Biology_of_Turtles

a contralateral synchronization (Davenport et al., 1984). At higher current velocities, they employed simultaneous foreflipper beating. At 25°C, the transition between swimming modes occurred at a sustained speed of 1.32 BLs-1 and the mean maximum sustained swimming speed using the flapping foreflippers was 3.31 BLs-1. In this case, the synchronized lift-based flapping gave a 2.5-fold advantage over dogpaddle (Davenport et al., 1984), and the morphological configuration (shell shape, streamlining) was identical in the two conditions.

According to Pace et al. (2001), the evolution of aquatic specialization appears to have followed two distinct paths. Good swimming ability is correlated with forelimb hypertrophy and the use of flapping limb kinematics in sea turtles and carettochelyids. In contrast, in softshell turtles good swimming ability is correlated with rowing limb kinematics, in which the thrust-generating surface area of the limbs is maximized and drag minimized. In comparison to these specialized paths, the more functionally and morphologically generalized freshwater turtles, such as emydids, can be predicted to exhibit systems of propulsion with a greater capacity for both aquatic and terrestrial performance.

5.3.3Coordination of the Limbs in Aquatic Locomotion

There are important differences in the limb coordination of turtles during aquatic locomotion. Zug (1971) analyzed sequences of limb movement during swimming of 15 species of cryptodires, mainly including semiaquatic or highly aquatic forms of four families: Chelydridae, Kinosternidae, Emydidae, and Trionychidae. He showed that all animals adopted in-water gaits equivalent to fast walks or moderate runs when the retraction phase of rowing was considered as equivalent to the stance phase of vertebrate terrestrial locomotion (they both are the propulsive stage of the limb cycle). In several cycles of swimming of each species, calculation of the percentage of stride during which the left fore-footfall follows the left hind foot, according to Hildebrand’s (1966) method of symmetrical gait analysis, enabled us to characterize swimming as equivalent to a diagonal sequence of walk or running trot (Figure 5.17). In these gaits, in the displacement of a diagonal couplet the beginning of retraction of a hindlimb occurs before the beginning of retraction of the diagonally opposed forelimb. Zug (1971) estimated that this overlapping diagonal sequence would produce a minimal amount of yaw. This reinforces the idea that generation of consistent amounts of thrust can be produced from diagonally opposed limb propulsive phases operating more-or-less at the same time. In swimming, the variation of timing of limb movement is great, caused by variation in the ipsilateral pairs. In contrast, in swimming adult sea turtles (Figure 5.18) the foreflippers flap simultaneously. In hatchlings, this simultaneous movement is not perfect, but most swimming results from near-synchronous beating of the foreflippers. However, hatchling marine turtles can also swim with a type of rowing, termed “dogpaddling,” occurring when diagonally opposite limbs move together, (Davenport et al., 1997). This is similar to the swimming gaits of freshwater turtles except that the limbs move in the horizontal and vertical planes to a greater extent. Marine turtle hatchlings (Caretta caretta) can also adopt a propulsive rear flipper kicking by simultaneously beating the hindlimbs; in this case, the forelimbs are tucked against the shell and take no part in locomotion (Davenport & Pearson, 1994).

Differences also appear when semiaquatic or highly aquatic turtle forms walk on land and are therefore forced to move under gravitational constraints. Zug (1971) studied the terrestrial locomotion of aquatic species and noticed a basic change (Figure 5.17): the slower gaits fall, in Hildebrand’s analysis system (1966), between the walk and the trot of the lateral sequence. In each diagonal couplet, the beginning of retraction of the forelimb started before the beginning of retraction of the opposite hindlimb. In gaits characterized by this lateral sequence, the footfall of the hindlimb is followed by the footfall of the ipsilateral forelimb. In the diagonal sequence, the hindlimb footfall is, in contrast, followed by the diagonally opposite forelimb footfall.

The gait of terrestrial forms (Testudinidae) is characterized by this lateral sequence alone (Figure 5.19). These data signify that semiaquatic and highly aquatic freshwater turtles (such as Trionychidae) that have “discovered” a diagonal sequence of moving when they progress in water have

B

Carettochelys

Figure 5.18  Simultaneous beating of the fore limbs during the swimming of Dermochelys coriacea (A) and suggestion of the same limb synchrony in Carettochelys. (Used with permission from Pritchard, 1979.)

(B) The numbers indicate successive stages of the two phases of the forelimb cycle: upstroke (from 1 to 5) and downstroke (from 6 to 10) (used with permission from Renous & Bels, 1993).

and contribute to the equilibrium of the body. These turtles have lost the basic terrestrial gait and have acquired, by minimal adjustment of the swimming mode, a type of energy-saving system appropriate for moving a heavy mass forward under gravity.

5.3.4A History of Limb Coordination

Patterns of limb coordination are vital to understand the evolution of locomotion in aquatic turtles and the selection of ecological strategies (Hendrickson, 1980). We believe that coordination guides the structural modifications of the limbs. Particularly interesting are the questions posed by the existence of synchronous foreflipper action in locomotion in diverse extant groups, some of which have rigid flippers (Cheloniidae, Demochelyiidae), others not (Carettochelyiidae). Even among the still living species, it seems that rigid foreflippers are not a prerequisite for synchronization.

Recently, a study of large tetrapod track ways with exceptional fossilization (Figure 5.21) from Cerin (Upper Jurassic, France) (Bernier, 1985; Roman et al., 1994) suggests that giant aquatic turtles used simultaneous movements of contralateral limbs (Gaillard et al., 2003). These extinct turtles moved in shallow waters of a lagoon (though we do not know whether they also swam in deeper waters). Interpretation of the footprints suggests that the limbs differed from extant marine turtles in the presence of free claws and an interdigital web. The fossilized animals did not possess true rigid

Figure 5.19  Patterns of propulsion and limb coordination to progress in water and on land in adult extant chelonians (adapted from Renous et al., 1999) ant.ap.prop., anterior appendicular propulsion; post.ap.prop., posterior appendicular propulsion (which can be shared with the anterior limb in the better swimmers). The marine swimmers are divided in small and large forms. The white rectangle indicates the synchronous periodic movements of the limbs introducing a novelty in the swimming adaptation. The grey rectangle indicates the action field of gravity.

flippers and so must have acquired the synchronous pattern of limb coordination before the selection of highly modified rigid fins. This important difference raises questions about the locomotor mechanisms of primitive groups of Eurysternidae and allied forms also present at the site, as well as primitive Protostegidae. Besides, the hindlimbs ought to have been longer than the forelimbs. Synchronous movement of the limbs evidently appeared several times as far as the Jurassic forms of Cheloniidae-Dermochelyiidae and Carettochelyidae are concerned. As we can deduce from Zug (1971) for non-marine and non-carettochelyid species, no phylogenetic relationships between these groups can be resolved from their limb coordination patterns.

So what drove the evolution of synchronous foreflipper action? Large size does not seem an adequate explanation; large individuals of riverine species (e.g., Batagur, Podocnemis expansa) may overlap in body size with small individuals of extant marine turtles but employ the diagonal pattern associated with rowing in most aquatic turtles, which is probably retained for its flexibility that permits maneuverability and regular use of the terrestrial habitat.

We suggest that synchronous forelimb movement is the primary feature that allows a true pelagic marine life in modern forms—it is seen in its most highly developed form in those large turtles that, as adults, carry out extensive high speed oceanic migrations (Chelonia, Dermochelys). Synchronous limb movement was already used by other species during the Jurassic (Cerin and probably Bavaria and Canjuers), possibly also by the first South American protostegids. At the same time, it was found used with in freshwater environments (as suggested by the extant Carettochelys);

the oldest recorded Carettochelyidae are either from the late Jurassic or early Cretaceous (Young & Chow, 1953) but they were probably already well present at Jurassic times, according to their wide distribution during the Lower Cretaceous (Lapparent de Broin, 2004) and new discoveries in the Cretaceous from France. This implies an early separation from the Trionychidae, their sister group, probably during the late Jurassic. The carettochelyid humerus is known from the early Cretaceous from Laos and complete limbs are described from the Eocene (Harrassowitz, 1922); they are identical to those of the extant forms. The carettochelyid paddle had therefore characterized all members of the family at its appearance and was probably associated to synchronous movement of the limbs. The ability of extant Carettochelyidae to penetrate estuaries and salt waters is known in New Guinea and Australia, thus it is not impossible that the track ways from Cerin was made by a carettochelyid, if not a protostegid or an eurysternid.

The challenge is to identify the environmental or life history features that have selected for synchronous forelimb movement in non-pelagic situations and at relatively small sizes. The very shallow environment of the Cerin trackways and the occasional synchronous action of forelimbs during mud digging in some extant Trionyx species may give a clue; perhaps synchronous limb action aids semi-terrestrial movement in shallow water overlying extensive soft substrata? More study is clearly required.

5.4Conclusion

In chelonians, phylogenetic studies and cladograms constructed with combined molecular and morphological data have revealed satisfactory correlations between the great diversity of habitats occupied by turtles and the major patterns of locomotion that they exhibit. This is an important first step toward improving our understanding of the evolutionary processes of this terrestrial group of vertebrates. In reality, all chelonians are “terrestrial” animals by virtue of their major physiological functions, especially respiratory physiology and principal behaviors (notably, egg-laying). At regular periods in their lives, at least females of all turtle species must return to land. In spite of secondary adaptations to aquatic habitats that introduced changes in the mode of coordination of limb movements (from lateral sequences on land under the influence of gravity to diagonal sequences and synchronous sequences in water), they all must use some kind of limb-based locomotion to move on land.

Different evolutionary scenarios and locomotor strategies have been selected in the chelonians. It is likely that the basic structural “turtle pattern” arose in terrestrial forms solely in the context of a terrestrial life—most likely to provide maximum body protection. Subsequent adaptations to a semiaquatic or highly aquatic way of life were rapidly adopted, allowing the development of a mode of locomotion that was able to work within the constraints of the two environments, aquatic and terrestrial. This mode retained an alternating limb function, even if it underwent some further adjustments when the environment changed. In this context, the morphological-functional features of turtles cannot be solely attributed to the constraints of a single environment. However, we must take into account the fact that the constraints of the terrestrial environment are strong, with animals simultaneously benefiting from and being compromised by these constraints (e.g., structural integrity of the body versus lightening of the body). Nevertheless, even in the context of this “polyvalent” adaptation requiring a diagonal coordination of the limbs, as more time is spent swimming in the aquatic environment the more turtles exhibit true “challenges” when returning to the terrestrial environment (even to lay eggs). The result is a decrease of speed and other locomotor parameters and the induction of greater energetic cost. The use of synchronous limb movement was probably adopted very early before the forelimbs were highly modified into rigid flippers (e.g., Jurassic turtles from Cerin), whereas the hindlimbs were flexible elements, similar to those of some extant forms (Carettochelyidae). This pattern of coordination, present in Cerin (an environment distinct from the sea and characterized by shallowness, great evaporation, and high salinity, hence selecting for hypertrophied salt glands), for example, was the first that permitted the eventual full conquest of

the marine environment. Subsequently, the differentiation of rigid foreand hindlimbs, longer in the case of the foreflippers, has favored a more complete adaptation to a pelagic marine life.

It is probable that the synchronous movement of the forelimbs in truly marine locomotion was the result of exaptation: perhaps performance enhancement that allowed greater thrust and speed, in prey capture or predator avoidance. The development of rigid limbs and their synchronous functioning in marine turtles facilitated the coverage of great distances in open sea, either to move between far-flung benthic food resources (coral reefs, sea grass) or to seek out ephemeral pelagic food concentrations (e.g., jellyfish). These morphofunctional parameters strongly influence efficiency on land when the female turtles come back to lay eggs with a large body that is very heavy under the influence of gravity. Alternate functioning of the limbs could introduce added instability and greater energy expenditure. In contrast, the alternate limb movements exhibited by the great majority of semi-aquatic and highly aquatic turtles found their optimum efficiency in the exploitation of the great diversity of fresh water habitats, yet also allowed more extensive use of the terrestrial environment.

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