Mud snails move with fat tails: Experimental evidence for intrinsic Lévy signatures

Background/Question/Methods

When searching for unpredictably distributed food items, animals are presumed to pursue random search strategies, such as Ballistic motion, Brownian walks or Lévy walks. The presence of Lévy walks in nature remains, however, highly disputed. Evidence from tracked free-living animals is open to alternative interpretation, and it remains debated whether such complex multi-scale movement strategies are an evolutionary adaptation or an emergent property of the animal reacting to its habitat. In this study we present a novel approach of direct assessment of the search movement of mud snails (Hydrobia ulvea) in an experimental setup.

Results/Conclusions

While the classical analysis of move length distributions provided only weak support for the presence of Lévy walks, local search cluster analyses uncovered clear signatures of complex, fat tailed movement behavior: clusters of small movement steps and frequent turns form both in response to and independent of food encounter, and cluster size follows a truncated power law. A comparison of tracks in experimental landscapes showed that interactions with food patches modulated the observed movement to more Brownian walk-like searches, indicating that interaction with the landscape can obscure innate complex, multi-scale movement behavior. Finally, our experiments point out that the observed search strategy is most efficient in finding food patches in fractal landscapes, which complies with the distribution of food in the snails’ natural habitat. Thus, our study provides clear experimental evidence that in mud snails, Lévy-like, complex scale-free movement strategies are an intrinsic adaption to habitat conditions rather than an emergent property of the animal responding to its environment.