Fig. 4. Scaling of gestation periods for the same sample of 429 placental mammal species, following subdivision into species with altricial neonates and those with precocial neonates. Best-fit lines fitted separately to the two grades have approximately similar slopes that are both markedly lower than the overall slope for the single distribution shown in Fig. 3 and are generally close to 0.15. (For altricial mammals: least-squares regression, 0.158; major axis, 0.160; reduced major axis, 0.198; rotation line, 0.176. For precocial mammals: least-squares regression, 0.135; major axis, 0.136; reduced major axis, 0.168; rotation line, 0.133.) (After Martin, 1989.) Gestation (days); body mass (g).