When to Go Against the Flow: Examining Patterns of Performance Over Multiday Migration Events in the Hawaiian Stream Fish, 'O'opu Nopili (Sicyopterus stimpsoni).

Many animals migrate across regions of their geographic range as part of extended events, with groups of individuals proceeding through areas of travel on several successive days. Early migrating individuals may have an advantage over late migrating individuals by gaining early access to the resources at the eventual destination. For situations where early access to resources would provide an advantage, specific sets of locomotor traits might be found among individuals that are earlier migrators. We tested for associations between migration timing and traits related to escape responses, climbing, and morphology in the amphidromous Hawaiian stream goby, 'o'opu nopili (Sicyopterus stimpsoni). In this species, juvenile fish migrate in pulses over several days immediately following flash floods. We collected daily measurements of escape responses and waterfall climbing from juvenile fish arriving at streams from the ocean. We found that escape performance showed mainly stochastic variation across migrating individuals tested on successive days. In contrast, some metrics of climbing performance decrease over successive pulses during a migration event. We also found more variation in body shape among fish from early pulses during migration events compared to later in pulses. These results could have implications for guiding conservation efforts, identifying critical time windows for protection as periods with the greatest likelihood of successful migrants.

Forelimb muscle activation patterns in American alligators: Insights into the evolution of limb posture and powered flight in archosaurs.

The evolution of archosaurs provides an important context for understanding the mechanisms behind major functional transformations in vertebrates, such as shifts from sprawling to erect limb posture and the acquisition of powered flight. While comparative anatomy and ichnology of extinct archosaurs have offered insights into musculoskeletal and gait changes associated with locomotor transitions, reconstructing the evolution of motor control requires data from extant species. However, the scarcity of electromyography (EMG) data from the forelimb, especially of crocodylians, has hindered understanding of neuromuscular evolution in archosaurs. Here, we present EMG data for nine forelimb muscles from American alligators during terrestrial locomotion. Our aim was to investigate the modulation of motor control across different limb postures and examine variations in motor control across phylogeny and locomotor modes. Among the nine muscles examined, m. pectoralis, the largest forelimb muscle and primary shoulder adductor, exhibited significantly smaller mean EMG amplitudes for steps in which the shoulder was more adducted (i.e., upright). This suggests that using a more adducted limb posture helps to reduce forelimb muscle force and work during stance. As larger alligators use a more adducted shoulder and hip posture, the sprawling to erect postural transition that occurred in the Triassic could be either the cause or consequence of the evolution of larger body size in archosaurs. Comparisons of EMG burst phases among tetrapods revealed that a bird and turtle, which have experienced major musculoskeletal transformations, displayed distinctive burst phases in comparison to those from an alligator and lizard. These results support the notion that major shifts in body plan and locomotor modes among sauropsid lineages were associated with significant changes in muscle activation patterns.

Limb bone strains during climbing in green iguanas (Iguana iguana): testing biomechanical release as a mechanism promoting morphological transitions in arboreal vertebrates.

Across vertebrate diversity, limb bone morphology is typically expected to reflect differences in the habitats and functional tasks that species utilize. Arboreal vertebrates are often recognized to have longer limbs than terrestrial relatives, a feature thought to help extend the reach of limbs across gaps between branches. Among terrestrial vertebrates, longer limbs can experience greater bending moments that might expose bones to a greater risk of failure. However, changes in habitat or behavior can impose changes in the forces that bones experience. If locomotion imposed lower loads in trees than on the ground, such a release from loading demands might have produced conditions under which potential constraints on the evolution of long limbs were removed, making it easier for them to evolve in arboreal species. We tested for such environmental differences in limb bone loading using the green iguana (Iguana iguana), a species that readily walks over ground and climbs trees. We implanted strain gauges on the humerus and femur, and then compared loads between treatments modeling substrate conditions of arboreal habitats. For hindlimbs, inclined substrate angles were most correlated with strain increases, whereas the forelimbs had a similar pattern but of lesser magnitude. Unlike some other habitat transitions, these results do not support biomechanical release as a mechanism likely to have facilitated limb elongation. Instead, limb bone adaptations in arboreal habitats were likely driven by selective pressures other than responses to skeletal loading.

Integrating biomechanics in evolutionary studies, with examples from the amphidromous goby model system.

The functional capacities of animals are a primary factor determining survival in nature. In this context, understanding the biomechanical performance of animals can provide insight into diverse aspects of their biology, ranging from ecological distributions across habitat gradients to the evolutionary diversification of lineages. To survive and reproduce in the face of environmental pressures, animals must perform a wide range of tasks, some of which entail tradeoffs between competing demands. Moreover, the demands encountered by animals can change through ontogeny as they grow, sexually mature or migrate across environmental gradients. To understand how mechanisms that underlie functional performance contribute to survival and diversification across challenging and variable habitats, we have pursued diverse studies of the comparative biomechanics of amphidromous goby fishes across functional requirements ranging from prey capture and fast-start swimming to adhesion and waterfall climbing. The pan-tropical distribution of these fishes has provided opportunities for repeated testing of evolutionary hypotheses. By synthesizing data from the lab and field, across approaches spanning high-speed kinematics, selection trials, suction pressure recordings, mechanical property testing, muscle fiber-type measurements and physical modeling of bioinspired designs, we have clarified how multiple axes of variation in biomechanical performance associate with the ecological and evolutionary diversity of these fishes. Our studies of how these fishes meet both common and extreme functional demands add new, complementary perspectives to frameworks developed from other systems, and illustrate how integrating knowledge of the mechanical underpinnings of diverse aspects of performance can give critical insights into ecological and evolutionary questions.

Anatomy informs geology: Hydrodynamic dispersal of alligator bones, with implications for taphonomic interpretations of fossil deposits of crocodylians, dinosaurs, and other morphologically novel taxa.

Distinctive anatomical features of bones can influence not only how these structures perform in living animals but also the tendency of elements to be transported by flowing water after death. Such transport can be critical in the concentration of fossils from animals that live near freshwater habitats, providing important context for interpreting the composition of paleocommunities. Measurements of the tendency of flowing water to disperse skeletal elements have been collected for diverse taxa, including mammals, turtles, and birds. However, these extant models may not be entirely appropriate for many morphologically distinct extinct lineages, such as non-avian dinosaurs. To expand the range of models available for evaluating the influence of hydrodynamic transport on the assembly of fossil deposits, we used a flow tank to measure the water speeds that disperse bones from a subadult American alligator (Alligator mississippiensis), with the skull and mandible tested in multiple starting orientations. Alligator bones are sorted into three main dispersal groups: early (vertebrae, most girdle elements), intermediate (ribs, most limb bones), and late (pubis, femur), with the skull and mandible varying between intermediate and late depending on orientation. Late dispersing elements tended to be heavy or very flat. These results can refine interpretations of the taphonomic context for deposits of fossil crocodylians and morphologically similar taxa (e.g., choristoderes, phytosaurs) and provide an additional comparative model for deposits of non-avian dinosaurs. Moreover, variation in hydrodynamic sorting across lineages highlights how distinctive anatomical features can influence the concentration of fossils, shaping understanding of assemblage composition and paleofaunal evolution.

Kinematic comparisons between mudskipper fins and salamander limbs during terrestrial locomotion.

Terrestrial and aquatic environments present drastically different challenges, yet amphibious behaviors evolved multiple times in vertebrates. Terrestrial salamanders are often used to model the locomotion of crownward stem tetrapods, but amphibious fishes may model earlier evolutionary stages as vertebrates became terrestrial. For instance, some early tetrapods may have moved on land with a mudskipper-like gait. Previously published kinetic data found that the ground reaction forces produced by the pectoral fins of mudskippers (Periophthalmus barbarus) were more medial than the limbs of tiger salamanders (Ambystoma tigrinum), which might elevate bending stresses in the fins. However, kinematic data are needed to explain these kinetic differences. Therefore, we quantified the three-dimensional kinematics of mudskipper pectoral fins and compared these to published data on tiger salamander limbs. We found that mudskipper pectoral fins generally remained more retracted, extended, and adducted compared to salamander limbs. Kinematic patterns in mudskipper pectoral fins were aligned with published kinetic data and shared a restricted range of motion found in early tetrapods. Our findings demonstrate that mudskipper pectoral fins provide weight support and propulsion but have lower mobility in the proximal versus distal elements, for which greater flexibility in the latter might compensate. Broadly, these data provide new insights into the biomechanics of using fins versus limbs for moving over land and factors that may favor the evolution of different terrestrial gaits.

Terrestrial force production by the limbs of a semi-aquatic salamander provides insight into the evolution of terrestrial locomotor mechanics.

Amphibious fishes and salamanders are valuable functional analogs for vertebrates that spanned the water-land transition. However, investigations of walking mechanics have focused on terrestrial salamanders and, thus, may better reflect the capabilities of stem tetrapods that were already terrestrial. The earliest tetrapods were likely aquatic, so salamanders that are not primarily terrestrial may yield more appropriate data for modeling the incipient stages of terrestrial locomotion. In the present study, locomotor biomechanics were quantified from semi-aquatic Pleurodeles waltl, a salamander that spends most of its adult life in water, and then compared with those of a primarily terrestrial salamander (Ambystoma tigrinum) and a semi-aquatic fish (Periophthalmus barbarus) to evaluate whether terrestrial locomotion was more comparable between species with ecological versus phylogenetic similarities. Ground reaction forces (GRFs) from individual limbs or fins indicated that the pectoral appendages of each taxon had distinct patterns of force production, but GRFs from the hindlimbs were comparable between the salamander species. The rate at which force is produced can affect musculoskeletal function, so we also calculated 'yank' (first time derivative of force) to quantify the dynamics of GRF production. Yank was sometimes slower in P. waltl but there were some similarities between the three species. Finally, the semi-aquatic taxa (P. waltl and P. barbarus) had a more medial inclination of the GRF compared to terrestrial salamanders, potentially elevating bone stresses among more aquatic taxa and limiting their excursions onto land.

Ontogenetic changes in limb posture, kinematics, forces and joint moments in American alligators (Alligator mississippiensis).

As animals increase in size, common patterns of morphological and physiological scaling may require them to perform behaviors such as locomotion while experiencing a reduced capacity to generate muscle force and an increased risk of tissue failure. Large mammals are known to manage increased mechanical demands by using more upright limb posture. However, the presence of such size-dependent changes in limb posture has rarely been tested in animals that use non-parasagittal limb kinematics. Here, we used juvenile to subadult American alligators (total length 0.46-1.27 m, body mass 0.3-5.6 kg) and examined their limb kinematics, forces, joint moments and center of mass (CoM) to test for ontogenetic shifts in posture and limb mechanics. Larger alligators typically walked with a more adducted humerus and femur and a more extended knee. Normalized peak joint moments reflected these postural patterns, with shoulder and hip moments imposed by the ground reaction force showing relatively greater magnitudes in the smallest individuals. Thus, as larger alligators use more upright posture, they incur relatively smaller joint moments than smaller alligators, which could reduce the forces that the shoulder and hip adductors of larger alligators must generate. The CoM shifted nonlinearly from juveniles through subadults. The more anteriorly positioned CoM in small alligators, together with their compliant hindlimbs, contributes to their higher forelimb and lower hindlimb normalized peak vertical forces in comparison to larger alligators. Future studies of alligators that approach maximal adult sizes could give further insight into how animals with non-parasagittal limb posture modulate locomotor patterns as they increase in mass and experience changes in the CoM.

Adhesive force and endurance of the pelvic sucker across different modes of waterfall-climbing in gobiid fishes: Contrasting climbing mechanisms share aspects of ontogenetic change.

Waterfall-climbing gobiids from oceanic islands use a suction-based adhesive mechanism formed by fused pelvic fins (pelvic sucker) and exhibit rock-climbing behavior during upstream migration. Although adhesion is a common feature of locomotion in these fishes, two distinct climbing styles - powerburst climbing and inching - have evolved. We compared the performance of the pelvic sucker during climbing across a range of body sizes between two species that use these different styles, collecting new data from the powerburst climber Lentipes concolor, and comparing these to published data for the inching climber Sicyopterus japonicus. Suction force for adhesion generated during continuous climbing did not differ between the species, with similar mean safety factors of 2.5-3.0. However, L. concolor engaged its pelvic sucker for a significantly longer duration of time (approximately 34 % longer per climbing cycle) than S. japonicus during continuous climbing. During sustained adhesion, both species exhibited non-linear scaling of fatigue time, with intermediate-sized individuals (e.g., large juveniles to small adults) showing the greatest endurance. However, the two species exhibited strikingly different maxima and variability in the endurance of their pelvic suckers. Maximum time to fatigue in L. concolor was less than half that of S. japonicus, but L. concolor showed more than double the variability of S. japonicus in time to fatigue. Our comparisons of these species reveal that despite differences in several aspects of their adhesive performance, some features of sucker function remain similar across climbing styles, including several related to how performance changes through ontogeny.

Wading through water: effects of water depth and speed on the drag and kinematics of walking Chilean flamingos, Phoenicopterus chilensis.

Wading behaviours, in which an animal walks while partially submerged in water, are present in a variety of taxa including amphibians, reptiles, mammals and birds. Despite the ubiquity of wading behaviours, few data are available to evaluate how animals adjust their locomotion to accommodate changes in water depth. Because drag from water might impose additional locomotor costs, wading animals might be expected to raise their feet above the water up to a certain point until such behaviours lead to awkward steps and are abandoned. To test for such mechanisms, we measured drag on models of the limbs of Chilean flamingos (Phoenicopterus chilensis) and measured their limb and body kinematics as they walked and waded through increasing depths of water in a zoo enclosure. Substantial drag was incurred by models of both open- and closed-toed feet, suggesting that flamingos could avoid some locomotor costs by stepping over water, rather than through it, during wading. Step height was highest while wading through intermediate water depths and while wading at a faster speed. Stride length increased with increasing water depth and velocity, and the limb joints generally flexed more while moving through intermediate water depths. However, movements of the head and neck were not strongly correlated with water depth or velocity. Our results show a wide range of kinematic changes that occur to allow wading birds to walk through different water depths, and have implications for better understanding the locomotor strategies employed by semi-aquatic species.

Bendy to the bone: Links between vertebral morphology and waterfall climbing in amphidromous gobioid fishes.

Locomotor force production imposes strong demands on organismal form. Thus, the evolution of novel locomotor modes is often associated with morphological adaptations that help to meet those demands. In the goby lineage of fishes, most species are marine and use their fused pelvic fins to facilitate station holding in wave-swept environments. However, several groups of gobies have evolved an amphidromous lifecycle, in which larvae develop in the ocean but juveniles migrate to freshwater for their adult phase. In many of these species, the pelvic fins have been co-opted to aid in climbing waterfalls during upstream migrations to adult habitats. During horizontal swimming, forces are produced by axial musculature pulling on the vertebral column. However, during vertical climbing, gravity also exerts forces along the length of the vertebral column. In this study, we searched for novel aspects of vertebral column form that might be associated with the distinctive locomotor strategies of climbing gobies. We predicted that stiffness would vary along the length of the vertebral column due to competing demands for stability of the suction disk anteriorly and flexibility for axial thrust production posteriorly. We also predicted that derived, climbing goby species would require stiffer backbones to aid in vertical thrust production compared to non-climbing species. To test these predictions, we used microcomputed tomography scans to compare vertebral anatomy (centrum length, centrum width, and intervertebral space) along the vertebral column for five gobioid species that differ in climbing ability. Our results support our second prediction, that gobies are more flexible in the posterior portion of the body. However, the main variation in vertebral column form associated with climbing ability was the presence of larger intervertebral spaces in Sicyopterus stimpsoni, a species that uses a distinctive inching behavior to climb. These results build on past kinematic studies of goby climbing performance and lend insights into how the underlying vertebral structure of these fishes may enable their novel locomotion.

Sticking to it: testing passive pull-off forces in waterfall-climbing fishes across challenging substrates.

The pelvic sucker of Hawaiian waterfall climbing gobies allows these fishes to attach to substrates while climbing waterfalls tens to hundreds of meters tall. Climbing ability varies by species and may be further modulated by the physical characteristics of the waterfall substrate. In this study, we investigated the influence of surface wettability (hydrophobic versus hydrophilic surface charges) and substrate roughness on the passive adhesive system of four species of gobies with different climbing abilities. Overall, passive adhesive performance varied by species and substrate, with the strongest climbers showing the highest shear pull-off forces, particularly on rough surfaces. Thus, differences in passive adhesive performance may help to explain the ability of some species to migrate further upstream than others and contribute to their ability to invade new habitats.

Functional correlations of axial muscle fiber type proportions in the waterfall-climbing Hawaiian stream fish Sicyopterus stimpsoni.

Assessing the factors that contribute to successful locomotor performance can provide critical insight into how animals survive in challenging habitats. Locomotion is powered by muscles, so that differences in the relative proportions of red (slow-oxidative) vs. white (fast-glycolytic) fibers can have significant implications for locomotor performance. We compared the relative proportions of axial red muscle fibers between groups of juveniles of the amphidromous gobiid fish, Sicyopterus stimpsoni, from the Hawaiian Islands. Juveniles of this species migrate from the ocean into freshwater streams, navigating through a gauntlet of predators that require rapid escape responses, before reaching waterfalls which must be climbed (using a slow, inching behavior) to reach adult breeding habitats. We found that fish from Kaua'i have a smaller proportion of red fibers in their tail muscles than fish from Hawai'i, matching expectations based on the longer pre-waterfall stream reaches of Kaua'i that could increase exposure to predators, making reduction of red muscle and increases in white muscle advantageous. However, no difference in red muscle proportions was identified between fish that were either successful or unsuccessful in scaling model waterfalls during laboratory climbing trials, suggesting that proportions of red muscle are near a localized fitness peak among Hawaiian individuals.

Variation in limb loading magnitude and timing in tetrapods.

Comparative analyses of locomotion in tetrapods reveal two patterns of stride cycle variability. Tachymetabolic tetrapods (birds and mammals) have lower inter-cycle variation in stride duration than bradymetabolic tetrapods (amphibians, lizards, turtles and crocodilians). This pattern has been linked to the fact that birds and mammals share enlarged cerebella, relatively enlarged and heavily myelinated Ia afferents, and γ-motoneurons to their muscle spindles. Both tachymetabolic tetrapod lineages also possess an encapsulated Golgi tendon morphology, thought to provide more spatially precise information on muscle tension. The functional consequence of this derived Golgi tendon morphology has never been tested. We hypothesized that one advantage of precise information on muscle tension would be lower and more predictable limb bone stresses, achieved in tachymetabolic tetrapods by having less variable substrate reaction forces than bradymetabolic tetrapods. To test this hypothesis, we analyzed hindlimb substrate reaction forces during locomotion of 55 tetrapod species in a phylogenetic comparative framework. Variation in species means of limb loading magnitude and timing confirm that, for most of the variables analyzed, variance in hindlimb loading and timing is significantly lower in species with encapsulated versus unencapsulated Golgi tendon organs. These findings suggest that maintaining predictable limb loading provides a selective advantage for birds and mammals by allowing energy savings during locomotion, lower limb bone safety factors and quicker recovery from perturbations. The importance of variation in other biomechanical variables in explaining these patterns, such as posture, effective mechanical advantage and center-of-mass mechanics, remains to be clarified.

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.

The Translation of Movement From the Equine to Rider With Relevance for Hippotherapy.

Although horseback riding is a well-established means of rehabilitation therapy for a variety of human patients, there are few data on the biomechanical relationships between horse and rider during such hippotherapy. We simultaneously tracked the movements of a horse with several different novice riders, under conditions similar to hippotherapy, to evaluate whether horses pass the same motion to different riders while being lead at a walk. Riders were outfitted with a goniometric data collection system that recorded the angles of flexion and extension and lateral bending of the thoracic and lumbar spine, as well as the flexion, extension, abduction, and adduction of each hip. We found consistent rhythmic motion in the horse's hock, shoulder, and knee. There were no significant differences in equine movement across six different riders, supporting the conclusion that horses can be used as a reproducible rehabilitation platform for riders. Moreover, although riders demonstrated different baseline postures, there were no significant differences in their ranges of motion in each joint. These results indicate that a horse can reproducibly influence a population of participants in a hippotherapy situation.

Evidence of local adaptation in a waterfall-climbing Hawaiian goby fish derived from coupled biophysical modeling of larval dispersal and post-settlement selection.

BACKGROUND: Local adaptation of marine and diadromous species is thought to be a product of larval dispersal, settlement mortality, and differential reproductive success, particularly in heterogeneous post-settlement habitats. We evaluated this premise with an oceanographic passive larval dispersal model coupled with individual-based models of post-settlement selection and reproduction to infer conditions that underlie local adaptation in Sicyopterus stimpsoni, an amphidromous Hawaiian goby known for its ability to climb waterfalls. RESULTS: Our model results demonstrated that larval dispersal is spatio-temporally asymmetric, with more larvae dispersed from the southeast (the Big Island) to northwest (Kaua'i) along the archipelago, reflecting prevailing conditions such as El Niño/La Niña oscillations. Yet connectivity is nonetheless sufficient to result in homogenous populations across the archipelago. We also found, however, that ontogenetic shifts in habitat can give rise to adaptive morphological divergence when the strength of predation-driven post-settlement selection crosses a critical threshold. Notably, our simulations showed that larval dispersal is not the only factor determining the likelihood of morphological divergence. We found adaptive potential and evolutionary trajectories of S. stimpsoni were greater on islands with stronger environmental gradients and greater variance in larval cohort morphology due to fluctuating immigration. CONCLUSIONS: Contrary to expectation, these findings indicate that immigration can act in concert with selection to favor local adaptation and divergence in species with marine larval dispersal. Further development of model simulations, parameterized to reflect additional empirical estimates of abiotic and biotic factors, will help advance our understanding of the proximate and ultimate mechanisms driving adaptive evolution, population resilience, and speciation in marine-associated species.

Comparative limb bone scaling in turtles: Phylogenetic transitions with changes in functional demands?

Several terrestrial vertebrate clades include lineages that have evolved nearly exclusive use of aquatic habitats. In many cases, such transitions are associated with the evolution of flattened limbs that are used to swim via dorsoventral flapping. Such changes in shape may have been facilitated by changes in limb bone loading in novel aquatic environments. Studies on limb bone loading in turtles found that torsion is high relative to bending loads on land, but reduced compared to bending during aquatic rowing. Release from torsion among rowers could have facilitated the evolution of hydrodynamically advantageous flattened limbs among aquatic species. Because rowing is regarded as an intermediate locomotor stage between walking and flapping, rowing species might show limb bone flattening intermediate between the tubular shapes of walkers and the flattened shapes of flappers. We collected measurements of humeri and femora from specimens representing four functionally divergent turtle clades: sea turtles (marine flappers), softshells (specialized freshwater rowers), emydids (generalist semiaquatic rowers), and tortoises (terrestrial walkers). Patterns of limb bone scaling with size were compared across lineages using phylogenetic comparative methods. Although rowing taxa did not show the intermediate scaling patterns we predicted, our data provide other functional insights. For example, flattening of sea turtle humeri was associated with positive allometry (relative to body mass) for the limb bone diameter perpendicular to the flexion-extension plane of the elbow. Moreover, softshell limb bones exhibit positive allometry of femoral diameters relative to body mass, potentially helping them maintain their typical benthic position in water by providing additional weight to compensate for shell reduction. Tortoise limb bones showed positive allometry of diameters, as well as long humeri, relative to body mass, potentially reflecting specializations for resisting loads associated with digging. Overall, scaling patterns of many turtle lineages appear to correlate with distinctive behaviors or locomotor habits.

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.