Biology_of_Turtles

Figure 6.13  Comparison of representative mean activity patterns for three focal hindlimb muscles during swimming (shaded bars) and walking (unshaded bars) for Trachemys scripta (left column) and Apalone spinifera (right column) for (A, replotted from Figure 6.9) adults and (B) juveniles. Vertical lines demarcate cycles into recovery (R) and thrust (T) phases for swimming and swing (SW) and stance (ST) phases for walking; these demarcations are both indicated with appropriate labels on muscle activity graphs. All error bars indicate 1 SE.

studies of turtle locomotion can provide a model for workers on other animal systems who are seeking to understand general principles of how the musculoskeletal system works and how its function can diversify through the course of evolution.

Several future directions for research on turtle locomotion are suggested by our results. First, measurement of EMG patterns from additional muscles could test for distinctions in the roles of muscles that appear—based on anatomical position—to have redundant functions across particular joints. Such studies could evaluate potential correlations of muscle motor patterns with distinctions in, for example, muscle fiber types (Laidlaw et al., 1995) or moment arms (Kargo & Rome, 2002), providing further insight into the versatility of motor control in turtle limbs despite the potential constraints placed on locomotion by the shell. Second, some proposed functions of muscles could be examined through direct measurement of muscle length change using sonomicrometry (Olson & Marsh, 1998; Gillis & Biewener, 2000). Among other questions, such measurements could address the potential for limb muscles to be operating at suboptimal lengths during terrestrial locomotion in aquatic specialist species. A third area warranting study is the production of propulsive forces during both terrestrial and aquatic environments. Although force platform studies have provided insight into the maintenance of slow speeds and the locomotor energetics of turtles (Jayes & Alexander, 1980; Zani et al., 2005), the implications of external locomotor forces for muscular force production and loading of the limb skeleton (Biewener, 1983; Blob & Biewener, 2001) remain to be examined. Efforts to measure the net force produced by all limbs during swimming have been made in turtles (Davenport et al., 1984), but measurements of forces produced by individual limbs—for

example, using particle image velocimetry (Drucker & Lauder, 1999; Blob et al., 2003)—could inform evaluations of functional divergence between the fore and hindlimbs. A fourth potential area of focus for future research on turtle limb function is an expansion of the locomotor situations in which limb function is evaluated. Additional data on limb function in turtles under a variety of conditions, including variations in temperature, external load, and locomotor slope (Muegel & Claussen, 1994; Wren et al., 1998; Claussen et al., 2002, 2004), and in different behaviors, such as aquatic maneuvering (Rivera et al., 2006), could aid understanding of the full locomotor capacity of these animals and the impact it has on their survival (Biewener & Gillis, 1999; Reilly & Blob, 2003). Efforts to further examine forelimb function (Pace et al., 2001; Blob et al., 2004) and interlimb coordination (Renous & Bels, 1996; Blob et al., 2003) could also substantially improve understanding of turtle functional morphology, evolution, and ecology.

Examinations of any of these issues in single turtle species would be worthwhile, but because the turtle lineage includes species with a diverse range of habits and reasonably well resolved phylogenetic relationships, comparative studies of limb function in turtles hold especially high promise. Despite containing less than 300 species, turtles include species with diverse locomotor habits, including multiple evolutions of highly terrestrial groups (tortoises, box turtles), different types of highly aquatic groups (bilateral forelimb flapping sea turtles and carettochelyids, forelimb rowing trionychids, bottom walking kinosternids and chelydrids), and species (e.g., many emydids) with intermediate habits (Zug, 1971; Walker, 1973; Ernst & Barbour, 1989). The proliferation of recent research on evolutionary relationships of turtle species has provided an important framework for studies of this diversity (Shaffer et al., 1997; Van der Kuyl et al., 2002; Stephens & Wiens, 2003; Engstrom et al., 2004; Krenz et al., 2005). With data from a sufficiently broad ecological and phylogenetic range of species, it should be possible to evaluate primitive and derived character states for limb function in turtles, allowing tests of adaptive hypotheses through the application of phylogenetic comparative methods (Westneat, 1995; Butler & King, 2004; Garland et al., 2005). Thus, with continued expansion from efforts made to date, studies of turtle locomotion should provide an important model for future research on functional diversity, adaptation, and evolution.

Acknowledgments

We are grateful to the editors for their invitation to contribute to this volume, and to J. Wyneken and anonymous reviewers for helpful comments on an early version of this chapter. We also thank Cinnamon Pace and Erin Scanga for assistance with data collection for many of the experimental results reported in this chapter, Jeff Walker for access to QuickImage® and QuickSAND® software used for kinematic analysis, and Stephen Gosnell for pointing out helpful literature. Field collection of animals for this work was authorized by Illinois Scientific Permit No. A99.0550. This research was supported by NIH (1F32NS10813-01 and 5F32NS10813-02 to RWB) and the Office of Naval Research (N000149910184 to MWW and J. Walker). Support during final analyses and manuscript preparation was provided by NSF (IOB 0517340, to RWB), NIH (2 R01 DC005063-06A1 to E. Peterson, Ohio University; subaward UT10853 to RWB), and the Clemson Department of Biological Sciences.

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members of the genus Apalone are voracious predators that will actively strike at elusive prey under water (Dalrymple, 1977; Ernst & Barbour, 1989).

7.2Materials and Methods

7.2.1Anatomical Studies

The anatomy of the cervical vertebrae, joint structures, and cervical musculature of Chelodina longicollis was studied by means of dissection of a preserved specimen (K.B.I.N., R.G.nr.4566). Additionally, an adult C. reimani (cadaver obtained through the commercial pet trade) was used to study the cervical anatomy. One individual of the species Apalone ferox and one A. spiniferus were dissected to investigate the anatomy of the cervical system in a typical cryptodire. Animals were obtained through the commercial pet trade, sacrificed by means of an overdose of Nembutal and preserved in a 10% aqueous formaldehyde solution for 24 hours. Next, animals were rinsed extensively and transferred to a 70% aqueous ethanol solution.

Cervical morphology was studied by means of dissections. In the morphological descriptions, vertebrae are indicated by capital C or D (cervical and dorsal vertebrae, respectively) followed by their serial number. C1 is closest to the head. Joints are labeled by the number of the adjacent vertebrae: C(n)-(n − 1), with n the number of the more caudal vertebra and (n − 1) the number of the more cranial one as suggested by Heidweiller (1991). For example, the joint between vertebra 5 and 6 is defined as “joint C6-5.”

7.2.2Kinematic Analyses

To study the kinematics of snorkeling, two live adult specimens of Chelodina longicollis were used for the experiments (one female of 730 g, 18 cm carapace length and one male of 520 g, 15 cm carapace length). The animals were obtained with the help of the Antwerp Zoo and were housed in a glass aqua-terrarium on a 12-hour light/dark cycle. One live Apalone ferox and one live A. spiniferus, obtained through the commercial pet trade, were used to study the mobility of the cervical vertebrae in these long-necked cryptodires. The water temperature was kept at 28°C for all species. Twice a week, the turtles were fed with meat, mice, and small invertebrates (crickets, grasshoppers).

Snorkeling movements (neck movements in the vertical plane) of C. longicollis were recorded by means of cineradiography in lateral view using a Polydoros 80S generator equipped with a Siemens Siregraph D40 x-ray flash apparatus at 66 kV. The digital cineradiographic recordings (depending on the sequence 4 or 6 frames per second, Fluorospot H) were printed on Scopix laserfilm (35 × 43 cm) by means of an Agfa laser printer. During the recordings, the animals were restrained by means of a body-shaped corselet. This corselet was mounted under the water surface on a fixed frame.

The sequences were projected frame by frame and digitized. Digitization of the position of the joints allowed the calculation of several kinematical parameters (joint angles, elevation of the head, head position) in a turtle-bound frame. The same terminology of joint rotations is used as described for the neck movements in the horizontal plane, i.e., clockwise rotations are defined as positive.

Mobility of the cervical vertebrae in A. ferox and A. spiniferus was studied by means of CT scanning. The animal was anesthetized by means of intramuscular injection of Ketamine (150 mg/ kg body mass). CT scans were recorded using a CT-highlight Advantage scanner at the University of Antwerp Hospital. Of 10 different static neck positions, ranging from fully extended to fully retracted, 3-s long recordings resulting in 1.5 mm thick slices through the vertebral column were made at 140 kV, 140 mA. Scans were printed on Scopix LT2B-100 NIF x-ray film.