1917] The Ecology of Bubonic Plague 203 



C. Rats.. 



During November to February, the winter season at Punjab, 

 rat breeding was at its lowest; this is a pre-epizootic period. 

 During the rest of the year the portion of pregnant females 

 to non-pregnant ones was always at or above the mean. Breeding 

 was most vigorous during April, September and October. The 

 plague season was from February to June. At Bombay, the 

 brown rat, M. decumanus, breeds the year round, least from 

 December to February, a pre-epizootic period. During March, 

 July, August and October, breeding was most vigorous. The 

 plague epizootic among the rats begins in January, rages during 

 February and March and rapidly declines in April. The same 

 held true with the black rat, M. rattits. 



The effects of plague are very evident at first among the 

 large numbers of the rats that die; this means there is a super- 

 abundance of rats non-immune to plague. But later on in 

 the stage, there appear quite a number of immune rats, and 

 these furnish the start for the next increase, plenty of young, 

 susceptible rats for the next plague rise. The influence of 

 plague in the rat association is in the nature of a radical dis- 

 turbance of equilibrium. The reports show very nicely how, 

 after a large percentage of the rats had succumbed to plague, 

 there came a vigorous breeding spell. This sudden breeding-fit 

 is but a natural effort to re-establish again a relative equilibrium. 

 The habitat remained favorable throughout. 



Jennings (1910) found 2.20 fleas per each norway rat 

 {Mus norvegicus) he examined, and 3.61 fleas per each black 

 rat {M. rattus). The difference is not due to the texture of the 

 fur in the two species, but rather to the nesting habits of the 

 two species. The norway rat is more ferocious and its bur- 

 rowing habit is more pronounced. It constructs its tunnels 

 anywhere it can, mostly wher^ it is moist. The black rat, on 

 the other hand, builds its nests above the ground, in the walls 

 of buildings, etc., consequently in a drier habitat. Moisture 

 is inimical to the flea larva and adult, and therefore the greater 

 number of fleas on the black rat. 



Heavy rains affect rats and fleas. They drive them from 

 the wet or submerged burrows into drier situations, and this 

 means closer contact with people as well as increased flea 

 breeding. 



