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J Comp Physiol A Neuroethol Sens Neural Behav Physiol
2026 Jul 21; doi: 10.1007/s00359-026-01826-0.
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Otolithic organs and vibratory sensitivity in frogs: do saccular otoconial volumes vary according to lifestyle?
Mason MJ, Goutte S, Seal PJ, Kluonis M, Christensen-Dalsgaard J.
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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.
Fig. 1. Reconstructions of left inner ear structures of the frog Lithobates pipiens, seen from approximately a posterolaterally, b anteromedially, c dorsally and d ventrally. The endolymph-filled otic labyrinth (translucent grey), sensory nerve branches (yellow) and sensory epithelia (red) were reconstructed using Stradview 7.321, based on serial sections obtained as part of a previous study (Mason et al. 2015). The lagenar (L), saccular (S) and utricular (U) otoliths were not visible in the sections owing to dissolution; their shapes and positions were estimated based on containing structures and CT scans of the same species, also described in Mason et al. (2015). They are shown in white. AP = amphibian papilla; AS = anterior semicircular canal; BP = basilar papilla; CA = crista ampullaris; LS = lateral semicircular canal; PS = posterior semicircular canal; RL = ramus lagenae; RS = ramus sacculi (saccular nerve). Scale bar 2.5 mm
Fig. 2 Schematic cross-section of the frog sacculus. The orientation of the macula is almost vertical, and the kinocilia of the hair cells (red) are coupled to the otolithic membrane through pores in the membrane (insert). HC, hair cell; NF, nerve fibre; OM, otolithic membrane.
Fig. 3. a: Responses of a vibration sensitive VIIIth nerve fibre in Leptodactylus albilabris to substrate vibrations. The figures are phase histograms showing the phase locking to vibration stimuli at different acceleration amplitudes at 130 Hz, bin width 3.6 degrees. b & c: Spike rate response b and synchronization responses c of a saccular fibre in Rana temporaria stimulated at frequencies from 10–200 Hz. d: Histogram of preferred phases of 95 single fibres in R. temporaria. a reprinted with permission from Narins, P.M. & Lewis, E.R. (1984) The vertebrate ear as an exquisite seismic sensor. Journal of the Acoustical Society of America 76: 1384–1387. Copyright 1984, Acoustical Society of America. b-d redrawn from Christensen-Dalsgaard, J. & Jørgensen, J.M. (1988) The response characteristics of vibration-sensitive saccular fibers in the grassfrog, Rana temporaria. Journal of Comparative Physiology A 162: 633–638, Springer Nature, reproduced with permission from SNCSC
Fig. 4. CT reconstructions of the skulls of five frog specimens. a: Ptychadena delphina SB310; b: Xenopus laevis XL14 (male); c: Leptopelis rugosus SB558; d: Sclerophrys arabica SB1328; e: Hyperolius viridiflavus SB180. Columellae are coloured orange; otoliths are shown in white and are superimposed onto the skulls. Scale bar 10 mm
Fig. 5. Transverse tomograms through the heads of five frog specimens. The sections were chosen to pass through the centres of the saccular otoliths, which are visible as the white, rounded structures on each side. a: Ptychadena delphina SB310; b: Xenopus laevis XL14 (male); c: Leptopelis rugosus SB558; d: Sclerophrys arabica SB1328; e: Hyperolius viridiflavus SB180. Scale bar 5 mm
Fig. 6. Reconstructions (above) and transverse tomograms (below) through the heads of three highly fossorial frog species. The tomogram sections were chosen to pass through the centres of the saccular otoliths. a: Hemisus guineensis (CAS: HERP:258533); b: Rhinophrynus dorsalis (CAS: HERP:71766); c: Nasikabatrachus sahyadrensis (CES-F-203). In the reconstructions, opercula (in a and b) and columellae (in c) are coloured orange. Otoliths, shown in white, are superimposed onto the skulls. Scale bar 10 mm
Fig. 7. Reconstructions of the left otoliths of five species of frogs. a: Leptopelis diffidens SB110; b: L. xeniae SB188; c: Sclerophrys arabica SB1328; d: Xenopus laevis XL13 (male); e, f: Ptychadena delphina SB310. All shown from a rostral view except f, which is from a posterior view. The lagenar and utricular otoliths are not shown in the Ptychadena reconstructions; the narrow projections of the saccular otolith head in this specimen might reflect dense soft tissue close to the otoconial mass. H = head of saccular otolith; L=lagenar otolith; S=saccular otolith; U=utricular otolith. Not to scale
Fig. 8. Relationship between the volumes of the saccular otoliths and maximum skull widths in the frog specimens examined. Otolith volumes are averages of left and right sides. a: regular axes; b: logarithmic axes. In a, each individual specimen is shown. In b, each species is represented by a single averaged point except for Xenopus, for which the average points representing males and females are shown separately. AC = Afrixalus clarkei; HG = Hemisus guineensis; HV = Hyperolius viridiflavus; LG = Leptopelis rugosus; LS = Leptopelis shebellensis; NS = Nasikabatrachus sahyadrensis; PD = Ptychadena delphina; RD = Rhinophrynus dorsalis; SA = Sclerophrys arabica. Solid line = OLS regression line; dashed line = PGLS regression line. The equation is for the OLS regression. Regression lines were calculated from all points shown in b
Fig. 9. Relationship between the volumes of the saccular otoliths and skull volumes in the frog specimens examined. Otolith volumes are averages of left and right sides. Skull volumes include mandibles but exclude stapes, operculum and otoliths. a: regular axes; b: logarithmic axes. In a, each individual specimen is shown. In b, each species is represented by a single averaged point except for Xenopus, for which the average points representing males and females are shown separately. AC = Afrixalus clarkei; HG = Hemisus guineensis; HV = Hyperolius viridiflavus; LG = Leptopelis rugosus; LS = Leptopelis shebellensis; NS = Nasikabatrachus sahyadrensis; PD = Ptychadena delphina; RD = Rhinophrynus dorsalis; SA = Sclerophrys arabica. Solid line = OLS regression line; dashed line = PGLS regression line. The equation is for the OLS regression. Regression lines were calculated from all points shown in b
CT reconstructions of the skulls of f Xenopus laevis XL14 (male). Columellae are coloured orange; otoliths are shown in white and are superimposed onto the skulls. Scale bar 10 mm
Transverse tomograms through the head of a X. laevis (male). The sections were chosen to pass through the centres of the saccular otoliths, which are visible as the white, rounded structures on each side. See for article for scale
