14 BULLETIN 1090, U. S. DEPARTMENT OE AGRICULTURE. 



All the factors which have been considered — constitutional vigor, 

 heredity, season, age of dam, and interval since preceding litter — if 

 combined, determine the size of litter only to a small extent, prob- 

 ably less than one-tenth. The determination of the size of any par- 

 ticular litter must be due largely to rather temporary conditions. 

 The immediate direction of change in the condition of the dam at a 

 critical period, for example, may be the important factor. From 

 the standpoint of the condition of the female, during an appreciable 

 period of time, it appears that variations in size of litter are largely 

 a matter of chance. The most vigorous female may have a litter of 

 1 under what seems the best of conditions, and a litter of 4 may be 

 born when everything seems opposed. 



MORTALITY AMONG THE YOUNG. 



Guinea pigs may be born dead for a variety of reasons. A large 

 percentage of those classified as born dead are born prematurely, 

 and the average weight is much less than that of those born alive. 

 There are, however, not infrequent cases of animals which are unu- 

 sually large at birth, but are found dead apparently because of dif- 

 ficulties in parturition. Many of those classified as born dead un- 

 doubtedly were born alive but died before being recorded. In gen- 

 eral the percentage born alive obviously depends largely on the 

 health of the dam. Unfavorable environmental conditions act in 

 the main indirectly on the young. The inherent vigor of the young, 

 however, plays some part, as is shown by the improvement in the 

 percentage born alive when inbred females are mated with unre- 

 lated males, instead of with their brothers, a point which will be 

 discussed in a bulletin to follow. 



The percentage of the young which is raised among those born 

 alive also depends much on the health of the dam, but to a less ex- 

 tent than the percentage born alive. There have been cases in 

 which the young reached 33 days largely through their own efforts, 

 the mother having died a few days after their birth. Environmental 

 influences act on the young directly as well as indirectly in this case. 

 The inherent vigor of the young counts for much. Thus we find 

 that the percentage raised of those born alive shows a more marked 

 improvement when inbred females are crossed with unrelated males 

 instead of with brothers than is the case with the percentage born 

 alive. 



Both of these percentages reflect closely the changes in environ- 

 mental conditions. Inadequate or inferior green feed causes a large 

 number of stillbirths and an increased mortality among the living 

 young. A change from alfalfa to timothy- hay has been observed 

 to have the same effects. The records are usually much better in 

 both respects in summer than in winter. In years in which one of 



