10 BULLETII<J" 1090, U. S. DEPAEniEXT OF AGKICULTUEE. 



Under the head of fertihty, both the size and frequency of litters 

 have been considered. Little attempt has been made to distinguish 

 complete and partial sterility, as cases of complete sterihty have been 

 rare and uncertain in all stocks. The failure of a mating to produce 

 litters has usually been due to the early death of one member of the 

 pair. 



The data on the rate of growth up to the age of weaning (33 days) 

 are naturally much more extensive than those on later growth, and 

 have been studied in more detail. The principal characters which 

 are used in this connection are the weight at birth of all of the young 

 bom, the birth weight of those which survive to 33 days, and the gain 

 between birth and 33 days. 



The losses among the young are considered under two heads, death 

 at or before birth and death between birth and weaning. The char- 

 acters used are the percentage born and found alive and the percent- 

 age raised to 33 days of those born alive. The product of these two, 

 namely, the total percentage raised, is also used. 



LIFE HISTORY. 



Guinea pigs are born in litters of 1 to 9. Litters of from 2 to 4 are 

 most common, and litters of more than 6 have been decidedly rare in 

 the present work. The young are bom in a very advanced state of 

 development, with thick fur, open eyes, and the ability to run about 

 at once. They soon begin nibbling at the leaves of cabbage or other 

 green food in the pen. They are, in short, rather better able to take 

 care of themselves from the time of birth than the young of any other 

 familiar domesticated mammal. They grow rapidly and reach about 

 half the adult weight when between 2 and 3 months of age. The final 

 weight is nearly reached at a year, but there is slow growth for a 

 longer time. Guinea pigs are in their prime between 1 and 3 years. 

 After reaching 3 years there seems to be a distinct decline. The 

 present experiments, however, have not been designed to study 

 longevity, as matings have often been disposed of to make room for 

 those of a later generation. The oldest dam recorded had reached 

 47 months. The average age of dams has been between 14 and 1^ 

 months. One female is reported to have died at an age of 59 months. 

 Her last litter was born at 38 months. 



Sexual maturity is reached early. In nearly every family and ex- 

 periment there have been a few cases in which females have had 

 young when about 100 days old. As the gestation period is about 68 

 days, this means that these litters were sired by the dam's sire before 

 she was weaned at 33 days. The minunum age at which a male 

 may sire a litter seems to be about 60 days, although we have one 

 apparently reliable record at about 48 days (litter born at 117 days). 



