Mandibular shape variation in mainland and insular hylobatids

Area of Science:

• Primate morphology
• Comparative anatomy
• Evolutionary biology

Background:

• Hylobatids (gibbons) are the most diverse living apes, yet their morphological variation is poorly understood.
• Understanding variation in extant primates is crucial for interpreting the primate fossil record.

Purpose of the Study:

• To assess mandibular shape variation in two gibbon species: white-handed gibbons (Hylobates lar) and black-handed gibbons (Hylobates agilis).
• To examine interspecies, intraspecies, and geographic patterns of mandibular shape variation.

Main Methods:

• Utilized 71 three-dimensional landmarks to quantify mandibular shape.
• Employed generalized Procrustes analysis, Procrustes analysis of variance, and principal components analysis.
• Analyzed a mixed-sex sample of adult H. lar and H. agilis.

Main Results:

• Hylobates lar exhibited greater mandibular shape variation compared to Hylobates agilis.
• Both species showed similar allometric patterns in mandibular shape.
• A geographic pattern was identified: insular hylobatids possess distinct mandibular condyle morphology compared to mainland populations.

Conclusions:

• Significant interspecies and geographic variation exists in gibbon mandibular morphology.
• Geographic patterns may reflect functional demands during mastication or island evolutionary pressures.
• Further research on Hylobates morphological variation is essential for interpreting primate fossil evidence.