Abstract
The evolution of elaborate secondary sexual traits (i.e., ornaments) is well‐studied in males but less so in females. Similarity in the appearance of ornaments between males and females supports the view that female ornaments arise as a neutral byproduct of selection on male traits due to genetic correlation between sexes, but recent research suggests an adaptive function of female ornaments in at least some contexts. Information on the degree to which production of ornaments differs between the sexes can shed light on these alternative perspectives. We therefore characterized the structural underpinnings of melanin‐based plumage production in males and females of two closely related passerine bird species (genus Malurus). Importantly, both ornamented and unornamented phenotypes in each sex are present between these two species, providing an opportunity to test the null expectation of equivalent modes of production in male and female ornamented phenotypes. In Malurus alboscapulatus, ornamented females are qualitatively similar to males, but we describe a distinctive ornamented female phenotype that differs from that of males in lacking a blue sheen and in lower feather barbule density. In M. melanocephalus, unornamented males and females are also similar in appearance, and we describe a similarity between unornamented phenotypes of males and females in both color and underlying feather barbule structure and pigment composition. Unornamented male M. melanocephalus can flexibly transition to the ornamented phenotype in weeks, and we found extreme differences in color and feather structure between these two alternative male phenotypes. These results contradict the idea that female ornaments have evolved in this system following a simple switch to male‐like plumage by demonstrating greater complexity in the production of the ornamented phenotype in males than in females.
Highlights
There is growing awareness that a comprehensive understanding of sexual selection depends upon better understanding the female perspective as well as that of males, in part because the selective pressures experienced by females may differ from those experienced by males (Amundsen, 2000; Clutton-Brock, 2007; Rosvall, 2011; Tobias, Montgomerie, & Lyon, 2012)
In cross section, barbules in ornamented males had a higher density of melanosomes, a thick outer melanosome layer, and thinner keratin layer compared to both unornamented males and females
Given the overall similarity in both color and feather structure among unornamented male Red-backed Fairywren (RBFW) and unornamented female RBFW, we propose that genetic correlation between male and female traits likely plays an important role in determining these characteristics
Summary
There is growing awareness that a comprehensive understanding of sexual selection depends upon better understanding the female perspective as well as that of males, in part because the selective pressures experienced by females may differ from those experienced by males (Amundsen, 2000; Clutton-Brock, 2007; Rosvall, 2011; Tobias, Montgomerie, & Lyon, 2012). Careful examination has revealed differences between the sexes in subtle features of color (e.g., in colors in the ultraviolet range; Hunt, Bennett, Cuthill, & Griffiths, 1998) and structural components (Shawkey, Estes, Siefferman, & Hill, 2005) that may imply sex-specific selection pressures (Heinsohn, 2005) Such studies remain relatively rare, and a better understanding of the proximate sources of color variation can provide important insights into how selection acts on male and female ornaments (Gluckman, 2014; Maia, Rubenstein, & Shawkey, 2013). These findings demonstrate that the mechanisms of ornament production are similar among males of distinct species, but differ among males and females of a single species These findings suggest that that the flexible transition when molting from unornamented to ornamented male phenotypes is associated with substantial structural changes
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