In biology, the term precocial refers to species in which the young are relatively mature and mobile from the moment of birth or hatching. The opposite developmental strategy is called "altricial" where the young are born or hatched helpless. Extremely precocial species may be called "superprecocial". These three categories form a continuum, without distinct gaps between them. Precocial species are normally nidifugous, meaning that they leave the nest shortly after birth or hatching.