North American Jumping Mice (Genus Zapus) Part 4

FIGS. 38 and 39. _Zapus trinotatus._

FIGS. 40 and 41. _Zapus princeps._

FIGS. 42 and 43. _Zapus hudsonius._]

BACULUM.--The general shape and dimensions of the baculum (os p.e.n.i.s) provide characters of taxonomic value for the species of _Zapus_ (see figs. 23-25 and figs. 28-30).

Three measurements--length, transverse diameter at the base, and transverse diameter at the tip--are easily obtained and are diagnostic.

The bacula of all species are somewhat curved. The measurement of length used by me does not represent the actual length of the bone, but instead the chords of the arcs involved.

SKULL.--Some of the structures useful for separating taxonomic ent.i.ties may have little or no biological significance to the animals in nature.

Characters mentioned by me are chosen simply for their significance taxonomically. The zygomata vary in degree of lateral bowing, being widely bowed in _Z. princeps_ and _Z. trinotatus_, and less so in _Z.

hudsonius_. Differences in zygomatic breadth owing to the degree of bowing are an aid in differentiating subspecies. The length of the skull from the occipital condyles to the tip of the longest nasal bone is useful in separating _Z. hudsonius_ from _Z. trinotatus_ and _Z.

princeps_. The narrowness of the base of the zygomatic process of the squamosal is useful in distinguishing between _Z. hudsonius_ and _Z.

princeps_, but shows no variation of subspecific worth. The shape and dimensions of the incisive foramina provide specific and subspecific characters. The position of the anterior margin of the postpalatal notch, in relation to the last molars, provides subspecific characters in _Z. princeps_. In the species _Z. princeps_ the median projection on the inferior ramus of the zygomatic process of the maxillary is absent in some subspecies, small in others, and large in some. Shape and inflation of the auditory bullae, shape of the pterygoid fossae, and shape of the nasals are useful in determining specific and subspecific relationships.

TEETH.--The alveolar length of the upper maxillary tooth-rows aids in distinguishing _Z. hudsonius_ from _Z. princeps_ and _Z. trinotatus_.

Nearly parallel versus anteriorly divergent upper tooth-rows is a subspecific difference in _Z. princeps_. Variations in the dimensions of P4 and M1 aid in estimating the relationships of species. The occlusal pattern shows little variation and was of no use in separating species.

NONGEOGRAPHIC VARIATION

A knowledge of variation resulting from age, individual, or secondary s.e.xual differences, as opposed to geographic variation between two or more populations of a single species is important in determining the reliability of taxonomic characters.

The largest population-sample of _Zapus_ available to me for the study of nongeographic variation was 63 individuals from various localities in Keweenaw and Menominee counties, Michigan. Thirty-nine were females and 24 were males. It is on these specimens that this discussion is based.

Age Variation

TEETH.--The teeth provide a valuable standard for age determination in that they wear at a measurable rate. The molars erupt in sequence from front to back, and wear shows first on M1 and last on M3. The peglike permanent P4, of which I have not seen the deciduous precursor, receives wear at the same time that the molars are being worn. Wear proceeds at approximately the same rate in the teeth of both the upper jaws and lower jaws.

In order to be more nearly certain that specimens used in making racial comparisons were comparable as to age, six age-groups were established, from youngest to oldest. These groups were based on the degree of wear on the occlusal surface of the upper cheek-teeth, and are as follows: group 1, in which M1 and M2 have not reached full and equal height and show no occlusal wear, and M3 has not erupted or is just breaking through the alveolus; group 2, in which M1 and M2 have reached full and equal height and show slight wear, and M3 may be almost or quite equal in height to M1 and M2 and, when equal, sometimes shows slight wear; group 3, in which M1 and M2 show wear on all cusps but cusps are visible, and M3 shows slight wear; group 4, in which P4 shows slight wear, M1 has cusps and re-entrant folds between cusps mostly gone, M2 shows considerable wear but re-entrant folds are visible, and M3 has most re-entrant folds and cusps gone; group 5, in which P4 shows considerable wear, M1 has cusps completely worn away, M2 has re-entrant folds and cusps worn away, and M3 lacks occlusal pattern except for one or two lakes; group 6, in which all upper cheek-teeth are without occlusal pattern.

These groupings are based on continuously variable features, and, therefore, when the teeth are at certain stages of wear a specimen is difficult to place in one of two groups.

Age group 1 and 2 include juvenal and subadult animals. Animals of age groups 3 through 6 are considered adult. Individuals of age groups 3 through 5, including as they do the great majority of the adult population, were the only age cla.s.ses used in measuring geographic variation.

Quimby's (1951:69) data indicate that some mice produce litters at the age of approximately 2 months, when four-fifths grown. Therefore, s.e.xual maturity is not always synonymous with morphological maturity.

MEASUREMENTS OF EXTERNAL PARTS.--Data presented here on _Z. hudsonius_ are those recorded by Quimby (1951) on specimens from Anoka County, Minnesota, and those obtained by me from museum specimens from Menominee and Keweenaw counties, Michigan.

According to Quimby (1951:65-66) the mean length [= body length] for three newly born _Z. hudsonius_ is 24.8 mm (24.0-25.5); at the end of the fourth week of growth the mean length averaged 64.4 mm and at the 13th week 77.6 mm. Rapid growth occurs during the first four weeks, with the mean length increasing approximately 2.6 times the size at birth.

After the fourth week of development, growth proceeds at a slower rate; the mean length at 13 weeks is only 3.1 times greater than the mean length at birth.

In specimens a.s.signed to age groups 1 and 2 the length of the body averaged 70 and 74.8 mm, respectively. The individuals of both groups are less than 13 weeks old if we a.s.sume that growth proceeds at the same rate in Michigan as it does in Minnesota.

In the specimens from Michigan of age groups 3, 4, 5, and 6 the average length of the body is 80.9, 83.7, 89.0, and 83.6, respectively.

According to Quimby (_loc. cit._), the average length of the tail for three _Z. hudsonius_ at birth was 9.2 mm. (8.5-10.0). During the first four weeks of development the tail grew rapidly and reached an average length of 92.0 mm, which was 10 times the length at birth. By the end of 13 weeks of development the average length of the tail for these three individuals was 119.6 mm or 12 times the average length at birth. The most rapid growth was early in development: 80 per cent of the growth of the tail occurred during the first month, after which growth proceeded at a much slower rate.

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. a.s.suming 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, a.s.signed 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 s.e.x 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 dent.i.tion 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.