Almost a decade ago, Manami Hara published a wonderful series of papers about species differences in the cell composition and cytoarchitecture of pancreatic islets (e.g. Steiner et al., 2010, Islets 2:135). The differences are really striking: cat and horse islets look like inverted mouse islets. Rodent islets are extreme in their high proportion of beta cells, whereas hyena islets mostly contain alpha cells. Human and non-human primate look alike, but then are also similar to bat islets. It is clear from Dr. Hara's studies that there is no prototypical islet design upon which evolution sculpted all other islet models. However, because the mouse has been such a successful animal model, researchers in the islet biology field insist that islets have to follow the mouse islet model.