No. 538] REPRODUCTION IN THE RROWN RAT 635 



Sexual maturity is attained much sooner than full 

 growth. One female of litter No. 11 conceived on the day 

 she was four months old, hence giving birth to her first 

 young in her fifth month. The males are sexually mature 

 as soon as the females, and I have seen some indications 

 that they may be mature early in their fourth month. 



Conclusions 



1. The brown rat breeds in every month of the year. 



2. The gestation period is 23^ to 25^ days. 



3. The number of young per litter varies from six to 

 nineteeen with an average of between ten and eleven. 



4. Five or six litters may be reared by a single pair in 

 a year. 



5. Seven litters were born in seven months by one 

 female and, presumably, twelve would be produced in the 

 course of a year when all the young perish at birth. 



6. There is very little courtship among the brown rats. 



7. Odor is the primary factor in sex recognition; the 

 aggressiveness of the male is second. 



8. Brown rats in captivity eat almost fifty per cent, of 

 their young at birth. Most of the ydung eaten, if not all, 

 are eaten by the females. 



9. Full growth is attained not under eighteen months. 



10. Sexual maturity is reached at least by the end of 

 the fourth month in both sexes. 



REFERENCES 



Blue, B., '08. The Underlying lTiii.-i,,les of Anti-plague Measures. Cali- 



Plague from San Francisco," 1909, p. 217. 

 Boelter, W. B., '09. The Bat Problem, pp. 6 (note), 88. 

 Bostock, F. B., '04. The Training of Wild Animals, pp. 40, 50^ ^ ^ 



Journal of Experimental Zoology, Vol. 9, No. 4, p. 868. 

 Lantz, D. E., '10. Natural History of the Bat. Public Health and Marine 



116-122. 



