e Biological Surveys Collection
(USNM). We are grateful also to persons in charge of the following
collections for allowing one of us (Jones) to examine Nebraskan
specimens of _R. megalotis_ in their care: University of Michigan
Museum of Zoology (UMMZ); University of Nebraska State Museum (NSM);
and U.S. National Museum (USNM). A research grant from the Society
of the Sigma Xi facilitated travel to the institutions mentioned.
Specimens not identified as to collection are in the Museum of
Natural History of The University of Kansas. All measurements are in
millimeters, and are of adults (as defined by Hooper, 1952:12)
unless otherwise noted.

Secondary Sexual Variation

Hooper (1952) did not accord separate treatment to males and females in
taxonomic accounts of Latin American harvest mice because (p. 11): "In
no species ... does sexual dimorphism in the measurements, if present at
all, appear to be sufficient to warrant separating the sexes in the
analysis." Hooper did not statistically test the validity of treating
the sexes together in _R. megalotis_. He did test a series of _R.
sumichrasti_ from El Salvador, in which he found no basis for separate
treatment of males and females.

Some authors (Verts, 1960:6, for instance) have recorded females of _R.
megalotis_ as larger than males in external measurements, whereas others
(Dalquest, 1948:325, for instance) have recorded males as the larger. In
order to learn something of secondary sexual variation, and to decide
whether or not to separate the sexes in our study, we compared adult
males and females from the southern part of the Panhandle of Nebraska
(Cheyenne, Keith, Kimball, Morrill and Scotts Bluff counties) in four
external and twelve cranial measurements (see Table 1). The external
measurements are those customarily taken by collectors and were read
from the labels of the specimens; cranial measurements were taken to the
nearest tenth of a millimeter by means of dial calipers, and are those
described by Hooper (1952:9-11). Females from our sample averaged larger
than males in all external and several cranial measurements, but
individual variation greatly exceeded secondary sexual variation in each
of these measurements and in no case was the greater size of females
statistically significant. Therefore, and because we found no
qualitative external or cranial differences between the sexes, males and
females have been con