1Department of Anthropology, University of Pennsylvania, 2Department of Anthropology, George Washington University, 3PACEA, Université de Bordeaux
April 18, 2020 , Platinum Ballroom
Orangutans exhibit intrasex bimaturism, a trait rare among primates. Males exist in two morphs: flanged, with large bidiscoid cheek pads on their face and a laryngeal throat pouch, and unflanged, lacking secondary sexual characteristics and displaying “developmental arrest.” Flanged males in captivity are shown to have higher levels of testosterone and cortisol than unflanged males. However, research on wild orangutan remains has a greater potential to inform differences in early life stress experiences, as zoo orangutans rarely remain developmentally arrested. Here, we use associated orangutan skins and skulls to assess flanging status, canine height (n=37), and measure an early life stress indicator, linear enamel hypoplasia (LEH) defect depth, using confocal profilometry (n=7). Of the 24 defects measured, flanged male defects were deeper (range 24.7-126.6, mean 73.4) than in unflanged adult males (range 12.1-63.5, mean 31.04) (t test p=0.026). While the majority of population-level variation in defect depth is related to how rapidly canines grow in height, when assessing intrasex variation, evidence from great apes suggests that deeper defects might reflect more severe stress events during development. Flanged and unflanged males likely have similar canine development given that canine projective crown heights are similar across all males regardless of morph (p=0.47) with no overlap between males and females (p<0.001) (means: flanged 23.15, unflanged 24.55, female 14.75). Therefore, our results may indicate an adaptive benefit to arresting development is avoiding chronic stress and its associated physiological impacts. Future directions include examining differences in microanatomical growth between morphs via histologic and surface analyses.
This project received funding from the European Union’s Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant agreement No798117, Penn Museum, and NSF Graduate Research Internship Program (GRIP).