Tympanum and vocal apparatus structure of male and female white-lipped frog, Hylarana raniceps (Peters, 1871) (Amphibia: Anura: Ranidae) in Sarawak, Malaysian Borneo

BACKGROUND: Acoustic communication in anurans involves specialised structures associated with sound production and reception, including the vocal apparatus and tympanum, respectively. Sexual differences in these structures may reflect variation in overall body size or sex-specific structural differentiation. However, information regarding sexual dimorphism in acoustic-related morphology remains limited for Bornean ranids, particularly in Hylarana raniceps. METHODS: This study investigated sexual dimorphism in the tympanum and vocal apparatus morphology of H. raniceps from Sarawak, Malaysian Borneo. A total of 24 individuals collected from Gunung Mulu National Park, Gunung Gading National Park, Ranchan Recreational Park, Kubah National Park, and Simunjan Forest Reserve were examined. Morphological observations and morphometric measurements were obtained for the tympanum, snout-vent length (SVL), and tibia length (TL). Five linear measurements of the vocal apparatus were obtained after histological processing using Haematoxylin and Eosin staining. RESULTS: Male H. raniceps exhibited larger vocal apparatus structures, whereas females possessed larger body size, tibia length, and tympanum dimensions. The sexual dimorphism index (SDI) indicated male-biased vocal apparatus measurements and female-biased body size and tympanum measurements. Histological observations revealed apparently greater development of the surrounding musculature in males; however, muscle thickness was not directly quantified. ANCOVA showed that tympanum morphology was strongly associated with SVL (p < 0.001), with significant sex × SVL interactions indicating sex-specific allometric relationships. Several vocal apparatus traits (PVPM and CPEP) remained significantly different between sexes after accounting for body size variation, providing evidence of sexual shape dimorphism. Principal component analysis showed that PC1 explained 92.17% of the total morphological variation and was mainly associated with overall body size, particularly TL, while tympanum measurements contributed additional variation. Discriminant function analysis classified 95.83% of individuals using raw morphological variables and 91.67% using PCA scores. CONCLUSIONS: This study demonstrates that sexual dimorphism in H. raniceps involves both size-dependent and sex-specific morphological variation. Female-biased body size contributed substantially to tympanum variation, whereas male-biased vocal apparatus morphology showed stronger evidence of sex-specific structural differentiation. Although these morphological differences may be associated with acoustic communication, further studies incorporating advertisement calls, hearing sensitivity, and behavioural data are required to confirm their functional significance. These findings provide baseline information for understanding acoustic-related sexual dimorphism in Bornean ranid frogs.