Part 8

Quimby (loc. cit.) records an average dimension of 4.7 mm (4.5-5.0) for the length of the hind foot in three newly born Z. hudsonius. The hind foot grew rapidly in length and by the fourth week had increased 5.6 times in its length and averaged 26.3 mm. Growth was much less rapid from the fourth to the thirteenth week when the hind foot averaged 27.7 mm, only five per cent more than in mice four weeks old. Assuming the average length of the hind foot of the adults to be 29.0 mm, the hind foot in individuals 13 weeks old is 96 per cent of the adult size.

According to Quimby (loc. cit.), the pinna of the ear at birth is small and folded over the external auditory meatus. The length of the ear increases proportionately more (29 per cent) than any other external dimension after the first four weeks of growth.

If the average length of the ear (measured from the crown) of adults is 14.7 mm, the animals from Michigan in age groups 1 and 2 are 91.8 per cent and 96.5 per cent as large as adults.

TABLE 1.--Average Dimensions (in Millimeters) for Specimens of Z. h. hudsonius of Various Ages (Specimens from Michigan).

==============+=======+=======+=======+=======+=======+======= Age groups | 1 | 2 | 3 | 4 | 5 | 6 --------------+-------+-------+-------+-------+-------+------- No. examined | 4 | 13 | 33 | 12 | 3 | 3 --------------+-------+-------+-------+-------+-------+------- Body | 70.0 | 74.8 | 80.9 | 83.7 | 89.0 | 83.6 --------------+-------+-------+-------+-------+-------+------- Tail | 113.8 | 118.5 | 122.9 | 125.0 | 125.0 | 118.3 --------------+-------+-------+-------+-------+-------+------- Hind foot | 28.8 | 28.6 | 28.9 | 29.1 | 28.9 | 29.3 --------------+-------+-------+-------+-------+-------+------- Ear | 13.5 | 14.2 | 14.7 | 14.8 | 15.0 | 14.3 --------------+-------+-------+-------+-------+-------+-------

From these data, concerning growth of external parts, it seems that: growth is most rapid during the four weeks following parturition; specimens from Michigan, assigned to age groups 1 and 2 on the basis of tooth wear, are less fully developed and probably younger than mice from Minnesota, with a known age of 13 weeks; individuals with sufficient wear on the teeth to be placed in age group 3, if they were obtained in the late fall, may be young from the first litters of the year or, if they were obtained in early spring, may be at least one year old; individuals in age groups 4, 5, and 6 are at least one year old.

SKULL.--The post-embryonic development of the skull is rapid. Animals in age groups 1 and 2 have skulls which average more than 80 per cent of the size that is here considered adult (an average size obtained from age groups 3, 4, and 5). The actual increase in size of certain cranial elements for various age groups is given in table 2.

In age group 1 the rostrum is relatively short as it is in Neotoma micropus (J. A. Allen, 1894:235) and juveniles of Peromyscus truei (Hoffmeister, 1951:7). The rostrum lengthens rapidly and there is a general increase in actual and relative size of the entire preorbital region; the increase after age group 3 is slower and of lesser magnitude. Changes with age in the size of the braincase are slight. In age group 1 the average depth of the braincase is 99.6 per cent of the adult size; the average breadth of the braincase is 98 per cent of the adult size, and the average width across the mastoid region is 96.4 per cent of the adult size. These dimensions indicate that the braincase reaches full size early. The zygomatic arch, however, undergoes change with age; there is a gradual increase in breadth owing to lateral bowing and a gradual lengthening which is in keeping with a general elongation of the skull anterior to the braincase.

The incisive foramina in age group 1 are short (4.0 mm), broad (2.2 mm in the middle), and taper to a point at each end. In age group 2 the foramina have elongated (4.2 mm) and are less pointed posteriorly, but there is no change in breadth. In age groups 3, 4, 5, and 6 the foramina become progressively longer (4.5 mm in age group 6), have a relatively constant breadth (2.2 mm), and become more nearly truncate anteriorly.

TABLE 2.--Average and Extreme Measurements (in Millimeters) of Skulls of Six Age-groups in Specimens of Zapus hudsonius from Michigan.

===============+============+============+============ Age groups | 1 | 2 | 3 ---------------+------------+------------+------------ Number | | | examined | 4 | 13 | 33 | | | Occipitonasal | 20.5 | 21.2 | 22.0 length | 20.0 21.2 | 20.8 21.8 | 21.5 23.2 | | | Mastoid | 9.8 | 10.04 | 10.12 breadth | 9.7 10.0 | 9.6 10.4 | 9.5 10.5 | | | Length of | 8.07 | 9.02 | 9.07 zygomatic arch | 8.0 8.2 | 8.5 9.3 | 8.5 9.4 | | | Breadth of | 3.36 | 3.33 | 3.37 palate at P4 | 3.3 3.5 | 3.1 3.4 | 3.1 3.8 | | | Breadth of | 2.4 | 2.55 | 2.66 palate at M3 | 2.3 2.6 | 2.3 2.7 | 2.3 3.2 | | | Palatal | 8.67 | 8.98 | 9.38 length | 8.4 9.1 | 8.8 9.2 | 9.3 9.8 | | | Distance from | | | incisors to | 8.53 | 8.98 | 9.08 postpalatal | 8.4 8.7 | 8.5 9.5 | 9.0 9.8 notch | | | | | | Interorbital | 4.25 | 4.19 | 4.2 breadth | 4.2 4.3 | 4.0 4.4 | 4.0 4.4 | | | Average length | | | of upper | 3.2 | 3.2 | 3.21 molar series | 3.2 3.4 | 3.2 3.4 | 2.9 3.5 | | | Breadth of | 9.5 | 9.58 | 9.61 braincase | 9.3 9.7 | 9.2 9.7 | 9.1 10.0 | | | Zygomatic | 10.33 | 10.49 | 10.55 breadth | 10.0 10.7 | 10.4 10.9 | 10.1 11.2 | | | Condylobasal | 16.9 | 18.33 | 18.80 length | 16.6 17.1 | 17.4 19.2 | 18.2 19.5 ---------------+------------+------------+------------

===============+============+============+============ Age groups | 4 | 5 | 6 ---------------+------------+------------+------------ Number | | | examined | 14 | 3 | 3 | | | Occipitonasal | 22.7 | 22.9 | 23.0 length | 21.8 23.4 | 22.7 23.3 | 22.4 23.7 | | | Mastoid | 10.12 | 10.3 | 10.36 breadth | 9.6 10.7 | 10.0 10.8 | 10.1 10.8 | | | Length of | 9.25 | 9.5 | 9.35 zygomatic arch | 9.2 9.4 | 9.5 9.5 | 9.1 9.6 | | | Breadth of | 3.44 | 3.66 | 3.45 palate at P4 | 3.1 3.7 | 3.6 3.7 | 3.4 3.5 | | | Breadth of | 2.74 | 3.11 | 2.77 palate at M3 | 2.5 3.0 | 3.0 3.2 | 2.6 2.9 | | | Palatal | 9.59 | 9.73 | 9.8 length | 9.0 10.0 | 9.5 9.9 | 9.6 10.1 | | | Distance from | | | incisors to | 9.68 | 9.73 | 9.80 postpalatal | 9.2 10.0 | 9.5 9.9 | 9.6 10.1 notch | | | | | | Interorbital | 4.2 | 4.23 | 4.2 breadth | 4.0 4.4 | 4.1 4.4 | 4.2 4.2 | | | Average length | | | of upper | 3.22 | 3.2 | 3.16 molar series | 2.9 3.5 | 3.2 3.2 | 3.1 3.2 | | | Breadth of | 9.68 | 9.83 | 9.63 braincase | 9.3 10.0 | 9.5 10.2 | 9.3 9.9 | | | Zygomatic | 10.80 | 11.0 | 11.25 breadth | 10.7 11.2 | 10.5 11.5 | 11.2 11.3 | | | Condylobasal | 19.33 | 19.6 | 19.9 length | 18.5 19.9 | 19.4 19.8 | 19.5 20.3 ---------------+------------+------------+------------

Individual Variation

Measurements of external parts in Zapus are more variable than are measurements of most parts of the skull. As Hoffmeister (1951:16) points out for Peromyscus truei, this variation in external features results in part from "the difficulties in accurately measuring soft parts of the anatomy" and also from inconsistencies on the part of collectors in making these measurements.

A comparison of coefficients of variation (see table 3) for cranial measurements between populations of like age and sex for the species Z. hudsonius, Z. princeps, and Z. trinotatus shows that variation of approximately the same degree is recorded in corresponding elements in all species; that is to say, structures which are most variable individually in Z. princeps are also most variable in Z. trinotatus and Z. hudsonius.

Individual variation in the occlusal pattern of the molariform teeth is slight. In several specimens, however, the re-entrant fold is absent from the lingual surface of M1. Teeth in addition to the normal number were recorded for five specimens. In all instances they are in the upper dentition and usually at the posterior end of the maxillary tooth-row. In each of four specimens (KU No. 34852, KU No. 32852, MVZ No. 52105, all Z. princeps, and USBS No. 22921, Z. hudsonius), there is only a single additional tooth. One individual (USBS No. 264388, Z. princeps) possessed two extra molars, one in each maxillary tooth-row. The extra teeth vary in size from those which are only slightly smaller than the adjacent normal molars to those which are simple, peglike structures. In four of the five animals the extra teeth are posterior to the normal M3; in the fifth (MVZ No. 52105) the added tooth is anteriormedial to M3.

TABLE 3.--Coefficients of Variation for Dimensions of Corresponding Parts of the Skull of Three Species of Zapus. The Specimens of Zapus hudsonius are from Menominee and Keweenaw counties, Michigan, the Zapus princeps are from the Vicinity of Encampment, Wyoming, and the Zapus trinotatus from Huntingdon, British Columbia.

==============================+=============+============+============== | Z. h. | Z. p. | Z. t. Species | hudsonius | princeps | 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).

==============================+=========+=========+===================== | | | Per cent difference, Sex | Male | Female | 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 ------------------------------+---------+---------+---------------------

CHECK-LIST OF THE SPECIES AND SUBSPECIES OF THE GENUS ZAPUS

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