Rise and spread of a social tradition of interspecies abduction

Cultural traditions lacking clear function are exceptionally common in humans, partially explained by our hyper-reliance on social learning¹. In non-human animals, it is unclear whether the same ecological and social conditions drive the emergence of both seemingly adaptive and non-adaptive traditions. Here, we describe the origins and spread of a tradition of interspecies abduction in the wild. We documented carrying of eleven different infant howler monkeys (Alouatta palliata coibensis) by five immature male white-faced capuchin monkeys (Cebus capucinus imitator) over 15 months on Jicarón island, Coiba National Park, Panama. All cases occurred in one capuchin group, which has been studied since 2017 for their localized tradition of habitual stone tool-use, unique to Cebus². We captured the origin of this 'howler abduction' tradition, starting with one subadult male innovator, and its spread to four subadult and juvenile males (Figure 1 and Supplemental information; see also interactive timeline: https://www.ab.mpg.de/671374/Capuchin-tool-use/interspecies-abduction-tradition). We argue that the same conditions which favored adaptive cultural innovations like tool-use on Jicarón also underlie this non-adaptive tradition.