16 



THE BROWN EAT IN THE UNITED STATES. 



captivity gave birth to 11 young when only eight weeks old. As 

 gestation in rats occupies three weeks, this animal must have bred 

 when only five weeks old. Kolazy records instances of but 25 days 

 between successive litters of well fed albino rats. a 



The known facts concerning the reproduction of the brown rat in 

 temperate latitudes may be briefly stated as follows: The animals 

 breed from three to five times a year, each time bringing forth from 

 6 to 20 young. After a gestation period of 21 days, the female gives 

 birth to her young in nests in underground burrows or under floors, 

 stacks, lumber, wood piles, or other shelter. The young are blind 

 and naked when born, but grow rapidly, and the females are capable 

 of breeding when less than three months old. 



If we assume that the animals breed three times a year and that 

 the average litter is ten, and suppose that a pair and their progeny 

 breed uninterruptedly at this rate for three years, with no deaths, 

 the result would be as follows : 



Theoretical results of three years' (nine generations) uninterrupted breeding of a pair of 



brown rats. 





Generation. 





I. 



II. 



III. 



IV. 



V. 



VI. 



VII. 



VIII. 



IX. 



Number of pairs breeding 



Number of young produced 



Total number of individuals 



1 

 10 

 12 



6 

 60 



72 



36 



-360 



432 



216 

 2,160 

 2,592 



1.296 

 12,960 

 15, 552 



7,776 

 77,760 

 93,312 



46, 656 

 466, 560 

 559, 872 



279, 936 

 2,799,360 

 3, 359, 232 



1,679,616 

 16, 796, 160 

 20,155,392 



Of course, such results never occur in nature. Apparently not 

 nearly half the rats born are females; at least, among mature rats 

 the males greatly predominate. Then, too, the life of young rats, 

 as well as that of the old, is a continuous struggle for existence. 

 Disease, the elements, natural enemies, the devices and cunning of 

 man, and even cannibalism are continually at work to reduce their 

 numbers. 



MIGRATIONS AND INVASIONS. 



Migrations of rats have often been recorded. Pallas narrates that 

 in the autumn of 1727 the brown rat arrived at Astrakhan in southern 

 Russia from the east in such numbers and in so short a time that 

 nothing could be done to oppose them. They crossed the Volga 

 in large troops. The cause of the migration was attributed to an 

 earthquake; but since similar movements of this species often occur 

 unattended by earth disturbance, it is probable that only the food 

 problem was involved in the migration which first brought the brown 

 rat to Europe. 



aVerh. Zool. Botan. Gesel., Wien, pp. 731-734, 1871. 



