One challenge in evolutionary biology is that fossils rarely contain DNA, and so it can be tricky to classify fossil animals without the ability to travel back in time and see them in the wild. Scientists have begun exploring how other methods, like quantifying skeletal morphology, are useful for taxonomic assessment. They test these methods by looking at morphologic differences between closely related living species. 

New research by a team of anthropologists investigated whether there were differences in the mandibular shape of gibbons. They studied three subspecies of white-handed gibbon (Hylobates lar) one species of black-handed gibbon (Hylobates agilis) from Indonesia, Thailand, and Myanmar. They tested whether mandible (jaw) shape was influenced by allometry (or, the relationship of body size to shape) and geography.  

Using novel geometric morphometric methods, the researchers found that there were differences in mandibular shape between the two species, and that the white-handed gibbons had a more variable mandible than the black-handed gibbon. This was not due to allometry, or body size differences, but could be attributed to where they live. The gibbons from Indonesian islands had a different mandibular shape than the gibbons from mainland Thailand and Myanmar. The researchers hypothesized that these differences are related to behavioral differences such as different diet compositions. For example, island species might eat more fruits than mainland species.  

Results from this study suggest that we can learn about taxonomic differences in closely related species from studying their morphology. This is an important clue for better understanding primate evolution, as it highlights how testing specific groups, like gibbons, will help us to better understand primate taxonomy and evolution.