No. 550] 



597 



sive breeding* of mice. But to rear mice successfully 

 further requires a practical knowledge of their breeding 

 habits. 



III. Breeding Habits of Mice 



Copulation in mice is a well-defined act which usually 

 follows only upon the persistent efforts of the male. It is 

 marked by a period of union which lasts for several 

 seconds (ten to twenty-five) and is followed by an interval 

 of more or less complete rest. 



Practical questions for the breeder are : 1. When will 

 copulation occur— that is, when is the period of heat? 2. 

 What is the duration of this period? 3. How often does 

 the period of heat recur ? To the last of these questions 

 my experience can offer no answer ; and to the second my 

 observations add little. In one case, however, after a 

 double copulation had been observed in the evening copu- 

 lation again took place on the following morning. The 

 fact that the beginning of heat is shown in some as early 

 as five hours after parturition and is delayed in others as 

 long as thirty-six hours thereafter makes it difficult to 

 determine with exactness the duration of this period. 



As to the first question— When may copulation be 

 secured?— two periods can be determined with consider- 

 able accuracy. One of these closely succeeds parturition, 

 the other follows upon a period of rest. 



In the first case, if the female has given birth to young, 

 copulation will usually take place, if she is put with the 

 male, within from five to twenty-four hours. 4 In my own 

 experience I have found that the greater number of births 

 take place in the early morning, and that copulation will, 

 in such a case, occur from seven to eight o'clock m the 

 evening. The female is not invariably in heat at this 

 time, however, as has been shown by a considerable 

 number of cases which I have observed. 



The second period in which copulation may be expected 

 is after a mother has gone through an interval of rest, 



