Underwater calling behavior in two high-altitude frog species

Underwater calls of frogs are poorly documented, probably because they are usually weak and difficult to detect out of water because the impedance contrast at the water-air interface limits sound transmission. The tympanic middle ear and vocal sacs, which may not be essential for underwater acoustic communication, are often reduced in high-elevation anurans. In addition to the Andean Telmatobius and Sierra Nevada Rana sierrae, many other highland frog species may vocalize underwater rather than be mute as previously thought. We report evidence from the eastern Tibetan Plateau supporting this speculation. In two species of the megophryid genus Scutiger, breeding males emitted trill-like calls in series under submerged rocks at elevations of approximately 3700 m a.s.l. Because the few high-elevation anurans known to vocalize underwater are all highly aquatic, this strategy is possibly used because the signal receivers usually remain in water during the breeding season. The finding of underwater calling in Scutiger characterized by reduced middle ears was partly guided by co-occurrence of loose skin and underwater vocalization in some frog species and constitutes another independent case of such co-occurrence. We suggest that the presence/absence of loose skin folds should be routinely included in the morphological descriptions of anurans, because an increased surface/volume ratio facilitates effective cutaneous respiration that is required for metabolically costly underwater behaviors which are usually cryptic to human observers.

Automated summary

Underwater calls of frogs are rarely documented due to the limitations in sound transmission. However, our study found evidence of underwater vocalization in high-elevation frogs, which is associated with their reduced middle ears and vocal sacs.