4 MAMMALS IN THEIR RELATION TO SPOTTED FEVER. 



a theory of its possible connection with some native mammal or 

 mammals living solely on that side ; but a study of the range of the 

 mammals of Bitterroot Valley has failed to show that a single 

 species is restricted to the western side. Nevertheless the two sides 

 differ physically so widely as to influence greatly the distribution 

 of mammals. On the west side the mountains rise abruptly, forests 

 or brush covering much of the land except the cultivated tracts; 

 while on the east side a strip of rolling, treeless, sage-covered bench 

 land lies between the river and the mountains. Thus the west side 

 has a somewhat more humid climate than the east, with heavier 

 growth of brush, which furnishes good cover for most of the small 

 mammals, and hence favors the presence of ticks. But on the oppo- 

 site or less humid side good " tick country " occurs only around 

 those ranches which nestle close up to the mountains. Ground 

 squirrels, chipmunks, woodchucks, and pine squirrels — animals 

 which were found to be the principal hosts of the nymphs and seeds 

 of the fever tick {Dermacentor venustus) — are mainly absent from 

 the broad strip of sage-covered bench land ; but as this land is now 

 being rapidly brought under cultivation, some of these small mam- 

 mals may be expected sooner or later to occupy it. Hence, though 

 the east side of Bitterroot Valley is at present less suited as a 

 habitat for the several wild mammal hosts of the fever tick, a study 

 of the habits of the mammals of the west side discloses no apparent 

 reason why the spotted fever should not in time be carried to the 

 east side. 



In the course of our- investigations more than 500 mammals were 

 collected in and around the valley and 20 species Avere found to carry 

 ticks either in the immature or adult stage. The hosts of fever ticks 

 fall naturally into two groups, those that harbor chiefly adult ticks 

 and those that harbor the younger stages. In the former class be- 

 long mountain goats, bears, coyotes, badgers, woodchucks, and possi- 

 bly elk, deer, mountain sheep, rabbits, and domestic stock, as horses, 

 cattle, and sheep. Those of the second class, mainly rodents, com- 

 prise ground squirrels, woodchucks, chipmunks, pine squirrels, mice, 

 and wood rats. 



DOMESTIC STOCK. 



Unquestionably the great bulk, of fever ticks which become en- 

 gorged in the Bitterroot Valley do so on domestic stock. These ani- 

 mals are much more numerous on the west side of the valley than on 

 the east and they obtain the ticks from pasture and other unculti- 

 vated land infested by wild mammals. It is obvious, therefore, that 

 if the domestic animals in the valley are rendered tick free by dip- 

 ping, spraying, or by some other equally effective method, the chances 

 of the infection of human beings will be vastly lessened. 



[Cir. 82] 



