12 



Rats and Mice 



that, as in other Muridae, the urethra of the female passes through 

 the prominent cUtoris ; in mature does, the vagina is a conspicuous 

 orifice between the chtoris and the anus ; but in immature females 

 the vagina is sealed and quite invisible externally, and such 

 specimens can frequently be mistaken for males ; distinction by 

 mere inspection is, however, possible, because the distance 

 between the clitoris and the anus is less than between the penis 

 and the anus. 



The number of litters per doe per annum and the number of 

 young per litter are dependent upon many factors, of which the 

 most important are food supply, the age and condition of the female 

 pairing, and, in a much lesser degree, climate and temperature. 

 B. rattus probably shows a higher fecundity in warm temperate 

 countries than in cold temperate lands ; B. norvegiciis, on the 

 other hand, is more fecund in cold temperate climates. Increased 

 nutrition not only leads to larger and more frequent litters, but it 

 almost certainly results in decreasing the mortality among very 

 young rats. Although very good and careful mothers, as a rule, 

 female rats eat their offspring in certain conditions, such as when 

 overcrowding leads to disturbance or to too keen competition for 

 supplies. In Newton Miller's experiments with B. norvegicus in 

 captivity 50 per cent, of the young born were thus devoured. 



The embryos of the pregnant females among 12,000 rats 

 trapped at Bombay were counted, with the following results : — 



B. rattus. E. norvegicus. 

 Average number of young per litter 5*2 8-1 

 Maximum number of young per litter 9 14 



In Europe B. rattus is described as bearing from four to 

 eleven young in a litter; probably the normal number per litter 

 in Britain is between six and eight. 



As regards B. norvcgims, twenty-three litters examined by Mr. 

 A. H. Cocks, at Great Marlow and Poynetts, Bucks, gave a range 

 of from six to twelve per litter, the average number being nearly 

 eight per litter. This may perhaps be taken as a measure of fecundity 

 in rural conditions.'"' But much larger litters are frequently observed, 

 particularly in towns. Litters of seventeen, nineteen, twenty-two, 



* Between January 16 and February 14, 1911, Petrie and Macalister 

 examined 6,071 specimens of R. norvegicus collected in Suffolk and Essex 

 during the period specified ; of these 3,273 were males, 2,724 females, and of 

 74 the sex was not recorded; 290 or 10-6 per cent, of the females were 

 pregnant, the average number of foetuses being 9 (7, p. 60). 



