morphological data in general. However, the osteology of cowbirds has not yet been studied under a quantitative phylogenetic framework and it is uncertain whether their skulls carry phylogenetic signal. In order to test this, we assembled a data matrix, including continuous and discrete characters of the skull of cowbirds and allies, and analyzed it with two complementary approaches under the evolutionary framework provided by molecular phylogenies. We first took a multivariate approach, exploring the occupation of a phylomorphospace based on skull data and estimating the amount and significance of phylogenetic signal by calculation of the Kmult statistic. The second approach relied on a Maximum Parsimony optimization of characters on a scaffold tree. Our results indicate that, although some homoplasy exists, the skull of cowbirds and allies carries significant phylogenetic signal and provides useful characters to diagnose Molothrus and other still poorly diagnosed clades. This first comparative approach is promising and opens the possibility of integrating data from fossils, while encouraging further osteological analyses in cowbirds and other icterids.