Orangutans are typically described as exhibiting high levels of sexual dimorphism, but they also show pronounced within-sex size variation. Most males develop secondary sex characteristics during puberty called flanges, but not always. Some males delay the development of flanges for a few years up to their entire lifetimes. These phenotypically “immature” but reproductively capable males are referred to as ‘unflanged males’ and are typically described as exhibiting a reproductive strategy of using smaller body sizes to “sneak” and force copulations. The morphology of unflanged males has also been described as a less-than-ideal response to stress, but much remains unknown about them. To improve our understanding of unflanged males, I developed a novel osteobiographic methodology for identify orangutan flanging status in museum collections. This approach allowed me to identify eighteen unflanged males in twelve museums across five countries. My results revealed that adult unflanged males are not strictly female-sized but have skeletal measures that range between those of adult females and flanged males. I also observed that unflanged males are not an unusual response to high stress, as they had less severe stress markers when compared to flanged males. This osteobiographic approach allowed for a re-examination of the locomotor behavior of orangutan females and males and may prove useful in the study of ape remains research more broadly. Synthesized together, results show adult unflanged males are part of the natural range of orangutan morphology and behavior and that previous understandings of unflanged males as evolved in response to stress and “sneaking” as female sized both require revision. In conversation with queer theory, I explore the ways in which we project gendered thinking onto the skeleton of our non-human primate relatives, complicating binary understandings of orangutan body size in a genus described as having some of the largest levels of sexual dimorphism among apes. This rethinking, in turn, interrogates the appropriateness of the term sexual dimorphism to describe orangutan variation and promotes a broader understanding of features which vary according to biological sex in our ape cousins, hominin ancestors, and ourselves.