More severe stress markers in the teeth of flanged versus unflanged orangutans (Pongo spp.)

Abstract

Objectives: We compared an early life stress indicator, linear enamel hypoplasia (LEH), in the canine teeth of two male orangutan (Pongo spp.) morphs. Flanged males have large bi-discoid cheek pads and a laryngeal throat pouch, and they exhibit either the same or higher levels of the stress hormone cortisol throughout development compared with unflanged males, which lack secondary sexual characteristics. Such "developmental arrest" is hypothesized to either reflect a response to experienced high stress (Hypothesis 1), or an adaptation to avoid elevated stress levels and/or having experienced lower stress levels (Hypothesis 2) during early life. As LEH defect depth has been shown to reflect the severity (i.e., intensity and/or duration) of early life stress events, we examined whether unflanged males have shallower LEH defects than flanged males.Materials and methods: Flanging status was assessed by measuring the faces of preserved skins. Canine height (N = 37) was measured in the same individuals to assess commonality between morphs. LEH defect depths were analyzed using a standardized confocal profilometry method (N = 34).Results: Flanged males have significantly deeper LEH defects than unflanged adult males. Canine projected crown heights are similar across males regardless of morph.Conclusions: Evidence from great apes shows that, when comparing canines with similar growth patterns, deeper defects reflect more severe stress events during development. Thus, our results suggest that "developmental arrest" of unflanged males is not a response to having experienced stress, but rather an adaptation to avoid the physiological impacts associated with chronic stress and/or experiencing lower stress levels.

Specimens as individuals: Four interventions and recommendations for great ape skeletal collections research and curation

“…Researchers might correctly categorize an adult unflanged male specimen as an adult male when they only study the skull, but mistake it for a juvenile when they only examine the postcrania. This is because, unlike typical adult primates, adult unflanged males have unfused long bones 119–123 . This pattern has been found 18 times across five museums, with six males confirmed as adult unflanged males showing the full pattern in skins, skulls, and skeletons, and 13 individuals identified as likely adult unflanged males as evidenced by the pattern in their skulls and skeletons 123 .…”

“…This is because, unlike typical adult primates, adult unflanged males have unfused long bones. [119][120][121][122][123] This pattern has been found 18 times across five museums, with six males confirmed as adult unflanged males showing the full pattern in skins, skulls, and skeletons, and 13 individuals identified as likely adult unflanged males as evidenced by the pattern in their skulls and skeletons. 123 This mosaic osteology appears to have been missed by researchers for around 100 years and exacerbated by the separate housing of different elements.…”

“…63,187 Had we followed the usual protocol of removing "captive" individuals or outliers from the data set, and not sought out associated records about her life, we might have missed the opportunity to understand how major stressors can permanently leave their mark in the form of major enamel defects. By incorporating her life story, we were able to provide complementary data to the experimental studies showing that more severe stress forms larger defects in sheep, 188 and ultimately allowing us to better interpret these same features in individuals without associated records, like wild apes 121 and fossil hominins. 187 In piecing together two studies, 56,117 we see how the four recommendations can be put into action.…”

“…The holistic approach of analyzing all body parts together and treating the remains as coming from one individual who lived a life with a narrative and identity makes it possible to identify unique morphologies and growth patterns that might otherwise be missed, such as the mosaic pattern of osteological development in adult unflanged male orangutans. [119][120][121] While holistic approaches are known to be difficult to accomplish well, they are considered to be a challenge worth accepting. 74 both the population and species levels.…”

Beyond Dimorphism: Body Size Variation Among Adult Orangutans Is Not Dichotomous by Sex

“…Accordingly, studies began to refer to these sexually active males without flanges not as “subadults” but instead as unflanged males (e.g., Kuze et al 2005 ; Rayadin and Saitoh 2009 ; Pradhan et al 2012 ; Marty et al 2015 ), avoiding the ambiguity inherent in the term “subadult” ( Setchell and Lee 2004 ; Utami Atmoko and van Hooff 2004 ). Following this practice, we also avoid using “subadult” in this context and instead adopt the convention used by Kralick and McGrath (2021 ) and refer to sexually active males with unflanged faces in their adolescent years as “young adult unflanged males.” In this study, we refer to adult unflanged males and young adult unflanged males collectively as “unflanged males”. This feature is not necessarily dichotomous, though, as flange development occurs over a widely variable period of time ( Maggioncalda 1999 ) and some older individuals have diminished flanges, known as “past prime” males ( Knott 2009 ).…”