Comparative post-natal growth in five species of the genus <i>Sigmodon</i>. II. Cranial character relationships

Abstract

At birth, the catton rat skull is oval and the braincase continues to expand during the first 20 days to near the adult size. Subsequently with the development of the dorsal area and the extension of the bones at the base of the braincase, the rounded form is transformed into the typical long-snouted appearance of the adult. Skull measurements demonstrate the initial rapid expansion of the braincase and the subsequent extension of the rostrum. Adult skull size is attained within the first 100 days of life.

Normally, during the first days after birth, height of the braincase is greater than the length of the post-palatal area. However, at 20 to 30 days growth in the latter is greatest and its dimensions surpass those of the former. In three species, Sigmodoll hispidru, Sigmodon alleni and Sigmodon ochrognathus, the rostral length exceeds the height of the braincase only after the animals are over 100 days old.

At 40 days palatal and maxillary tooth-row lenglbs are ncar the maximum adult size, while tbe diastema is 50 to 67 percent (depending on the species). At birth the zygomatic arches are small but rapidly expand laterally in comparison with the slower-growing braincase. This process, together with the lengthening of the rostral region is related to weaning and the development of the masticatory and facial muscles. The change of growth from an accelerated phase (1 to 40 days) to a much slower rate seems correlated with attainment oí sexual maturity.