The co-occurrence of loose skin and underwater calling in frogs-further evidence from Amolops ricketti and its implications

Underwater vocalizations of frogs are usually inaudible to the human ear and have been poorly documented. Some authors believe that they are more common than currently recognized, yet there have been no new records at the genus level for nearly 30 years. I report on the recordings of underwater calling by the ranid species Amolops ricketti (Boulenger, 1899) from a mountain stream in southern China. Males emitted 2.2-3.5 calls per minute under submerged rocks, which were inaudible to the human ear. Most of the recorded calls lasted 0.73-2.09 s and contained a weak initial note with fewer pulses (1-5) and one to three subsequent notes (3-13 pulses each) repeated at a rate of 0.82-1.99 notes center dot s(-1), with an internote interval lasting 0.36-1.19 s. The dominant frequency of calls ranged from 2.1 to 3.0 kHz, and the notes exhibited increasing intensity and slight upward frequency modulation. Comparisons between individuals of different sexes and breeding stages reveal that the loose skin of the flanks and thighs is a male nuptial trait in A. ricketti. The finding extends the phylogenetic distribution of underwater vocalization to a distinct lineage and constitutes another instance of loose-skin frogs calling underwater, which can also been identified in Pelobates (Pelobatidae), Leptobrachium (Megophryidae), Telmatobius (Telmatobiidae), Rana italica and Rana sakuraii (Ranidae). These independent cases may allow the proposal of an evolutionary correlation of the two traits in anurans, as a large surface/volume ratio facilitates high levels of cutaneous respiration required for the effective performance of underwater calling. An awareness of this possibility may aid in increasing our understanding of the natural history of anurans.