m. 645] FECUNDITY IN THE GUINEA PIG 351 



tween attempted matings were shortened, with the idea 

 that heat might be recurring unnoticed, but mating never 

 occurred at other intervals. It is doubtful whether any- 

 definite rhythm is maintained by old guinea pigs, for pig 

 No. 9, which was observed to be in heat December 27, 

 was not in heat again until 49 days later. Animal No. 

 20 was in heat October 17 and heat did not return until 

 91 days later. In another instance heat returned after 

 118 days. However, since the age of these animals is not 

 known, it is impossible to be sure that these irregularities 

 are due to senility. 



Bischoi¥, '44, stated that copulation in the guinea pig 

 occurs within 3 hours after parturition. In four cases in 

 which he prevented copulation heat returned after inter- 

 vals of 40, 50, 51, and 51 days. Hensen, '76, and Eein, '83, 

 claimed that the most favorable time for copulation is 

 within one hour after parturition. I observed copulation 

 in 12 animals immediately after parturition. Matings 

 were attempted at one-hour intervals for six hours after- 

 ward. In four cases I was unable to mate the females at 

 this time. They were found in heat again 34, 36, 81, and 

 120 days later. The first two animals were about six 

 months old. The last two were very old, judging by their 

 teeth. Two females mated 1 hour after parturition, 2 

 after 2 hours, 1 after 3 hours, 1 after 5 hours, and 2 after 

 6 hours. In three cases no pregnancy resulted and heat 

 returned in 31, 31 and 29 days. 



Many writers have reported that females refuse the 

 male shortly after the first copulation. The inference is 

 that some nervous mechanism automatically terminates 

 heat soon after copulation. Instances have been reported 

 in which the female refused the male 20 minutes after the 

 first copulation. In observations extending over nearly a 

 year, however, three cases were observed in which the 

 female mated again eight hours after the first copulation. 

 In the majority of cases the female permitted copulation 

 three hours after the first mating. One animal mated 13 

 times in an interval of 8 hours. It seems that a female 

 accepts the male at any time during the first stage of heat 

 regardless of any previous intercourse, but apparently 



