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 "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 accompanying 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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