Doctoral Dissertation Research: The Biomechanics of Vertical Clinging and Grasping in Primates

Duke University, Durham NC

Investigators

Abstract

Doctoral student Laura Johnson (Duke University), under the supervision of Dr. Daniel Schmitt, will investigate the biomechanics of a specific set of primate postures - those adopted while vertically clinging to tree trunks. The research focuses on a key adaptive shift associated with the evolutionary origins of primates, that of the 'invasion' of the complex arboreal environment. Early primates evolved grasping hands and feet with nails, rather than keeping claws, such as found in other tree-dwelling species (e.g., squirrels). This evolutionary event of loss of claws raises a critical paradox for biomechanists, that of understanding how arboreal primates maintain an effective grip on large, vertical tree trunks without claws. The investigation of this question addresses morphological adaptation to an arboreal niche in both living and ancestral primates. Not being able to take advantage of claws in the arboreal habitat, primates must hold onto the trees by using postures that will minimize muscular work. Prior research on mammalian biomechanics suggests hypothesized variation within three contexts involving differing musculoskeletal anatomies, substrate sizes, and body weights. To examine these variables, a sample of eight primate species from the Duke Lemur Center which differ in anatomies and body weights are observed in a laboratory setting. Individuals are prompted to vertically cling and grasp to substrates of four increasing sizes, relative to their body weight. The ground squirrel, Scurius caroliniensis, is included as a clawed non-primate mammalian outgroup. Video data are collected using an eight-camera system to document postures in three dimensions, and data on single-limb loading patterns are collected utilizing force plates inserted into the substrate. Results from this study will shed light on the evolutionary pressures that led primates to lose claws in favor of nails, an adaptation that may be related to primate body size increase, and is a necessary precursor to human hand grasping. The project also incorporates undergraduate training and broad dissemination of data to other researchers and the public, including local K-12 schools, and will find application in zoo design and primate conservation.