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.
Aquatic vertebrates display a variety of control surfaces that are used for propulsion, stabilization, trim and maneuvering. Control surfaces include paired and median fins in fishes, and flippers ...and flukes in secondarily aquatic tetrapods. These structures initially evolved from embryonic fin folds in fishes and have been modified into complex control surfaces in derived aquatic tetrapods. Control surfaces function both actively and passively to produce torque about the center of mass by the generation of either lift or drag, or both, and thus produce vector forces to effect rectilinear locomotion, trim control and maneuvers. In addition to fins and flippers, there are other structures that act as control surfaces and enhance functionality. The entire body can act as a control surface and generate lift for stability in destabilizing flow regimes. Furthermore, control surfaces can undergo active shape change to enhance their performance, and a number of features act as secondary control structures: leading edge tubercles, wing-like canards, multiple fins in series, finlets, keels and trailing edge structures. These modifications to control surface design can alter flow to increase lift, reduce drag and enhance thrust in the case of propulsive fin-based systems in fishes and marine mammals, and are particularly interesting subjects for future research and application to engineered systems. Here, we review how modifications to control surfaces can alter flow and increase hydrodynamic performance.
Re-invasion of the aquatic environment by terrestrial vertebrates resulted in the evolution of species expressing a suite of adaptations for high-performance swimming. Examination of swimming by ...secondarily aquatic vertebrates provides opportunities to understand potential selection pressures and mechanical constraints, which may have directed the evolution of these aquatic species. Mammals and birds realigned the body and limbs for cursorial movements and flight, respectively, from the primitive tetrapod configuration. This realignment produced multiple solutions for aquatic specializations and swimming modes. Initially, in the evolution of aquatic mammals and birds, swimming was accomplished by using paired appendages in a low-efficiency, drag-based paddling mode. This mode of swimming arose from the modification of neuromotor patterns associated with gaits characteristic of terrestrial and aerial locomotion. The evolution of advanced swimming modes occurred in concert with changes in buoyancy control for submerged swimming, and a need for increased aquatic performance. Aquatic mammals evolved three specialized lift-based modes of swimming that included caudal oscillation, pectoral oscillation, and pelvic oscillation. Based on modern analogs, a biomechanical model was developed to explain the evolution of specialized aquatic mammals and their transitional forms. Subsequently, fossil aquatic mammals were described that validated much of the model. However, for birds, which were adapted for aerial flight, fossil evidence has been less forthcoming to explain the transition to aquatic capabilities. A biomechanical model is proposed for birds to describe the evolution of specialized lift-based foot and wing swimming. For both birds and mammals, convergence in morphology and propulsive mechanics is dictated by the need to increase speed, reduce drag, improve thrust output, enhance efficiency, and control maneuverability in the aquatic environment.
Attempts to measure the propulsive forces produced by swimming dolphins have been limited. Previous uses of computational hydrodynamic models and gliding experiments have provided estimates of thrust ...production by dolphins, but these were indirect tests that relied on various assumptions. The thrust produced by two actively swimming bottlenose dolphins (Tursiops truncatus) was directly measured using digital particle image velocimetry (DPIV). For dolphins swimming in a large outdoor pool, the DPIV method used illuminated microbubbles that were generated in a narrow sheet from a finely porous hose and a compressed air source. The movement of the bubbles was tracked with a high-speed video camera. Dolphins swam at speeds of 0.7 to 3.4 m s(-1) within the bubble sheet oriented along the midsagittal plane of the animal. The wake of the dolphin was visualized as the microbubbles were displaced because of the action of the propulsive flukes and jet flow. The oscillations of the dolphin flukes were shown to generate strong vortices in the wake. Thrust production was measured from the vortex strength through the Kutta-Joukowski theorem of aerodynamics. The dolphins generated up to 700 N during small amplitude swimming and up to 1468 N during large amplitude starts. The results of this study demonstrated that bubble DPIV can be used effectively to measure the thrust produced by large-bodied dolphins.
One of the great transformations in evolution of vertebrates has been the return to the aquatic environment after the conquest of terrestrial ecosystems. With structural and physiological ...characteristics adapted to function on land, the various non-piscine taxa had to modify these characteristics to perform in water. Secondary aquatic vertebrates successfully transformed mechanisms for feeding, locomotion, osmoregulation, and sensory systems to function and thrive in an aqueous environment. This symposium emphasized the changes that had to be acquired to operate in the water with morphologies previously evolved to function on land. It brought together researchers working on different aspects of functional biology and on various taxa in order to illustrate the diversity in the required adaptations: the numerous convergences as well as the specific adaptive traits. The collection of talks, posters, and of the contributions to this special volume highlights recent advances in the understanding of the functional adaptations associated to secondary adaptation to an aquatic lifestyle in vertebrates.
The manta is the largest marine organism to swim by dorsoventral oscillation (flapping) of the pectoral fins. The manta has been considered to swim with a high efficiency stroke, but this assertion ...has not been previously examined. The oscillatory swimming strokes of the manta were examined by detailing the kinematics of the pectoral fin movements swimming over a range of speeds and by analyzing simulations based on computational fluid dynamic potential flow and viscous models. These analyses showed that the fin movements are asymmetrical up- and downstrokes with both spanwise and chordwise waves interposed into the flapping motions. These motions produce complex three-dimensional flow patterns. The net thrust for propulsion was produced from the distal half of the fins. The vortex flow pattern and high propulsive efficiency of 89% were associated with Strouhal numbers within the optimal range (0.2-0.4) for rays swimming at routine and high speeds. Analysis of the swimming pattern of the manta provided a baseline for creation of a bio-inspired underwater vehicle, MantaBot.
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 (
), 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
and 2.05±1.26 m, respectively. Such turning maneuvers fall within the range of performance exhibited by swimmers with rigid bodies.
Dolphins have become famous for their ability to perform a wide variety of athletic and acrobatic behaviors including high-speed swimming, maneuverability, porpoising and tail stands. Tail stands are ...a behavior where part of the body is held vertically above the water's surface, achieved through thrust produced by horizontal tail fluke oscillations. Strong, efficient propulsors are needed to generate the force required to support the dolphin's body weight, exhibiting chordwise and spanwise flexibility throughout the stroke cycle. To determine how thrust production, fluke flexibility and tail stroke kinematics vary with effort, six adult bottlenose dolphins (Tursiops truncatus) were tested at three different levels based on the position of the center of mass (COM) relative to the water's surface: low (COM below surface), medium (COM at surface) and high (COM above surface) effort. Additionally, fluke flexibility was measured as a flex index (FI=chord length/camber length) at four points in the stroke cycle: center stroke up (CU), extreme top of stroke (ET), center stroke down (CD) and extreme bottom of stroke (EB). Video recordings were analyzed to determine the weight supported above the water (thrust production), peak-to-peak amplitude, stroke frequency and FI. Force production increased with low, medium and high efforts, respectively. Stroke frequency also increased with increased effort. Amplitude remained constant with a mean 33.8% of body length. Significant differences were seen in the FI during the stroke cycle. Changes in FI and stroke frequency allowed for increased force production with effort, and the peak-to-peak amplitude was higher compared with that for horizontal swimming.
The lifestyle of spinosaurid dinosaurs has been a topic of lively debate ever since the unveiling of important new skeletal parts for Spinosaurus aegyptiacus in 2014 and 2020. Disparate lifestyles ...for this taxon have been proposed in the literature; some have argued that it was semiaquatic to varying degrees, hunting fish from the margins of water bodies, or perhaps while wading or swimming on the surface; others suggest that it was a fully aquatic underwater pursuit predator. The various proposals are based on equally disparate lines of evidence. A recent study by Fabbri and coworkers sought to resolve this matter by applying the statistical method of phylogenetic flexible discriminant analysis to femur and rib bone diameters and a bone microanatomy metric called global bone compactness. From their statistical analyses of datasets based on a wide range of extant and extinct taxa, they concluded that two spinosaurid dinosaurs (S. aegyptiacus, Baryonyx walkeri) were fully submerged "subaqueous foragers," whereas a third spinosaurid (Suchomimus tenerensis) remained a terrestrial predator. We performed a thorough reexamination of the datasets, analyses, and methodological assumptions on which those conclusions were based, which reveals substantial problems in each of these areas. In the datasets of exemplar taxa, we found unsupported categorization of taxon lifestyle, inconsistent inclusion and exclusion of taxa, and inappropriate choice of taxa and independent variables. We also explored the effects of uncontrolled sources of variation in estimates of bone compactness that arise from biological factors and measurement error. We found that the ability to draw quantitative conclusions is limited when taxa are represented by single data points with potentially large intrinsic variability. The results of our analysis of the statistical method show that it has low accuracy when applied to these datasets and that the data distributions do not meet fundamental assumptions of the method. These findings not only invalidate the conclusions of the particular analysis of Fabbri et al. but also have important implications for future quantitative uses of bone compactness and discriminant analysis in paleontology.
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
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