Characterization of the sexual dimorphism in oilbirds (Steatornis caripensis)

Abstract

The extent to which males and females of monomorphic species differ from each other is still an ecological and evolutionary puzzle. In these species it has been proposed that slight differences in selection pressures can result in phenotypic differences between sexes. However, such differences may be very subtle or may be exhibited in traits that are not perceived by humans, such as coloration. For example, in blue tits (Cyanistes caeruleus) males differ from females in the UV reflection of the crown patch. Here, I aimed to identify the morphological differences between sexes in a monomorphic specie, the oilbird (Steatornis caripensis). In this species, males are slightly larger and grayish than females, making it difficult to determine the sex of individuals by means other than molecular sexing, an invasive technique. To characterize phenotypic differences between males and females, I measured five morphological traits on 75 museum individuals and 16 wild individuals, and the feather coloration in 33 museum individuals and 11 wild individuals. I used these measures to quantify the degree of sexual dimorphism and identify the traits that differentiate males from females. I found that S. caripensis is mostly monomorphic showing subtle differences in the width and the length of the bill, which is larger in males compared to females. I also found subtle differences in plumage coloration suggesting that color may play an important role in the differentiation of the sexes, and possibly in mate choice. My results indicate that despite the slight differences in beak shape between males and females, the sexes in oilbirds seem to be indistinguishable.