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The Convergent Evolution of Suspensory Posture and Locomotion in Tree Sloths

Abstract

Recent phylogenetic analyses imply a distant relationship and long separated evolution of two-toed sloths (Choloepus) and three-toed sloths (Bradypus). No known fossil sloth is interpreted to have been suspensory. As a consequence, the suspensory posture and locomotion of the extant genera likely evolved convergently in both lineages, forming a new framework for the analysis of functional aspects of the locomotor apparatus of extant tree sloths. The suspensory posture and locomotion has altered functional demands from the phylogenetically plesiomorphic non-suspensory pronograde situation. Here, anatomical traits that have been argued to be of adaptive significance for quadrupedal suspensory locomotion are reviewed and the evolution of these traits is discussed in light of the new framework. Experimental data are largely limited to Choloepus, but help to deduce functional aspects of the anatomy in Bradypus as well. The most important adaptive traits are hands and feet modified into relatively rigid hook-like appendages, great mobility of all joints proximal to the midcarpal and transverse tarsal joints, relatively long arms with a relatively short scapula, a rounded thorax with a small diameter, a highly mobile sterno-clavicular articulation, and emphasis on powerful flexion in the proximal limb joints via advantageous lever arms. Despite these changes, patterns of limb kinematics remained conservative during the course of evolution in the lineages leading to extant tree sloths, and it is suggested here that this also applies to the pattern of neuromuscular control of limb movements during locomotion. Morphological ‘solutions’ to altered functional demands posed by inversed orientation of the body differ in the two genera of extant tree sloths, thereby corroborating the proposed diphyly. Convergent evolution in tree sloths may be attributed to functional constraints posed by fossorial adaptations in early Xenarthra that canalized sloths to adopt a suspensory posture and locomotion in the arboreal habitat.

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Acknowledgements

I would like to thank the organizers of the Xenarthra Symposium at the 9th International Conference on Vertebrate Morphology (ICVM) at Punta del Este, Uruguay for the invitation to submit this review paper. The ICVM meetings in Paris and Punta del Este have been critical for my dissertation and gave me the opportunity to get in touch with the researchers of the field. I would further like to thank my supervisor, Martin S. Fischer, for giving me the opportunity to do research on these fascinating animals. The project would not have been possible without the constant help of my colleagues; in particular I am indebted to my fellow Ph.D. candidates, some of which—of course—already earned their degrees. Bettina Hesse, John Wible, and two anonymous reviewers helped to improve earlier versions of the manuscript. Last but not least I want to thank my family for the encouragement and support to commence and finish my dissertation.

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Correspondence to John A. Nyakatura.

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Nyakatura, J.A. The Convergent Evolution of Suspensory Posture and Locomotion in Tree Sloths. J Mammal Evol 19, 225–234 (2012). https://doi.org/10.1007/s10914-011-9174-x

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Keywords

  • Xenarthra
  • Convergent evolution
  • Choloepus
  • Bradypus
  • Locomotion
  • Functional morphology