Growth of the Skull in the Cottontail (Sylvilagus floridanus) and Its Application to Age-Determination

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Abstract
Skulls of 125 cottontails, Sylvilagus floridanus, of ages determined from lens-weight, were available from Iroquois and Champaign counties, east-central Illinois. An attempt was made to find features of the skull that might be useful in placing these rabbits into age-groups. Later, the lens-weight was correlated with these groups. In very young animals, less than 21 days old, the teeth are not fully erupted; group 1 animals, 21-92 days lens-age, have no fusion anywhere along the exoccipital-supraoccipital suture; group 2 animals, 93-105 days lens-age, have the suture clearly visible, but there is some fusion along it, especially near one end; in group 3 animals, 106-170 days lens-age, the suture is fused along its entire length but not obliterated, although it may be seen in some cases only with a lens or microscope; group 4 animals, more than 170 days lens-age, have the suture no longer discernible. Growth of the skull and some of its cranial elements from age 5 days (approxi-mate) to and beyond 260 days are graphed. Using the age-groups and the graphs, the age of the skull can be ascertained with fair accuracy. Ninety % of adult size is reached by 125 days of age, 96.5% by 170 days. The most rapid growth occurs during the first 60 days. Al- though there is no actual decrease in size in any part of the skull measured, there is a relative decrease in zygomatic breadth, breadth of braincase, postorbital breadth, and parietal breadth.

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