Geographic Varieties through Morphological Assessment in the Orangutan, and Implications for the Conservation Program

Abstract

Deforestation is a major threat to orangutans and has resulted in a significant decrease in population numbers. Bornean orangutans alone experienced a loss of around 100,000 individuals between 2009 and 2015. Hunting also causes a relatively high number of yearly losses. Some orangutans that survive deforestation are placed in rehabilitation centres with a view towards reintroduction within their historical distribution range. It is important to understand orangutan morphological traits that evolved specifically in different environments to aid efforts in reintroducing them to an appropriate population. This study investigates the variations in cranial, mandibular and dental morphology of subadult and dentally mature orangutans sampled from six regions in Borneo and Sumatra. Four hundred and seventy-two individuals with detailed provenance were studied and 51 measurements were taken: 22 cranial, 10 mandibular and 19 from the dentition. Analyses focused on inter- and intraspecific variations in sagittal crest development, patterns of sexual dimorphism, dental morphology and masticatory apparatus, including attention to evidence of the types of food orangutans consume. I also investigated evidence suggesting that orangutans are hunted as bushmeat by examining damage to the cranial base. This study finds interpopulation differences in the patterns of sexual dimorphism among orangutans. The facial skeleton is more sexually dimorphic than the neurocranium, and Bornean orangutans (Pongo pygmaeus) tend to be more sexually dimorphic than Sumatran orangutans (Pongo abelii). Study findings show that increases in masticatory equipment size are associated with larger sagittal crests. Both sexes of Bornean orangutans are wider across the incisors than Sumatran orangutans. Sumatran females have a longer palate than Bornean females, but the opposite pattern is observed in males. In both sexes, southwest Kalimantan orangutans have the largest dimensions, followed by North Sumatran and north Kapuas River. vii viii For the dentition, Bornean and Sumatran males do not differ in canine size. Bornean females have thicker and shorter lower canines than females. Females of southwest Kalimantan have the largest postcanine teeth of all the populations sampled, with the exception of the M3. For mesio-distal measurements, females of North Sumatra have similar molar size sequences (MSS) to males. However, males of North Sumatra have different MSS than other males as well as females. My study findings show that the targeting of orangutans for bushmeat is related to the orangutan’s size and sex class. This is likely a result of hunters’ preference for targeting the prey that are easiest to kill. Orangutan ecology and behaviour may play a role in how exposed orangutans are to hunters. In this thesis, I show that orangutan morphological variation is species- specific, with some populations showing population-specific morphologies. These findings should be considered in orangutan rehabilitation and reintroduction programs. It is especially important to take into account each individual’s population-specific morphological affinities to identify whether the population to which it is being reintroduced is appropriate in the context of the individual’s specific adaptations to its environment. This will ensure that each individual orangutan is reintroduced into its proper environment.