Applying an ecomorphological framework to the study of orangutan positional behaviour and the morphological variation within non-human apes.

Establishing relationships between morphology and behaviour in response to environmental selection pressures are crucial to understand the evolution of diversity within groups such as the hominoids. Muscle architecture (fascicle length and physiological cross-sectional area) from the fore and hindlimbs in the non-human apes were compared, with the result that it did not differ substantially, likely reflecting their characteristic use of orthograde behaviours. At the micro-architecture level, significant differences in the proportions of fast and slow muscle fibres of the triceps surae were found between orangutans and chimpanzees, reflecting subtle differences in locomotion and habitat use. As the largest, predominantly arboreal ape, orangutans were expected to have specific behavioural adaptations to the complex arboreal habitat. A new method was developed, Sutton Movement Writing and was successfully applied to record the subtle variations in positional behaviour and compliant support use in orangutans under field conditions. Finally, postural specialisations used during feeding in the terminal branch niche were identified. Overall, this thesis shows that although the non-human apes appear to share overall behaviours and morphology, more subtle variations in micro-architecture and behaviour are present in orangutans in response to their habitat, and reflects key adaptations since their split from the last common-ape ancestor