618 



THE MONTHLY BULLETIN. 



squirrels had been removed. Other nests were found to be infested 

 with fleas, though at least one-half of the nests examined were free from 

 these parasites. In certain localities squirrels were taken that were 

 to casual appearance absolutely free from fleas, while in other localities 

 squirrels taken were invariably infested to a greater or less extent. 

 The species of flea that infests ground squirrels is not the species that 

 commonly attacks human beings. While ground squirrels are their 

 preferred hosts, we found that the former did not object to human 

 society when their squirrel hosts had died. At least two methods are 

 used by the squirrels to rid themselves of these uninvited guests. The 

 first, or dust-bath method, is that of suffocation of the fleas which hide 

 in their fur by thorough wallowing in especially dusty places. The 

 second is by digging a new burrow and making a new nest, thereby 

 leaving the bulk of the fleas behind. 



Some years ago it was discovered that the fleas harbored by the 

 California Ground Squirrel carried the bacillus of bubonic plague. A 

 vigorous campaign of extermination was waged against the squirrels 

 by the United States Public Health Service and they were practically 

 eliminated from many areas, locally, in the San Francisco Bay region. 



Fig. 12. Diagram showing extent and height of breeding season in the California 

 Ground Squirrel. Heavy line shows percentage of females pregnant, for weekly periods 

 from January to May. Based on record of embryos found in over 10,000 females 

 examined by the United States Public Health Service (McCoy, 1912, p. 1070). 



Soon after the efforts against the squirrels were relaxed the latter 

 began to 4 ' spill in" from adjacent areas until now in places they are as 

 numerous as ever. Nevertheless the prime object was attained, that of 

 eliminating the foci of dissemination of the disease. 



It is extremely important to know definitely the season and rate of 

 breeding of any economically important rodent. Fortunately for our 

 purpose, there is available for the California Ground Squirrel abundant 

 data, supplied through the records of the United States Public Health 

 Service (see McCoy, 1912, p. 1070). As will be seen from the accom- 

 panying diagram (fig. 12) based on over 10,000 females examined, the 

 breeding season is restricted to a comparatively brief period of the 

 year. Pregnant female ground squirrels have been taken in the Bay 

 region as early as the first week in January, but the main breeding 

 season does not begin until February, and it is practically concluded 

 by the middle of April. The largest per cent of pregnant females is to 

 be found during the last week in February. By June 4 only two-tenths 

 of one per cent (0.2) of females examined contained embryos. At higher 



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