336 Dr. F. N. White. Variations in Sex Ratio o/Mus rattus 



mistakes under this head were very unlikely to occur. A young female is 

 not always easily differentiated from a young male by external inspection only. 



Under ordinary circumstances M. rattus would appear to be slightly 

 polygamous. Some observers have stated that it is very markedly so, and 

 explain the fact that about equal numbers of males and females are usually 

 caught by the alleged shyness of the adult female. This point I have 

 carefully gone into and believe the allegation has no foundation in fact. 

 As will be shown later, about equal numbers of males and females appear 

 to be born under normal conditions. When adult age is reached there is 

 either a somewhat enhanced mortality amongst males as compared with 

 females or else males are more wary and so less readily caught. It should 

 be noted also that females are as readily caught at the height of the breeding- 

 season as at other times. 



In the presence of severe plague a condition of marked polygamy is some- 

 times met with. I believe this to be chiefly due to the fact that plague is a 

 more fatal disease to male than to female rats.* When polygamy is marked 

 rats are scarce or difficult to trap ; on the other hand, when the rat population 

 is very large the numbers of the two sexes trapped appear to be more nearly 

 equal. In support of these statements Tables II-V have been produced. 



(1) Ballia district : Here plague is always present and rats, probably in 

 consequence, are very difficult to catch. Out of 4525 M. rattus caught 

 during 10 months 2550 were females, a proportion of 77 males for every 

 100 females. If the rats be separated into two groups, young and adult, 

 considering half the rats of 90 grm. and all those of lesser weight as young 

 and the remainder as adults, the degree of polygamy prevailing amongst the 

 rats of Ballia is seen to be even more marked than the above figures indicate. 

 Of the 1875 young rats 943 were male and 932 female, whereas of the 

 2650 adult rats 1618 were female and only 1032 male. In other words, 

 there were only 63 adult males for every 100 adult females (see Table V). 

 The rats were obtained from places scattered all over the district. 



(2) Coimbatore Town : Has suffered from repeated but not very severe 

 epidemics of plague. M. rattus appears to be scarce ; the catches were very 

 small. Here of 3889 M. rattus 2072 were females, 88 females for every 

 100 males (see Table IV). Here again the excess of females over males 

 affects adults only. 



(3) Cawnpore : Here rats were extraordinarily numerous. When our 

 observations started a mild epidemic of plague was drawing to a close. No 

 acute rat plague was, however, met with. . During the previous few years 



* See 3rd Plague Eeport, 1907, ' Journ. Hygiene,' vol. 7, p. 750, and 7th Plague Eeport, 

 1912, 'Journ. Hygiene,' vol. 12, p. 265, Table VII. 



