12 



KOBTH AMERICAN FAUNA. 



[No. 36. 



Although in most places harvest mice are only moderately numer- 

 ous, in a few regions they occur in great abundance. 



PELAGES. 



The mice of this genus usually molt but once a year, normally in 

 the late fall (at least in northern latitudes), but the changes produced 

 in the pelage by wear and fading are so great that the summer pelage 

 of adults differs markedly from their winter pelage. The young are 

 usually less ochraceous and in some species darker than adults, but 

 the difference between young and adults is less marked than in 

 Peromyscus and other related genera. Of the forms examined, 

 R, humulis merriami presents the greatest differences between young 

 and adults. 



Immature or subadult individuals taken in summer and fall always 

 present a fresh, unworn appearance, the color usually being decidedly 

 paler than in fresh winter pelage. Adults taken in midsummer or 

 early fall are almost always in a more or less worn condition — often so 

 much so as to appear ' 'ragged" — but occasionally one may be found 

 in summer in fresh pelage. The fresh, full pelage is retained during 

 most of the winter, and little evidence of wear is seen before March 

 or April. As the tips of the hairs wear off, the pelage in most species 

 becomes redder, and by midsummer the evidences of fading are often 

 pronounced, individuals and local colonies, however, showing marked 

 differences in the amount of wear and the date of its appearance. 

 Excessively worn individuals of certain species (particularly R. mega- 

 lotis) often become very pale and gray. 



Specimens showing clearly the process of molting are comparatively 

 rare, and in the majority of cases the molt seems to progress insidi- 

 ously, new hairs coming in simultaneously over all parts of the body. 

 In a few cases, however, the line of demarcation is plainly evident, 

 new pelage appearing first on the hinder back and along the sides, 

 the fore part of the back being the last portion to be invested. This 

 is equally true of young and adults.^ 



While the fall molt is doubtless normal in northern latitudes, occa- 

 sional exceptions have been noted, indicating the occurrence (prob- 

 ably abnormal, at least rare) of a spring or midsummer molt. A 

 young adult male specimen of R. megalotis dycTiei, from Meadow, 

 Wyo., taken June 28, 1909, has nearly completed a full molt, the 

 new pelage covering the whole of the anterior portion of the body to 

 the rump, where the old pelage, decidedly redder than the new, 

 remains. A specimen of R. fulvescens aurantius (also a young adult 

 male), from Sour Lake, Tex., taken July 18, 1902, shows a fresh, long 



1 Specimens showing this molt are as follows: 



R. humulis merriami, 9 juv., Lexington, Ky., November 19. 

 R. megalotis longicauda, cf ad., Stanford University, Gal., November 7. 

 R. megalotis aztecus, 9 subadult. Grand Junction, Colo., November 2. 

 R. montanus, $ ad., Medano Ranch, Colo., November 4. 



