Assortative mating by multiple skin color traits in a seabird with cryptic sexual dichromatism

Most seabirds are monochromatic in plumage, yet many of them show multiple colored integuments, typically modulated by sex steroids such as testosterone, which can reflect individual quality and may be under mutual sexual selection. In the Masked Booby Sula dactylatra, both sexes exhibit multiple color traits that vary in their expression either in color (feet ranging from olive to orange, yellowish bill and black mask) or size (mask). Here, we report sexual differences in skin color traits and evaluate whether their expression is related to individual body condition and plasma testosterone, whether different color traits co-vary, and whether Masked Boobies mate assortatively by color. Our results show that Masked Boobies are sexually dichromatic in foot color and monochromatic in bill and mask color. After controlling for body size and testosterone levels, mask size was larger in females than in males. We found a positive relationship between body condition and plasma testosterone in males and females, suggesting a condition-dependence of plasma androgen levels. Interestingly, foot color covaries positively with testosterone levels in individuals with good body condition and negatively in individuals with poor body condition, whereas mask size was positively related with body condition. Taken together, these results suggest that only individuals in good condition could produce more conspicuous traits. Also, we found a negative relationship between mask color and condition, probably due to the fact that melanin-based traits can pleiotropically vary with other functions, including food intake patterns. Finally, within-individual foot color and mask size were positively correlated and pairs mated assortatively with respect to foot color and mask color and size. We suggest that such a mating pattern may result from a scenario where both sexes choose mates based on traits that convey reliable information of individual quality.Research was founded by Consejo Nacional de Ciencia y Tecnología (CONACYT 129774), Universidad Nacional Autónoma de México (UNAM, PAPIIT-IN206713) and Posgrado en Ciencias Biológicas, Universidad Autónoma de Tlaxcala (UATx). I.L.R was supported by a postdoctoral fellowship from UNAM (DGAPA), and a postdoctoral fellowship from CONACYT at the UATx.Peer Reviewe