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 maximum 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 appears 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, and the only safe statement, in the light of my observations, is that the mother dances less than usual for a few days after a litter is born to her.
The development of the young, as I have observed it in the cases of twenty litters, for ten of which (Table I) systematic daily records were kept, may be sketched as follows. At birth the mice have a rosy pink skin which is devoid of hair and perfectly smooth; they are blind, deaf, and irresponsive to stimulation of the vibrissae on the nose. During the first week of post-natal life the members of a litter remain closely huddled together in the nest, and no dance movements are exhibited. The mother stays with them most of the time. On the fourth or fifth day colorless hairs are visible, and by the end of the week the body is covered with a coat which rapidly assumes the characteristic black and white markings of the race. For the first few days the hind legs are too weak to support the body weight, and whatever movements appear are the result of the use of the fore legs. As soon as the young mice are able to stand, circling movements are exhibited, and by the end of the second week they are pronounced. Somewhere about the tenth day the appearance of the teats in the case of the females serves to distinguish the sexes plainly. Between the tenth and fifteenth days excitability, as indicated by restless jerky movements in the presence of a disturbing condition, increases markedly; the auditory meatus opens, and, in the case of some individuals, there are signs of hearing. On or after the fifteenth day the eyes open and the efforts to escape from the nest box rapidly become more vigorous. About this time the mother resumes her dancing with customary vigor, and the young, when they have opportunity, begin to eat of the food which is given to her. They now dance essentially as do the adults. From the end of the third week growth continues without noteworthy external changes until sexual maturity is attained, between the fourth and the sixth week. For several weeks after they are sexually mature the mice continue to increase in size.
TABLE I
DEVELOPMENT OF THE YOUNG
NUMBER JERKY REACT IN HAIR TEATS MOVE- EARS TO EYES PARENTS LITTER VISIBLE VISIBLE MENTS OPEN SOUND OPEN M F APPEAR
152+151 5 0 4th day — 13th day 14th day 14th day 16th day 152+151 1 3 4th day 9th day 10th day 12th day 13th day 15th day 410+415 4 1 5th day 11th day 14th day 15th day 15th day 17th day 410+415 2 4 5th day 10th day 13th day 14th day 14th day 16th day 420+425 0 2 4th day 10th day 12th day 14th day 14th day 16th day 210+215 4 1 — — 17th day 13th day 17th day 15th day 210+215 3 3 5th day 11th day 11th day 14th day No 16th day 212+211 1 3 4th day 10th day 15th day 14th day No 15th day 220+225 2 4 4th day 10th day 16th day 14th day No 15th day 220+225 3 3 4th day 10th day 17th day 13th day No 15th day
A course of development very similar to that just described was observed by Alexander and Kreidl (3 p. 565) in three litters of dancing mice which contained 3, 5, and 7 individuals respectively. These authors, in comparing the development of the dancer with that of the common mouse, say that at birth the young in both cases are about 24 mm. in length. The young common mouse grows much more rapidly than the dancer, and by the ninth day its length is about 43 mm. as compared with 31 mm. in the case of the dancer. According to Zoth (31 p. 148) the adult dancer has a body length of from 7 to 7.5 cm., a length from tip of nose to tip of tail of from 12 to 13 cm., and a weight of about 18 grams. The movement of the dancer from the first tends to take the form of circles toward the middle of the nest; that of the common mouse has no definite tendency as to direction. When the common mouse does move in circles, it goes first in one direction, then in the other, and not for any considerable period in one direction as does the true dancer. Neither the young dancer nor the common mouse is able to equilibrate itself well for the first few days after birth, but the latter can follow a narrow path with far greater accuracy and steadiness than the former. The uncertain and irregular movements of the common mouse are due to muscular weakness and to blindness, but the bizarre movements of the young dancer seem to demand some additional facts as an explanation.
A brief account of the development of the dancer given by Zoth (31 p. 149) adds nothing of importance to the description given by Alexander and Kreidl. As my own observations disagree with their accounts in certain respects, I shall now give, in the form of a diary, a description of the important changes observed from day to day in a normal litter. The litter which I have selected as typical of the course of development in the dancer grew rapidly under favorable conditions. I have observed many litters which passed through the various stages of development mentioned in this description anywhere from a day to a week later. This was usually due to some such obviously unfavorable condition as too little food or slight digestive or bowel troubles. According to the nature of the conditions of growth the eyes of the dancer open anywhere from the fourteenth to the twentieth day. This statement may serve to indicate the degree of variability as to the time at which a given stage of development is reached by different litters.