_Specimens examined._--Total, 396: North end Mesa Verde National
Park, 7000 ft., 76004-76100; Prater Canyon, 7600 ft., 76101-76144,
MV 7839/507, 7840/507; Upper Well, Prater Canyon, 7575 ft.,
69328-69329; Morfield Canyon, 7600 ft., 76145-76184; Park Point,
8525 ft., 69330-69342, 69344-69360; 1-1/2 mi. E Waters Cabin, 6400
ft. (labels on some specimens read "West Bank Mancos River,
Northeast side Mesa Verde National Park"), 69361-69376,
76185-76204; Sect. 27, head of east fork Navajo Canyon, 7900 ft.,
69377-69380, 69422-69426; 3 mi. N Rock Springs, 8200 ft.,
69403-69410; 2 mi. NNW Rock Springs, 7900 ft., 69411-69412; 1 mi.
NNW Rock Springs, 7600 ft., 69413-69418; 1/2 mi. NNW Rock Springs,
7500 ft., 69419-69421; Far View Ruins, 7700 ft., 69386-69402; Far
View Point, 76530-76531, 79221 and 90 uncatalogued specimens in
preservative; Mancos River, 6200 ft., 69382-69385; back of Park
Museum, 6930 ft., MV 7857/507; Mesa Verde, 25 mi. [by road] SW
Mancos, 149093 USNM; Cornfield, MV 7878/507.
The most abundant mammal is the ubiquitous deer mouse. Series of
specimens taken in August (by Anderson in 1956), in September (by
Shepherd in 1958 and 1959), and in November (by Alcorn in 1957) make
possible the following comparisons of age, reproductive conditions, and
molts.
The specimens obtained in August and November were placed in five
categories according to age (as judged by wear on the teeth). These
categories correspond in general to those used by Hoffmeister (1951:1)
in studies of _Peromyscus truei_. From his descriptions I judge that
wear in _Peromyscus maniculatus_ differs from wear in _Peromyscus truei_
in that the last upper molar is not worn smooth before appreciable wear
appears on the first two molars, and the lingual and labial cusps wear
more nearly concurrently. The five categories differ as follows:
category 1, last upper molar in process of erupting, showing no wear;
category 2, some wear apparent on all teeth, but most cusps little worn;
category 3, greater wear on all teeth, lingual cusps becoming rounded or
flattened; category 4, lingual cusps worn smooth, labial cusps show
considerable wear; category 5, all cusps worn smooth. The condition of
the pelage was noted for each prepared skin. Hoffmeister (_op. cit._: 4)
summarized changes in pelage that he observed in _Peromyscus truei_, and
he summarized earlier work by Collins with _Peromyscus maniculatus_. In
_P. maniculatus_ a grayish juvenal pelage is replaced by a postjuvenal
pelage in which the hairs are longer and have longer, pale, terminal or
subterminal bands giving a paler and more buffy or ochraceous hue to the
dorsal pelage. The postjuvenal pelage is replaced by an adult pelage
that is either brighter or, in some cases, is not distinguishable with
certainty from the postjuvenal pelage. Not only is the juvenal pelage
distinguishable from the postjuvenal pelage, but the sequence of
ingrowth of postjuvenal pelage follows a regular pattern that is usually
different from that of subsequent molts. The loss of juvenal hair is
less readily observed than the ingrowth of new postjuvenal hair on
account of the greater time required for the growth of any individual
hair than for the sudden loss of a hair.
Molt was observed in some individuals no longer having juvenal pelage;
some new pelage was observed on the skins of seven mice collected in
August. Each of these was in category 4 or 5 and probably had been born
in the previous calendar year. These seven molting individuals make up
nearly 17 per cent of 42 individuals that had completed the juvenal to
postjuvenal molt. In November, 80 per cent of individuals (92 of 115)
that had previously obtained their postjuvenal or adult pelage were
molting. These mice were in age-categories 3, 4, and 5. Some of the
individuals in category 3 were developing new hair beneath a relatively
unworn bright pelage that I judge to be an adult pelage rather than a
postjuvenal pelage. If this judgment be correct and if the relatively
unworn dentition (category 3) means that these animals are young of the
year, we must conclude that individuals born in early summer may molt
from juvenal to postjuvenal, then to adult pelage, and finally in the
autumn into another adult pelage. Other individuals, six in number and
of categories 2 and 3, are simultaneously completing the juvenal to
postjuvenal molt and beginning the postjuvenal to adult molt. The
juvenal to postjuvenal molt begins, as has been described by various
authors, along the lateral line and proceeds dorsally and ventrally and
anteriorly and posteriorly, and the last patch to lose the gray juvenal
color is the top of head and nape, or less frequently the rump. In some
individuals a gray patch on the nape remained but emerging hair was not
apparent; perhaps the molt had been halted just prior to completion. The
progressing band of emerging hair is narrow in most specimens but in
some up to one-fifth of the circumference of the body has hair at the
same degree of emergence. Subsequent molts, both from postjuvenal to
adult pelage and between adult pelages, are less regular in point, or
points, of origin, width of progressing molt, and amount of surface
molting at one time. Half or more of the dorsum is oftentimes involved
in the same stage of molt at once. In some specimens the molt begins
along the lateral line, and in others in several centers on the sides.
In some skins distinct lines of molt are visible without parting the
hair, and in some others the molt is patchy in appearance. Growth of new
hair is apparent at various times of the year as a result of injury such
as that caused by bot fly larvae, cuts, scratches, or bites of other
mice. Abrasion, wear, irritation by ectoparasites, and other kinds of
injury to the skin may play a part in the development of a patchy molt.
Both breeding and molting are sources of considerable stress, and the
delay of the peak of molting activity until November when breeding
activity has decreased seems of benefit to the mice. A change in the
ratio of young mice (categories 1, 2, and 3) to old mice (categories 4
and 5) between August and November was noted. In August, 29 per cent of
the population is composed of old mice, and in November only 6 per cent.
This change results from birth of young as well as death of old mice,
but may indicate that a mouse in November has less than one chance in
ten of being alive the following November. Some females born early in
the reproductive season breed in their first summer or autumn. For
example, a female of category 2, taken on August 12, and probably in
postjuvenal pelage, had placental scars. Undoubtedly the young of the
year contribute to the breeding population, especially late in the
season.
[Illustration: FIG. 3. Frequency distributions, according to size, of
_Reithrodontomys megalotis_ and _Peromyscus maniculatus_ in three
samples taken in August, September, and November. Sexes and pregnancy or
nonpregnancy of females are indicated. See discussion in text.]
In Figure 3 the proportion of females bearing embryos in August,
September, and November is shown. Of the females trapped in August, 11
of 32 that were more than 144 mm. in total length contained embryos; an
additional 14 females were lactating or possessed placental scars or
enlarged uteri. Therefore, approximately 80 per cent of the larger
females were reproducing in August. In September two females were
pregnant and an additional sixteen of the 44 females examined showed
other evidence of reproduction; these eighteen females make up 41 per
cent of those more than 144 mm. in total length. The only reproductive
data available for November pertain to the presence or absence of
embryos. No female was pregnant although 35 females more than 144 mm. in
total length were examined. Some of the skins show prominent mammae
indicative of recent nursing, and juveniles less than a month old were
taken. The reproductive activity of deer mice on the Mesa Verde seems to
be greatly reduced in autumn.