126 ANIMAL LIFE IN THE TOSEMITE 



blackish juvenal pelage but were otherwise immature. It seems safe to 

 assume that the youngest of these mice was not much over two months of 

 age when it began to breed, and that both had been bom during February 

 or March. 



In Yosemite Valley during the fall months more than half the popu- 

 lation of meadow mice was found to consist of obviously young animals — 

 at least this was so indicated by our trapping records. Of 43 individuals 

 obtained October 8 to 10, 1914, 23 were juveniles. 



Over the entire northern hemisphere meadow-inhabiting mice are to be 

 found wherever the ecologic niche of this group is represented, but under 

 very diverse climatic conditions. The Lemmings of the Arctic regions 

 and the Meadow Mice of the Great Basin are extreme examples. From 

 time to time great fluctuations in the populations of these mice are to be 

 noted. Sometimes the animals are so scarce as to be found only by diligent 

 search; in other years they are extremely plentiful. A case of the first 

 sort is related of the Mariposa Meadow Mice at Bean Creek east of Coulter- 

 ville in 1915 ; on Tuolumne Meadows the same year the Yosemite Mice were 

 so numerous that 5 or 6 holes per square yard were counted in places. 

 Under certain favoring conditions, regarding the nature of which we still 

 have much to learn, meadow mice may increase enormously, even far 

 beyond the population indicated for Tuolumne Meadows. Then a 'mouse 

 plague' results. It was a race closely related to the Yosemite Meadow 

 Mouse which over-ran Humboldt Valley, Nevada, in 1907 and 1908 and 

 caused great damage to agricultural interests there by destroying prac- 

 tically all the surface vegetation and even the roots of the alfalfa. 



California Meadow Mice. Microtus calif ornicus (Peale)^" 



Field characters. — Body size between that of House Mouse and House Rat ; tail short, 

 less than one-half head and body; pelage long, soft, and dense; ears short, nearly buried 

 in the fur. (For measurements see footnote.) General coloration above dark brown, 

 middle of back red tinged; under surface blue-gray to whitish. Workings: Runways 

 1 to 1^/^ inches wide through grass, connecting with round holes in earth. 



10 Two subspecies of California Meadow Mice occur on the west slope of the Yosemite 

 section. These inhabit different life zones and can best be distinguished on the basis 

 of distribution. 



TuLE Meadow Mouse, Microtus californicu-s aestuarinus Kellogg, a subspecies which 

 inhabits the Sacramento and San Joaquin basins and other lowland districts in central 

 California, is common in the bottom lands near Snelling and Lagrange. 



Mariposa Meadow Mouse, Microtus californiciis moriposae Kellogg, a form found 

 in the foothills along the west side of the Sierra Nevada, has been recorded at IMesusant 

 Valley and thence eastward to Smith Creek (6 miles east of Coulterviiie), to ('ascades, 

 and to Sweetwater Creek. This subspecies is distinguished from the Tule Meadow 

 Mouse by its brighter, more reddish coloration and somewhat larger size. 



Measurements: Acstuarimis : head and boily 4 to f)!/.. inches (99-139 mm.), tail 

 1% to 2% inches (44-G7 mm.), himl foot nearly 1 inch {22-2r).5 mm.), ear from crown 

 Mi to % inch (14-17 mm.), weight 1>/, to 1% ounces (38.3-49.2 grams). Mariposac: 

 head and body 5 to 5% inches (128-145 mm.), tail 1% to 2% inches (48-64 mm.), hind 

 foot nearly 1 inch (21-25 mm.), ear from crown about Ms inch (10-14 mm.), weight 

 2>/j to 2ya ounces (64.0-73.8 grams). 



