Fossil evidence from Colombia shows that an ancient howler monkey ancestor shifted to a leaf diet 13.6 million years ago, enabling larger size and less competition with fruit-eating primates.

Fossil evidence from Colombia reveals that Stirtonia victoriae, an extinct ancestor of howler monkeys, shifted to a leaf-based diet around 13.3 to 13.6 million years ago, enabling larger body size and reduced competition with fruit-eating primates. Jaw fossils show shearing molars and a deep, wide jaw structure, suggesting adaptations for tough plant material and possibly the early development of the enlarged hyoid bone linked to howler monkeys' loud calls. The findings, based on 3D modeling and analysis, highlight a key evolutionary transition during the Miocene as the proto-Amazonian rainforest formed, shedding light on primate diversification in South America.