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.