Evolution of color production mechanisms

I am using the incredible diversity of mechanisms that produce animal coloration to understand the evolution of signal production. Through a multi-university collaboration, I was able to help uncover how carotenoid-based coloration (e.g. typically the reds and yellows in birds) evolved through an ordered evolutionary pathway of carotenoid modification and deposition into bird feathers in Fringillid finches (Ligon, Simpson, et al. 2015). Now, I am using scanning and transmission electron microscopy to measure the surface and internal structures of hummingbird feathers and test how these feather structures vary among species, predict variation in feather color, and possibly co-evolved with courtship behaviors.

My other research themes:
1. Sensory and evolutionary ecology of animal signals and their diversity
2. Mechanisms and evolution of signal interactions

Behavioral and environmental modulation of signals

While communicating, animals can use behaviors and the environment to modulate their signals, either to increase their effectiveness or potentially create unique signal properties. For example, during hummingbird courtship a male’s iridescent throat plumage is oriented and positioned towards females and the sun in specific ways through their courtship dances to alter how their plumage appears throughout their dance. Thus, the male’s behavior and signaling environment modulate the appearance of that male’s colorful plumage to the female creating a unique signal property (color appearance) not possible outside of this context. Using six North American hummingbird species as my study system, my research in this theme focuses on understanding the mechanics of behavioral and environmental signal modulation, how signal modulation varies among species, and how male plumage, signal environment, courtship dances, and their interactions co-evolved.

Through a novel display re-creation method, I developed, where I mapped the orientation-and-positional movements of video-recorded hummingbird courtship dances combined with full-spectrum photography (Simpson & McGraw 2018a), I quantified male color appearance, thus directly measuring how males modulate their signals with behavior and the environment. I discovered that male color appearance is not solely tied to the color of his ornaments (brighter feathers does not mean brighter appearance), due to behavioral modulations of appearance (Simpson & McGraw 2018b, Simpson & McGraw 2019a). These findings demonstrates the need to break with traditional, static-snapshot color measurements and instead study animal coloration as a dynamic trait or behavior, while also illustrating the importance of signal modulation in communication. Among species, I found that color appearance evolved through two divergent evolutionary pathways alongside exaggeration in plumage or behavioral displays (Simpson & McGraw 2019b).

I am continuing to study behavioral modulation of signals in Peruvian hummingbird species, to test if other hummingbird lineages use similar or unique behaviors and environments to modulate their plumage in completely different geographic locations.

My other research themes:
1. Drivers of signal evolution and diversity
2. Evolution of signal production mechanisms

Drivers of Signal Evolution and Diversity

I am studying how the natural history and environment shape the evolution and diversity of animal signals. The bulk of my work in this theme has focused on color evolution in wood warblers (Family: Parulidae). Wood warblers vary considerably in song and color among species, occur throughout a wide variety of habitats (such as pine forests, swamps, and meadows), and differ in multiple ecological traits, such as migratory behavior and species range overlap. I previously found that evolutionary changes in migratory distance were driving the evolution of sexual differences in coloration in this group likely due to losses of female coloration, which is contrary to previous ideas on how sexual color differences were primarily driven by changes in male coloration (Simpson et al. 2015). I also uncovered how both natural and sexual selection act together on different aspects of warbler plumage coloration to shape color diversity in both males and females (Simpson et al. 2020). Currently, I am studying how species range overlap influences signal divergence between species in both song and plumage coloration.

My other research themes:
1. Behavioral and environmental modulation of signals
2. Evolution of signal production mechanisms