740 THE SMALL-MAMMAL PROBLEM 



maintained by the pressure of competition or by extreme 

 persecution ; the greater the loss inflicted upon the rat 

 population the more rapid the rate of recovery. 



Rodier, dealing primarily with the rabbit pest in Australia, 

 has advanced an ingenious plan for controlling the numbers 

 of rodents. He thinks we should aim at producing a vast 

 excess of males. Rabbits, rats, or mice should be trapped 

 alive ; the females should be killed and the males given their 

 liberty. By this proceeding a great disparity in the numbers 

 of the sexes will be produced in due course, and a keen 

 competition will arise among the males for the possession of 

 the surviving females. The males will fight each other con- 

 tinuously, and they will, at all times, relentlessly pursue and 

 harass the females. The nursing does will be unable to rear 

 their families, and any species attacked by this system will 

 become rare if not extinct. The present system of indis- 

 criminate trapping and poisoning, according to Rodier, has 

 a directly opposite effect ; it ensures the destruction of the 

 surplus males, and results in fertile unions for all females. 



Rodier's scheme has been advocated recently by Mr G. 

 Jennison,^ who has been experimenting for some years at 

 the Manchester Zoological Gardens. He states that "our 

 present system of destruction helps the rat in the struggle 

 for existence. The more rats killed, the more food for the 

 remainder ; the more males killed, the greater the chance for 

 the doe to breed quietly and raise her offspring. These two 

 facts together neutralise all the good effects of indiscriminate 

 slaughter. The rats can be reduced quickly to a certain 

 point beyond which it is almost impossible to make further 

 progress, and from wTiich they soon reach their former numbers 

 if at all neglected, e.g., Copenhagen caught 100,000 in four 

 months, 8th August to 8th December 1904; they could still 

 catch 99,000 in the three months of July quarter 1908, under 

 the new rat law." Applying the Rodier system to the Bellevue 

 Gardens, Manchester, Jennison reduced the number of rats 

 caught there from about 34 per month at the end of 1915, 

 to 18.5 per month in the first six months of 1920. He 

 says "the best plan for rat destruction appears to me plain. 



1 G. Jennison, " Rat Repression by Sexual Selection," /. R. San. Inst, xli., 358. 



