1903.] 



Observations on the Sex of Mice. 



39 



whether the percentages of male and female young would have varied. 

 Some little information may, however, be obtained from the record of 

 8, who when he was mated with five does of an average age of 7 months, 

 produced 65 per cent, of male offspring, while with six does averaging 



2 months old the male offspring was only 53 per cent. Though the 

 numbers here are small (eighty-three young in all), the record certainly 

 supports the suggestion that an adult doe is more likely to bring 

 forth an excess of male offspring than a very young one. 



With regard to the effect of the age of the buck we are unfortu- 

 nately unable to give any definite opinion, since the bucks from which 

 we chiefly bred were apparently fully grown when purchased, though 

 we had no means of actually determining their age. It will be seen, 

 on looking at the record of oc, that in his later offspring females pre- 

 dominated, but there three factors at least are concerned : (1) The 

 advanced age of the buck; (2) the predisposition of C 1 and her 

 descendants to produce females; and (3) the possibility that the 

 excessive inbreeding to which these mice were subjected may have 

 led to an increase of females. 



The last factor we propose to consider at the present time is that 

 of external temperature. The mice were kept in an unheated 

 greenhouse in which the temperature usually ranged between 80° 

 and 100° F. during the day-time in summer, while in winter it 

 often descended several degrees below freezing point. It may be 

 interesting to contrast the records of the young born in July, 

 August and September with those born in December, January, 

 February and March, and both of these with the total records for the 

 whole 15 months over which our experiments extended. During the 



3 hot months 136 young were born, of which 75 (55 per cent.) were 

 males, and 61 (45 per cent.) females, while during the 4 cold months 

 127 young were born, of which 65 (51 per cent.) were males and 62 

 (49 per cent.) females. 



<?. ?. 



Three hot months 55 per cent. 45 per cent. 



Four cold months 51 „ 49 ,, 



Total records for 15 months... 53*3 ,, 46*7 „ 



These results do not seem to indicate that temperature or time of 

 year exerts any marked effect on the proportion of the sexes in the 

 young. 



In conclusion it must be confessed that we have learnt comparatively 

 little from this 15 months' experimental work on mice, but we are 

 hopeful that the labour has not been entirely expended in vain, and 

 that at least certain clues have been obtained which may usefully be 

 followed up, both by ourselves and other breeders. So far as our 

 experiments have gone, the chief points of interest would seem 

 to be : — 



