SQUIRRELS OF MEXICO AND CENTRAL AMERICA 21 



colimcnsis on the coast of Colima, and S. apache and S. diirangi 

 in the Sierra Madre. No such antagonism exists between the 

 large species (subgenus Echinoscmriis) and the smaller ones 

 (subgenera Gaerlingucttis and Microscmrus) which frequently 

 occupy the same ranges. 



Many species change their environment by periodical migra- 

 tions in search of food, moving from one locality to another with 

 the ripening of fruits or seeds upon which they subsist. This 

 is most marked on high mountains where a species may have a 

 vertical range of many thousand feet. Dr. Buller obtained a 

 specimen of Sciiwiis poliopus cervicalis among the pines on the 

 Sierra Nevada de Colima at an altitude of 12,000 feet, but when 

 we visited this mountain at another season, gnawed pine cones 

 were abundant near the summit, but the squirrels had de- 

 scended to lower levels and were feeding on wild figs and acorns 

 at an altitude of 4000-6000 feet. In eastern Queretaro and 

 northern Hidalgo, S. aureogaster^ a tropical species which 

 usually ranges below 4000 feet, was found in winter among the 

 oaks and pines above 8000 feet alt. S. deppei likewise at 

 times wanders high above its normal range. 



The effect of climate on the character of the pelage is so 

 marked that it is possible to tell with considerable certainty 

 whether a species belongs to the tropics or to the higher moun- 

 tains. Tropical species have thin pelage, short thin under fur, 

 and coarse, stiff, or almost bristly dorsal hairs ; those of the 

 Transition and Boreal zones have thick soft pelage with long 

 dense under fur. These differences are sometimes strikingly 

 shown in subspecies of the same squirrel : thus the tropical 

 Sciurtis aurcogaster hypopyrrhiis has thin coarse pelage, while 

 S. aiireogaster Jrtmientor, which ranges between the altitudes 

 of 6000 and 8000 feet on a cold mountain slope, has dense soft 

 pelage. Species of the hot coasts of Central America are char- 

 acterized by peculiarly coarse, shining, bristly dorsal hairs. 

 Seasonal differences in pelage are usually slight, since there is 

 no area of heavy snow fall or long continued cold weather ex- 

 cept in the Sierra Madre of Durango and Chihuahua. Indi- 

 vidual variation, on the other hand, is often excessive and ren- 

 ders some species extremely difficult to describe. The large 



