Forelimb kinematics during swimming in the pig-nosed turtle,Carettochelys insculpta, compared with other turtle taxa: rowing versus flapping, convergence versus intermediacy

Summary Animals that swim using appendages do so by way of rowing and/or flapping motions. Often considered discrete categories, rowing and flapping are more appropriately regarded as points along a continuum. The pig-nosed turtle, Carettochelys insculpta, is unusual in that it is the only freshwater turtle to have limbs modified into flippers and swim via synchronous forelimb motions that resemble dorsoventral flapping, traits that evolved independently from their presence in sea turtles. We used high-speed videography to quantify forelimb kinematics in C. insculpta and a closely related, highly aquatic rower (Apalone ferox). Comparisons of our new data to those previously collected for a generalized freshwater rower (Trachemys scripta) and a flapping sea turtle (Caretta caretta) allow us to (1) more precisely quantify and characterize the range of limb motions used by flappers versus rowers, and (2) assess whether the synchronous forelimb motions of Carettochelys insculpta can be classified as flapping (i.e. whether they exhibit forelimb kinematics and angles of attack more similar to closely related rowing species, or more distantly related flapping sea turtles). We found that the forelimb kinematics of previously recognized rowers (T. scripta and A. ferox) were most similar to each other, but that those of Carettochelys were more similar to rowers than to flapping Caretta. Nevertheless, of the three freshwater species, Carettochelys was most similar to flapping Caretta. "Flapping" in Carettochelys is achieved through very different humeral kinematics than in Caretta, with Carettochelys exhibiting significantly more anteroposterior humeral motion and protraction, and significantly less dorsoventral humeral motion and depression. Based on several intermediate kinematic parameters and angle of attack data, Carettochelys may in fact represent a synchronous rower or hybrid rower-flapper, suggesting that traditional views of Carettochelys as a flapper should be revised.