I shall discuss the challenges faced by living organisms
trying to locate and move towards a source of nutrients,
odors, pheromones, etc., i.e. substances emitted by the
source and randomly transported by the environmental medium.
Microorganisms, such as bacteria performing chemotaxis, can
rely on local concentration gradients to guide them towards
the source, yet they have to cope with the stochastic nature
of the microscopic world and must reliably infer local
gradients from noisy series of detections. Macro-organisms,
such as insects and birds, are spared by molecular noise but
they lack local cues pointing towards the location of the
source because mixing in a flowing medium breaks up regions
of high concentration into random and disconnected patches,
carried by winds and currents. Thus macroscopic animals,
e.g. sensing odors in air or water, detect them only
intermittently as patches of odor sweep by, and they must
devise a strategy of movement based upon sporadic cues and
partial/missing information. Understanding of the strategies
evolved by living organisms also has technological
applications to robotics.
