An Effective Hydrodynamic Description of Marching Locusts
A fundamental question in complex systems is how to relate interactions between individual components ("microscopic description") to the global properties of the system ("macroscopic description"). Another fundamental question is whether such a macroscopic description exists at a...
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Main Authors | , , , , , , |
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Format | Journal Article |
Language | English |
Published |
03.08.2023
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Subjects | |
Online Access | Get full text |
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Summary: | A fundamental question in complex systems is how to relate interactions
between individual components ("microscopic description") to the global
properties of the system ("macroscopic description"). Another fundamental
question is whether such a macroscopic description exists at all and how well
it describes the large-scale properties. Here, we address these questions using
as a canonical example of a self-organizing complex system - the collective
motion of desert locusts. One of the world's most devastating insect plagues
begins when flightless juvenile locusts form "marching bands". Moving through
semiarid habitats in the search for food, these bands display remarkable
coordinated motion. We investigated how well physical models can describe the
flow of locusts within a band. For this, we filmed locusts within marching
bands during an outbreak in Kenya and automatically tracked all individuals
passing through the camera frame. We first analysed the spatial topology of
nearest neighbors and found individuals to be isotropically distributed.
Despite this apparent randomness, a local order was observed in regions of high
density with a clear second neighbor peak in the radial distribution function,
akin to an ordered fluid. Furthermore, reconstructing individual locust
trajectories revealed a highly-aligned movement, consistent with the
one-dimensional version of the Toner-Tu equations, which are a generalization
of the Navier-Stokes equations for fluids, used to describe the equivalent
macroscopic fluid properties of active particles. Using this effective Toner-Tu
equation, which relates the gradient of the pressure to the acceleration, we
show that the effective "pressure" of locusts increases as a linear function of
density in segments with highest polarization. Our study thus demonstrates an
effective hydrodynamic description of flow dynamics in plague locust swarms. |
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DOI: | 10.48550/arxiv.2308.02589 |