530 
Journal of Agricultural Research voi. xxvn, No. a 
CARE OF YOUNG 
The young are hairless and weigh only about 3 gm. each when born, 
with closed eyes and ears and no trace of teeth. They grow rapidly, 
however, gaining after the first few days about 1 gm. a day until over 
half grown. Their dark-colored fur begins to appear as soft velvet in 
five or six days, their incisor teeth about the fifth, the molar teeth about 
the seventh, and their eyes and ears open on or near the eighth day. As 
soon as their eyes are open they are quick to run and hide if disturbed, 
and a few days later are out of the nest searching for food, following the 
trails, and, if in cages, running on the wheels, playing and pushing for 
their rights. 
When about 12 days old, the young are weaned, but they remain with 
their mother, occupying the old nest and holding together in friendly 
family relations until time for the next installment of young, when the 
mother seeks or builds a new nest and leaves her previous family to care 
for itself. If food were abundant, they would remain contentedly 
together for an indefinite time but for the disturbing sexual forces which 
before they are full grown impel the females to seek new homes for pro¬ 
spective offspring and the males to wander constantly in search of one 
mate after another. 
FACTORS MODIFYING BREEDING 
Factors modifying breeding activities are food, weather, cover, 
proximity, and contentment. A group of 9 mice, 5 females of breeding 
age and 4 males, kept in roomy cages and starved for 36 days until 
hungry, thin, cross, and squealing like half-fed pigs, showed no signs of 
breeding until at the close of the period when the quantity and quality 
of their food had been brought up to a satisfying point. The females, 
fighting the males away, kept them constantly cowed and in fear of their 
lives and actually killed and partly ate one old male, the largest animal in 
the cage. They also killed and ate one quarter-grown young at the time 
When famine was sorest. As their food was increased and changed to 
more nutritious quality the females rapidly yielded to male attention 
and all were quiet and contented again. Even for this short period, 
others in cages where well fed had raised one family of young and had the 
next well under way. It seems highly probable that a natural shortage 
of food in a dry year or some other cause might retard breeding for a 
much longer or for an indefinite period. The three litters of young 
afterward bom to the starved females numbered only 3, 4, and 5, while at 
the same time, in cages well supplied with food, 5, 6, and 8 were the 
usual numbers. 
Weather apparently has an effect on the rate of breeding, for while 
under certain conditions the mice breed in winter, their breeding activi¬ 
ties are generally much retarded then or entirely suspended. Of great 
numbers of specimens collected at all seasons of the year, few females are 
found to contain embryos in winter except in warm southern climates or 
Where there is unusual protection. In Minnesota tiny naked young are 
sometimes found in nests under haystacks in midwinter, but in less 
Sheltered localities with only the normal winter food they are rarely 
found. While the reduced winter food supply may be partly responsible 
for the general suspension of winter breeding, the excessive cold seems 
undoubtedly a potent factor. The continuous hot weather of the Tropics 
