| Species | Z. h. hudsonius | Z. p. princeps | Z. t. trinotatus |
| No. examined | 52 | 46 | 19 |
| Mastoid width | 2.85 | 1.98 | 2.21 |
| Occipitonasal length | 2.64 | 1.37 | 1.20 |
| Incisors to postpalatal notch | 3.02 | 2.56 | 2.56 |
| Interorbital constriction | 2.75 | 3.66 | 3.22 |
| Zygomatic breadth | 2.74 | 2.54 | 1.94 |
| Maxillary tooth-row | 4.50 | 4.44 | 3.82 |
The size and shape of certain cranial elements vary individually even between right and left sides of the same animal. The paired parietal bones in some animals are nearly square and identical. In other animals these bones are approximately equal and straight on three sides with the fourth side forming an anterolateral projection; this projection may be slightly or greatly produced, and opposite elements in a single individual differ in this respect.
The interparietal also is variable; the lateral arms may be blunted and not included in the fusion of the squamosal, parietal, and occipital elements, or the interparietals may be elongated and fused with these elements. Posterior and anterior borders of the interparietal may be straight, produced anteriorly, produced posteriorly, or produced anteriorly and posteriorly.
There is frequently variation in the degree of taper of the nasals. They may be parallel sided, narrowed distally, or narrowed proximally. There is some variation in the degree of inflation, in the size, and in the shape of the frontal bones. The anterior surface of the postpalatal notch varies individually and may be truncate, anteriorly convex, or anteriorly concave.
Individual variation in the color of the pelage of animals that are in the same stage of molt or non-molt is by my observation slight. The presence of oil in the hair results in a false impression of sleekness and seemingly darker pigmentation. Abnormal white-spotting dorsally occurs as does yellow and melanistic coat color. These mutations are considered in the discussion concerning pelage.
Secondary Sexual Variation
In specimens of the two sexes from similar age groups of hudsonius from Michigan, the mean values for each measurement for the two sexes differ only slightly or are essentially the same (see [table 4]). In no species has secondary sexual variation been found to be greater than individual variation.
Table 4.—Mean Measurements for Adult Male and Female Z. hudsonius of Age Group 2 and Per Cent Difference of Females to Males (Specimens from Michigan).
| Sex | Male | Female | Per cent difference, females to males | |
| No. examined | 18 | 15 | ||
| Total length | 202.85 | 202.88 | 0.02% | larger |
| Hind foot | 122.85 | 122.10 | 0.60% | smaller |
| Mastoid width | 10.10 | 10.28 | 1.50% | larger |
| Occipitonasal length | 22.15 | 22.03 | 0.55% | smaller |
| Incisors to postpalatal notch | 9.39 | 9.33 | 0.64% | smaller |
| Zygomatic breadth | 10.47 | 10.57 | 0.95% | larger |
| Maxillary tooth-row length | 3.52 | 3.60 | 0.23% | larger |