We present an original approach which consists in analysing natural innovation in animal digestive tracts in order to improve anaerobic digestion processes. The first step of this approach consists in a large descriptive inventory of animal digestive tracts. Each digestive system can be divided into elementary steps which could be translated in terms of process engineering: cutting and grinding compartments; Continuous Stirred Tank Reactors (CSTR) or Plug Flow Reactor (PFR); chemical, enzymatic or biological reactors and press filter. This paper presents only the two examples of cow and termite digestive systems, which are known as the most efficient to transform not easily degradable matter. The key elements of these two systems were optimised and then put together into two lab-scale semi-continuous pilots fed with waste activated sludge. The performance of both processes is compared with the performance of lab references and of conventional WWTP digesters.