An effective hydrodynamic description of marching locusts

Area of Science:

• Complex Systems
• Collective Behavior
• Biophysics

Background:

• Understanding the relationship between microscopic interactions and macroscopic properties is a key challenge in complex systems.
• The existence and descriptive power of macroscopic models for biological systems remain unclear.

Purpose of the Study:

• To investigate the validity of a macroscopic description for a complex biological system.
• To use the collective motion of desert locusts (Schistocerca gregaria) as a model for fluid dynamics.

Main Methods:

• Filming locusts in marching bands during a Kenyan outbreak and automatically tracking individuals.
• Analyzing the spatial distribution and nearest neighbor topology of locusts.
• Reconstructing individual trajectories to compare with theoretical models like the Toner-Tu equations.

Main Results:

• Locusts exhibited isotropic distribution but showed local ordering in high-density regions, similar to an ordered fluid.
• Individual locust trajectories aligned, consistent with the one-dimensional Toner-Tu equations.
• Effective locust 'pressure' increased linearly with density in highly polarized segments.

Conclusions:

• An effective hydrodynamic description, specifically the Toner-Tu equations, can capture the flow dynamics of desert locust marching bands.
• This study demonstrates the applicability of macroscopic physical models to complex biological systems like insect swarms.