, 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.

Peromyscus difficilis nasutus (J.A. Allen) Rock Mouse

_Specimen_: 1 mi. NNW Rock Springs, 7600 ft., 69413, a young
individual completi