This study brings a new perspective to the study of primate origins by examining the influence of very small body size on the evolution of primate locomotor adaptations. Living primates exhibit a diverse array of specialized locomotor behaviors, but most include quadrupedalism in their locomotor repertoire. The form of quadrupedal walking used by primates is unusual among mammals, and is thought to have arisen as an evolutionary innovation when the earliest primates diverged from their mammalian ancestor. Current consensus states that primate quadrupedal features are a biomechanical complex that gave early primates exclusive access to resources available in an arboreal "fine branch niche," by providing mechanisms for balance on small and unstable branches.
One important issue often overlooked in research on the evolution of primate quadrupedalism is the interaction of body size with substrate (e.g., branch) size. Clearly, the locomotor challenges presented by the "fine branch niche" cannot be assumed to be the same for small and large primates. We cannot understand the evolution of primate quadrupedal locomotion without considering body size relative to substrate size. Recent debate on primate ancestral size includes the view that ancestral primates may have been much smaller than the smallest living primates (mouse lemurs), perhaps as small as 10-15g, yet few studies have considered the effects of body sizes of 100g or less on primate quadrupedalism.
This study will use locomotor growth and development as a means to investigate the influence of very small body size on primate locomotor evolution. Quadrupedal movement on a variety of substrates will be compared between and within infant and adult mouse lemurs (Microcebus murinus) in conjunction with a study of locomotor development in two small-bodied marsupials, the arboreal sugar glider (Petaurus breviceps) , and the terrestrial gray short-tailed opossum (Monodelphis domestica). Posturally stable infants will be used to test hypotheses regarding the effects of reduced body size and substrate type on quadrupedal movement. In addition, developmental transitions in morphology and locomotion from youngest infancy to adulthood will be used to test hypotheses about the biomechanical effects of body shape on quadrupedalism.
The influence of very small body size on primate quadrupedalism and in the context of the "fine branch niche" remains largely unexplored in either primates or marsupials. This study will be the first to combine developmental analysis with an examination of primate quadrupedal locomotion from the perspective of very small body size, and the first to address locomotor development in Microcebus, Petaurus, and Monodelphis from an ecological and evolutionary perspective.
This study will enhance undergraduate education at UT Austin, as the sugar gliders and mouse lemurs will be housed in an observation room utilized by students collecting behavioral data for anthropology and biology courses. Research training will be enhanced for a postdoctoral associate and students participating in the project. Results will be incorporated into an educational electronic lab module, and raw data will be made available to other researchers. Collaboration between UT Austin, Southwest National Primate Research Center, and Duke University will be strengthened.
