626 THE AMERICAN NATURALIST [Vol. XL V 



cember and January, it is safe to say that they produce 

 young in January, also, which gives their breeding 

 period as the whole year. 



The gestation period was found to be 23| to 25^ days, 

 counting from the time of the first copulation. If 23| 

 days is the shortest time, then females Nos. 1 and 7 

 must have mated on February 4 and June 30, respec- 

 tively, the days they gave birth and also ate their young. 



Frequently a female eats her young and when she 

 does so, she may breed immediately. It is seen from 

 the table that female No. 7, which had the habit of de- 

 vouring her young, gave birth to seven litters at inter- 

 vals of about thirty days ; i. e., taking February 2 as the 

 date of conception for her first offspring, she produced, 

 seven litters in seven months. 



if the young are reared, a second litter may be pro- 

 duced in two months. Some do not give birth so soon, 

 which may be due, largely to the fact that a female is 

 not sure of conceiving at any given copulation. Daniel's 

 experiment with white mice seems to have some bearing 

 on this point. He found that a female suckling a large 

 litter, carried the second longer, as much as ten days 

 over the average gestation time. According to my data, 

 the brown rat does not conceive before the last ten days 

 of the lactation period, which should not have so marked 

 an effect on the gestation period as is found in the case 

 of the white mouse, which becomes pregnant as soon as 

 a litter is born. 



The number of young in the eight litters which I was 

 able to count varied from seven to twelve, with an aver- 

 age of 10.5. Boelter quotes Brehm as recording a litter 

 of twenty-one. Lantz gives two cases where seventeen 

 embryos were found in one female and nineteen in 

 another. He also thinks that for temperate regions the 

 average litter is not under ten. From the data at hand, 

 we can conclude that a pair of adult rats is capable of 

 rearing fifty to sixty young per year. Zuschlag's theo- 

 retical table, as quoted by Boelter, gives sixty-four as 



