Thirteen Million Year Old Fossils Reveal Howler Monkey Ancestors Developed Leaf Eating Diet

New research on 13 million year old fossils shows how leaf eating triggered the evolution of larger body sizes and iconic calls in South American primates.

By: AXL Media

Published: Mar 13, 2026, 7:30 AM EDT

Source: Information for this report was sourced from Johns Hopkins Medicine

Ancient Jaw Fossils Unveil Evolutionary Pivot in Neotropical Rainforests

A team of international researchers has identified a critical turning point in primate evolution occurring approximately 13 million years ago in what is now the Tatacoa desert of Colombia. By analyzing two mandible fossils from the extinct Stirtonia victoriae, scientists have pinpointed the era when ancestors of the modern howler monkey transitioned to a leaf based diet. This shift, according to Siobhán Cooke, an associate professor at the Johns Hopkins University School of Medicine, marks the earliest evidence of folivory among South American primates. The findings suggest that this dietary expansion allowed these monkeys to minimize competition for resources while thriving in the lush, proto Amazonian landscape.

Anatomical Shifts Enable Dietary Diversity and Physical Growth

The physical evidence found in the molars of Stirtonia victoriae indicates a highly specialized adaptation for grinding tough vegetation. According to researchers, the lower molars featured sharp protrusions that functioned as shears to break down complex carbohydrates found in leaves. This anatomical development was a departure from the fruit heavy diets of earlier, smaller primates in the region. By accessing a plentiful and consistent food source like foliage, the species was able to undergo a significant increase in body mass. Estimates derived from the fossils suggest these monkeys weighed between 17 and 22 pounds, representing a substantial leap in size compared to their contemporaries in the fossil record.

The Structural Origins of the Loudest Land Mammal

Beyond dietary habits, the deep and wide structure of the discovered jawbones provides clues regarding the origin of the howler monkey's signature vocalizations. The research team notes that the mandibular body of Stirtonia victoriae appears to have had sufficient space to accommodate a specialized hyoid bone. In modern howler monkeys, this bone acts as a resonating chamber that produces their guttural call, which can be heard for miles through dense canopy. While Cooke cautions that researchers cannot be certain if these ancient primates engaged in the same vocal behaviors, the skeletal framework necessary for such a "ballooning" neck bone was clearly present millions of years ago.