Ontogeny of hallucal metatarsal rigidity and shape in the rhesus monkey (Macaca mulatta) and chimpanzee (Pan troglodytes) |
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| Authors: | Biren A. Patel Jason M. Organ Tea Jashashvili Stephanie H. Bui Holly M. Dunsworth |
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| Affiliation: | 1. Department of Integrative Anatomical Sciences, Keck School of Medicine, University of Southern California, Los Angeles, CA, USA;2. Human and Evolutionary Biology Section, Department of Biological Sciences, University of Southern California, Los Angeles, CA, USA;3. Department of Anatomy and Cell Biology, Indiana University School of Medicine, Indianapolis, IN, USA;4. Department of Biomedical Engineering, Indiana University – Purdue University Indianapolis, Indianapolis, IN, USA;5. Molecular Imaging Center, Department of Radiology, Keck School of Medicine, University of Southern California, Los Angeles, CA, USA;6. Department of Geology and Paleontology, Georgian National Museum, Tbilisi, Georgia;7. Department of Sociology and Anthropology, University of Rhode Island, Kingston, RI, USA |
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| Abstract: | Life history variables including the timing of locomotor independence, along with changes in preferred locomotor behaviors and substrate use during development, influence how primates use their feet throughout ontogeny. Changes in foot function during development, in particular the nature of how the hallux is used in grasping, can lead to different structural changes in foot bones. To test this hypothesis, metatarsal midshaft rigidity [estimated from the polar second moment of area (J) scaled to bone length] and cross‐sectional shape (calculated from the ratio of maximum and minimum second moments of area, Imax/Imin) were examined in a cross‐sectional ontogenetic sample of rhesus macaques (Macaca mulatta; n = 73) and common chimpanzees (Pan troglodytes; n = 79). Results show the hallucal metatarsal (Mt1) is relatively more rigid (with higher scaled J‐values) in younger chimpanzees and macaques, with significant decreases in relative rigidity in both taxa until the age of achieving locomotor independence. Within each age group, Mt1 rigidity is always significantly higher in chimpanzees than macaques. When compared with the lateral metatarsals (Mt2–5), the Mt1 is relatively more rigid in both taxa and across all ages; however, this difference is significantly greater in chimpanzees. Length and J scale with negative allometry in all metatarsals and in both species (except the Mt2 of chimpanzees, which scales with positive allometry). Only in macaques does Mt1 midshaft shape significantly change across ontogeny, with older individuals having more elliptical cross‐sections. Different patterns of development in metatarsal diaphyseal rigidity and shape likely reflect the different ways in which the foot, and in particular the hallux, functions across ontogeny in apes and monkeys. |
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| Keywords: | ape climbing cross‐sectional geometry grasping hallux monkey |
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