Causes and Consequences of a Sexually Selected Signal, the Chest Patch, in Male Geladas

Abstract

In species with intense male competition, selection favors conspicuous signals that allow rival males to display their strength or condition, reliably indicating some aspect of “quality” that predicts their ability to win a physical fight. These sexually selected conspicuous signals ensure only evenly matched males engage in fights which resolves conflicts at the lowest cost to both males as fighting is energetically costly and exposes both the winner and loser to injury and infection. In primates, conspicuous red skin signals occur in several male catarrhine species with a strong reproductive skew towards dominant males. Red skin has been associated with aspects of male competition such as rank or rates of aggression in these species. This research examines a putatively sexually selected trait used in rival assessment, the brilliant red chest patch in male geladas (Theropithecus gelada). Although previous research has established variation in chest redness across males, little is known about the mechanisms underlying chest redness or the functional consequences associated with a red chest patch. I aim to answer the following question: what are the causes and consequences of variation in the red chest patch for leader male geladas? To achieve this, I use long-term demographic, hormonal, photogrammetric, and climatic data accompanied by transcriptomic and photogrammetric data collected during annual capture-and-release events from a population of wild geladas living in the Simien Mountains National Park, Ethiopia. In Chapter 2, I first sought to identify the best method of objective color measurement for working with gelada chest skin redness. I compared the accuracy and consistency of multiple objective color measurement methods and selected the Adobe Photoshop method for all subsequent color measurement analyses based on the results. I then examined physical correlates of chest redness and found that redder chests were associated with increased physical activity and higher chest skin surface temperature. In Chapter 3, I tested whether male and female geladas exhibit differences in (1) chest redness under natural, free-ranging conditions and while under anesthesia during a capture-and-release program, and (2) chest skin gene expression patterns. While both males and females overlapped substantially in chest redness, males exhibited a wider within-individual range in chest redness under natural conditions. I detected sex differences in chest skin gene expression where higher expression in males was associated with angiogenesis, blood pressure, and blood vessel maintenance. Together, the results from Chapters 2 and 3 provide evidence that gelada chest redness is linked to increased blood flow near the surface of the skin, specifically through increased blood vessel branching in the chest skin. In Chapter 4, I investigated whether higher circulating androgens predict redder chests and found a weak positive relationship between chest redness and fecal androgen metabolites (fAMs) suggesting circulating androgens may work alongside a blood flow mechanism to increase chest redness. Lastly, I assessed whether chest redness predicts how long an adult male gelada is the dominant male in a reproductive unit. I found that leaders with redder chests trend towards longer tenures. This potentially suggests that a redder chest wards off rival males and thus indirectly contributes to reproductive success by allowing leaders a longer time to sire more offspring. Together, these chapters advance our understanding of a unique sexually selected signal of male competitive ability in a non-human primate.