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[CM ICTP - Trieste]

JOINT ICTP/SISSA STATISTICAL PHYSICS SEMINAR



Tuesday, 11 March    -    12:30 hrs.




Seminar Room-ICTP Main Building-1st floor


A. CAVAGNA  ( Università di Roma 'La Sapienza' )



" Starling flocks: An empirical study of the interaction ruling 
collective animal behaviour "



Abstract

Numerical models indicate that collective animal behaviour may emerge 
from simple local rules of interaction among the individuals.   
However, very little is known about the nature of such interaction, so 
that models and theories mostly rely on aprioristic assumptions.  By 
reconstructing the three-dimensional position of individual birds in 
airborne flocks of few thousands members, we prove that the interaction 
does not depend on the metric distance, as most current models and 
theories assume, but rather on the topological distance.  In fact, we 
discover that each bird interacts on average with a fixed number of 
neighbours (six-seven), rather than with all neighbours within a fixed 
metric distance.  We argue that a topological interaction is 
indispensable to maintain flock's cohesion against the large density 
changes caused by external perturbations, typically predation.  We 
support this hypothesis by numerical simulations,  showing that a 
topological interaction grants significantly higher cohesion of the 
aggregation compared to a standard metric one.