[VARIATION]
Variation with Age
The kind of variation which results from increasing age has been dealt with extensively for the skull (of the Old World Mustela erminea) by Hensel (1881) and for the external features and to some extent for the skull by Hamilton (1933) in the North American forms M. erminea cicognanii and M. frenata noveboracensis.
The young of both erminea and frenata are hairless and blind at birth. In M. frenata noveboracensis, the eyes open on approximately the 37th day. When 2 to 4 months old, the tail is pointed at the tip. This is because the terminal hair of the tail, including the black tip, is short and lies flat on the tail. In subadults and adults the hair on the terminal part of the tail is as long as that on the basal part, and the tail appears to be of uniform diameter all the way out to the end.
In the western subspecies of M. frenata, and in its tropical subspecies, animals so young as to have pointed tails commonly have the underparts of the body more intensely colored than do adults. The young may have salmon-colored instead of yellowish fur on the underparts.
Otherwise, in animals that have attained approximately adult proportions—which appears to be at approximately 6 months of age in males—there are no variations which are ascribable to increasing age in the color-pattern or pelage that cause the systematist to confuse species or subspecies.
Of the several parts of the skull in juvenal animals, the braincase and width of the posterior part of the palate are most nearly of the size attained in the adult, the facial part of the skull at birth is the least developed, and the interorbital region is, in relation to its ultimate adult size, intermediate in stage of development. The permanent teeth are acquired when the animal is approximately eleven weeks old.
Four age groups, based on characters of the dentition and skull, have been recognized. They are: