A STUDY OF INFLUENCES WHICH MAY AFFECT THE 



SEX-RATIO OF THE DEER-MOUSE 



(PEROMYSCUS). 



FRANCIS B. SUMNER, MARY E. McDANIEL AND RALPH R. HUESTIS. 



INTRODUCTION. 



For the past eight years the senior author has been conducting 

 breeding experiments upon California deer-mice, chiefly subspe- 

 cies of Pcromyscns maniculatus. 1 Throughout this period, fairly 

 complete records have been kept of the births, with a view to the 

 ultimate use of these data in a study like the present. 2 The 

 work of tabulating the crude data and of computing the values 

 herein presented has been chiefly performed by the junior authors. 

 The senior author has, however, supervised the work throughout, 

 and assumes responsibility for the accuracy of these various 

 figures. 



The data upon which this report is based are not in all respects 

 as complete as might be desired for the study of sex-ratios, though 

 we do not believe that their value is seriously affected by these 

 limitations. Thus the number of individuals in a brood was fre- 

 quently not determined until the expiration of some days, or even 

 as much as two weeks after birth. As a rule, the brood was 

 discovered on the day of its birth, or within one or two days 

 thereafter. The number of young was commonly recorded at 

 that time, in cases where it was possible to do so without seriously 

 disturbing the mother. At latest, the number was recorded about 

 16 days after birth, at which time we have regarded it as safe to 

 clean out the cage and change the nesting material. 



Record of the number of each sex in a brood was not usually 

 made at the time of this first count. In some cases it was made 



1 Sumner, 1920, and papers therein cited. 



2 There are here included a few hundred mice from the records of Mr. 

 Huestis, and a few hundred others from the records of Mr. H. H. Collins. 

 We are indebted to Mr. Collins for permission to use these last. 



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