Figure 1.

The vocal phylogeny of Xenopus (adapted from Tobias et al. [12]). (a) Xenopus advertisement calls can be classified as one of four types: click (yellow), burst (orange), trill (red) and biphasic (blue), based on the number of clicks per call, interval between calls, intensity modulation of clicks within a call and number of phases in a call. (b) Call types mapped onto the Xenopus phylogeny (colour scheme as in panel (a); grey indicates no data). Reticulation in the phylogeny (e.g. in the species group containing X. boumbaensis) reflects past instances of speciation via hybridization [10,11]. Maximum parsimony analysis suggests that the ancestral call type is a burst, and the simplified click call type is evolutionarily derived [12]. Xenopus borealis and X. boumbaensis (in bold) are two distantly related click-type callers examined in this study.