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

2013, Journal of Experimental Biology

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. more common and ancestral form of swimming in turtles (Joyce and Gauthier, 2004) and has 1 been reported as the exclusive swimming mode for all but one freshwater species (Fig. 1). In 1 rowing turtles, the forelimb of one side moves essentially in phase with the contralateral 1 hindlimb, so that forelimbs (and hindlimbs) of opposite sides move asynchronously (Pace et al., 1 2001; Rivera et al., 2006; Rivera and Blob, 2010; Rivera, G. et al., 2011). Rowing species also 1 tend to possess moderate to extensive webbing between the digits of the forelimb and hindlimb 1 (Pace et al., 2001) [i.e. distally expanded and paddle-shaped (Walker and Westneat, 2002b)] 1 (Fig. 2). Synchronous flapping is a much rarer locomotor style among turtles, definitively 1 employed by the seven extant species of sea turtle (Wyneken, 1997) (Fig. 1). Flapping turtles swim via synchronous motions of forelimbs that have been modified into flat, elongate, semi-1 rigid flippers [i.e. distally tapering wing-like appendages (Walker and Westneat, 2002b)] (Fig. 1 2). Foreflippers may produce thrust on both upstroke and downstroke, but the hindlimbs have a 1 negligible propulsive role (Walker, 1971, 1973; Davenport et al., 1984; Renous and Bels, 1993; of T. scripta) and dorsoventral (less than a third that of T. scripta) motions of the forelimb (Pace 1 et al., 2001). These findings indicate that in addition to differences in kinematics between modes 1 of locomotion (i.e. flapping versus rowing), significant variation can also exist within locomotor 1 modes. Finally, the lack of data on the angle of attack of limbs, for all but a few species of sea 1 turtle (Davenport et al., 1984) , limits the ability to fully interpret the hydrodynamic significance 1 of kinematic results found in the literature. 1 The goals of this study were to (1) examine forelimb kinematics within and between 1 locomotor modes across turtle species to more precisely quantify and characterize the range of 1 limb motions used by flappers and rowers, and (2) determine how Carettochelys insculpta uses 1 synchronous forelimb movements to swim, allowing us to evaluate whether the limb motions 1 displayed by this distinctive freshwater species are more strongly correlated with its phylogenetic 1 relationships to other species, or its locomotor mode (i.e. synchronous use of the foreflippers).