150 THE OREGON SPORTSMAN 



RELATIVE NUMBERS IN SEXES OF THE TREE MOUSE. 



BY 

 WALTER P. TAYLOR, 

 Curator of Mammals, Museum of Vertebrate Zoology, University of California 



Of the many significant discoveries made by field natural- 

 ists during the last few years the "rediscovery" of the tree 

 mouse (Phcnacomys longicaudus — True), is acknowledged to be 

 one of the most interesting. It w T as the good fortune of the 

 writer, in connection with his regular duties as leader of the 

 North Coast Counties expedition of the Museum of Vertebrate 

 Zoology of the University of California in 1913, to be able to 

 study the habits of this species. Upon examination of the speci- 

 mens collected it immediately becomes apparent that the females 

 are much more numerous than the males. In fact, in a series of 

 eight adult specimens, six are females and only two are males. 

 This difference in numbers immediately prompts the question as 

 to why such a condition should obtain. 



There are at least two possibilities to be considered: (1) that 

 the mice practice polygamy, and that there are actually more 

 females than males; (2) that the adult females remain more 

 closely in the nests than the males do, and so were taken in great 

 numbers by our methods of capture, which involved the dissec- 

 tion of the nests. 



Of the young specimens available eight are males and thir- 

 teen females. It appears that among the young individuals the 

 proportion of females to males is well below two to one, while 

 among the adults the proportion is four to one. If males and 

 females are born in equal numbers, a non-selected series of young 

 should divide up equally between the sexes. Since our specimens 

 do not divide in this way, one is tempted to the conclusion, on 

 the one hand, that the females are actually about twice as 

 numerous as the males. On the other hand, the difference in 

 proportions of females to males in the series of young and adults 

 respectively seems to indicate that the females do remain more 

 closely in the nests than the males and so were taken in greater 

 numbers by the methods of capture employed. 



It is realized that the numbers of specimens examined in this 



