Article begins

One type of male orangutan has perplexed scientists because they run counter to long-held binary expectations of sex differences in the skeleton.

Orangutans are said to have the largest sexual dimorphism in body size of any ape, with adult males typically twice the size of adult females. But not all males are quite as large. There are smaller sexually active adult males who look like fully grown adolescents. The existence of these surprising male orangutans has puzzled scientists for centuries.

The wide spectrum of size variation in adult male orangutans perplexed Alfred Russel Wallace, the co-conceiver of the theory of evolution through natural selection. When Wallace examined orangutan skeletons in the 1850s, he was so baffled by the smaller males that he suggested they must be of a different species than the larger males.

Scientists have since learned a great deal more about orangutans. We now know that these two types of males are not only the same species, but often even the same individual, at times starting off one way and then changing to be the other. Yet in great ape skeletal biology the narrative remains that males are consistently and substantially larger than females. My research on orangutan within-sex skeletal variation rethinks such narratives and the ways in which we project gendered thinking onto the skeleton. Normative assumptions about male body size persist in today’s science—they are misleading and in need of a twenty-first-century update.

Subadult or unflanged males discovered

In the 1970s, the two types of males were no longer described as different species, as Wallace had proposed. Instead, they were thought to be different stages of male development throughout an individual’s lifetime. Around age 8 or 10 an orangutan male was said to leave the juvenile phase and enter the “subadult” phase. Subadults were sexually active but lacking secondary sex characteristics, two things rarely combined in biology. These males of interest were called subadults because they had delayed puberty only a few years and were still in their adolescent years. When a male reached puberty around age 14 or 15, they would become full adults and get the full suite of secondary sex characteristics: large visible bidiscoid pads on their faces and a throat sac under their chin that they use to call to potential mates. These males are called flanged males after the characteristic cheek flanges on their face.

Credit: Meredith L. Bastian, Sungai Lading Orangutan Project
Photograph of an orangutan in a tree
An unflanged male orangutan.

The two male types were described as having alternative reproductive strategies. Flanged males are known for calling and waiting for females to come to them and mating with the most fertile females, a strategy called “sit and wait.” Flanged males are thought to be large because of mate selection pressures like female mating choice and the need to defend home ranges aggressively. Flanged males are considered dominant and unflanged males subordinate because they flee from flanged males and so relinquish their place in fruit trees to these larger males. Subadult males are said to experience reduced mate competition and be on the receiving end of far less aggression from larger males. They are more likely to be resisted in mating attempts than flanged males, so are said to employ a strategy of forcing copulations described as “sneak and rape.”

In the 1980s, Biruté Galdikas discovered much older subadults: male orangutans who lacked flanges and were sexually active up to five years past the average age of puberty. In the 1990s, Irenaueus J. A. te Boekhorst and others discovered males who had delayed flanging 10 years or more, well into adulthood. Then, in the 2000s, males were discovered who remained without flanges into their thirties, possibly never flanging. Since you can hardly call an orangutan almost at the end of his adulthood subadult, Sri Suci Utami Atmoko suggested we consider them full adults and instead call them unflanged males after their flange-less faces. Despite this big revelation and the change in their names, their biology had yet to be reevaluated.

Unflanged males have long been said to be female sized. When unflanged males were called subadults this size claim made sense because they were still in that adolescent/pubertal window. But researchers have discovered that males can stay unflanged well into adulthood, if not their entire lives. Are all unflanged males female sized? Would these elderly unflanged males also be female sized, sneak, and force copulations? Is it possible that adult unflanged males look and behave differently than those in their adolescent years?

Credit: Meredith L. Bastian, Sungai Lading Orangutan Project
Photograph through leaves of an orangutan in a tree
Flanged males have all of their secondary sex characteristics (cheek pads on the sides of the face and a throat pouch) while unflanged males (pictured above) lack those characteristics.

Identifying unflanged males in museum collections

A skeletal study of unflanged male body sizes in light of this twenty-first-century knowledge was still missing from primatological work.

The difference between flanged and unflanged skeletons is hard to study because cheek pads on faces are not visible on the skull. Wallace knew what their faces looked like because he personally observed many of the orangutans alive in the trees before he killed and studied them. But scientists today are using collections in museums from individuals who died in the wild long ago, a rare and irreplicable resource.

Orangutan skeletons in museums do have faces, rendered lifeless long ago when collectors like Wallace sent remains to museums, but they exist in the form of taxidermy skins. Skins are an easily overlooked part of the collections because they are typically housed in completely different cabinets, rooms, or even sections of the museum from the skeleton. As an undergraduate I had spent a lot of time at the mammals collection in the Smithsonian National Museum of Natural History in Washington, DC, so I knew that there were orangutans whose remains in the collection included both skins with visible faces and associated skulls and skeletons stored in a different room. With the help of this collection, I could identify which males were flanged and which were unflanged and then look at the differences in their skeletons.

Opening the drawers that contained the orangutan skins at the Smithsonian, it was immediately clear that it was possible to differentiate flanged and unflanged faces. There were big flanges on the flanged male faces missing from unflanged faces. The skulls associated with each face were in nearby cabinets, and all had the features one would expect in adult males: fully adult dentition, large canines, and some level of dental wear with age. But when I went to the room with the rest of their skeletons, that’s when things got interesting.

Old males with juvenile bodies

All primate juveniles have open growth plates at the ends of their long bones that allow for growth. These growth plates fuse shut at puberty when individuals reach their adult heights. The orangutans whose skulls were clearly adult male and whose faces were flanged had limb bones you’d expect for an adult male: the ends of their long bones were fused shut. But the ones whose skulls were clearly adult male but whose faces were unflanged had something surprising: unfused long bones. Adult unflanged males, despite having fully adult dentition, had long bones that looked like they came from a juvenile.

These seemingly old males with juvenile bodies were quite unexpected at first, but on reflection they began to make a lot of sense. Adult unflanged males lacked their secondary sex characteristics, so hadn’t really gone through puberty yet, which is typically when bones fuse. Since some unflanged males go on to flange later, it’s possible they stayed unfused to allow for the chance that they might flange later and in turn fuse their long bones.

When evolutionary narratives only describe large, strong males as desired by females, they miss the diversity of body shapes and sizes that males come in.

Still, I wanted to make sure this wasn’t some sort of mix up. To make sure it wasn’t an error, I sought out as many orangutan skeletons as I could find. It took time, and sometimes rescheduled trips due to omicron surges, but in the end, I made it to 14 museums across five countries (United States, United Kingdom, Germany, Switzerland, and France). This included a visit to the very orangutan skeletons Wallace studied, some of which are now housed at the Natural History Museum in London. Sure enough, I found 18 unflanged males with male-sized canines and adult dentition but unfused long bones.

Measuring unflanged male body sizes

Once I had these males identified, I could start to look at differences in the skeletons of flanged and unflanged males. I measured features in the skeleton that correlate to body size, both as mass and as stature, in close to 100 adult orangutans. To estimate mass, I examined features that typically correlate with weight (femoral head diameter, pelvic bi-iliac breadth, long bone cross-sectional area), and I also looked at reported weight at death cataloged in the records by the collector of many of the Smithsonian orangutans, W. L. Abbott. I estimated stature using long bone length.

As expected, the young adult unflanged males, still in their adolescent years, had female-sized correlates of mass. Yet their stature could even be larger than adult females. The correlates of mass for males who remained unflanged well into adulthood were typically intermediate between female sized to flanged male sized. When it came to stature, their long bone lengths ranged between female sized and flanged male sized, but some were as large as the flanged male range. While you will be able to read the results in far more detail in my forthcoming Integrative and Comparative Biology manuscript; overall, unflanged male body size values ranged from female sized to flanged male sized. Orangutans show us that a feature that varies according to biological sex is more complicated than a large/small dichotomy. Features typically thought of as highly sexually dimorphic can vary on a spectrum.

Rethinking the sneak

Adult unflanged males were not necessarily sneaking as female sized. This idea was interwoven with the narrative of them as a sneaky sexual predator. While forced copulations can happen in orangutans, these occurrences are not unique to unflanged males. Flanged males also force copulations and unflanged males can also be successful siring offspring with females who have not had a baby before or in times of dominance hierarchy instability. Just because they are at the time small and subordinate, does not mean that they are some unwanted deviant sneaking with their female sizes and forcing copulations. When evolutionary narratives only describe large, strong males as desired by females, they miss the diversity of body shapes and sizes that males come in. We can do better to explain all male orangutans that exist in all their varied sizes and behaviors.

Towards a more inclusive primatology

This shift in perspective can lead to a more accurate and more inclusive primatology. Improving how we talk about primates can impact how we talk about humans too. There are peer-reviewed journal articles which use the higher reproductive success of large, strong primate males to substantiate the idea that female apes and humans all evolved to desire large, strong males. As if desire takes one shape or form. But if ape males instead evolved a diversity of body shapes and sizes, and there is no one ideal male size, then we can begin to expand how we think about who or what is most desired. Adult unflanged males break down our gendered expectations of who is “normal” and how male bodies evolve. A diversity of body shapes, sizes, and desires exist, and our evolutionary narratives are capable of reflecting that if we let them.

Reevaluating sexual dimorphism

Human bodies too come in all sorts of shapes and sizes. Any feature that varies according to biological sex, be it height, weight, pelvic breadth, genes, hormones, chromosomes, behavior, aggression, or desire, exists on a spectrum and not a dichotomy. There are differences in group averages, but focusing on such differences would mean ignoring the huge range of overlap. It would mean ignoring intersex folks who make up around two percent of the population. It would also mean missing all the times that people fail to fit the binary cleanly and clearly. Queer folks know this well.

While recent work has been challenging these dichotomies in humans, showing that features in human skeletons are not always either masculine or feminine, a similar shift in understanding has yet to take place in primatology. In the absence of obviously apparent intersex primates speaking up that they exist, it’s hard for researchers to notice that biological sex could be any more complicated. It’s high time to start reevaluating the term sexual dimorphism in humans and primates and the assumptions of sex as a binary that undergirds it. Orangutan body sizes sit on a spectrum and recognizing this can help us shift our expectations of biological sex in humans and primates alike.

Sex is not a strictly binary phenomenon. My hope is that we can let go of pathologizing the bodies of those who do not fit clearly or cleanly into male or female categories in every way and instead seek evolutionary explanations that can capture the diversity of body shapes and sizes we see―whether we are speaking about apes or ourselves.


Alexandra Kralick

Alexandra Kralick is a PhD candidate at the University of Pennsylvania set to graduate in May 2023. She has written about skeletons and the sex binary for SAPIENS and has forthcoming papers in review in the journals Evolutionary Anthropology and Integrative and Comparative Biology as well as a co-organized symposium on queer biological anthropology coming up at the American Association of Biological Anthropologists annual conference. Follow her work at and reach her at [email protected].

Cite as

Kralick, Alexandra. 2023. “When Ape Sex Isn’t Simple.” Anthropology News website, March 14, 2023.