Alexandra E. Kralick, Ph.D.

Orangutans are typically described as exhibiting high sexual dimorphism between males and females, although they also show pronounced within-sex size differences. All males eventually become flanged males, with large cheek pads (flanges) and throat pouches. Most males develop flanges during puberty, but not always. Some males delay the maturation of flanges for anywhere from a few to 20 years. These phenotypically “immature” but reproductively capable males are called adult unflanged males. Orangutan skeletons from wild-shot individuals in museum collections offer a unique opportunity to measure features that correlate with body size, such as long bone length, strength, and bi-iliac breadth. We measured adult orangutan skeletons from museums across the USA and Europe. Our results show that adult unflanged male sizes ranged between those of adult flanged males and adult females. These findings indicate that orangutans are not a sexually dimorphic species per se, and raise intriguing questions about the factors that shape male orangutan phenotypic and sexual maturation in the wild.

Manuscript avaialble here

I organized an American Anthropological Association (AAA) invited podium symposium that focused on decolonial theory, contextual reflexivity, and animal ethics in great ape skeletal remains research. The panelists and I developed a manuscript for Evolutionary Anthropology that discusses best practices for improving standards of research on and curation of ape remains. The paper currently out in Evolutionary Anthropology encourages scholars to consider their own positionality when studying ape remains and provides recommendations for making access to remains more equitable, such as centering and amplifying voices in the Global South in conversations and actions, including digital repatriation and co-ownership of remains. The resulting recommendations include contextualizing the individual, piecing individuals back together, challenging/questioning the captive-wild dichotomy, and collaborative international conversations. This serves as one example of how we can be decolonizing our respective research within biological anthropology.

Differences in habitat continuity and predators have been posited as explanations for higher rates of terrestrial locomotion in Bornean compared to Sumatran orangutans. However, it is unclear whether greater terrestriality in Bornean orangutans is due to recent environmental changes to their habitats. Orangutan skeletons collected a century ago may shed new light on this question as habitat continuity on both islands would have been greater in the past. In this study, cross-sectional geometry ratios were calculated for orangutans from CT scans. Both orangutan taxa show significantly less relative leg-to- arm strength than all gorillas, even those that climb the most (i.e., western lowland gorillas and infant mountain gorillas). Moreover, Bornean orangutans display limb bone strength proportions that are consistent with more terrestriality than their Sumatran counterparts. Thus, recent habitat discontinuity in Borneo does not explain greater terrestriality in Bornean orangutans, as also suggested by camera trap data, and is most likely due to the absence of tigers on Borneo.