Comparative study of the relationship between genetic mating system, testes size and body-size dimorphism in swallows (Hirundinidae)

Sexual selection can occur in two ways: it can favor the competitive ability of the individuals of one sex to gain access and fertilize individuals of the other, or it can favor the development of characters in individuals of one sex that are attractive to the other sex. The objectives of this work are: 1) to compare testes size (gonadal volume as a primary sexual character that might be an indicator of intrasexual competition) and body size (a secondary sexual character of males competition) in swallows (Fam. Hirundinidae) that differ in their extra-pair paternity (EPP); and, 2) to analyze if gonadal volume and body size dimorphism are good predictors of the level of EPP in the species studied. We found information for a total of nine species from the “Museo Argentino de Ciencias Naturales” and in the database VertNet. Overall, species differed in their body size dimorphism and gonadal volume. However, this difference did not seem to be related to their level of EPP. When we looked at the relationship between EPP and gonadal volume and sexual dimorphism in body size we did not find the expected relationship between intensity of sexual selection (measured in terms of EPP), and these variables. It is possible that the relationship between these characters does not exist, or that it exists but with these data we could not detect it. Anatomical changes might respond to sustained ecological differences in mating systems, and differences in reproductive anatomy might become visible at deeper evolutionary relationships.