344 University of California Publications in Zoology  |Vou.9 
It has already been shown that the percentage of fat in the 
guinea-pig’s milk is high, but this does not necessarily prove 
that the young receive a great amount of fat, for the amount of 
milk might be small. From the above table, however, it is quite 
evident that each young one in a litter of two receives about 
30 grams of fat from the mother’s milk, if, as we assume, all the 
accumulated fat leaves the body in the milk. Besides this it gets 
fat from the same source which supphes the fatty food utilized 
by the mother. So the relative amount of fat received by the 
nursing guinea-pig is much greater than that received by an 
adult. 
It remains to be shown why the young guinea-pig needs more 
fat than the young of such animals, for example, as the rabbit. 
Rabbits, rats, and mice are born in large litters and so the 
young help to keep each other warm. These animals also build 
nests in which the mother, for purposes of suckling the young, 
stays a large part of the time. On the other hand the guinea-pig is 
rarely born in htters of more than two or three and quite fre- 
quently there is only one. The young are provided with a coat 
of fur at birth and run about at once, the mother building no 
nest before they are born. They derive little heat from the body 
of the mother, for they spend a very small portion of the time 
suckling, but the milk which they do get is almost half fat, a 
food of ‘‘high ealorifie value.’’ 
Another point in which guinea-pigs differ from most mammals 
is pointed out by Minot (1891, p. 118) who says ‘‘Male guinea- 
pigs lose weight as do new-born children for a variable period of 
’ 
a few days after birth.’’ Females do not lose weight, as may 
be observed by inspection of table III, giving the average weights 
of male and female guinea-pigs for the first three weeks. 
TABLE III 
Number of 0 1 2 3 
observations days day days days 
Males 15 76.6gm. 75.0gm. 76.6gm. 79.8 gm. 
Females 13 68.0gm. 718gm. 75.0gm. 76.7 gm 
It may be said that there are cases in which the male does 
not lose weight after birth; indeed I have seen individual cases 
in which there was an actual gain in the weight of the male. 
