Viscoelastic burrowing locomotion in nematodes reveal transition to complex postural dynamics

Despite being one of the most well studied model organisms in biology, little is known about the locomotion of C. elegans in natural settings. In the wild, these animals contend with heterogenous environments like rotten fruit, soil and the backs of insects; however, the majority of behavioral studies of C. elegans consider swimming in Newtonian fluids or crawling on the surface of agarose. In these environments, C. elegans moves via regular traveling waves of body curvature, with wavelengths and frequencies that decrease with increasing fluid viscosity. We observed C. elegans burrowing within non-Newtonian gels with tunable viscoelasticity - models of conditions in rotting fruit. In these viscoelastic environments, previously identified wavelength/frequency trends persist, however the motion becomes complex and irregular. Principal component analysis reveals that similar quasi-sinusoidal component waveforms account for the majority of postural variation in each regime (swimming, crawling and viscoelastic burrowing). However, where swimming and crawling produce smooth, regular orbits in the amplitude space of the components, burrowing reveals jagged, complex trajectories, accounting for highly irregular body postures in both turning and forward locomotion.