Aquatic turning performance of painted turtles (Chrysemys picta)and functional consequences of a rigid body design

Turtle Girdles: Comparing the Relationships Between Environment and Behavior on Forelimb Function in Loggerhead Sea Turtles (Caretta caretta) and River Cooters (Pseudemys concinna)

Locomotion in water and on land impose dramatically different demands, yet many animals successfully move in both environments. Most turtle species perform both aquatic and terrestrial locomotion but vary in how they use their limbs. Freshwater turtles use anteroposterior movements of the limbs during walking and swimming with contralateral fore- and hindlimbs moving in synchrony. In contrast, sea turtles swim primarily with "powerstroke" movements, characterized by synchronous forelimb motions while the hindlimbs act as rudders. High-speed video has been used to study powerstroking, but pectoral girdle movements and long-axis rotation (LAR) of the humerus are likely both key components to turtle locomotor function and cannot be quantified from external video. Here, we used XROMM to measure pectoral girdle and humeral movements in a sea turtle (loggerhead, Caretta caretta) compared to the freshwater river cooter (Pseudemys concinna) during terrestrial and aquatic locomotion. The largest difference among species was in yaw of the pectoral girdle during swimming, with loggerheads showing almost no yaw during powerstroking whereas pectoral girdle yaw in the cooter during rowing was over 30°. The magnitude of humeral LAR was greatest during loggerhead powerstroking and the temporal pattern of supination and pronation was opposite from that of cooters. We hypothesize that these kinematic differences are driven by differences in how the limbs are used to power propulsion. Rotations at the glenoid drive the overall patterns of movement in freshwater turtles, whereas glenohumeral LAR in loggerheads is used to direct the position and orientation of the elbow, which is the joint that determines the orientation of the thrust-generating structure (the flipper) in loggerheads.

Multiple behaviors for turning performance of Pacific bluefin tuna (Thunnus orientalis)

Tuna are known for exceptional swimming speeds, which are possible because of their thunniform lift-based propulsion, large muscle mass and rigid fusiform body. A rigid body should restrict maneuverability with regard to turn radius and turn rate. To test if turning maneuvers by the Pacific bluefin tuna (Thunnus orientalis) are constrained by rigidity, captive animals were videorecorded overhead as the animals routinely swam around a large circular tank or during feeding bouts. Turning performance was classified into three different types: (1) glide turns, where the tuna uses the caudal fin as a rudder; (2) powered turns, where the animal uses continuous near symmetrical strokes of the caudal fin through the turn; and (3) ratchet turns, where the overall global turn is completed by a series of small local turns by asymmetrical stokes of the caudal fin. Individual points of the rostrum, peduncle and tip of the caudal fin were tracked and analyzed. Frame-by-frame analysis showed that the ratchet turn had the fastest turn rate for all points with a maximum of 302 deg s-1. During the ratchet turn, the rostrum exhibited a minimum global 0.38 body length turn radius. The local turn radii were only 18.6% of the global ratchet turn. The minimum turn radii ranged from 0.4 to 1.7 body lengths. Compared with the performance of other swimmers, the increased flexion of the peduncle and tail and the mechanics of turning behaviors used by tuna overcomes any constraints to turning performance from the rigidity of the anterior body morphology.

Kinematic and hydrodynamic analyses of turning manoeuvres in penguins: body banking and wing upstroke generate centripetal force

Penguins perform lift-based swimming by flapping their wings. Previous kinematic and hydrodynamic studies have revealed the basics of wing motion and force generation in penguins. Although these studies have focused on steady forward swimming, the mechanism of turning manoeuvres is not well understood. In this study, we examined the horizontal turning of penguins via 3D motion analysis and quasi-steady hydrodynamic analysis. Free swimming of gentoo penguins (Pygoscelis papua) at an aquarium was recorded, and body and wing kinematics were analysed. In addition, quasi-steady calculations of the forces generated by the wings were performed. Among the selected horizontal swimming manoeuvres, turning was distinguished from straight swimming by the body trajectory for each wingbeat. During the turns, the penguins maintained outward banking through a wingbeat cycle and utilized a ventral force during the upstroke as a centripetal force to turn. Within a single wingbeat during the turns, changes in the body heading and bearing also mainly occurred during the upstroke, while the subsequent downstroke accelerated the body forward. We also found contralateral differences in the wing motion, i.e. the inside wing of the turn became more elevated and pronated. Quasi-steady calculations of the wing force confirmed that the asymmetry of the wing motion contributes to the generation of the centripetal force during the upstroke and the forward force during the downstroke. The results of this study demonstrate that the hydrodynamic force of flapping wings, in conjunction with body banking, is actively involved in the mechanism of turning manoeuvres in penguins.

The role of California sea lion (Zalophus californianus) hindflippers as aquatic control surfaces for maneuverability

California sea lions (Zalophus californianus) are a highly maneuverable species of marine mammal. During uninterrupted, rectilinear swimming, sea lions oscillate their foreflippers to propel themselves forward without aid from the collapsed hindflippers, which are passively trailed. During maneuvers such as turning and leaping (porpoising), the hindflippers are spread into a delta-wing configuration. There is little information defining the role of otarrid hindflippers as aquatic control surfaces. To examine Z. californianus hindflippers during maneuvering, trained sea lions were video recorded underwater through viewing windows performing porpoising behaviors and banking turns. Porpoising by a trained sea lion was compared with sea lions executing the maneuver in the wild. Anatomical points of reference (ankle and hindflipper tip) were digitized from videos to analyze various performance metrics and define the use of the hindflippers. During a porpoising bout, the hindflippers were considered to generate lift when surfacing with a mean angle of attack of 14.6±6.3 deg. However, while performing banked 180 deg turns, the mean angle of attack of the hindflippers was 28.3±7.3 deg, and greater by another 8-12 deg for the maximum 20% of cases. The delta-wing morphology of the hindflippers may be advantageous at high angles of attack to prevent stalling during high-performance maneuvers. Lift generated by the delta-shaped hindflippers, in concert with their position far from the center of gravity, would make these appendages effective aquatic control surfaces for executing rapid turning maneuvers.

Inferring cost of transport from whole-body kinematics in three sympatric turtle species with different locomotor habits

Chelonians are mechanically unusual vertebrates as an exoskeleton limits their body wall mobility. They generally move slowly on land and have aquatic or semi-aquatic lifestyles. Somewhat surprisingly, the limited experimental work that has been done suggests that their energetic cost of transport (CoT) are relatively low. This study examines the mechanical evidence for CoT in three turtle species that have differing degrees of terrestrial activity. Our results show that Apolone travels faster than the other two species, and that Chelydra has higher levels of yaw. All the species show poor mean levels of energy recovery, and, whilst there is considerable variation, never show the high levels of energy recovery seen in cursorial quadrupeds. The mean mechanical CoT is 2 to 4 times higher than is generally seen in terrestrial animals. We therefore find no mechanical support for a low CoT in these species. This study illustrates the need for research on a wider range of chelonians to discover whether there are indeed general trends in mechanical and metabolic energy costs.

Patterns of fluctuating asymmetry in the limbs of freshwater turtles: Are more functionally important limbs more symmetrical?

Understanding how selective forces influence patterns of symmetry remains an active area of research in evolutionary biology. One hypothesis, which has received relatively little attention, suggests that the functional importance of morphological characters may influence patterns of symmetry. Specifically, it posits that for structures that display bilateral symmetry, those with greater functional importance should display lower levels of asymmetry. The aim of this study was to examine the patterns of fluctuating asymmetry (FA) present in the limb bones of freshwater turtles in the family Emydidae. Aquatic emydid turtles of the subfamily Deirochelyinae employ a hindlimb-dominant swimming style, suggesting that hindlimbs should display lower levels of FA. Consistent with the morpho-functional hypothesis of symmetry, we found a strong, clade-wise pattern of humeral-biased FA in aquatic Deirochelyinae. In contrast, some emydids of the subfamily Emydinae possess more terrestrial tendencies. As terrestrial locomotion places more equal importance on fore- and hindlimbs, we predicted that such behaviors may minimize differences in FA. No clade-wise pattern was detected in the subfamily Emydinae. We also detected a phylogenetic signal in FA within the femur and discovered that FA has evolved at vastly different rates between the fore- and hindlimbs.

Advantages of aquatic animals as models for bio-inspired drones over present AUV technology

Robotic systems are becoming more ubiquitous, whether on land, in the air, or in water. In the aquatic realm, aquatic drones including ROVs (remotely operated vehicles) and AUVs (autonomous underwater vehicles) have opened new opportunities to investigate the ocean depths. However, these technologies have limitations related to shipboard support, programing, and functionality in complex marine environments. A new form of AUV is being developed to become operational. These drones are based on animal designs and capabilities. Biological AUVs (BAUVs) promise to improve performance in the varied environments of the ocean. Comparison of animal swimming performance with conventional AUVs and BAUVs demonstrates that natural systems still have swimming capabilities beyond the current state of AUV technology. However, the performances of aquatic animals with respect to swimming speed, efficiency, maneuverability, and stealth can serve as benchmarks to direct the development of bio-inspired AUV technology with enhanced capabilities.

Pectoral and pelvic girdle rotations during walking and swimming in a semi-aquatic turtle: testing functional role and constraint

Pectoral and pelvic girdle rotations play a substantial role in enhancing stride length across diverse tetrapod lineages. However, the pectoral and pelvic girdle attach the limbs to the body in different ways and may exhibit dissimilar functions, especially during locomotion in disparate environments. Here, we tested for functional differences between the forelimb and hindlimb of the freshwater turtle Pseudemys concinna during walking and swimming using X-ray reconstruction of moving morphology (XROMM). In doing so, we also tested the commonly held notion that the shell constrains girdle motion in turtles. We found that the pectoral girdle exhibited greater rotations than the pelvic girdle on land and in water. Additionally, pelvic girdle rotations were greater on land than in water, whereas pectoral girdle rotations were similar in the two environments. These results indicate that although the magnitude of pelvic girdle rotations depends primarily on whether the weight of the body must be supported against gravity, the magnitude of pectoral girdle rotations likely depends primarily on muscular activity associated with locomotion. Furthermore, the pectoral girdle of turtles rotated more than has been observed in other taxa with sprawling postures, showing an excursion similar to that of mammals (∼38 deg). These results suggest that a rigid axial skeleton and internally positioned pectoral girdle have not constrained turtle girdle function, but rather the lack of lateral undulations in turtles and mammals may contribute to a functional convergence whereby the girdle acts as an additional limb segment to increase stride length.

Swimming Turned on Its Head: Stability and Maneuverability of the Shrimpfish (Aeoliscus punctulatus)

The typical orientation of a neutrally buoyant fish is with the venter down and the head pointed anteriorly with a horizontally oriented body. However, various advanced teleosts will reorient the body vertically for feeding, concealment, or prehension. The shrimpfish (Aeoliscus punctulatus) maintains a vertical orientation with the head pointed downward. This posture is maintained by use of the beating fins as the position of the center of buoyancy nearly corresponds to the center of mass. The shrimpfish swims with dorsum of the body moving anteriorly. The cross-sections of the body have a fusiform design with a rounded leading edge at the dorsum and tapering trailing edge at the venter. The median fins (dorsal, caudal, anal) are positioned along the venter of the body and are beat or used as a passive rudder to effect movement of the body in concert with active movements of pectoral fins. Burst swimming and turning maneuvers by yawing were recorded at 500 frames/s. The maximum burst speed was 2.3 body lengths/s, but when measured with respect to the body orientation, the maximum speed was 14.1 body depths/s. The maximum turning rate by yawing about the longitudinal axis was 957.5 degrees/s. Such swimming performance is in line with fishes with a typical orientation. Modification of the design of the body and position of the fins allows the shrimpfish to effectively swim in the head-down orientation.

Body and Pectoral Fin Kinematics During Routine Yaw Turning in Bonnethead Sharks (Sphyrna tiburo)

Maneuvering is a crucial locomotor strategy among aquatic vertebrates, common in routine swimming, feeding, and escape responses. Combinations of whole body and fin movements generate an imbalance of forces resulting in deviation from an initial path. Sharks have elongate bodies that bend substantially and, in combination with pectoral fin rotation, play a role in yaw (horizontal) turning, but previous studies focus primarily on maximal turning performance rather than routine maneuvers. Routine maneuvering is largely understudied in fish swimming, despite observations that moderate maneuvering is much more common than the extreme behaviors commonly described in the literature. In this study, we target routine maneuvering in the bonnethead shark, Sphyrna tiburo. We use video reconstruction of moving morphology to describe three-dimensional pectoral fin rotation about three axes to compare to those previously described on yaw turning by the Pacific spiny dogfish. We quantify kinematic variables to understand the impacts of body and fin movements on routine turning performance. We also describe the anatomy of bonnethead pectoral fins and use muscle stimulation to confirm functional hypotheses about their role in actuating the fin. The turning performance metrics we describe for bonnethead sharks are comparable to other routine maneuvers described for the Pacific spiny dogfish and manta rays. These turns were substantially less agile and maneuverable than previously documented for other sharks, which we hypothesize results from the comparison of routine turning to maneuvering under stimulated conditions. We suggest that these results highlight the importance of considering routine maneuvering in future studies. Cinemática del Cuerpo y de las Aletas Pectorales Durante el giro en el eje Vertical en la Cabeza del Tiburón Pala (Sphyrna tiburo) (Body and Pectoral Fin Kinematics During Routine Yaw Turning in Bonnethead Sharks [Sphyrna tiburo]).

Variation in Morphology and Kinematics Underlies Variation in Swimming Stability and Turning Performance in Freshwater Turtles

Among swimming animals, stable body designs often sacrifice performance in turning, and high turning performance may entail costs in stability. However, some rigid-bodied animals appear capable of both high stability and turning performance during swimming by propelling themselves with independently controlled structures that generate mutually opposing forces. Because such species have traditionally been studied in isolation, little is known about how variation within rigid-bodied designs might influence swimming performance. Turtles are a lineage of rigid-bodied animals, in which most species use contralateral limbs and mutually opposing forces to swim. We tested the stability and turning performance of two species of turtles, the pleurodire Emydura subglobosa and the cryptodire Chrysemys picta. Emydura subglobosa exhibited both greater stability and turning performance than C. picta, potentially through the use of subequally-sized (and larger) propulsive structures, faster limb movements, and decreased limb excursions. These data show how, within a given body design, combinations of different traits can serve as mechanisms to improve aspects of performance with competing functional demands.

The impact of keels and tails on turtle swimming performance and their potential as models for biomimetic design

Stability and turning performance are two key metrics of locomotor performance in animals, and performance in both of these metrics can be improved through a variety of morphological structures. Aquatic vehicles are often designed with keels and rudders to improve their stability and turning performance, but how keels and rudders function in rigid-bodied animals is less understood. Aquatic turtles are a lineage of rigid-bodied animals that have the potential to function similarly to engineered vehicles, and also might make use of keels and rudders to improve their stability and turning performance. To test these possibilities, we trained turtles to follow a mechanically controlled prey stimulus under three sets of conditions: with no structural modifications, with different sized and shaped keels, and with restricted tail use. We predicted that keels in turtles would function similarly to those in aquatic vehicles to reduce oscillations, and that turtles would use the tail like a rudder to reduce oscillations and improve turning performance. We found that the keel designs we tested did not reduce oscillations in turtles, but that the tail was used similarly to a rudder, with benefits to both the magnitude of oscillations they experienced and turning performance. These data show how variation in the accessory structures of rigid-bodied animals can impact swimming performance, and suggest that such variation among turtles could serve as a biomimetic model in designing aquatic vehicles that are stable as well as maneuverable and agile.

Kinematics of swimming of the manta ray: three-dimensional analysis of open-water maneuverability

For aquatic animals, turning maneuvers represent a locomotor activity that may not be confined to a single coordinate plane, making analysis difficult, particularly in the field. To measure turning performance in a three-dimensional space for the manta ray (Mobula birostris), a large open-water swimmer, scaled stereo video recordings were collected. Movements of the cephalic lobes, eye and tail base were tracked to obtain three-dimensional coordinates. A mathematical analysis was performed on the coordinate data to calculate the turning rate and curvature (1/turning radius) as a function of time by numerically estimating the derivative of manta trajectories through three-dimensional space. Principal component analysis was used to project the three-dimensional trajectory onto the two-dimensional turn. Smoothing splines were applied to these turns. These are flexible models that minimize a cost function with a parameter controlling the balance between data fidelity and regularity of the derivative. Data for 30 sequences of rays performing slow, steady turns showed the highest 20% of values for the turning rate and smallest 20% of turn radii were 42.65±16.66 deg s^-1 and 2.05±1.26 m, respectively. Such turning maneuvers fall within the range of performance exhibited by swimmers with rigid bodies.

Central pattern generators in the turtle spinal cord: selection among the forms of motor behaviors

Neuronal networks in the turtle spinal cord have considerable computational complexity even in the absence of connections with supraspinal structures. These networks contain central pattern generators (CPGs) for each of several behaviors, including three forms of scratch, two forms of swim, and one form of flexion reflex. Each behavior is activated by a specific set of cutaneous or electrical stimuli. The process of selection among behaviors within the spinal cord has multisecond memories of specific motor patterns. Some spinal cord interneurons are partially shared among several CPGs, whereas other interneurons are active during only one type of behavior. Partial sharing is a proposed mechanism that contributes to the ability of the spinal cord to generate motor pattern blends with characteristics of multiple behaviors. Variations of motor patterns, termed deletions, assist in characterization of the organization of the pattern-generating components of CPGs. Single-neuron recordings during both normal and deletion motor patterns provide support for a CPG organizational structure with unit burst generators (UBGs) whose members serve a direction of a specific degree of freedom of the hindlimb, e.g., the hip-flexor UBG, the hip-extensor UBG, the knee-flexor UBG, the knee-extensor UBG, etc. The classic half-center hypothesis that includes all the hindlimb flexors in a single flexor half-center and all the hindlimb extensors in a single extensor half-center lacks the organizational complexity to account for the motor patterns produced by turtle spinal CPGs. Thus the turtle spinal cord is a valuable model system for studies of mechanisms responsible for selection and generation of motor behaviors. NEW & NOTEWORTHY The concept of the central pattern generator (CPG) is a major tenet in motor neuroethology that has influenced the design and interpretations of experiments for over a half century. This review concentrates on the turtle spinal cord and describes studies from the 1970s to the present responsible for key developments in understanding the CPG mechanisms responsible for the selection and production of coordinated motor patterns during turtle hindlimb motor behaviors.

Foot-propelled swimming kinematics and turning strategies in common loons

Loons (Gaviiformes) are arguably one of the most successful groups of swimming birds. As specialist foot-propelled swimmers, loons are capable of diving up to 70 meters, remaining underwater for several minutes, and capturing fish. Despite the swimming prowess of loons, their undomesticated nature has prevented prior quantitative analysis. Our study used high-speed underwater cameras to film healthy common loons (Gavia immer) at the Tufts Wildlife Clinic in order to analyze their swimming and turning strategies. Loons swim by synchronously paddling their feet laterally at an average of 1.8 Hz. Combining flexion-extension of the ankle with rotation at the knee, loon swimming resembles grebe swimming and likely generates lift forces for propulsion. Loons modulate swimming speed by altering power stroke duration and use head-bobbing to enhance underwater vision. We observed that loons execute tight but slow turns compared to other aquatic swimmers, potentially associated with hunting by flushing fish from refuges at short range. To execute turns, loons use several strategies. Loons increase the force produced on the outside of the turn by increasing the speed of the outboard foot, which also begins its power stroke before the inboard foot. During turns, loons bank their body away from the turn and alter the motion of the feet to maintain the turn. Our findings demonstrate that foot-propelled swimming has evolved convergently in loon and grebes, but divergently from cormorants. The swimming and turning strategies used by loons that allow them to capture fish could inspire robotic designs or novel paddling techniques.

Turning performance of brief squid Lolliguncula brevis during attacks on shrimp and fish

Although squid are generally considered to be effective predators, little is currently known quantitatively about how squid maneuver and position themselves during prey strikes. In this study, high-speed video and kinematic analyses were used to study attacks by brief squid Lolliguncula brevis on both shrimp and fish. Squid attack success was high (>80%) and three behavioral phases were identified: (1) approach, (2) strike and (3) recoil. Lolliguncula brevis demonstrated greater maneuverability (i.e., a smaller length-specific turning radius) and employed more body adjustments (i.e., mantle angle posturing) during approaches toward shrimp versus fish. Squid exhibited higher linear approach/strike velocities and accelerations with faster swimming fish prey compared to slower shrimp prey. Agility (i.e., turning rate) during prey encounters was comparable to performance extremes observed during non-predatory turns, and did not differ according to prey type or distance. Despite having the ability to modulate tentacle extension velocity, squid instead increased their own swimming velocity rather than increasing tentacle velocity when targeting faster fish prey during the strike phase, but this was not the case for shrimp prey. Irrespective of prey type, L. brevis consistently positioned themselves above the prey target prior to the tentacle strike, possibly to facilitate a more advantageous downward projection of the tentacles. During the recoil, L. brevis demonstrated length-specific turning radii similar to those recorded during the approach despite vigorous escape attempts by some prey. Clearly, turning performance is integral to prey attacks in squid, with differences in attack strategy varying depending on the prey target.

"On the Fence" versus "All in": Insights from Turtles for the Evolution of Aquatic Locomotor Specializations and Habitat Transitions in Tetrapod Vertebrates

Though ultimately descended from terrestrial amniotes, turtles have deep roots as an aquatic lineage and are quite diverse in the extent of their aquatic specializations. Many taxa can be viewed as "on the fence" between aquatic and terrestrial realms, whereas others have independently hyperspecialized and moved "all in" to aquatic habitats. Such differences in specialization are reflected strongly in the locomotor system. We have conducted several studies to evaluate the performance consequences of such variation in design, as well as the mechanisms through which specialization for aquatic locomotion is facilitated in turtles. One path to aquatic hyperspecialization has involved the evolutionary transformation of the forelimbs from rowing, tubular limbs with distal paddles into flapping, flattened flippers, as in sea turtles. Prior to the advent of any hydrodynamic advantages, the evolution of such flippers may have been enabled by a reduction in twisting loads on proximal limb bones that accompanied swimming in rowing ancestors, facilitating a shift from tubular to flattened limbs. Moreover, the control of flapping movements appears related primarily to shifts in the activity of a single forelimb muscle, the deltoid. Despite some performance advantages, flapping may entail a locomotor cost in terms of decreased locomotor stability. However, other morphological specializations among rowing species may enhance swimming stability. For example, among highly aquatic pleurodiran turtles, fusion of the pelvis to the shell appears to dramatically reduce motions of the pelvis compared to freshwater cryptodiran species. This could contribute to advantageous increases in aquatic stability among predominantly aquatic pleurodires. Thus, even within the potential constraints of a body plan in which the body is encased by a shell, turtles exhibit diverse locomotor capacities that have enabled diversification into a wide range of aquatic habitats.

Pelvic girdle mobility of cryptodire and pleurodire turtles during walking and swimming

Movements of the pelvic girdle facilitate terrestrial locomotor performance in a wide range of vertebrates by increasing hind limb excursion and stride length. The extent to which pelvic movements contribute to limb excursion in turtles is unclear because the bony shell surrounding the body presents a major obstacle to their visualization. In the Cryptodira, which are one of the two major lineages of turtles, pelvic anatomy indicates the potential for rotation inside the shell. However, in the Pleurodira, the other major suborder, the pelvis shows a derived fusion to the shell, preventing pelvic motion. In addition, most turtles use their hind limbs for propulsion during swimming as well as walking, and the different locomotor demands between water and land could lead to differences in the contributions of pelvic rotation to limb excursion in each habitat. To test these possibilities, we used X-ray reconstruction of moving morphology (XROMM) to compare pelvic mobility and femoral motion during walking and swimming between representative species of cryptodire (Pseudemys concinna) and pleurodire (Emydura subglobosa) turtles. We found that the pelvis yawed substantially in cryptodires during walking and, to a lesser extent, during swimming. These movements contributed to greater femoral protraction during both walking and swimming in cryptodires when compared with pleurodires. Although factors related to the origin of pelvic-shell fusion in pleurodires are debated, its implications for their locomotor function may contribute to the restriction of this group to primarily aquatic habits.

Shoulder girdle rotation, forelimb movement and the influence of carapace shape on locomotion in Testudo hermanni (Testudinidae)

Studies into the function of structures are crucial for making connections between morphology and behaviour of organisms, but are still rare for the terrestrial Testudinidae. We investigated the kinematics of shoulder girdle and forelimb motion in Hermann's tortoise Testudo hermanni using biplanar X-ray fluoroscopy with a twofold aim: firstly, to understand how the derived shapes of shoulder girdle and carapace together influence rotation of the girdle; and, secondly, to understand how girdle rotation affects forelimb excursion. The total degree of shoulder rotation in the horizontal plane is similar to a species with a less domed shell, but because of the long and nearly vertically oriented scapular prong, shoulder girdle rotation contributes more than 30% to the horizontal arc of the humerus and nearly 40% to the rotational component of step length. The antebrachium and manus, which act as a functional unit, contribute roughly 50% to this component of the step length because of their large excursion almost parallel to the mid-sagittal plane. This large excursion is the result of the complex interplay between humerus long-axis rotation, counter-rotation of the antebrachium, and elbow flexion and extension. A significant proportion of forelimb step length results from body translation that is due to the propulsive effect of the other limbs during their stance phases. Traits that are similar to other tortoises and terrestrial or semi-aquatic turtles are the overall slow walk because of a low stride frequency, and the lateral-sequence, diagonally coupled footfall pattern with high duty factors. Intraspecific variation of carapace shape and shoulder girdle dimensions has a corresponding effect on forelimb kinematics.

Body side-specific control of motor activity during turning in a walking animal

Animals and humans need to move deftly and flexibly to adapt to environmental demands. Despite a large body of work on the neural control of walking in invertebrates and vertebrates alike, the mechanisms underlying the motor flexibility that is needed to adjust the motor behavior remain largely unknown. Here, we investigated optomotor-induced turning and the neuronal mechanisms underlying the differences between the leg movements of the two body sides in the stick insect Carausius morosus. We present data to show that the generation of turning kinematics in an insect are the combined result of descending unilateral commands that change the leg motor output via task-specific modifications in the processing of local sensory feedback as well as modification of the activity of local central pattern generating networks in a body-side-specific way. To our knowledge, this is the first study to demonstrate the specificity of such modifications in a defined motor task.

DOI:http://dx.doi.org/10.7554/eLife.13799.001

Walking along a curve or turning is a complex manoeuvre for the nervous system, as it must coordinate different leg movements on each side of the body. Rhythmic processes such as walking are controlled by networks of neurons called central pattern generators. The resulting movements can be adjusted by feedback from sense organs in response to environmental conditions. For example, sensory feedback that provides information about the load placed on each leg, allows the animal to control the duration of a stance. How the nerve cells, or neurons, involved in these processes work together to produce complex, flexible movements such as turning is largely unknown.

Previous work on how the brain negotiates turning movements has been carried out mostly in animals that swim or fly. To understand what happens during walking, Gruhn et al. monitored stick insects that walked in a curve on a slippery surface, and recorded the electrical activity within the animals' nervous system as they turned.

By comparing the activity of the nervous system on each side of the body while the insects walked a curve, Gruhn et al. found that the nervous system uses at least three different mechanisms to produce the different movements on the inside and outside. Firstly, the sensory feedback signals that communicate the load on the leg are processed in the legs on the outside of the curve to support forward steps, while they are processed on the inside legs to support forward, sideward, and backward steps. Secondly, the motor activity produced by the central pattern generator is modulated to be stronger for the muscle that moves the leg backward on the outside of the curve. At the same time, this activity is stronger for the muscle that moves the leg forward on the inside of the curve. Thirdly, signals from a front leg influence the movement of the other legs on the same side of the body. This influence is strong on the inside and weak on the outside of the curve.

Together or separately, these three mechanisms could provide the animal with the means to perform turns in all their different curvatures. Future work will need to work out exactly which local neurons process the signals sent from the brain to control movement.

DOI:http://dx.doi.org/10.7554/eLife.13799.002

Turning performance in squid and cuttlefish: unique dual mode, muscular hydrostatic systems

Although steady swimming has received considerable attention in prior studies, unsteady swimming movements represent a larger portion of many aquatic animals' locomotive repertoire and have not been examined extensively. Squids and cuttlefishes are cephalopods with unique muscular hydrostat-driven, dual mode propulsive systems involving paired fins and a pulsed jet. These animals exhibit a wide range of swimming behavior, but turning performance has not been examined quantitatively. Brief squid Lolliguncula brevis and dwarf cuttlefish Sepia bandensis were filmed during turns using high-speed cameras. Kinematic features were tracked, including the length specific radius of the turn (R/L), a measure of maneuverability, and angular velocity (ω), a measure of agility. Both L. brevis and S. bandensis demonstrated high maneuverability, with (R/L)min values=3.4x10−3±5.9x10−4 and 1.2x10−3±4.7x10−4 (mean±s.e.m.), respectively, which are the lowest measures of (R/L) reported for any aquatic taxa. Lolliguncula brevis exhibited higher agility than S. bandensis (ωamax=725.8° s−1 vs. ωamax=485.0° s−1), and both cephalopods have intermediate agility when compared with flexible-bodied and rigid-bodied nekton of similar size, reflecting their hybrid body architecture. In L. brevis, jet flows were the principal driver of angular velocity. Asymmetric fin motions played a reduced role, and arm wrapping increased turning performance to varying degrees depending on the species. This study indicates that coordination between the jet and fins is important for turning performance, with L. brevis achieving faster turns than S. bandensis and S. bandensis achieving tighter, more controlled turns than L. brevis.

Home is where the shell is: predicting turtle home range sizes

Home range is the area traversed by an animal in its normal activities. The size of home ranges is thought to be tightly linked to body size, through size effect on metabolic requirements. Due to the structure of Eltonian food pyramids, home range sizes of carnivores are expected to exceed those of herbivorous species. The habitat may also affect home range size, with reduced costs of locomotion or lower food abundance in, for example, aquatic habitats selecting for larger home ranges. Furthermore, home range of males in polygamous species may be large due to sexual selection for increased reproductive output. Comparative studies on home range sizes have rarely been conducted on ectotherms. Because ectotherm metabolic rates are much lower than those of endotherms, energetic considerations of metabolic requirements may be less important in determining the home range sizes of the former, and other factors such as differing habitats and sexual selection may have an increased effect. We collected literature data on turtle home range sizes. We used phylogenetic generalized least squares analyses to determine whether body mass, sex, diet, habitat and social structure affect home range size. Turtle home range size increases with body mass. However, body mass explains relatively little of the variation in home range size. Aquatic turtles have larger home ranges than semiaquatic species. Omnivorous turtles have larger home ranges than herbivores and carnivores, but diet is not a strong predictor. Sex and social structure are unrelated to home range size. We conclude that energetic constraints are not the primary factor that determines home range size in turtles, and energetic costs of locomotion in different habitats probably play a major role.

Underwater turning movement during foraging in Hydromedusa maximiliani (Testudines, Chelidae) from southeastern Brazil

A type of locomotor behavior observed in animals with rigid bodies, that can be found in many animals with exoskeletons, shells, or other forms of body armor, to change direction, is the turning behavior. Aquatic floated-turning behavior among rigid bodies animals have been studied in whirligig beetles, boxfish, and more recently in freshwater turtle, Chrysemys picta. In the laboratory we observed a different kind of turning movement that consists in an underwater turning movement during foraging, wherein the animal pivoted its body, using one of the hindlimbs as the fixed-point support in the substratum. We describe, analyze and quantify this movement during foraging in Hydromedusa maximiliani, using observations made in the laboratory. We studied 3 adult specimens (2 males, 1 female) and 2 non-sexed juveniles of H. maximiliani. They were kept individually in an aquarium filled with water and small fish. They were filmed, in dorsal view, at 30 frames per second. Sequences were analyzed frame by frame and points were marked on limbs and shell to enable analysis of variation in limb flexion and extension, as well as rotation movements. While foraging, turtles frequently turned their bodies, using one hind leg as the pivot point. This underwater turning movement, in addition to slow movements with the neck stretched, or staying nearly immobile and scanning the surroundings with lateral movements of the neck (in arcs up to 180°), and fast attacks of neck, may increase prey capture rates.

Sensory-evoked turning locomotion in red-eared turtles: kinematic analysis and electromyography

We examined the limb kinematics and motor patterns that underlie sensory-evoked turning locomotion in red-eared turtles. Intact animals were held by a band-clamp in a water-filled tank. Turn-swimming was evoked by slowly rotating turtles to the right or left via a motor connected to the shaft of the band-clamp. Animals executed sustained forward turn-swimming against the direction of the imposed rotation. We recorded video of turn-swimming and computer-analyzed the limb and head movements. In a subset of turtles, we also recorded electromyograms from identified limb muscles. Turning exhibited a stereotyped pattern of (1) coordinated forward swimming in the hindlimb and forelimb on the outer side of the turn, (2) back-paddling in the hindlimb on the inner side, (3) a nearly stationary, "braking" forelimb on the inner side, and (4) neck bending toward the direction of the turn. Reversing the rotation caused animals to switch the direction of their turns and the asymmetric pattern of right and left limb activities. Preliminary evidence suggested that vestibular inputs were sufficient to drive the behavior. Sensory-evoked turning may provide a useful experimental platform to examine the brainstem commands and spinal neural networks that underlie the activation and switching of different locomotor forms.

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

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 with 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 C. 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 C. insculpta were more similar to rowers than to flapping C. caretta. Nevertheless, of the three freshwater species, C. insculpta was most similar to flapping C. caretta. 'Flapping' in C. insculpta is achieved through humeral kinematics very different from those in C. caretta, with C. insculpta 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, C. insculpta may in fact represent a synchronous rower or hybrid rower-flapper, suggesting that traditional views of C. insculpta as a flapper should be revised.

Forelimb kinematics and motor patterns of swimming loggerhead sea turtles (Caretta caretta): are motor patterns conserved in the evolution of new locomotor strategies?

Novel functions in animals may evolve through changes in morphology, muscle activity or a combination of both. The idea that new functions or behavior can arise solely through changes in structure, without concurrent changes in the patterns of muscle activity that control movement of those structures, has been formalized as the neuromotor conservation hypothesis. In vertebrate locomotor systems, evidence for neuromotor conservation is found across evolutionary transitions in the behavior of terrestrial species, and in evolutionary transitions from terrestrial species to flying species. However, evolutionary transitions in the locomotion of aquatic species have received little comparable study to determine whether changes in morphology and muscle function were coordinated through the evolution of new locomotor behavior. To evaluate the potential for neuromotor conservation in an ancient aquatic system, we quantified forelimb kinematics and muscle activity during swimming in the loggerhead sea turtle, Caretta caretta. Loggerhead forelimbs are hypertrophied into wing-like flippers that produce thrust via dorsoventral forelimb flapping. We compared kinematic and motor patterns from loggerheads with previous data from the red-eared slider, Trachemys scripta, a generalized freshwater species exhibiting unspecialized forelimb morphology and anteroposterior rowing motions during swimming. For some forelimb muscles, comparisons between C. caretta and T. scripta support neuromotor conservation; for example, the coracobrachialis and the latissimus dorsi show similar activation patterns. However, other muscles (deltoideus, pectoralis and triceps) do not show neuromotor conservation; for example, the deltoideus changes dramatically from a limb protractor/elevator in sliders to a joint stabilizer in loggerheads. Thus, during the evolution of flapping in sea turtles, drastic restructuring of the forelimb was accompanied by both conservation and evolutionary novelty in limb motor patterns.

Effects of shell morphology on mechanics of zebra and quagga mussel locomotion

Although zebra mussels (Dreissena polymorpha) initially colonized shallow habitats within the North American Great Lakes, quagga mussels (Dreissena bugensis) are becoming dominant in both shallow- and deep-water habitats. Shell morphology differs among zebra, shallow quagga and deep quagga mussels but functional consequences of such differences are unknown. We examined effects of shell morphology on locomotion for the three morphotypes on hard (typical of shallow habitats) and soft (characteristic of deep habitats) sedimentary substrates. We quantified morphology using the polar moment of inertia, a parameter used in calculating kinetic energy that describes shell area distribution and resistance to rotation. We quantified mussel locomotion by determining the ratio of rotational (K(rot)) to translational kinetic energy (K(trans)). On hard substrate, K(rot):K(trans) of deep quagga mussels was fourfold greater than for the other morphotypes, indicating greater energy expenditure in rotation relative to translation. On soft substrate, K(rot):K(trans) of deep quagga mussels was approximately one-third of that on hard substrate, indicating lower energy expenditure in rotation on soft substrate. Overall, our study demonstrates that shell morphology correlates with differences in locomotion (i.e. K(rot):K(trans)) among morphotypes. Although deep quagga mussels were similar to zebra and shallow quagga mussels in terms of energy expenditure on sedimentary substrate, their morphology was energetically maladaptive for linear movement on hard substrate. As quagga mussels can possess two distinct morphotypes (i.e. shallow and deep morphs), they might more effectively utilize a broader range of substrates than zebra mussels, potentially enhancing their ability to colonize a wider range of habitats.

Hydrodynamic stability of the painted turtle (Chrysemys picta): effects of four-limbed rowing versus forelimb flapping in rigid-bodied tetrapods

Hydrodynamic stability is the ability to resist recoil motions of the body produced by destabilizing forces. Previous studies have suggested that recoil motions can decrease locomotor performance, efficiency and sensory perception and that swimming animals might utilize kinematic strategies or possess morphological adaptations that reduce recoil motions and produce more stable trajectories. We used high-speed video to assess hydrodynamic stability during rectilinear swimming in the freshwater painted turtle (Chrysemys picta). Parameters of vertical stability (heave and pitch) were non-cyclic and variable, whereas measures of lateral stability (sideslip and yaw) showed repeatable cyclic patterns. In addition, because freshwater and marine turtles use different swimming styles, we tested the effects of propulsive mode on hydrodynamic stability during rectilinear swimming, by comparing our data from painted turtles with previously collected data from two species of marine turtle (Caretta caretta and Chelonia mydas). Painted turtles had higher levels of stability than both species of marine turtle for six of the eight parameters tested, highlighting potential disadvantages associated with 'aquatic flight'. Finally, available data on hydrodynamic stability of other rigid-bodied vertebrates indicate that turtles are less stable than boxfish and pufferfish.

Forelimb kinematics and motor patterns of the slider turtle (Trachemys scripta) during swimming and walking: shared and novel strategies for meeting locomotor demands of water and land

Turtles use their limbs during both aquatic and terrestrial locomotion, but water and land impose dramatically different physical requirements. How must musculoskeletal function be adjusted to produce locomotion through such physically disparate habitats? We addressed this question by quantifying forelimb kinematics and muscle activity during aquatic and terrestrial locomotion in a generalized freshwater turtle, the red-eared slider (Trachemys scripta), using digital high-speed video and electromyography (EMG). Comparisons of our forelimb data to previously collected data from the slider hindlimb allow us to test whether limb muscles with similar functional roles show qualitatively similar modulations of activity across habitats. The different functional demands of water and air lead to a prediction that muscle activity for limb protractors (e.g. latissimus dorsi and deltoid for the forelimb) should be greater during swimming than during walking, and activity in retractors (e.g. coracobrachialis and pectoralis for the forelimb) should be greater during walking than during swimming. Differences between aquatic and terrestrial forelimb movements are reflected in temporal modulation of muscle activity bursts between environments, and in some cases the number of EMG bursts as well. Although patterns of modulation between water and land are similar between the fore- and hindlimb in T. scripta for propulsive phase muscles (retractors), we did not find support for the predicted pattern of intensity modulation, suggesting that the functional demands of the locomotor medium alone do not dictate differences in intensity of muscle activity across habitats.

Is sexual body shape dimorphism consistent in aquatic and terrestrial chelonians?

Comparisons between aquatic and terrestrial species provide an opportunity to examine how sex-specific adaptations interact with the environment to influence body shape. In terrestrial female tortoises, selection for fecundity favors the development of a large internal abdominal cavity to accommodate the clutch; in conspecific males, sexual selection favors mobility with large openings in the shell. To examine to what extent such trends apply in aquatic chelonians we compared the body shape of males and females of two aquatic turtles (Chelodina colliei and Mauremys leprosa). In both species, females were larger than males. When controlled for body size, females exhibited a greater relative internal volume and a higher body condition index than males; both traits potentially correlate positively with fecundity. Males were more streamlined (hydrodynamic), and exhibited larger openings in the shell providing more space to move their longer limbs; such traits probably improve mobility and copulation ability (the males chase and grab the female for copulation). Overall, although the specific constraints imposed by terrestrial and aquatic locomotion shape the morphology of chelonians differently (aquatic turtles were flatter, hence more hydrodynamic than terrestrial tortoises), the direction for sexual shape dimorphism remained unaffected. Our main conclusion is that the direction of sexual shape dimorphism is probably more consistent than sexual size dimorphism in the animal kingdom.

Consequences of buoyancy to the maneuvering capabilities of a foot-propelled aquatic predator, the great cormorant (Phalcrocorax carbo sinensis)

Great cormorants are foot-propelled aquatic divers utilizing a region of the water column where their underwater foraging behavior is affected by their buoyancy. While swimming horizontally underwater, cormorants use downward lift forces generated by their body and tail to overcome their buoyancy. Here we assess the potential of this swimming strategy for controlling maneuvers in the vertical plane. We recorded the birds swimming through a submerged obstacle course and analyzed their maneuvers. The birds reduced swimming speed by only 12% to maneuver and were able to turn upward and then downward in the sagittal plane at a minimal turning radius of 32±4 cm (40% body length). Using a quasi-steady approach, we estimated the time-line for hydrodynamic forces and the force-moments produced while maneuvering. We found that the tail is responsible for the pitch of the body while motions of the body, tail, neck and feet generate forces normal (vertically) to the swimming direction that interact with buoyancy to change the birds' trajectory. Vertical maneuvers in cormorants are asymmetric in energy cost. When turning upward, the birds use their buoyancy but they must work harder to turn downward. Lift forces generated by the body were always directed ventrally. Propulsion improves the ability to make tight turns when the center of the turn is ventral to the birds. The neck produced only a small portion (10%) of the normal vertical forces but its length may allow prey capture at the end of pursuit, within the minimum turning radius.

Death roll of the alligator: mechanics of twist feeding in water

Crocodilians, including the alligator (Alligator mississippiensis), perform a spinning maneuver to subdue and dismember prey. The spinning maneuver, which is referred to as the `death roll',involves rapid rotation about the longitudinal axis of the body. High-speed videos were taken of juvenile alligators (mean length=0.29 m) performing death rolls in water after biting onto a pliable target. Spinning was initiated after the fore- and hindlimbs were appressed against the body and the head and tail were canted with respect to the longitudinal body axis. With respect to the body axis, the head and tail bending averaged 49.2° and 103.3°,respectively. The head, body and tail rotated smoothly and freely around their individual axes of symmetry at 1.6 Hz. To understand the dynamics of the death roll, we mathematically modeled the system. The maneuver results purely from conservation of angular momentum and is explained as a zero angular momentum turn. The model permits the calculation of relevant dynamical parameters. From the model, the shear force, which was generated at the snout by the juvenile alligators, was 0.015 N. Shear force was calculated to scale with body length to the 4.24 power and with mass to the 1.31 power. When scaled up to a 3 m alligator, shear force was calculated at 138 N. The death roll appears to help circumvent the feeding morphology of the alligator. Shear forces generated by the spinning maneuver are predicted to increase disproportionately with alligator size, allowing dismemberment of large prey.