An animal identification code is described comprising two numbers, one of which is encoded into a human-readable marking and the other of which is encoded into a machine-readable marking, where the two numbers and the two encodings are different. The combination of the two numbers, plus additional information not marked on a first animal, such as time of read, is looked up a first table to determine conditional validity and from there to a second table to determine a valid and unique animal ID associated with the animal, a primary key. The animal may be a rodent in a vivarium and the markings may be tattooed on the animal tail. The second marking may be a vine code with a spine where the spine is aligned with the animal tail. The first number may be unique within a first animal population such as an animal study. The combination of the first and second marking may be reused on a second animal wherein the lifetimes of the first and second animal do not overlap. The primary key is unique among all animals in a third population, dead or alive.