Abstract
Until recently, there has been little consensus as to the functional benefits of having vertically-shortened canines in the earliest humans. In an effort to resolve this problem, Hylander (2013) tested the hypothesis that canine height dimensions in catarrhines are linked to modifications in the amount of jaw gape . The data demonstrate that most adult male catarrhines have relatively larger canine overlap dimensions and relatively larger gapes than do conspecific females. Humans and hylobatids are the exceptions in that canine overlap is nearly the same between sexes, and so is relative gape, although humans have relatively small gape and hylobatids have relatively large gape. A correlation analysis demonstrated that a large portion of relative gape (maximum gape/projected jaw length) is predicted by relative canine overlap (canine overlap/jaw length). Relative gape is mainly a function of jaw muscle position and/or jaw muscle-fiber length. All things equal, more caudally positioned jaw muscles and/or longer muscle fibers increase the amount of gape. The net benefit for increasing gape in catarrhines is related to within species interactions as well as predation patterns. The cost, however, is to decrease bite force . In order to compensate for a decrease in bite force, jaw muscle mass must be increased so as to assure that the original bite force is maintained. On the other hand, and all things equal, more rostrally positioned jaw muscles and/or shorter muscle fibers decrease gape. The net benefit to decreasing gape is to increase bite force without a corresponding increase in muscle mass. Alternatively, the original bite force can be maintained whereas the costs of original muscle size can be reduced. Overall, the data support the hypothesis that canine reduction in early hominins is functionally linked to increased mechanical efficiency of the jaws. The purpose of this chapter is to review certain aspects of the original paper by Hylander (2013), as well as discussing additional implications of this study not previously considered. These include, but are not restricted to: (1) a review of recent developments about muscle mass and fiber lengths in a highly dimorphic model catarrhine primate, Macaca fascicularis ; (2) a discussion of the fact that relative canine overlap in male catarrhines do not mirror those in female catarrhines; and (3) based on the catarrhine data, interpretations are advanced as to relevance of the functional significance of the high mandibular condyle position in certain catarrhines, with a particular emphasis on the high condyle of robust australopithecines .
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Notes
- 1.
Changing the geometry of the mandible includes altering the position of the mandibular condyle relative to the occlusal place, as well as, e.g., shifting the tooth row rostrally or caudally.
- 2.
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Acknowledgments
I thank Matt Cartmill, Yoel Rak, Mike Plavcan, Carel van Schaik, Chris Vinyard, Christine Wall, Tim White and Richard Wrangham for their comments and suggestions on an earlier published version of this study (Hylander 2013). I also thank Andrea Taylor and Chris Vinyard for help and comments, as well as Matt Ravosa, Erella Hovers and Assaf Marom for their editorial suggestions, as well as the other reviewers of this manuscript for their helpful and cogent comments. Most importantly, I thank Yoel Rak for his continued friendship, support, and for sharing with me his keen eye for morphology, as well as his wonderful memory of his many and hilarious Israeli (Jewish) jokes. I also thank him for his wonderful and repeated hospitality on my many visits to Israel. His gracious and wonderful family (Ricka, Carmi, Benjamin and Ariel) always seemed to suffer under the delusion that I was a “real mensch”. I also thank Yoel for honoring my family for his many visits to Durham (NC), as well as to our cattle farm in the mountains of Virginia. My dear wife Linda and I, during his many visits, frequently wonder about how long one human needs to recover from jet lag. Over the past 15 years I have learned so much from Yoel. In addition, on many of our research trips, Yoel and I may not have not gotten much work done, but we did have a lot of fun. I also thank him for teaching me about Neandertal morphology, most notably during our many trips together to central and western Europe. Probably the best thing he taught me on these trips was where to find the best prosciutto and wine. Thanks Yoel, you dah man. We all love you.
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Hylander, W.L. (2017). Canine Height and Jaw Gape in Catarrhines with Reference to Canine Reduction in Early Hominins. In: Marom, A., Hovers, E. (eds) Human Paleontology and Prehistory. Vertebrate Paleobiology and Paleoanthropology. Springer, Cham. https://doi.org/10.1007/978-3-319-46646-0_7
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