THE BROWN OR COMMON RAT 627 



still, combined with trappers, professional or desultory, foxes, 

 owls, and other creatures, together with the plan of bacillus 

 infection, which appears to have met with some success 

 recently, a large toll is annually taken, but seems to have 

 no effect on their numbers. Sooner or later civilised man 

 will have to face the problem of totally destroying these pests, 

 but hitherto his efforts have met with practically no success. 

 In Japan alone several hundred thousand to a million rats 

 are said to be killed annually, but without producing any 

 sensible diminution of the numbers present.^ The cutting- 

 off of the chief sources of food-supply, thus reducing the 

 number of young, and the universal erection of rat-proof 

 dwellings, as recommended by Mr Lantz,^ if combined in a 

 systematic manner with trapping, may prove more effective 

 in the long run than the present desultory campaign. 



Rats are extremely prolific, and when living in houses in 

 warmth and plenty, will produce young at every season of the 

 year ; but this, of course, does not indicate that any particular 

 female will breed throughout the year. Those who live out of 

 doors and are more poorly fed have a sexual season varying 

 with their circumstances, but coinciding more or less with the 

 warmer six or eight months of the year.' Fertility is greatest 

 in countries of mild climate free from extremes of heat or cold, 

 but in exceptional cases winter litters are found even in severe 

 weather in the open country.* 



Darwin {Desc. of Man, ed. 2, 247) was informed that the 

 males are "in great excess," while John Sinclair reported 

 (Thompson, iv., 18) that 75 per cent, of the rats in litters he 

 examined were males. Bonhote, however, found that in 

 Egypt males were apparently fewer than females, constitut- 

 ing only 42 per cent, of those he examined ; and of eighty-four 

 examined at Kilmanock, on one occasion, only fourteen were 

 bucks. 



Tame female white rats are said to be capable of breeding 



' Professor Kitasako, quoted by C. Hart Merriam, U.S. Department Agric. Biol. 

 Survey, Bull., 23, 1909, letter of transmittal. 



' Ibid., 10. Much valuable information as to the best means of destroying rats or 

 of protecting property and food from their attacks is given in this paper (pp 36-54). 



' Heape found the dicestrous cycle to occupy about ten days. 



* J. C. B. Noble, Field, 26th November 1904, 950. 



