1912] Read: Suckling Period in the Guinea-Pig 345 
The weights of Bb and Be given in table IV, columns five and 
six, show that these two animals suffered no loss of weight. 
So it is observed that there are eases which do not conform to 
the rule. But there is in all mammals a post-natal retardation 
of growth while the newly born animal is adapting itself to its 
new environment. The point to be borne in mind is that this 
retardation is very slight in the case of the guinea-pig and 
amounts to an actual loss of weight in the case of the males only. 
Minot further observed that ‘‘ Birth retards growth from two 
to five days.’’ This being the case, a young guinea-pig, especially 
a male (which actually loses weight), would be at considerable 
disadvantage if weaned within this period of retardation, or even 
shortly after it. 
The normal time at which mice, rabbits, and guinea-pigs 
should be weaned is about three weeks. With the last-named 
animal, however, there is considerable variation in this respect. 
I have observed a mother which allowed a young one to suckle 
when it was four or five weeks old. Another animal was seen 
to kick the young away and refuse to let them suckle at sixteen 
days. My animals, except those considered in detail in this paper, 
were weaned at twenty-one days. For future comparison I will 
add that a suckling guinea-pig doubles its weight in about 
eighteen days after birth. 
In view of the fact that the young begin to eat solid food 
when three or four days old and that there seems to be no definite 
time at which the mother weans them, it was decided to determine 
how soon after birth the young could be weaned and live. 
The first young one studied (known as W) was a normal 
animal weighing at birth 77 grams, which is two grams above the 
average. Its weight upon successive days is shown in the second 
column of table IV. It was taken away from the mother at the 
end of ten days. Two days after weaning it had lost 2.4 per cent 
of its weight, but four days after weaning it was 3 grams (2.4 
per cent) heavier than when first separated from the mother. 
