Relevance of Metric-Free, "Topological" Interactions in Flocking Phenomena

Motivated by recent findings on animal and human collective motion, we show that the collective properties of self-propelled particles aligning with their "topological" (Voronoi) neighbors are {\it qualitatively} different from those of usual models where metric interaction ranges are used. Our results, obtained in a minimal setting, indicate the relevance of metric-free interactions and suggest that, indeed, realistic flocking models may have to incorporate them.