A Proposed Fluid Mechanical Basis for Anterior Propulsor Placement

Megan Leftwich, The George Washington University, Mechanical and Aerospace Engineering, Washington, DC, United States, Emma Lederer, Tufts University, United States, John Dabiri, Caltech, Graduate Aerospace Laboratories and Mechanical Engineering, Pasadena, CA, United States, Brad Gemmell, University of South Florida, Department of Integrative Biology, Tampa, United States, Sean Colin, Roger Williams University, Marine Biology and Environmental Science, Bristol, United States and Jack Costello, Providence College, Biology, Providence, United States
Abstract:
We show that across 245 species of swimming and flying animals—including birds, fish, cetaceans, bats, insects, and mollusks—the location of lateral maneuvering propulsors is
tightly constrained at a location approximately 1/3 of the body length from the anterior end (Fig. 1). A theoretical model for optimal leverage in turning is consistent with these observations, and it makes a further prediction of a non-trivial relationship between propulsor placement and body center of mass. That prediction is also confirmed by morphometric analyses, suggesting that a simple but robust physical rule governs animal design for effective maneuvering in air and water.

Among bilaterally symmetric metazoans, a common feature is a pair of lateral appendages that generate side forces to effect changes in heading (Fig. 1). The torque generated by those side forces and overall turning dynamics depend on the anteroposterior placement of the propulsors along the body. In principle, the lateral appendages could generate torque for maneuvering at any location along the animal body that does not pass through the center of mass. To quantify the actual placement of the propulsors, we conducted a morphometric analysis of 996 individuals from 245 species of insects, birds, bats, mollusks, cetaceans and fish (see Supplementary Information).

Images were collected from publicly available sources, primarily from the internet. By sampling 35-45 species from each taxonomic group, a diversity of average body sizes were incorporated in the analysis of each group. Within each species, 3-5 replicates were collected to mitigate artifacts related to the variable camera angles of the images. To explain these observations, we developed a theoretical model of turning dynamics, which predicts that the lever arm (i.e. the distance between the lateral appendages and the turning pivot point on the body) is maximized for anterior appendage placement in the observed narrow range

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