Fig. 7. Histograms showing residual values for gestation periods of placental mammals calculated using an exponent value of 0.15, subdivided into individual taxonomic groups. Although there is some variability within each taxon, it can be seen that there is a fairly clear separation between groups with altricial neonates (carnivores, lipotyphlan insectivores, lagomorphs, myomorph and sciuromorph rodents), which typically have negative residual values, and groups with precocial neonates (hystricomorph rodents, pinnipeds, artiodactyls, cetaceans, perissodactyls and primates), which typically have positive residual values.