Evolution of male and female release calls in African clawed frogs

In anurans, male clasps can elicit release calls from either sex. Male release calls have been observed in many anuran genera and this vocal response is thus highly conserved. Female release calls, however, are not as prevalent, suggesting that evolutionary trajectories for anuran release calls differ by sex. We analyzed male and female release calls in all available species of Xenopus, a fully aquatic African genus. Phylogenetic relationships in this genus include three species groups, two of which are clades and one of which is characterized by a reticulated phylogeny due in part to hybridizations between species with different ploidy levels (Evans et al., 2004; Evans, 2008). In all species, males produce release calls when clasped by another male. Females in the reticulated group do not produce release calls, but females in the rest of the genus do. Release calls consist of click trains of variable durations and inter-call intervals. In both sexes, inter-click interval divides the genus into groups with different click rates and these groups are phylogenetically related. In general, inter-click interval is shorter in male than in female release calls. Across species and sexes, release calls are characterized by a single, low (similar to 1000 Hz) dominant frequency. In X. laevis Congo and X. borealis, clasp duration is longer for male female than for male male pairs and clasp duration is correlated with the number, but not the duration, of release calls in male male pairs. We discuss evolutionary scenarios for release call traits as well as the sex difference in occurrence.