Feeding, Breeding, and Development 21 



The sex of individuals may be determined easily in most 

 cases, at the age of 10 to 12 days, by the appearance of teats 

 in the case of females. 



The period of gestation is from 18 to 21 days. The maxi- 

 mum number born by my dancers in any single litter was 

 9, the minimum number 3. In 25 litters of which I have 

 accurate records, 135 individuals were born, an average of 

 5.4. The average number of males per litter was precisely 

 the same, 2.7, as the number of females. 



On the birth of a litter it is well to see that the female has 

 made a nest from which the young are not likely to escape, 

 for at times, if the nest is carelessly made, they get out of it 

 or under some of the pieces of paper which are used in its 

 construction, and perish. Several times I have observed 

 nests so poorly built that almost all of the young perished 

 because they got too far away to find their way back to the 

 mother. It is surprising that the female should not take 

 more pains to keep her young safe by picking them up in 

 her mouth, as does the common mouse, and carrying them to a 

 place where they can obtain warmth and nourishment. This 

 I have never seen a dancing mouse do. For the first day 

 or two after the birth of a litter the female usually remains 

 in the nest box almost constantly and eats little. About the 

 second day she begins to eat ravenously, and for the next 

 three or four weeks she consumes at least twice as much 

 food as ordinarily. Alexander and Kreidl (3 p. 567) state 

 that the female does not dance during the first two weeks 

 after the birth of a litter, but my experience contradicts 

 their statement. There is a decreased amount of activity 

 during this period, and usually the whirling movement ap- 

 pears but rarely; but in some cases I have seen vigorous 

 and long-continued dancing within a few hours after the birth 

 of a litter. There is a wide range of variability in this matter, 



