Altricial and precocial neonates and the secondary altriciality of human babies: the legacy of Adolf Portmann

Introduction Most mammals are poorly developed at birth, with closed eyes and little hair. Others are well-developed with eyes open and a full coat of hair. The former are altricial and born in large litters after a brief gestation; the latter are precocial and from smaller litters, often singletons, born after a long gestation. Methods. This paper reviews the work of Adolf Portmann and his successors on this dichotomy and considers whether evolutionary transitions from altricial to precocial, or the reverse, are reflected in placental morphology. Results. Portmann emphasized that human babies underwent intrauterine development like precocial mammals yet differ from them in being fully dependent on parental care. Recently it was proposed that the ancestral type of placentation (discoid, labyrinthine and haemochorial) is well suited to the altricial strategy, whereas transition to the precocial type triggered a placental crisis that drives placental evolution. These ideas are further explored. Discussion. Transitions from altricial to precocial are seen in hystricognath rodents and strepsirrhine primates and from precocial to altricial in carnivores. In each instance the transition is accompanied by marked innovations in placental morphology.