L I T T E R SIZE, B I R T H WEIGHT, AND EARLY GROWTH RATE OF MICE ( m sMUSCULUS) W. H. GATES Louisiana S t a t e EnCGersity, B a t o n Rouge, Louisiana FOUR CHARTS The following work was done in the Department of Genetics of the Carnegie Institution, Cold Spring Harbor, Long Island, during the summer of 1922, a t the suggestion of Dr. C. C. Little, to whom thanks are due for helpful advice and the use of his material. Dr. TV. E. Castle also has kindly given many helpful suggestions concerning the analysis of the data. Credit is due Miss H. Ulman for valuable aid in the mechanical operations of weighing and recording. The mice involved in this experiment were from the laboratory stock of Doctor Little and included the litters born from the 21st of J u n e to the 1st of August, 1922, inclusive. The object of the experiment was to secure individual growth curves f o r the mice from the time of birth u p to the time of weaning, three weeks after birth. The method of procedure was as follows: All pens coiitaining pregnant mice were carefully gone over daily by Doctor Little or his assistants and all new litters recorded. The record of these was then handed to me. This record included all young born before 9 o’clock in the morning. Ti1 a few instances, where parturition was not complete by that hour, the litter was weighed and recorded later in the day. TThen the record of the litters born had been received, each litter was examined separately and each individual was marked and then weighed. Many different methods of marking were tried out before a satisfactory one mas found. Staining with various dyes 183 T H E ANATOMIC4L RECORD, VOL. 29, NO. 3 JANUARY, 192.5 184 W. H. GATES was found not to be satisfactory, as the stains were either worn off or licked off by the mother. Where the stain was sufficient to stay for a day or more, it was lost soon after, due to the shedding of the epidermis and the growth of the hair. Finally, branding with a hot iron was tried and found to be very satisfactory. I n fact, so much so, that several months later Doctor I\IacDowell, who took over many of Doctor Little’s mice, became greatly puzzled to find irregular spots of aberrant color occurring on the pelage of some of the mice, which were later found to have resulted from the branding. The number of each individual was indicated by branding at a certain place on the body. Zero of each litter did not receive a brand. One, two, and three were branded, respectively, on the left shoulder, left rib, and left hip; four, five, and six were branded on the right shoulder, rib, and hip; seven, eight, and nine between the shoulders, the middle of the back, and at the base of the tail; ten, eleven, and twelve were a t first branded in three successive positions on the tail, but this was later given up as sometimes the young would lose their tails, so that for these higher numbers double brands were made, on tu7o of the regular positions. The branding was done with a fine entomological scalpel, the tip of which had been tempered and bent a t right angles. This gave a small flat surface which could be touched t o the skin without danger of injuring too large a surface. The scalpel was heated almost to the red hot point on an alcohol lamp and the mouse was quickly touched at the required spot. I n nearly all cases the brand healed within twenty-four hours and left a clean scar, which, of course, remained visible throughout the life of the mouse. Frequently, as the mouse grew and hair covered the body, the brand became covered, but it could always be found by parting the hair. To facilitate identification, after the second week the hair was cut around the scar, so as to save the time in hunting the brand. Thus every individual in the experiment retained its identity until it was weaned or mated. G I t O W T H O F YOTiXG MICE 185 The experiment in this particular report included 106 litters containing over 700 mice. The young were weighed every other day-that is, at twoday intervals. Each individual was given a card on which was recorded the weights, date of birth, litter number, the number of dam, and also the total number in each litter, the number of generations of brother-and-sister matings and any other peculiarity o r abnormality. As any other interesting coridition developed, it was also recorded. Earlier writers, C u h o t and Schultze in mice, and C u h o t ( W ) , King ( %), King and Stotsenburg ( '15) in rats, have noted that in these rodents the sex ratio shows an excess of males varying from 106 to 110.9 males t o 100 females in mice, and from 104.6 t o 107.5 males to 100 females in the rat. King ('24) concludes that 105.2 : 100 -+ 2.00 is probably the normal f o r albino rats, and the average for all strains of 1.35. Nus norvegicus is 105.7 males to 100 females I n this experiment there were 324 males and 328 females, o r a ratio of 98.78 males to 100 females 5.21. The sex in every case mas recorded a t the first weighing and was later checked on the ninth o r eleventh day. As this experiment included litters born during a relatively short length of time, this ratio may not, arid probably does not, represent the normal f o r extended periods of time o r larger populations. The size of the litters shows considerable variation, the numher in each litter varying from two to twelve. The general average, however, of the 106 litters is 7.4 -+ .14. The size of litters with the highest frequency being six, seven, eight, nine, and ten; these size litters containing 79 per cent of all the individuals (table 1and fig. 1). I n computing mortality statistics, particular care was exercised not to include individuals which showed that death was due to accident o r some unnatural cause. I n reckoning time, the day of birth is considered as the first day. 186 U ' . IT. GATES A study of the mortality during growth (table 2) shows just what one might expect-a rather high rate during the first two days, then a slight decrease, followed by a gradual rise up to about the seventeenth day, followed by a rather sudden and decided drop up to weaning. This is shown graphically in figure 2, the data for which are found in table 2. The high mortality f o r a day or so immediately following birth is to be expected. The gradual rise in mortality during the next tu7o weeks is to be accounted for probably by the crowding of the young and the lack of sufficient nourishment TABLE I TABLE 2 Latter size Mortality ~~~ \ I Y B F K I N FAVH LITIFR h i v n b H OF I IT1 E.Rb 2 3 4 1 3 4 6 3 1 9 6 17 7 16 a 17 9 18 10 11 11 1 12 6 Average, 7.4 & .14 ) P E K CENT IJtYS 7 9 11 13 1.i 17 19 1' 678 674 671 0 10 656 6 16 62 7 611 ,579 3 .2i rill 496 471 4 11 16 24 34 13 4 0.0 1.4 0.5 0.8 2.4 1.7 2.6 4.1 6.1 2.9 0.8 f o r all. This seemed t o be the case particularly in the larger litters. A study of the mortality is interesting also from the fact that the young mice were liaiidled a good deal. On the day of birth the young were first taken out of the cage, examined closely by Doctor Little or his assistants, a little later they were taken to the weigliing room, branded, and weighed individually, and then returned. All of this more or less rough treatment and interfering with the duties of the mother did not sccm to affect the mortality, because if it were going to have any effect a t all it would be most likely t o show on the very young ; whereas, in reality, the highest mortality rate 187 GROWTH O F YOUNG MICE was at the end of the second week at a time when the young were certainly old enough and strong enough not to be af'fected by handling. The highest death rate came about the time the mice were beginning to take care of themselves and .-- LITTER SIZE Dota in table1 15 E s- 10 4 0 z0 5 2 3 4 5 6 7 NO in 0 9 1 0 1 1 1 2 1 3 litter Figiire 1 Deuths oL Mortality t o feed independently. They quiclrly became accustomed to the change in diet, however, and in a few days were on tlieii feet and began t o increase in weight. Stillbirths were not included in any p a r t of the work. 188 \V. 13. GATES The total average rate of growth is shown graphically in figure 3 (data in table 3). Note tlie rapid increase in weight daily up to the ninth day, then a gradual lessening up to tbc seventeeiitli day, and then a final pick up again, np to the time of weaning. Considering these increases from the standpoint of percentage of gain to that of weight, we find that the greatest per cent of iiicrcase is from the third to the fifth day (table 3, fig. 3 ) . As stated above, the probable reason f o r tlie subsequent decrease may be explained on the ground of insufficieiit food and perhaps crowding in some litters. Note the T41jLE ? Growth weaghis, totals ~ ~~ D LPCI ~ 51). I N D . I\ EIGHC GAIN ~ 1 3 > 7 9 11 13 13 17 19 21 1.3ti tii8 674 671 6.5ti G27 611 379 1.80 4.4.5 3.21 3.94 4.;i3 5-58 G.T,G .44 .Gti .76 .73 .61 .R7 .34 511 496 471 .>.so .34 G.37 6.89 .47 52 5.12 32 36 31 20, 13 12 G G 8 8 final increase both in actual weight and percentage of gain to body weight a s the animals begin to feed independently. Comparing the males aiid females, we find that the general average weight at birth is tlic same. Miller ('11) reports that tlie average weight a t birth of males of the Norway rat is in excess of that of the females. King ( ' G , '23) also reports a relative weight of 5.34 grams for males and 5.09 grams f o r females in the Norway rat, and a relative weight of 4.69 grams and 4.5 grams for the albino rat, male and female, respectively, at birth. This experiment shows a practical equality of weight at birth of the males and females in mice. GROWTH O F YOUXG MICE 189 190 W. H. GATES The weight recorded as at birth is that after the young liave nursed. The actual birth weight would be slightly lower. This weight, however, is taken because it represents a similar condition in all subsequent weighings. It is interesting to note, however (table 4 and fig. 4), that from the third day on the females maintain a slight advantage over the males not only in actual weight, but i n the amount of gain per day. From the nineteenth day, however, they begin to lose their advantage and the males tend t o overtake them. If the experiment had been carried further, no doubt the males wonld sooner or later have outweighed the females. 1 3 1 7 9 11 13 1 .? 1i 19 21 324 330 328 326 320 313 “99 286’ 266 262 243 1.38 1.79 2.46 319 3.89 4.50 3.06 3.49 3.80 6.22 6.78 .41 .67 .72 .70 .60 .53 .43 .31 .41 .56 29 37 29 21 15 12 8 328 323 325 320 306 298 278 266 ) 048 7 9 239 228 1.38 1.88 2.32 3.27 4.00 4.60 5.17 5.64 6.00 6.43 7.01 50 .63 .7n .72 .60 .57 .16 .33 .43 37 36 33 29 22 1.5 12 8 6 7 8 King (’1!9-’24) finds similar conclitio’ns in the early life of both the Norway and albino rat. Table 5 shows a n interesting relation between the weight of the young and the size of the litter. The smaller the litter, the greater is the first-day weight of the young. This may he due in p a r t to their having received a greater per capita amount of milk on the first day, in p a r t to their having received more nourishment in utero. The maximum average first-day weight is that f o r the single litter which consisted of two young. The first-day weight then decreases steadily, a s the number in the litter increases, until it reaches a minimum in litters of nine, beyond which point it is prac- G R O W T H O F YOUNG MICE 192 W. R. GATES tically stationary. Tlie average weight a t twenty-one days is also largely influenced by litter size, as one might expect from the combined influence of a greater amount of nourishment in utero and in lactation to individuals born in small litters. No differential mortality is observed between large and small litters. I n general, individuals born in small litters are larger at birth and at twenty-one days than those lion1 in large litters, but it is not t o be supposed that this initial advantage mould result in greater adult weight. (See Castle ( ' 2 2 ) , who has investigated this matter in the case of rabbits.) TABLE 5 Kclation between latter s w e and wezght, ut bzrth anti on twenty-first day -~ \CVRI~ILI\ ~ IIICIR ' ~~ 2 ~ 3 4 , 5 6 7 1 8 1 9 10 11 12 ~ I ' 1.87 j1.78 1.66 >kverngeaeiglrt 1st day Average atlight 21st day 6.85 ,8.55 8.76 Kumber yonng 1st day 2 11 21 Sumt)eryoung 21stdar . 2 6 1 11 Pcr cent diminution. . . . 0 I 45 47 I 1.42 7.35 40 30 25 1.42 1.41 1 33 1.27 1 34 1.33 6 $157.20 6.92 6.47 6.54 6 30 90 96 129 134 97 11 ti5 61 91 80 74 10 27 36 29 48 23 9 I 1.32 6 i9 70 40 42 SUMMARY The statistics in this paper involve 106 litters containing over 700 individuals of mice born during the summer of 1922. The sex ratio is practically 100 : 100. The relative birth weights of males and females are equal. The rate of growth of the females is slightly more rapid than that of the males, at least for the first three weeks. The mortality is greatest as the young begin to stop nursing. The rate of growth shows a gradual increase at first, followed by a slackening about the nineteenth day, and a final increase as the young become independent. Litter size influences weight of the individuals both on the first day and on the twenty-first day after birth, through the availahle food supply, but there is no differential mortality favoring either small or large litters. GElOWTH O F Y O U N G MICE 193 L I T E R A T U R E CITED CASTLE,W. E. 1!)22 Size inherit:ince in rabbit crosses. Pnbl. 110. 320. Carnegie Inst., Wash., Pt. I. h C x o r r , L. lS9!1 Sur 1:i (1ctennin:ition du s e w rliez les :inim:iux. Bull. Sci. de la France e t d e la Belgique, T. 32. KING,H E L ~ S 1)u.N 1918 The eff'ects of inbreeding on t h e growth and vari:iljility in the ljody weight of t h e nlbino rat. Jour. Exp. Zool., vol 2 6 . 1922 The growth and variability in the bocly weight of t h e Norway rat. Anat. Rec., vol. 25. 1924 Litter production and the sex ratio i n various str:iins of rats. Anat. liee., vol. 27. Kisc;, 1%.D., AND STOTSENBUBG, J . M. 1915 On the normal sex ratio and t h e size of the litter i n the albino rat. Anat. Rec., vol. 9. ~ I I L L EIf. R , 1911 Reproduction in the brown rat. Am. Nat., vol. 43.