791.

VARIATION

Secondary sexual variation in chipmunks is small; the females are
slightly larger than the males. This difference in size is so slight
that it can be ignored when making taxonomic comparisons, for, large
samples of males and females of like age and from the same locality
were compared and were found statistically not to be significantly
different. This is in agreement with Johnson (1943:70) and Hall
(1946:329).

Variations of taxonomic worth are treated in the accounts of species
and subspecies.

Individual variation is slight, for, the analyses of measurements of
the skulls of series of specimens of like age, reveal markedly low
coefficients of variability resembling those published by Larrison
(1949).

The age-categories here recognized are based primarily on the structure
of the skull.

_Juveniles._--Nasals proportionally shorter and more pointed anteriorly
than in other categories; zygomatic arches more appressed to cranium;
suture separating basisphenoid and presphenoid noticeably "open";
deciduous P4 and p4 show no wear through enamel; M3 and m3 not yet
erupted; peglike deciduous P3 strongly leaning posteriorly; molars show
no wear through enamel; parietals paperlike or thin; skull convex
dorsally; 1 to 1-1/2 months of age.

_Young._--Nasals of adult proportions; zygomatic arches still
noticeably appressed anteriorly to cranium; suture between basisphenoid
and presphenoid still "open"; nasals rounded, no longer so pointed as
in juveniles; deciduous P4 and p4 show wear through enamel layer, and
in some specimens, permanent P4 and p4 can be seen beneath; roots of
deciduous P4 and p4 clearly show erosion beneath; M3 and m3 fully
erupted; peglike deciduous P3 still present; parietals noticeably
thicker and less paperlike; skull flattened (not so convex dorsally),
but not so flattened as in adults; 1-1/2 to 4 months of age.

In both juveniles and young the P4 and p4 are deciduous and differ in
occlusal pattern from the permanent P4 and p4. In the deciduous P4 the
anterior cingulum is projected strongly anteriorly forming the apex of
the sharpest angle of a triangle, whereas the permanent P4 is
trapezoidal in occlusal pattern. In the deciduous p4 the protoconid and
metaconid are close together giving this tooth a triangular appearance
in occlusal pattern, whereas this pattern in permanent p4 is
trapezoidal (see Hall 1926:390).

_Subadults._--Adult configuration of skull reached; suture be