Divergent competitive phenotypes between females of two sex-role-reversed species

Abstract

Abstract: Divergent phenotypes between lineages in the early stages of speciation can promote or impede reproductive isolation. Although divergence in male competitive morphology and behavior has been explored for many hybridizing lineages, it is less known how divergence between females influences hybridization. Here, I compare competitive phenotypes between females of two hybridizing, sex-role-reversed jacana species in Panama. Previous work suggests Jacana spinosa females monopolize mating in the hybrid zone, potentially through a competitive advantage. I tested whether J. spinosa females have a more competitive phenotype than Jacana jacana females. I compared morphological traits related to territoriality and measured aggressive behavior using territorial intrusion simulations: the first aggression assay in a shorebird. I also quantified these traits in males, to confirm previous studies reporting males as smaller and less aggressive than females in both species. As predicted, J. spinosa females had larger body mass and longer wing spurs than J. jacana females. J. spinosa females were also more aggressive than J. jacana females. Male J. spinosa had longer wing spurs than male J. jacana, but there was no difference in male body mass between the species, and J. spinosa males were more aggressive than J. jacana males. Additionally, male J. spinosa was more aggressive than female J. spinosa, suggesting mixed support for females as competitively dominant to males and indicating the need for additional experimental work on sex differences in Jacana. Significance statement: Historically, research on the role of mating behavior in hybridization has focused on competitive males and choosy females. Although female-female competition is widespread and has important fitness consequences for many species across numerous taxa, little is known about the role of female competition in mediating hybridization between closely related lineages. I compare morphological and behavioral traits related to competition between two hybridizing species of jacanas, tropical shorebirds with sex-role reversal. I find that J. spinosa females have a more competitive morphological phenotype and higher aggression than J. jacana females, which may allow them a competitive advantage in obtaining and defending territories and mates in the hybrid zone. These patterns align with a pattern of asymmetrical introgression of J. spinosa mitochondrial DNA previously described in the hybrid zone, as well as findings from other hybrid zones in which male-male competition can potentially explain asymmetric patterns of gene flow from the more dominant species into the less dominant species.