time of the year or weather, but rather  surfaces now become clear white.
Thin
on the age and physical condition of  completes the growth of the nestling
the animal. This agrees with findings  pelage and all follicular activity
ceases
concerning pelage changes in Microtus  for approximately 10 days.
by Goin (1943), who stated that the   During the development of nestling
time of molt is probably largely deter- pelage, no patterns bearing a dorso-
mined by the age and physical condi- lateral stripe were found. In order,
the
tion of the animal.                    only patterns involved are III, 0,
Il
  Accordingly, a 90-day-old muskrat  and V. I patterns are completely ab-
in August will have a pelage of ap- sent during this early period and only
proximately the same quality and the  the entirely dark 0 phase can be found
same stage of development as a 90- IV patterns are also conspicuously ab-
day-old muskrat in December. Imma-  sent. The first complete pelage is gray-
ture muskrats are continuously pro- ish in color and has a fuzzy appear-
ducing pattern changes throughout   ance. After a short period of quie-
their developmental period. Develop- scence (about 10 days) following the
mental hair growth can be subdivided  completion of the nestling pelage at
into four general pelages based mainly  28 days of age, hair growth commenc-
on hair growth covers: Nestling, juv-  es once more.
enile, post-juvenile, and  subadult.  Juvenile. Our summer pelt collec-
These pelages can be correlated with  tion which fits the juvenile pelage
the first four growth covers. The   classification and the sheared animals
nestling pelage is formed by the first in this age group fail to indicate
any
growth cover, the juvenile pelage is  evidence of the occurrence of other
formed by the 2nd growth cover, post- than 0, II, III, and V patterns. Dor-
juvenile pelage results from the 3rd  solateral stripes have not yet made
growth cover and subadult pelage is their appearance, and only 0 and V
brought in by the 4th growth cover. phases are to be found instead of I
Succeeding growth covers are more   and IV. The beginning pattern is II,
irregular and adult-like in their forma-  and the changes probably progress
tion and are considered intermediate  through 0 to III and back to V (rest-
between subadult and adult pelages.  ing) once more. One of the pelts in
  Nestling. At birth, hair growth in  the summer collection was an estimat-
the muskrat kit is already beginning, ed 55 to 65 days of age and was found
as evidenced by the slate-blue pigmen- to be changing from a III to a V phase,
tation of the dorsal skin surface. Ham- thus indicating a likelihood that
the
ilton (1938) observed similar early  final change to the juvenile pelage
oc-
hair development in the northern pine  curs in this maner. The shearing ex-
mouse. Although the time span is dif- periments give evidence that such a
ferent, regions  experiencing  hair  change does take place at this age.
growth in early development and their  The juvenile pelage, unlike the nest-
sequence of change, is similar in both  uing pelage which starts mid-dorsally,
the pine mouse and the muskrat.     begins laterally just posterior to the
  Hair growth ends at approximately  forelegs. From there it spreads caudo-
28 days of age in all areas and the flesh  ventrally and laterally to the
mid-

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