Monkeys Swimming Across Rivers Refine Questions About the Riverine Barrier Hypothesis

The Riverine Barrier Hypothesis posits that species distributions are limited by large rivers acting as geographical barriers. Accordingly, large rivers have long been thought to be a major driver of the extensive speciation and high levels of biodiversity among platyrrhine primates in South and Central America. Direct observations of river crossings provide evidence that complements studies of genetic diversity that can together shed new light on this hypothesis. Here, we discuss recent video evidence of howler monkeys successfully swimming across the Rupununi River in Guyana and the Panama Canal. The footage clearly reveals that howler monkeys can and do cross such bodies of water. These video observations help to refine questions about the species and circumstances under which rivers are barriers to gene flow. We end by joining other scientists who call for increased collaborations with local people living near river crossing sites to improve our knowledge and understanding of the frequency, contexts, and traits of the rivers and animals that characterize river crossings. Local knowledge provides new answers and helps refine questions about the river barrier hypothesis.