Diversity and evolution of male secondary sexual characters in African squeakers and long-fingered frogs

The African frog genera Arthroleptis and Cardioglossa are unique among vertebrates in having males with extremely long third fingers. In some species, this sexual dimorphism is impressive, with male third fingers approaching 40% of body length. However, the diversity of this trait has not been documented thoroughly and several species appear to lack this trait. The present study documents the diversity of male secondary sexual traits in Arthroleptis and Cardioglossa, including elongate third fingers and digital and inguinal spines. Furthermore, it explores hypotheses of trait evolution, including explanations for the absence of male traits. Analyses of covariance suggest that the functional relationship between finger length and snout-vent length (SVL), both within and among species, is different for male finger III than for male fingers I, II, and IV, or for female finger III. Ancestral state reconstruction suggests that all male traits were present in the most recent common ancestor of Arthroleptis and Cardioglossa and that reduction or loss of traits occurred later. Across species, independent contrast analyses find no correlation between SVL and either male relative third digit length or dimorphism in relative third digit length. The number of spines on male fingers II and III are positively correlated but spine number is not correlated with SVL and only weakly correlated with relative third digit length. The diversity of male traits is evolutionarily labile and is not explained by simple hypotheses of character evolution. Arthroleptis and Cardioglossa may thus provide an interesting study system for understanding how changes in sexual selection forces produce male trait diversity. (C) 2009 The Linnean Society of London, Biological Journal of the Linnean Society, 2009, 96, 553-573.