Haley L. Kenyon and Paul R. Martin
Due to the costly nature of physical aggression (injury or death), interaction between members of the same species over resources (intraspecific competition) makes heavy use of color to signal dominance. It is unclear, however, if this also occurs between species competing over resources (interspecific competition). Dr. Kenyon and Dr. Martin sought to address this gap in our understanding by examining the relationship between coloration and dominance status among closely related bird species.
They hypothesized that differences in color were indicators of dominance among closely related bird species engaging in aggressive interactions over resources. To test this, the authors used scientific illustrations and written descriptions to measure the proportions of black, white, and carotenoid colors in areas associated with aggressive signaling (face, throat, and bill) and overall appearance for 445 species from 178 genera, examining whether the appearances of dominant species have more of these focal colors. Differences between dominant and subordinate species in the amounts of these colors were considered along with mass difference, evolutionary distance, and range overlap because each factor was expected to impact between-species signaling.
Among closely related species of birds, dominant species tend to have more black coloration in key regions commonly used for aggressive signaling compared to subordinate species. This association between dominance and black coloration is consistent across different bird lineages and may be especially important for species without a size advantage to signal dominance. However, the association between dominance and white or carotenoid colors was less consistent across bird species, suggesting that different color signals may vary in their association with dominance depending on the specific lineage of birds. Factors such as size differences and coloration percentage between species were found to influence the importance of focal colors for broadcasting between-species dominance. For instance, the presence of more white coloration may signal dominance when paired with differences in black coloration, while less white coloration may signal subordinance when paired with differences in carotenoid coloration.
Dr. Kenyon and Dr. Martin’s work highlights how color-based dominance signals may play a crucial role in mediating interspecific aggressive interactions and promoting coexistence among closely related species. Asking this question of other taxa will enable researchers to understand how color signaling influences between-species aggression.
