THE GROUND SQUIRRELS OF CALIFORNIA. 619 



altitudes, where warm weather comes on much later and more abruptly, 

 these breeding dates would be correspondingly later and the breeding 

 season still more restricted. 



While males and females occur in practically equal numbers, mating 

 seems to be promiscuous; there is no permanent pairing off. 



The number of young per litter, as ascertained from counts of 

 embryos, varies from 4 to 11. The average, from very extensive records 

 kept by the United States Public Health Service (McCoy, 1912, p. 

 1070), may be inferred to be very close to 7.2. The same records serve 

 further to show that there is some variation in size of litter from 

 month to month. The average for February is 6.9 ; for March, 7.3 ; 

 for April, 7.5 ; for May, 6.8. The tendency seems to be toward slightly 

 larger litters in April, which is beyond the date of maximum number 

 of pregnant females (see fig. 12). Number of mamma3 (nipples), 

 which is usually six pairs in this species, occasionally but five, is no 

 criterion for number of young per litter. All the evidence at hand 

 indicates that each female raises but one litter each year. A female 

 ground squirrel was taken on the University campus at Berkeley, on 

 March 13, 1918, which contained eleven embryos each of which meas- 

 ured three-fourths of an inch long. Eight of these were contained in 

 one branch of the uterus and three in the other. Another female 

 taken at the same time contained eight embryos, each of which measured 

 five-eighths of an inch long. Five of these were in one branch of the 

 uterus and three in the other. 



On April 6, 1918, G. E. Stewart and the junior author dug out two 

 female ground squirrels which had been previously gassed in their bur- 

 rows. One of these females was found in a nest with four small young 

 which we took to be about ten days old, since their eyes were not yet 

 open. These baby squirrels averaged 170 millimeters or 6f inches in 

 length; a typical one weighed 61 grams, or a little over 2' ounces. 

 They were well covered with hair, which already showed on the back 

 the characteristic dappled pattern of the adult squirrel. The tail, 

 however, was nearly round and showed little sign of the fringe of hairs 

 along the sides. Their stomach contents showed no sign of their having 

 eaten green vegetation or anything else than milk. Data from other 

 sources indicate that the young are not completely weaned until they are 

 at least half grown. The other female secured in an adjoining burrow 

 not over ten feet distant was found to contain seven small embryos each 

 of which measured three-eighths of an inch in length. These embryos 

 could not well have reached full development short of two or three 

 weeks, so we have a variation of nearly a month in time of birth at one 

 locality. 



Cases such as those just given are thought to be exceptional and may 

 serve in part to explain the occurrence of late litters such as have been 

 the basis of the claim that this animal has two litters a season. Litters 

 of young squirrels which sometimes appear very late in the season are, 

 too, likely to be merely the result of efforts to replace first litters of 

 young which have met an untimely death. Thus two litters might be 

 born in one season, though only one raised. 



Shaw (1916, p. 4) gives 24 or 25 days as the period of gestation in 

 the Columbian Ground Squirrel (Citellus columhianus) in the region 

 about Pullman, Washington. The period of gestation of the California 



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