|Animals have very short life spans normally. This spell takes advantage of that. By aging the animal to its point of death, the animal targeted has one round left to life, before it dies of old age. If the animal succeeds in a fortitude saving throw, it is affected as by the spell slow for two rounds.|
Note that certain animals, such as tortoises, actually have life spans comparable with or longer than humanoids; such animals are merely slowed for two rounds if they fail their save, and unaffected if they succeed.