252 CALIFORNIA ACADEMY OF SCIENCES [Proc. 4th Ser. 



(2) Preponderance of females 



Of all the sexed specimens of Phcnacomys available 10 are 

 males and 20 females. This preponderance of the females in 

 series of longicaudus is impressive, and the question imme- 

 diately arises, why should there be such a discrepancy in 

 numbers of the sexes ? 



Segregating the specimens according to age, we find that 

 of the eight adults, their age being determined on the basis 

 of the degree of emergence of their cheek teeth, only two are 

 males. 



Twelve specimens of Phcnacomys orophiliis are equally 

 divided between the sexes, there being six of each. Of four 

 fully adult orophilns, one is a female and three males. 



There are at least two possibilities to be considered : 

 ( 1 ) that the mice are polygamous or promiscuous, and that 

 there are actually more females than males; (2) that the 

 female adults remain more closely in the nests than the males 

 do, and are taken in greater numbers by our methods of cap- 

 ture, which involved the dissection of the nests. 



It may be that both these possibilities are effective. Of the 

 eight youngest examples at hand six are males and two 

 females, but of all the young of longicaudus available, elimi- 

 nating four alcoholics not sexed, eight are males and thirteen 

 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 the young, supposedly 

 non-breeding individuals, should have divided up equally 

 between the sexes. But since they did not, one is tempted to 

 the conclusion that females are actually about twice as num- 

 erous as 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. 



Orophilus and alhipcs, being taken by free trapping on the 

 ground, would probably not give the bias in the number of 



*The classification of the specimens into ad'jlt and young is an arbitrary one, and 

 several of the examples classed as young probably are breeders. 



