Otolithic organs and vibratory sensitivity in frogs: do saccular otoconial volumes vary according to lifestyle?

Of the three otolithic organs found in frogs, the sacculus is known to possess acute sensitivity to seismic vibrations. Simple models of otolith function suggest that increasing size of an otolith should improve vibratory sensitivity. In the first part of this paper, we provide a short review of the existing literature pertaining to the vibratory sensitivity of the otolithic organs in frogs. Given parallels in other vertebrate groups, we hypothesized that fossorial frogs would have larger saccular otoconial masses ('otoliths') than non-burrowing species, used for the detection of ground vibrations. In the second part of this paper, we tested this hypothesis by measuring otolith volumes in 15 species of frogs, based on CT reconstructions. Larger species tended to have larger otoliths. The fossorial frog Hemisus guineensis had the largest otoliths relative to skull size of all species examined, followed by a non-fossorial species, Ptychadena delphina. The otoliths of two other fossorial specialists were not notably enlarged, however, and volumes were relatively small in some semi-fossorial species. Pronounced sexual dimorphism was noted in the aquatic frog Xenopus laevis, in which males have larger saccular otoliths than females in absolute terms, despite their smaller body size. Otoconial dissolution represents a problem for studies which involve the examination of preserved specimens, but with this caveat in mind, we conclude that factors other than fossoriality might drive expansion of the saccular otoliths in frogs.