J01MAL OF ACMQiniiAL RESEARCB 
Vol. XXVIII Washington, D. C., June 7, 1924 No. 10 
INDIVIDUAL AND AGE VARIATION IN MICROTUS 
MONTANUS YOSEMITE 1 
By A. Brazier Howell 
Scientific Assistant, Division of Biological Investigations , Bureau of Biological 
Survey , United States Department of Agriculture 
INTRODUCTION 
While engaged in field work in Mono County, Calif., during July, 1922, the 
writer found that it was a year of extreme abundance for the common meadow 
mouse ( Microtus montanus yosemite Grinnell), which inhabits the open meadow 
land of this region, and decided to make a detailed study of the individual and 
age variation occurring within this single unit group of this genus. A large 
series of specimens was carefully collected for this purpose. 
It seems remarkable that despite the great activity displayed in the study of 
mammals in America during the past several decades no one has attempted a 
really comprehensive investigation to ascertain the amount of age and indi¬ 
vidual variation normal to any single group of small mammals. J. A. Allen ( 2 ) 3 
published a short paper on the subject in 1894, and almost every reviser of a 
genus or group has done what he could to gain an understanding of these differ¬ 
ences; but usually the material available has been of such a nature that no 
detailed and intensive study in this regard was practicable. Almost without 
exception series have been small, or, if large, the specimens were secured by 
different collectors; they came from different, even though closely grouped 
spots; their histories were not perfectly known; or they were collected with no 
view to a later study of a particularly intensive nature. 
The purpose of the present contribution is twofold—to furnish a detailed 
report of the above nature and to offer a fairly comprehensive study of the 
osteology of the skull of a small rodent that is not particularly specialized. Only 
the exterior aspects of the bones are treated, with the purpose of filling a long- 
felt need that every systematic mammalogist has experienced at some point in his 
career. Throughout the discussion the writer has endeavored to treat fully all 
details that might be considered in any way important; but an effort has been 
made to avoid lengthy discussion of minute variations which are regarded as 
relatively unimportant and which would prove most tedious to the reader, and 
from which more pertinent matter might be separated only with considerable 
difficulty. 3 
1 Received for publication Apr. 14, 1924—issued Nov., 1924. 
i Reference is made by number (italic) to “ Literature cited,” p. 1015. 
* The drawings, except those of the muscles and of the foot, were made by the writer. All details com¬ 
prising each figure are enlarged in the same proportion, but there was no effort to have the various sep¬ 
arate figures on the same scale. AH numbers of specimens herein mentioned relate to serial numbers in 
the writer’s private collection. 
Journal of Agricultural Research, 
Washington, D. C. 
Vol. XXVIII, No. 10 
June 7,1924 
Key No. Q-3 
96461—24f-1 
(977) 
