Increasing viscosity helps explain locomotor control in swimming Polypterus seneglus

Locomotion relies on the successful integration of sensory information to adjust brain commands and basic motor rhythms created by central pattern generators. It is not clearly understood how altering the sensory environment impacts control of locomotion. In an aquatic environment, mechanical sensory feedback to the animal can be readily altered by adjusting water viscosity. Computer modeling of fish swimming systems show that, without sensory feedback, high viscosity systems dampen kinematic output despite similar motor control input. We recorded muscle activity and kinematics of six Polypterus senegalus in four different viscosities of water from 1 cP (normal water) to 40 cP. In high viscosity, P. senegalus exhibit increased body curvature, body wavespeed and body and pectoral fin frequency during swimming. These changes are the result of increased muscle activation intensity and maintain voluntary swimming speed. Unlike the sensory deprived model, intact sensory feedback allows fish to adjust swimming motor control and kinematic output in high viscous water but maintain typical swimming coordination.

Body and Tail Coordination in the Bluespot Salamander (Ambystoma laterale) During Limb Regeneration

Animals are incredibly good at adapting to changes in their environment, a trait envied by most roboticists. Many animals use different gaits to seamlessly transition between land and water and move through non-uniform terrains. In addition to adjusting to changes in their environment, animals can adjust their locomotion to deal with missing or regenerating limbs. Salamanders are an amphibious group of animals that can regenerate limbs, tails, and even parts of the spinal cord in some species. After the loss of a limb, the salamander successfully adjusts to constantly changing morphology as it regenerates the missing part. This quality is of particular interest to roboticists looking to design devices that can adapt to missing or malfunctioning components. While walking, an intact salamander uses its limbs, body, and tail to propel itself along the ground. Its body and tail are coordinated in a distinctive wave-like pattern. Understanding how their bending kinematics change as they regrow lost limbs would provide important information to roboticists designing amphibious machines meant to navigate through unpredictable and diverse terrain. We amputated both hindlimbs of blue-spotted salamanders (Ambystoma laterale) and measured their body and tail kinematics as the limbs regenerated. We quantified the change in the body wave over time and compared them to an amphibious fish species, Polypterus senegalus. We found that salamanders in the early stages of regeneration shift their kinematics, mostly around their pectoral girdle, where there is a local increase in undulation frequency. Amputated salamanders also show a reduced range of preferred walking speeds and an increase in the number of bending waves along the body. This work could assist roboticists working on terrestrial locomotion and water to land transitions.

Fish-inspired flexible protective material systems with anisotropic bending stiffness

Biological structures integrate morphometry (shape-based rules) with materials design to maximize organism survival. The exoskeleton of the armored fish, Polypterus senegalus, balances flexibility with protection from predatory and territorial threats. Material properties of the exoskeleton are known; however, the geometric design rules underlying its anisotropic flexibility are uncharacterized. Here, we show how scale shape, articulation, and composite architecture produce anisotropic mechanics using bio-inspired, multi-material 3D-printed prototypes. Passive loading (draping) shows that compliant connections between the scales contribute to mechanical anisotropy. Simulated and experimental active loading (bending) show orientation-dependent stiffness ranging over orders of magnitude, including ‘mechanical invisibility’ of the scales where they do not add stiffness to the exoskeleton. The results illustrate how morphometry provides a powerful tool to tune flexibility in composite architectures independent of varying constituent materials composition. We anticipate that introducing morphometric design strategies will enable flexible, protective systems tuned to complex shapes and functions.