There are scientific publications about the physiological, ethological and ecological needs of many species in their natural environment. However scientific literature indicating that those needs cannot or only partially be respected in a domestic household environment, are likely more challenging to find. It will highly depend on the assessment criteria chosen, whether or not assessors are able to find reliable, state of the art scientific data to assess a species on those criteria. In case of insufficient or inconclusive scientific data, the precautionary principle can be applied. This ensures a “no, unless” approach. In the Belgian Positive List, this principle is enshrined in the fifth assessment criterion: in the event of conflicting data or information, the animal is given the benefit of the doubt and isn’t included in the Positive List.