338 Dr. F. N. White. Variations in Sex Ratio q/'Mus rattus 



more difficult to catch than males ; on the contrary, it is possible that males 

 are slightly more wary than females. As stated above the two sexes are 

 normally produced in equal numbers, though adult females are usually in 

 slight excess of adult males. 



It may also be objected that the parallel assumption, that failure to catch 

 young males signifies that no males are born, is not warranted by facts. 

 This is admitted, but the only other explanation that I can offer is that 

 the males were destroyed by their parents soon after birth (at a lesser 

 weight than 10 grm., when it becomes just possible to trap them with 

 the traps we employed, i.e., at about a week old). That the parents should 

 have destroyed only the male offspring is, to me, less easy of credence than 

 that only females were produced. 



It is a matter of regret that my observations did not succeed in throwing 

 any light on the causes of the female mortality. Plague was certainly present 

 until April, 1911, amongst the Lucknow rats, but it was not severe or wide- 

 spread, and, as has been pointed out, plague is more fatal to the male than to 

 the female. Whatever the cause was it was a widespread one in the city. 

 The rats, caught from all parts of Lucknow, represented as fair a sample as 

 could be obtained. 



Further speculation on this interesting topic would not prove fruitful. The 

 rapid readjustment of the sex ratio after so grave disturbance is, to my mind, 

 the fact of chief interest. In May and June when hardly any females were 

 produced there must have been an extreme degree of polyandry. 



From a study of weight frequency curves of pregnant females, I have 

 concluded that for practical purposes 90 grm. represents a fair dividing line 

 between young and adult rats of Lucknow (Table VI). Half the rats of the 

 weight of 90 grm. can be considered young and half adult. Employing this 

 approximation, there appears to be an interesting correlation between the 

 excess of adult males over adult females and the excess of young females over 

 young males for the same month* 



In spite of the absence of any explanation of the facts the phenomenon 

 described appears to me to be of sufficient interest to warrant its publication. 



* Mr. Major Greenwood, Jr., Statistician to the Lister Institute, has kindly supplied 

 me with the approximate correlations between the sex ratios of mature and immature 

 rats for the same month and also for certain sequences. The groupings appear in 

 Table VII and the coefficients in Table VIII. It will be seen that the negative 

 correlation for data derived from one month's records is slightly larger than when the 

 records of successive months are combined. A possible explanation is that overlapping 

 of different affected colonies produces an apparent synchronism of cause and effect, but 

 the figures as they stand do not warrant any inference. The enormous excess of young 

 females in May and June is a statistically inexplicable fact. 



