Locomotion Energetics of the Ghost Crab: II. Mechanics of the Centre of Mass During Walking and Running

TLDR

Terrestrial locomotion with appendages evolved independently in vertebrates and arthropods, and differences in locomotor design can constrain energetics, yet muscle properties may preserve conservative mechanical and energetic characteristics. The study measured centre‑of‑mass energy fluctuations in ghost crabs by integrating ground‑reaction forces during sideways locomotion. Ghost crabs use a pendulum‑type energy exchange during walking, exhibit two running gaits with a speed‑dependent transition matching the trot‑gallop threshold of similarly sized mammals, and maintain speed‑independent, bird‑ and mammal‑level mass‑specific mechanical energy, indicating comparable energy‑conserving mechanisms and locomotor efficiency.

Abstract

ABSTRACT Terrestrial locomotion involving appendages has evolved independently in vertebrates and arthropods. Differences in the mechanical design of the locomotor apparatus could impose constraints on the energetics of locomotion. The mechanical energy fluctuations of the centre of mass of an arthropod, the ghost crab Ocvpode quadrata (Fabricius), were examined by integrating the ground reaction forces exerted during sideways locomotion. Crabs used a pendulum-type energy exchange mechanism during walking, analogous to an egg rolling end over end, with the same effectiveness as birds and mammals. Moreover, ghost crabs were found to have two running gaits. A switch from a slow to a fast run occurred at the same speed and stride frequency predicted for the trot-gallop transition of a quadrupedal mammal of the same body mass. In addition, the mass-specific mechanical energy developed over a unit distance was independent of speed and was within the limits measured for birds and mammals. Despite the obvious differences in mechanical design between crabs and mammals, energy-conserving mechanisms and the efficiency of locomotion were remarkably similar. These similarities may result from the fact that the muscles that generate forces during terrestrial locomotion have relatively conservative mechanical and energetic properties.