“Macroevolutionary Origin and Adaptive Function of a Polymorphic Female Signal Involved in Sexual Conflict” by Beatriz Willink, M. Catherine Duryea, and Erik Svensson published in the November 2019 issue of The American Naturalist is my choice for the 2020 ASN Presidential Award.
This exciting work captured my attention because the authors shine a spotlight on an aspect of intrasexual conflict that is rarely considered—the role of females traits. Exemplary of the best American Naturalist papers, the authors build their case with multiple lines of evidence: three years of field observations in 16 populations of damselflies in the genus Ishnura, a manipulative experiment altering the focal female trait, and phylogenetic comparative analyses across 41 species. In their study populations, males and female “male mimics” resemble each other in their abdominal color patterns. Interestingly, the bright blue abdominal patch of immature “heterochromic females” also mimics male patterns but its brightness decreases through development, appearing dark when these females reach sexual maturity. Since superfluous mating attempts are costly to females, female traits that reduce mating male attempts prior to female sexual maturity could increase female fitness. Willink and collaborators propose that the color mimicry expressed in immature heterochromic females could reduce the fitness cost of unsought mating attempts by making them less obvious to males. Field observations and results from an experiment in which the bright abdominal patch of heterochomic females was artificially darkened (or maintained as bright) clearly support the fitness costs to females of superfluous mating attempts. In addition, the comparative phylogenetic analysis shows that the developmental change in color pattern is found in other Ishnura species with heterochromic females, generalizing their result. Their findings support the idea that this female trait has evolved by exploiting Ishnura male bias against the blue abdominal patch of immature females. This paper underscores the importance of expanding the scope of sexual conflict studies beyond male signals to include the causes and consequences of the evolution of female signals. Well done!
Susan Kalisz, President of the American Society of Naturalists
p.s. It was a challenge to pick a single paper from of all those published in 2019. Am Nat continues to be an journal of outstanding importance and relevance to ecology and evolution with it high quality, high impact papers.