Phylogeography and niche modeling of night monkeys (Aotus) in northwestern South America

Cryptic species are genetically distinct lineages that are difficult to distinguish morphologically, which frequently occur in geographically complex regions where mountains and rivers act as isolating barriers. In American primates such as night monkeys (<i>Aotus</i>), cryptic speciation has been proposed but remains poorly understood due to limited sampling and subtle morphological differentiation. In northwestern South America, members of the <i>Aotus lemurinus </i>complex <i>(A. griseimembra, A. lemurinus</i>, and <i>A. zonalis)</i> have been assumed to occupy distinct elevations, although no clear geographic boundaries have been identified. To clarify their evolutionary relationships, we integrated mitochondrial DNA data from 93 sites, ecological niche modeling (ENM), and analyses of hair length variation. Phylogenetic analyses recovered four major clades—one cis-Andean and three trans-Andean—showing strong geographic structure largely associated with the Magdalena River. Riverine barriers, rather than elevation, appear to have played a primary role in lineage divergence. ENM revealed potential distributions on opposite riverbank across a broad elevational gradient, with a narrow zone of potential sympatry in the lower section of the Magdalena River. Niche similarity tests indicated no significant ecological divergence between clades, while hair length increased with elevation, suggesting convergent morphological responses. These findings challenge taxonomic hypotheses that separate <i>A. griseimembra</i> and <i>A. lemurinus</i> by elevation and instead emphasize the role of riverine isolation—particularly along the upper and middle sections of the Magdalena River—in shaping diversification. This integrative approach combining molecular, ecological, and morphological data provides new insights into cryptic diversification and phylogeographic structuring in American primates.