Caenorhabditis elegans alternate periods of forward movement with either large angle reorientation maneuvers called pirouettes or small angle turns. The trajectories of larger animals like nematodes and insect larvae are qualitatively similar ( Pierce-Shimomura et al., 1999 Luo et al., 2010). During runs, the bacteria swim in roughly straight lines, while during tumbles, the bacteria move erratically in place, ultimately picking the direction of a new run at random. A classic example is the swimming behavior of Escherichia coli ( Berg, 1993), which is characterized by an alternating sequence of runs and tumbles.
The trajectories of small organisms often involve stochastic transitions between distinct motor states. A key constraint on the behavioral strategies of these organisms appears to be the preservation of their capacity to continuously explore their environments in all directions even while moving toward favorable conditions. We find that the statistics of random exploration in any direction are little affected by directed movement along a stimulus gradient. How does directed navigation in a gradient modulate random exploration either parallel or orthogonal to the gradient? Here, we introduce methods originally used for analyzing protein folding trajectories to study the trajectories of the nematode Caenorhabditis elegans and the Drosophila larva in isotropic environments, as well as in thermal and chemical gradients.
In stimulus gradients, movement decisions are biased to drive directed navigation toward favorable environments. In these organisms, navigation in isotropic environments can be characterized as an essentially diffusive and undirected process. Many organisms-from bacteria to nematodes to insect larvae-navigate their environments by biasing random movements.
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