Female Bornean frogs have bigger ears, but males own the vocal gear

In the lowland forests of Sarawak, Malaysian Borneo, the white-lipped frog Hylarana raniceps carries its sex on the outside in more ways than one. A new morphological study of 24 adult frogs collected from five protected and disturbed forest sites shows that females grow larger bodies and larger tympana, the external eardrums that receive sound, while males pack larger vocal apparatus structures into their throats, the laryngeal machinery that produces their advertisement calls. Crucially, the male-biased differences in the vocal apparatus did not simply vanish once body size was taken into account, pointing to genuine sex-specific structural differentiation rather than a mere by-product of size.

Specimens came from Gunung Mulu National Park, Gunung Gading National Park, Ranchan Recreational Park, Kubah National Park and Simunjan Forest Reserve, surveyed at night between 1830 and 2300 hours using visual encounter surveys and acoustic detection. Males were identified by their advertisement calls, vocal sac openings and nuptial pads on the forelimbs, while females lacked these secondary sexual traits and were generally larger.

Every frog was measured with digital callipers to the nearest 0.01 millimetre. The team recorded snout-vent length, tibia length, body mass, and no fewer than eight tympanum diameters, taken in horizontal, vertical and two diagonal orientations on both the left and right eardrums. This unusually thorough approach reflects the fact that anuran tympana are not always perfectly circular and can be oval or irregular, so a single horizontal diameter may miss important shape variation. The tympanum of H. raniceps itself is a horizontal, nearly round structure sitting flush with the surrounding skin, posterior to the eyes, uncovered by a supratympanic skin fold and fully visible in lateral view, with a distinct tympanic annulus ringing the membrane.

To quantify the sound-producing side of the system, the researchers dissected the vocal apparatus of all 24 frogs and processed the tissue histologically. Samples were fixed in 10 percent formalin, embedded in paraffin, sectioned at 3.5 micrometres and stained with haematoxylin and eosin. Using a microscope at 4x magnification, the team measured five linear distances between six anatomical landmarks on the larynx: the pulvinar vocale, the cardiac, bronchial, lateral and oesophageal processes of the cricoid ring, and the posterior margin of the arytenoid cartilage. These structures sit between the lungs and the buccal cavity and are associated with the vocal cords and hyoids, forming the core of sound production in the frog.

The results split cleanly along sex lines. A sexual dimorphism index, calculated as the ratio of the female mean to the male mean minus one, ranged from minus 0.5030 for the cardiac-to-oesophageal distance to plus 0.4129 for tibia length. Negative values, spanning all five vocal apparatus measurements, meant males had the larger structures. Positive values on snout-vent length, tibia length and every single tympanum measurement meant females had the larger bodies and ears. Statistical tests confirmed these differences were significant for most variables, with the exception of the lateral-process-to-oesophageal-process distance, which did not differ significantly between the sexes.

The key analytical step was an analysis of covariance that used sex as a fixed factor and snout-vent length as a covariate, separating sexual size dimorphism from sexual shape dimorphism. For the tympanum, the verdict was unambiguous: all measurements showed strong associations with body size, with p-values below 0.001 except for the horizontal left diameter at 0.002, and once body size was accounted for, the main effect of sex disappeared entirely. Females simply have bigger ears because they have bigger bodies. However, every tympanum variable showed a significant sex-by-size interaction, meaning the scaling relationship between eardrum and body differs between males and females. Notably, males had a tympanum roughly similar in size to the eye diameter, a larger relative ear than females.

The vocal apparatus told a different story. Two measurements, the distance from the pulvinar vocale to the posterior margin of the arytenoid and the distance from the cardiac process to the oesophageal process of the cricoid, remained significantly different between the sexes even after controlling for body size, with p-values of 0.005 in both cases. This is the study’s strongest evidence for sexual shape dimorphism: male larynges are structurally different, not just bigger frogs with proportionally bigger voice boxes. Two other measurements, the bronchial-to-lateral and lateral-to-oesophageal distances, showed significant sex-by-size interactions instead, indicating sex-specific scaling rather than a simple sex effect. Histological sections also revealed that males appeared to have larger and more elongated circular striated muscle fibres around the arytenoids, although the authors stress that muscle thickness was not directly quantified, so this remains a qualitative observation.

Multivariate analyses reinforced the picture. Principal component analysis found that the first component alone explained 92.17 percent of total morphological variation and was dominated by tibia length, loading at 0.965, confirming that overall body size is the primary axis of variation. The second component, explaining 4.96 percent, was driven mainly by tympanum measurements. Discriminant function analysis using the raw morphometric variables classified 95.83 percent of individuals correctly, misclassifying only one of 11 males, and a version using principal component scores still reached 91.67 percent. The authors caution, however, that the overall discriminant function was not statistically significant with this small sample, and that these apparent classification accuracies need validation on larger, independent datasets.

What might these structural differences mean for the frogs themselves? The authors are careful to frame the functional interpretation as a hypothesis rather than a demonstrated result, because no advertisement calls were recorded and no hearing sensitivity was measured. Larger vocal apparatus structures in males are consistent with findings in other anurans, such as the leopard frog Rana pipiens, where males have larger larynges tied to acoustic communication. The musculature surrounding the arytenoid cartilages helps control vocal cord position, vibration and tension, and previous work in other species has shown males possess fast-twitch muscle fibres suited to rapid contractions during calling. The distance between the pulvinar vocale and the arytenoid margin has previously been linked to vocal cord vibration in Sarawak ranids. Whether these anatomical differences translate into differences in pitch, frequency or call complexity in H. raniceps remains to be tested.

The female-biased tympanum pattern is equally open-ended. Comparative studies show the relationship between eardrum size and hearing varies widely among frogs: female Odorrana tormata are more sensitive to frequencies between 2 and 7 kilohertz while males hear better between 3 and 15 kilohertz, yet male bullfrogs have tympana about 50 percent larger than females despite smaller bodies. The authors also note an ecological wrinkle: at Gunung Mulu National Park, H. raniceps lives alongside H. glandulosa and H. baramica, both loud callers, raising the possibility of heterospecific acoustic interference shaping communication in this community. For now, the study stands as a quantitative baseline for acoustic-related sexual dimorphism in Bornean ranids, and the authors call for future work integrating call recordings, hearing tests and ecological data to confirm what these differences in ear and voice box actually do for the frogs in the forest at night.

Subject of Research: Sexual dimorphism in the tympanum and vocal apparatus of the white-lipped frog Hylarana raniceps in Sarawak, Malaysian Borneo

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

Article References: Deka, E. Q., Lim, Z. Q., Zainudin, R., Abang Madzhi, D. S., & Abang Aimran, A. A. (2026). Tympanum and vocal apparatus structure of male and female white-lipped frog, Hylarana raniceps (Peters, 1871) (Amphibia: Anura: Ranidae) in Sarawak, Malaysian Borneo. BMC Zoology, 11(1), Article 36. https://doi.org/10.1186/s40850-026-00288-w

Image Credits: AI Generated

DOI: 10.1186/s40850-026-00288-w

Keywords: Hylarana raniceps, anurans, sexual dimorphism, tympanum, vocal apparatus, Borneo, Sarawak, morphometrics, histology, frog calls, Ranidae, ANCOVA