Predicting path from undulations for C. elegans using linear and nonlinear resistive force theory

A basic issue in the physics of behaviour is the mechanical relationship between an animal and its s urroundings. The nematode and model organism C. elegans provides an excellent platform to explore this relationship due to its anatom ical simplicity. Nonetheless , the physics of nematode crawling, in which the worm undulates its body to move on a wet sur face, is not completely understood and the mathematical models often used to describe this phenomenon are empirical . We confirm that linear resistive force theory , one such empirical model, is effective at predicting a worm’s path from its sequence of bod y postures for forward crawling, reversing, and turning and for a broad range of different behavioural phenotypes observed in mutant worms. However, agreement between the predicted and observ ed path is lost when using this model with recently measured val ue s of the drag anisotropy. A recently proposed nonlinear extension of the resistive force theory model also provides accurate predictions, but does not resolve the discrepancy between the parameters required to achieve good path prediction and the experi mentally measured parameters. This means that while we have good effective models of worm crawling that can be used in application s such as whole - animal simulations and advance d tracking algorithms, there are still unanswered questions about the precise n ature of the physical interaction between worms and their most commonly studied laboratory substrate.