Also, the tiger cannot change its stripes. One can't change one's essential nature. For example, He's a conservative, no matter what he says; the leopard cannot change its spots. These metaphoric expressions both originated in an ancient Greek proverb that appears in the Bible (Jeremiah 13:23): "Can the Ethiopian change his skin, or the leopard his spots?" It was first recorded in English in 1546.
American Heritage Idioms