The Guide mandates "...Use of appropriate species, quality, and number of animals. " It does not prescribe which animals to use; it takes a "performance" approach, leaving up to the scientist and his or her IACUC the decision as to what is "appropriate" in the local circumstances. The next-listed precept is "... Avoidance or minimization of discomfort, distress, and pain in concert with sound science." From these two requirements we are to infer that the lower the order of the subject animal, the less discomfort, distress, and pain will be experienced. Hence, the most "appropriate" animal model for research is the lowest order animal that provides the other requisite characteristics needed to perform sound science.
Under consideration in the process of selecting an animal model is the number of animals that will be required to provide statistically significant results. This is a matter of the sum of species characteristics and the statistical method being employed. A significantly larger number of a lower order animal may not be "appropriate" if a significantly smaller number of a somewhat higher order animal can be used instead.
Needless to say, appropriate species are those for which the institution has the requisite facilities and expertise.