Abstract
All animal behaviour must ultimately be governed by physical laws. As a basis for understanding the physics of behaviour in fruit fly larvae, we here develop an effective theory for the animals’ motion in three dimensions.
We first define a set of fields which quantify stretching, bending, and twisting along the larva’s anteroposterior axis, then perform a search in the space of possible theories that could govern these fields’ long-wavelength physics. Guided by symmetry considerations and stability requirements, we arrive at a unique, analytically tractable free-field theory with a minimum of free parameters.
Surprisingly, we are able to explain many features of larval behaviour by applying equilibrium statistical mechanics to this model. Our theory closely predicts the animals’ postural modes (eigenmaggots) as well as distributions and trajectories in the postural mode space, across several behaviours (crawling, rolling, self-righting, unbiased exploration).
We explain the low-dimensionality of postural dynamics via Boltzmann suppression of high frequency modes, and also propose and experimentally test novel predictions on the relationships between different behaviours. We show that crawling and rolling are dominated by similar symmetry properties, leading to identical dynamics/statistics in mode space, while rolling and unbiased exploration have a common dominant timescale. Together, our results demonstrate that relatively simple effective physics can be used to explain and predict animal behaviour.
Competing Interest Statement
The authors have declared no competing interest.
Footnotes
author list & author roles and acknowledgements updated; minor typographical errors fixed throughout; cosmetic changes to abstract; methods section updated to include details of self-righting behaviour experiments and analysis
