Orangutans serve as an important species in hominoid studies given their relatedness to the genus Homo, a flagship taxon chosen to represent forest conservation efforts in Indonesia, and an intriguing model for studying alternative reproductive tactics. Despite interest in the species and their importance to their environment, orangutans are exceptionally challenging to study in the wild and captivity. In this dissertation, I investigated the life history of great apes under human care and the growth/ developmental patterns of orangutans. Studbooks, records of all births and deaths of captive animals, compose the largest life history datasets for great apes. I compared the following life history variables for the four great ape species housed in the United States: age at first successful reproduction, interbirth intervals, prenatal death rate, twinning rate, number of offspring, age at last reproduction, and lifespan. Results showed species-specific differences in median age at first birth among females, age at first successful fertilization among males, and number of offspring sired by males and produced by females. The four great ape species had similar ages at last birth among females, age at last successful fertilization among males, and interbirth intervals. Overall, I found that great apes have similar reproductive stages of their lives and capabilities when living with low ecological risk despite some of the differences that exist in their life history parameters. These findings can be used in comparative studies of humans and their most recent living biological ancestors. Next, I investigated the process of flanging in male orangutans. Male bimaturism is one of the most intriguing aspects of orangutans’ biology, yet there is still much that is unknown about the flanging process. All males begin as an unflanged male with the potential to flange at a random timepoint in their lifetime. I conducted a longitudinal study of two males, Budi who flanged and Kembali who remained unflanged, housed at the Toronto Zoo. For Budi, flanging lasted approximately two years. In this time, he increased in body size by approximately 78%, increased his testosterone levels, grew cheek flanges, and began long calling. Throughout flanging, his cortisol levels also increased and his C-peptide of insulin levels fluctuated, both signs of the energetic cost associated with flanging. These findings highlight the extent to which flanging affects male orangutans in terms of their morphology, physiology, and behavior. Lastly, I investigated differences in muscle mass amongst the three adult morphs of orangutans (females, unflanged males, flanged males). Adult unflanged males are approximately the same body size as adult females whereas adult flanged males are approximately twice the size of adult unflanged males and females. Thus, I predicted flanged males would have the greatest estimated lean body mass (ELBM) followed by unflanged males and females. To test this hypothesis, I analyzed creatinine and specific gravity in a large dataset of urine samples from orangutans under human care from zoos in the United States and a smaller dataset from wild Bornean orangutans at Gunung Palung National Park (GPNP) in West Kalimantan, Indonesia. While my predictions were supported by mean ELBM values, I did not find that the different age-sex classes of adult orangutans were statistically different from one another. When we analyzed both datasets together, however, we found that flanged males had significantly higher ELBM compared to all other age-sex classes and that wild orangutans have significantly lower ELBM compared to their captive counterparts. Given the variation in body size amongst age-sex classes, I believe that sex differences in muscle mass will be distinguishable with a larger sample size. Collectively, these findings in my dissertation highlight differences amongst the life history patterns of great apes, which have classically been viewed as being largely similar to one another, and the distinctiveness of having three adult morphs in a primate species. / 2025-06-27T00:00:00Z