Secondary s.e.xual Variation

In specimens of the two s.e.xes from similar age groups of _hudsonius_ from Michigan, the mean values for each measurement for the two s.e.xes differ only slightly or are essentially the same (see table 4). In no species has secondary s.e.xual 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, s.e.x

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_

PAGE

_Zapus trinotatus_ 385 _Zapus trinotatus eureka_ A. B. Howell 389 _Zapus trinotatus monta.n.u.s_ Merriam 390 _Zapus trinotatus orarius_ Preble 391 _Zapus trinotatus trinotatus_ Rhoads 392

_Zapus princeps_ 394 _Zapus princeps cinereus_ Hall 399 _Zapus princeps curtatus_ Hall 400 _Zapus princeps idahoensis_ Davis 401 _Zapus princeps kootenayensis_ Anderson 404 _Zapus princeps luteus_ Miller 406 _Zapus princeps minor_ Preble 407 _Zapus princeps oregonus_ Preble 409 _Zapus princeps pacificus_ Merriam 412 _Zapus princeps princeps_ Allen 414 _Zapus princeps saltator_ Allen 416 _Zapus princeps utahensis_ Hall 418

_Zapus hudsonius_ 420 _Zapus hudsonius acadicus_ (Dawson) 432 _Zapus hudsonius alascensis_ Merriam 435 _Zapus hudsonius america.n.u.s_ (Barton) 436 _Zapus hudsonius campestris_ Preble 441 _Zapus hudsonius canadensis_ (Davies) 442 _Zapus hudsonius hudsonius_ (Zimmerman) 443 _Zapus hudsonius intermedius_ Krutzsch 447 _Zapus hudsonius ladas_ Bangs 449 _Zapus hudsonius pallidus_ c.o.c.krum and Baker 450 _Zapus hudsonius preblei_ Krutzsch 452 _Zapus hudsonius tenellus_ Merriam 453

Genus =Zapus= Coues

_Genotype._--_Dipus hudsonius_ Zimmerman.

EXTERNAL CHARACTERS.--Muriform in general appearance; forelimbs small, short; hind limbs greatly developed; hind feet long and narrow; tail tapering, attenuate, subcylindrical; head long and mouse-shaped; eyes small and situated midway between nose and ear; external ear somewhat longer than surrounding hair and provided with ant.i.tragal flap which can cover external auditory meatus, and in company with tragus completely close opening; upper lip without median groove; internal cheek-pouches well developed and opening at corners of mouth; mystacial vibrissae conspicuous; supercilliary vibrissae few; genal tuft absent; teats normally eight and arranged in pairs (one pectoral, two abdominal, and one inguinal); anterior and posterior pairs frequently undeveloped; general pelage coa.r.s.e; color of pelage varies somewhat in different species but always follows single basic pattern of broad dorsal band of some shade of brown or brownish-yellow darkened with brownish-black, sides of a lighter tone and slightly streaked with brownish-black, underparts snow-white, sometimes suffused with color of the sides and usually separated from color of sides by sharp line of clear brownish-yellow; backs of forefeet and hind feet grayish-white; tail distinctly bicolor, dark brown above and yellowish-white below; ears dark and narrowly edged with light color.

[Ill.u.s.tration: FIG. 44. Map showing distribution of the genus _Zapus_.]

CRANIAL CHARACTERS.--Skull short in relation to width, deep relative to other dimensions, somewhat convex; delicate, papery, without strong angularity; braincase relatively unexpanded; antorbital foramen obliquely oval and transmits ma.s.seter muscle of great size; foramen in inferior ramus of zygomatic process of maxillary for pa.s.sage of superior maxillary branch of trigeminal nerve small; zygomata not wide-spreading; underside of zygoma nearly horizontal, upper edge anteriorly rises prominently owing to extension of jugal upward along maxillary; jugal and lachrymal in contact; one ramus of zygomatic process of maxilla arises directly above other; rostrum thick basally and relatively attenuate distally; ends of nasals project noticeably beyond incisors; premaxillaries develop strong alveolar plate separating superior incisors for half their length; palatal bones shortened posteriorly, free edge often concave; incisive foramina long, broad, and separated by bulbose (except at posterior end) bony septum; mastoid bullae absent; auditory bullae short and transversely placed; pos...o...b..tal process never present; parietals nearly square, sometimes emarginate in front; angle of mandible flattened and bent inward; coronoid process weak, acute, and slopes strongly upward.

DENTAL CHARACTERS.--Dental formula

I 1, C O, P 1, M 3 -- -- -- -- -- -- -- -- = 18; i 1 c o p o m 3

upper incisors short, compressed, curved backward, and strongly grooved; lower incisors slender, curved backward, and ungrooved; both upper and lower incisors deep orange or yellow; four upper cheek-teeth present; premolar small, single rooted and, sometimes, non-functional; upper molars tri-rooted, sub-hypsodont, and with occlusal surface non-cuspidate (flat); enamel pattern, much complicated, consisting of one main re-entrant fold lingually and four re-entrant folds l.a.b.i.ally; three lower molars, bi-rooted, sub-hypsodont, flat crowned, with two outer and four inner re-entrant folds.

POSTCRANIAL CHARACTERS.--Neck short and weak; atlas large; axis separate from atlas; remaining (5) cervical vertebrae also free; thoracic (12) and lumbar (7) vertebrae strongly built; posterior lumbars with enlarged neural and anteriorly directed transverse processes; sacral vertebrae (7) as in murids; caudal vertebrae variable in number (average 36); clavicle long, slender, uniformly curved, convex outwardly; scapula with supraspinous and infraspinous fossae of equal size; forelimbs short, approximately half as long as hind limbs; hind limbs elongate, slender; femur with third trochanter; tibia and fibula fused slightly distal to middle of former; five elongate, separate metatarsals (first and fifth subequal, shorter than others).

ARTIFICIAL KEY TO THE SPECIES OF THE GENUS ZAPUS

A. Baculum with tip spade-shaped and tip wider than 0.43 mm; underfur with medullary pattern rectangular, cuticular scales small; coronoid process of mandible long and slender, angle of divergence from condyle broad; angle of mandible turned in and wide; pterygoid fossae wide; skull broad in relation to length; premolars with crescentine fold on occlusal surface.

_Zapus trinotatus_ p. 385

A'. Baculum with tip lanceolate (not spade-shaped) and tip less than 0.43 mm wide; underfur with medullary pattern square or rectangular; but, if rectangular, cuticular scales large; coronoid process short and broad, angle of divergence from condyle narrow; angle of mandible turned inward and small to medium; pterygoid fossae usually narrow; skull not broad in relation to length; premolars without crescentine fold on occlusal surface.

B(A'). Baculum less than 5.1 mm in total length; guard hair averaging 115 micra in diameter; underfur with rectangular medullary pattern, cuticular scales large; skull small; incisive foramina shorter than 4.6 mm; condylobasal length averaging less than 20 mm; length of maxillary tooth-row averaging less than 3.7 mm; palatal breadth at M3 less than 4.2 mm.

_Zapus hudsonius_ p. 420

B'. Baculum more than 5.1 mm in total length; guard hair averaging more than 140 micra in diameter; underfur with square medullary pattern, cuticular scales moderately large; skull large; incisive foramina longer than 4.7 mm; condylobasal length more than 21 mm; maxillary tooth-row averaging more than 3.8 mm; palatal breadth at M3 more than 4.4 mm.

_Zapus princeps_ p. 394

SYSTEMATIC ACCOUNTS OF SPECIES AND SUBSPECIES

=Zapus trinotatus= Rhoads

(Synonymy under subspecies)

_Range._--From southwestern British Columbia southward through western Washington and Oregon and in the humid coastal district of California almost to the Golden Gate (see fig. 45).

_Characters of the species_: _External._--Size medium to large (total length 221 mm to 238 mm); tail longer than head and body (131 mm to 149 mm) and bicolored, brown above, white to yellowish-white below; hind feet long (31 mm to 34 mm), grayish-white above; back various hues and tones of ochraceous and tawny; sides paler than back; lateral line separating sides from ventral surface usually distinct and bright; ventral coloration white, usually with suffusion of ochraceous; ears usually dark, sometimes flecked, and usually narrowly edged with color of sides; guard hairs average 141 microns (133u to 155u) in diameter; underhair with medullary pigment in narrow, hollow rectangles; cuticular scales of underhair smaller and more numerous than in other species.

_Baculum._--Size large (total length 6.7 mm to 7.4 mm); base broad (0.7 mm to 0.9 mm); tip broad (0.44 mm to 0.57 mm); spade-shaped in dorsal aspect and tilted upward, gradually tapering to thin-edged tip; shaft rounded, straight.

_Skull._--Large, broad and deep in relation to length; pterygoid fossa broad; anterior ramus of zygomatic process of maxillary relatively narrow; nasofrontal juncture relatively broad; coronoid process of mandible elongate. Upper premolars relatively large (averaging .70 mm in length and .75 mm in width), usually functional, occlusal surface with l.a.b.i.al re-entrant fold forming crescentine loop incompletely enclosing single central cusp; m3 relatively large, elongated; m1 elongated, broadly rounded anteriorly.

GEOGRAPHIC VARIATION

There are four subspecies currently recognized, all of which are confined to the Pacific coastal region of North America (See fig. 45).

The features that vary geographically are external size, color of pelage (shade and tone of upper parts and tint of lower parts), and dimensions of certain cranial structures (zygomata, braincase, incisive foramina, palatal bridge, auditory bullae, and pterygoid fossae).

External size is smallest in the southernmost geographic race (_Z. t.

orarius_) and largest in the northernmost geographic race (_Z. t.

trinotatus_). This decrease in size from north to south is clinal and is in keeping with Bergman's Rule which postulates that within one species the smallest individuals occur in the warmer parts of its geographic range.

[Ill.u.s.tration: FIG. 45. Map showing distribution of _Zapus trinotatus_.

1. _Z. t. eureka_ 3. _Z. t. orarius_ 2. _Z. t. monta.n.u.s_ 4. _Z. t. trinotatus_]

Coloration of pelage is geographically variable. There is a gradual change in the color of the pelage from north to south. Animals obtained in the northern part of the geographic range of _Z. trinotatus_ are generally darker dorsally (more tawny) with the ventral pelage usually pure white. Those individuals from the southern part of the geographic range of _Z. trinotatus_ have the dorsal pelage lighter (more reddish and yellow-brown) and ventrally the pelage is usually heavily suffused with reddish-brown. The crania also vary geographically; they are largest in the northernmost part of the range of the species and smallest in the southernmost part.

NATURAL HISTORY

_Habitat._--On the Olympic Peninsula, Washington, in 1931 Svihla and Svihla (1933:132) found this species equally abundant in alpine meadows near timberline, in open gra.s.sy areas, and in tall meadow gra.s.s and low blueberry bushes. All of the mice were in wet marshy places. Bailey (1936:232) reported that in Oregon, these mice live in meadows, marshes, under ferns and weeds in the woods, or near mountain brooks and streams.

Taylor (1922:221) found _Zapus_ in moderately moist meadows in the Hudsonian Life-zone at Mt. Rainier, Washington, and Dice (1932:49) found them in deciduous forest and in open, gra.s.sy, or sphagnum bogs. Dice records it as common also among the alders and willows in high, open, gra.s.sy parks. Merriam (1897b:223) found _Z. trinotatus_ abundantly in moist places grown-over with gra.s.s or weeds. Gra.s.s cuttings two to three inches long were left in small heaps at feeding sites and indicate the presence of these mice.