Kinematics of aquatic and terrestrial escape responses in mudskippers.
ABSTRACT Escape responses in fishes are rapid behaviors that are critical for survival. The barred mudskipper (Periophthalmus argentilineatus) is an amphibious fish that must avoid predators in two environments. We compared mudskipper terrestrial and aquatic escapes to address two questions. First, how does an amphibious fish perform an escape response in a terrestrial environment? Second, how similar is a terrestrial escape response to an aquatic escape response? Because a mudskipper on land does not have to contend with the high viscosity of water, we predicted that, if the same behavior is employed across environments, terrestrial escape responses should have 'better' performance (higher velocity and more rapid completion of movements) when compared with aquatic escape responses. By contrast, we predicted that intervertebral bending would be similar across environments because previous studies of escape response behaviors in fishes have proposed that vertebral morphology constrains intervertebral bending. High-speed digital imaging was used to record mudskipper escapes in water and on land, and the resulting images were used to calculate intervertebral bending during the preparatory phase, peak velocity and acceleration of the center of mass during the propulsive phase, and relative timing of movements. Although similar maximum velocities are achieved across environments, terrestrial responses are distinct from aquatic responses. During terrestrial escapes, mudskippers produce greater axial bending in the preparatory phase, but only in the posterior region of the body and over a much longer time period. Mudskippers also occasionally produced the 'wrong' behavior for a given environment. Thus, it appears that the same locomotor morphology is recruited differently by the central nervous system to produce a distinct behavior appropriate for each environment.
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ABSTRACT: To discover principles of flipper-based terrestrial locomotion we study the mechanics of a hatchling sea turtle-inspired robot, FlipperBot (FBot), during quasi-static movement on granular media. FBot implements a symmetric gait using two servo-motor-driven front limbs with flat-plate flippers and either freely rotating or fixed wrist joints. For a range of gaits, FBot moves with a constant step length. However, for gaits with sufficiently shallow flipper penetration or sufficiently large stroke, per step displacement decreases with each successive step resulting in failure (zero forward displacement) within a few steps. For the fixed wrist, failure occurs when FBot interacts with ground disturbed during previous steps, and measurements reveal that flipper generated forces decrease as per step displacement decreases. The biologically inspired free wrist is less prone to failure, but slip-induced failure can still occur if FBot pitches forward and drives its leading edge into the substrate. In the constant step length regime, kinematic and force-based models accurately predict FBot's motion for free and fixed wrist configurations, respectively. When combined with independent force measurements, models and experiments provide insight into how disturbed ground leads to locomotory failure and help explain differences in hatchling sea turtle performance. S Online supplementary data available from stacks.iop.org/BB/8/026007/mmedia (Some figures may appear in colour only in the online journal)Bioinspiration & Biomimetics 04/2013; 8(8):26007-26007. · 1.95 Impact Factor