Comparative skeletal anatomy of neonatal ursids and the extreme altriciality of the giant panda

Vertebrate neonates are born with a wide range of maturity at birth. Altricial newborns are born with limited sensory agency and require significant parental care, while precocial neonates are relatively mature physically and often capable of independent function. In extant mammals, placental newborns span this range, while marsupials and monotremes are all extremely altricial at birth. Bears (family Ursidae) have one of the lowest neonatal-maternal mass ratios in placental mammals and are considered to have the most altricial newborns among placentals. In particular, giant pandas (Ailuropoda melanoleuca) appear exceptionally altricial. Here we use μCT scanning to visualize and compare the neonatal skeletal maturity of ursids relative to other caniform outgroups. Most bear neonates resemble neonates of caniform outgroups in level of ossification; however, perinatal giant pandas have skeletal maturity more similar to that of fetal dogs. No bear exhibits any skeletal heterochronies seen in marsupial newborns.