100 GENETIC STUDIES ON A CAVY SPECIES CROSS. 



20. THE FECUNDITY OF THE FEMALE HYBRIDS. 



Almost every female hybrid in each generation was fertile in breeding. 

 The only exceptional generation was the | wild, in which the only 

 female was sterile. Occasionally a female hybrid was sterile, but such 

 cases were infrequent. Although no data were taken on sterility in 

 guinea-pigs, I am of the opinion that sterility in the female hybrids 

 was no more frequent than in these. There was at least one source of 

 data which gave information on the degree of fecundity in the female 

 hybrids — the average number of young per litter (see table 80). 



The wUd C. rufescens, bred in captivity, gave 46 offspring in 34 litters, 

 or an average of 1.35 per litter. We do not know what their average 

 per litter is in the wild habitat. The tame guinea-pigs, used as dams 

 in matings with wild sires to produce the | wild hybrids, gave 37 young 

 in 16 litters, or an average of 2.31 per litter. This shows that the wild 

 males impregnated the guinea-pigs just as successfully as a guinea-pig 

 male would have; for the average per litter in our guinea-pigs was 

 2.34. Minot (1891) found an average of 2.56 in his experiments with 

 tame guinea-pigs, but his numbers were smaller (see table 80). 



The Fi hybrids were intermediate, for they produced 83 young in 

 52 litters, or an average of 1.60. In fact, they were a little less fecund 

 than a theoretical midparental condition would demand, for this would 

 be 1.845. They were about as "wild," to all appearances, as the pure 

 wUd females, but were slightly more prolific. The Fz hybrid females, 

 the J wild, produced 217 young in 114 htters, or an average of 1.90 

 per litter. The F3 hybrid females produced 312 offspring in 152 litters, 

 or an average of 2.05. The subsequent hybrid generations did not show 

 an increased average, although they were produced by successive back- 

 crosses to the guinea-pig male. 



The analysis of these data is complicated by a number of conditions. 

 The guinea-pigs raised in our laboratory gave larger litters in summer 

 than in winter; for in summer they produced 218 young in 85 litters, 

 or an average of 2.56, whereas in winter they produced 266 young in 

 122 litters, an average of 2. 16 per litter. The young born from January 

 15 to July 15 were considered winter Utters in these data, because the 

 ovulations and conceptions corresponding to these births ranged from 

 about November 8 to May 8. Minot (1891) found a similar condition 

 in his experiments. 



Minot also found that the first litters were smaller than the average; 

 but first litters are usually borne by young females and it noay mean 

 that the smallness of first litters is entirely an effect of age. This may 

 account for the fact that our F4, Fs, and Fe females failed to show an 

 increased average per litter, since many of the female hybrids in these 

 generations were young, and the records contain a large proportion of 

 litters from such females. 



