# Uncovering Sexual Differences in the External Morphology, Appendicular Muscles, and Internal Organs of a Fossorial Narrow-Mouth Frog (Kaloula borealis)

## TL;DR

This study explores how male and female Kaloula borealis frogs differ in body size, muscles, and organs during the breeding season, revealing distinct adaptations for reproduction.

## Contribution

The study provides a detailed analysis of sexual dimorphism in a fossorial frog species, linking morphological differences to reproductive strategies.

## Key findings

- Females had larger snout-vent length and eye diameter, likely linked to fertility and body size.
- Males had greater head and thigh width, possibly for accessing food and improving movement.
- Males had heavier hearts and lungs, likely to meet higher metabolic and calling demands.

## Abstract

We investigated the sexual differences in the external morphological characteristics, appendicular muscle mass, and internal organ mass of Kaloula borealis during the breeding season. The snout-vent length and eye diameter were significantly larger in females than in males. Males exhibited significantly greater head width and thigh width than females. The dry mass of ten appendicular muscles showed significant differences between the sexes. In addition, males had significantly heavier hearts and lungs compared to females. This study presents the detail and comprehensive analyses of sexual dimorphism in a fossorial anuran species, enhancing our better understanding of the anuran reproductive strategies.

Sexual dimorphism is prevalent among animals, influencing both functional morphological traits and behavioral performances. In this study, we investigated the sexual differences in the morphological parameters of Kaloula borealis (Anura, Microhylidae) during the breeding season using 48 specimens. Our results reveal that among the 16 external morphological traits, females had significantly larger snout-vent length and eye diameter than males. The former presumably contributes to enhancing fertility, while the latter is associated with larger body size. Males exhibited significantly greater head width and thigh width than females, which may be related to accessing a wider range of food sources and enhancing their locomotor ability, respectively. Among the 32 appendicular muscles, 10 displayed significant sexual dimorphism in dry mass, suggesting divergent reproductive strategies between the sexes. Among the eight internal organs analyzed, males possessed significantly heavier hearts and lungs than females, which is likely an adaptation to higher metabolic demands and calling behavior. Collectively, our findings demonstrate that sex-specific differences in external morphology, muscle mass, and internal organ mass reflect distinct ecological and reproductive adaptations between males and females and contribute to the phenotypic diversities in Anura.