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THE AMERICAN NATURALIST [Vol.LVI 



she permits matings somewhat reluctantly after this. ' In- 

 stead of assuming the position for copulation when ap- 

 proached by the male she often runs around the cage and 

 resists vigorously. Unless the male is very persisten 

 and active copulation will not occur. One female resisted 

 a second coitus for fifteen minutes by kicking, snapping, 

 etc., only to stop suddenly and take the position for copu- 

 lation. This behavior of the female may be due to pre- 

 vious mating or it may simply mean that the period o: 

 heat is subsiding. I am inclined to the latter view, be 

 cause we have encountered many females among animals 

 which had not been previously mated, who resisted the 

 males vigorously for a time, only to yield in the end. The 

 time during which the females permitted copulation un- 

 hesitatingly was a relatively short one, but after this 

 phase had passed the animal might yet be mated if the 

 male was persistent. 



Stockard and Papanicolaou, '17, are of the opinion that 

 among domesticated guinea pigs only a slight seasonal 

 variation exists in the occurrence of heat, but in the pres- 

 ent series of guinea pigs the fall months were the most 

 favorable for matings, as shown by the following table: 



The males seemed to be partly responsible for this wide 

 variation. During the winter months they were lethargic 

 and indifferent. When placed in a pen with a female 

 known to be in heat, the male often ignored her, eating 

 unconcernedly instead. In many instances several males 

 had to be placed with such a pig, in succession, before 

 a mating took place. This is in marked contrast to the 

 customary behavior, for when placed in a pen with two 



