10 



INHERITANCE IN GUINEA-PIGS. 



24 times; litters of 1 have been recorded 18 times, litters of 3, 10 times, 

 and a litter of 4 once. Factors which influence size of litter are evi- 

 dently age and state of nourishment of the mother. Table 2 shows the 

 relation of age of mother to size of litter. Very young mothers (age 

 4 months or less) have only 1 young at a birth. The females become 

 sexually mature at a very early age, as do female guinea-pigs. Well- 

 nourished females may breed at 2 months of age, when they are less 

 than half -grown, full growth not being attained until they are 12 or 13 

 months old. Females over 4 months but under 12 months of age 

 produce usually 1 or 2 young at a birth, rarely 3 ; those which are 1 or 

 2 years old produce the maximum number of young, usually 2 or 3, 

 rarely 1 or 4. After the age of 2 years the number of young again 



TABLE 2. Relation between age of mother and size of litter, Cavia cutleri. 



decreases to 1 or 2. The oldest female known to have borne young 

 (one of the original stock) had at the time been in captivity over 2 

 years and her estimated age was 32 months. None of the females 

 born in captivity has given birth to young at a more advanced age 

 than 29 months. Our records accordingly indicate that females rarely 

 breed after they have attained the age of 1\ years. The duration of 

 the breeding period in the case of males is more extended. It is prob- 

 able that males do not attain sexual maturity quite so early as females, 

 for females may breed when less than 2 months old, but we have no 

 evidence that males can breed before they are 3 months old. 1 But the 

 capacity to breed once attained continues indefinitely. One male 

 (d"4) caught wild in December 1911 and estimated then to have been 

 6 months old is still siring young, more than 3 years after his capture, 

 being, it is estimated, nearly 4 years old. 



Females are capable of breeding again immediately after the birth 

 of a litter, but if they do so the number of young at the next birth is 



! Mr. Wright has called my attention to a record from his experiments which shows that a male 

 guinea-pig containing a slight infusion of rufescens blood must have been sexually mature at 2^ 

 months of age. This is the only record known to me of a guinea-pig male breeding when less 

 than 3 months old. 



