In the autumn and early winter, most of the specimens are subadults. Ordinarily the few adults obtained in these seasons can easily be segregated from the subadults because ontogenetic development in the twelve additional months of life of each of the older animals has obliterated the sutures on the rostrum, heightened (vertically) and lengthened (anteriorly) the sagittal crest, widened the rostrum, and produced still other changes in form that are revealed by direct comparison of specimens of the two ages.
The secondary sexual variation, which has been detected, is in size of the animal, relative length of the tail and shape of the skull. The female is the smaller. In the small _Mustela rixosa_ and apparently in _Mustela africana_ the secondary sexual difference in size is relatively slight. In _Mustela frenata_ and _Mustela erminea_, males are approximately twice as heavy as females, the degree of difference very definitely depending upon the subspecies. For example, in _M. e. richardsonii_ the recorded weights are 175 and 69 grams as opposed to 81 and 54 grams in _M. e. cicognanii_. In general, within one species the greatest difference in size of males and females is in those subspecies in which the animals are of large size. The secondary sexual variation in size is much more than the individual variation in either sex. The same is not true of secondary sexual difference in length of the tail (relative to the length of the head and body), which in eighteen subspecies of _M. erminea_ is from 1 to 7 per cent longer in males than in females. In two subspecies, _M. e. haidarum_ and _M. e. olympica_, the tail is a fraction of a per cent the longer in females if we may rely upon the few specimens for which collectors' measurements are available.
In both _M. erminea_ and _M. frenata_ the skull of the female is approximately 45 per cent lighter than that of the male, or put in the opposite way, the skull of the male is 83 per cent heavier than the skull of the female. The difference in this respect varies greatly depending on the subspecies. For example, the skull of the male is 127 per cent heavier than that of the female in _M. e. richardsonii_ but only 33 per cent heavier in _M. e. anguinae_. In _Mustela frenata_, the subspecies _noveboracensis_ shows most sexual dimorphism in weight of skull (3.6 and 1.7 grams) and _olivacea_ the least (5.3 and 3.8 grams). In general, the difference in this respect is less in subspecies the individuals of which are of small size.