Roberto Delgado: USC Department of Anthropology

Initial field observations and reports from a few short-term studies pointed to island differences between Bornean and Sumatran orangutans in their general appearance and behavioral ecology, implying meaningful taxonomic distinctions. However, upon further scrutiny at multiple sites and for longer periods of study, researchers have found population-specific differences across a wide array of features including dietary breadth, foraging strategies, life history parameters, molecular markers, morphological characters, reproductive tactics, sociality, tool-using abilities, vocalizations and other traits. Furthermore, the nature of this variation is not always in line with expectations based on a simple island (i.e. Borneo vs. Sumatra) dichotomy, making suspect previous assumptions about historical divergence patterns. Herewith, I review the extent of qualitative and quantitative differences documented and explore a theoretical framework for examining geographic variation and behavioral flexibility among orangutan populations. In particular, I address hypotheses invoking genetic differences, anatomical differences, ecological factors and opportunities for social learning to explain population-level differences. The available data to date suggest that differences in habitat productivity with its concomitant effects on demographic factors such as local population density can influence patterns of gregariousness as well as both the frequency and intensity of social learning within orangutan communities. By examining the occurrence and distribution of geographically varying characteristics such as subsistence skills, comfort skills, and signals, we can determine whether or not the observed patterns of variation are justified with a cultural interpretation when other (i.e. ecological, genetic) explanations are not adequate. Hence, identifying the underlying factors leading to geographic variation in behavior has implications for understanding the emergence of local traditions among great apes as well as the origins and evolution of culture within the human lineage.