Secondary Sexual Variation
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.
Therefore, as might be expected, the secondary sexual variation in weight of the skull is less in M. rixosa, individuals of which are of small size, than in M. erminea or than in M. frenata, in general of larger size. Nevertheless, in M. africana, in which the individuals are of large size, there appears to be less sexual dimorphism in weight of the skull than in M. frenata or than in M. erminea, although it should be remarked that there are too few data for M. africana to allow of forming a trustworthy conclusion concerning the amount of secondary sexual variation in that species.
The secondary sexual variation in shape of the skull consists of a slenderness in the female. In relation to the basilar length the spread of the zygomatic arches is more in males and, except in the one subspecies M. f. altifrontalis, the rostrum is broader. Also the interorbital region is relatively broader in males of most subspecies. In most subspecies of both M. frenata and M. erminea the tympanic bullae are relatively (to the basilar length) longer in females. The maximum sexual dimorphism occurs in M. erminea arctica and the minimum dimorphism in M. e. haidarum, M. e. anguinae and M. e. muricus. Taking into account all of the subspecies of each of the North American species, the shape of the skull differs most in M. erminea and least in M. frenata. In the latter species the greatest difference in shape of the skull, as was true also of its weight, is in the subspecies M. f. noveboracensis. In these two subspecies, M. f. noveboracensis and M. e. arctica, in addition to the secondary sexual variation already mentioned in the skull, females have the braincase smoother and more rounded, the postorbital-, mastoid-, and lacrimal-processes relatively smaller, and the ventral face of the tympanic bulla at its anterior margin more nearly flush with the floor of the braincase.
In the weasels, subgenus Mustela, the disparity in size of the two sexes is almost or quite as much as in any other fissiped carnivore. It is because of this large degree of difference that the skulls of the two sexes are described separately in the following systematic accounts. The need for such treatment was recognized by Reinhold Hensel (1881:127) more than sixty years ago when he wrote in the introduction to his "Craniologische Studien," of Mustela, as follows: ". . . die Geschlechtsdifferenzen am Schädel vieler Säugethiere . . . so gross sind, dass man diese wie Schädel verschiedener species behandeln muss, während in anderen Ordnungen (Rosores, Edentaten) die Schädel solche Unterschiede nichtzeigen." In the past, failure to appreciate the large amount of secondary sexual variation has resulted in erroneous deductions as regards characters of certain geographic races and has been the cause of some nomenclatural confusion, as for example, in Mustela frenata macrura, where the female was named as a separate species (Mustela jelskii).