Virtual design from nature: Kinematic modeling of the seahorse tail

Seahorses (belonging to the genus Hippocampus) and pipehorses are unique among fishes in being armed with robust body plates instead of scales, and in being able to bend their tail ventrally for use as a prehensile appendage. They lack a powerful caudal fin, making them very slow swimmers, which can hardly escape predatory fish. However, they have survived millions of years of natural selection, yielding a very well camouflaged fish with a highly specialized caudal skeleton. This skeleton consists of a central axis of articulating vertebrae, surrounded by jointed bony rings. The stiff bony plates form an armor, which likely makes the fish strongly resistant to bites and unappetizing for predators. However, the adaptive nature of the articulated segments may also be in relation to the formation of the flexible, prehensile tail, which can roll up ventrally over more than 360 degrees and enables the seahorse to grasp and hold onto a support. The tail bending mechanism of seahorses has been studied superficially in literature, but details of its functioning and constructional morphology are lacking. To gain a profound insight into the kinematics and mechanical interactions of the skeletal elements of the tail, the musculoskeletal system is modeled, combining pyFormex with Abaqus, using various features such as beam connectors (bony plates and vertebrae), cartesian, cardan and slot connectors (joints) and axial connectors (muscles). Such modeling may allow the design of nature-inspired structures, providing innovative solutions in engineering where high stiffness combined with high flexibility is needed.