Abstract
The kinematics of surface swimming of the Virginia opossum (Didelphis virginiana) and water opossum (Chironectes minimus) were studied to determine locomotor changes associated with the evolutionary transition from terrestrial to semiaquatic existence. Films of individual animals swimming across an aquarium were made and analyzed. Didelphis swam with a modified quadrupedal gait similar to a diagonal sequence run. Besides providing propulsive forces, this gait generated an upward force allowing the nasal apertures to remain above the water for breathing. In contrast, Chironectes paddled exclusively with the hind limbs in a manner similar to other semiaquatic mammals. The non-wettable fur of Chironectes seemed to provide sufficient buoyancy so that no additional forces were required to maintain the nares above the water surface. Transition to exclusive hind limb paddling in semiaquatic mammals appears to be associated with development of non-wettable fur for increased buoyancy and with increased swimming effectiveness resulting from lack of interference between forelimbs and hind limbs during the stroke cycle. Eliminating use of the forelimbs in paddling also permitted unrestricted movement of the hind limbs.
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