No. 538] REPRODUCTION IN THE BROWN RAT 627 



the possible number of young at the end of the yea]', the 

 product of a single pair. In this table Znschhig ;i-muiu- 

 eight to be the average number of young per litter, and 

 eight litters per year to be the product of a single pair. 

 On this basis the number of offspring at the end of the 

 second and succeeding years is far in excess to the num- 

 ber computed on experimental data. 



Rats begin mating, as a usual thing, about live o'clock 

 p.m.. and to obtain the period of gestation, males were 

 placed with the females every day at this hour. If a 

 female was in heat, she was removed to a separate caue 

 with one to three males. At first females were left with 

 the males not more than two hours, in which time many 

 copulations had taken place, but in no case did n preg- 

 nancy result. Later, they were left with the males 

 twelve to fifteen hours, and even then, failures to be- 

 come pregnant far outnumbered the pregnancies. 



T have not observed a single case of a female mating 

 with a male smaller than herself. It is not common for 

 an old female, even when in heat, to chase a young male 

 about the cage as though he were a female, not letting 

 him come near her. This same female, if placed with 

 a larger male, which could boss her, would mate with 

 him at once without any opposition. Mating in this case 

 seems to be, to some extent, dependent upon the domi- 

 nating ability of the male. 



The number of coitions during a single period of heat 

 is, apparently, great. In one case a female, placed with 

 four males, mated with them in such rapid succession 

 that fifty attempted coitions per half hour would be a 

 conservative estimate. It is impossible to say how many 

 of these attempts were successful, because the rat re- 

 quires such a short time (four seconds being a long 

 time) to perform the act, thereby making the details of 

 the process difficult of observation. 



The following table is a month's record of seven fe- 

 males kept to determine the frequency of the mating 

 periods. Such periods are indicated by an X. It is 



