Growth of the Albino rat with special reference to the influence of environment

Abstract

Observations were made on Albino rats over a period of four years (1933-37); 238 litters were obtained comprising 2043 young at birth; 1789 (872 males and 917 females) were reared and weighed weekly. PRENATAL GROWTH. (a) Oestrus and service. The length of the oestrous period was 4 days in 60 per cent of the cases observed, 3 days in 20 per cent, and 5 days in the remaining 20 per cent cases. The time that vaginal plugs were visible varied from 6 hours to 10 days after service, the average being 1 to 2 days. Nearly all females were served during the night or early morning. Quite a large number of the females that had bred, had infected uteri. Here the ovaries were inactive except when only one horn of the uterus was affected; then the ovaries were active but the females did not become pregnant when served. (b) Weights of pregnant females. Different classes of females were used in studying the changes in weight of pregnant females: (1) Those served when the vaginas opened, (2) old virgins 8 to 11 months old and weighing about 200 g. and (3) females that had already had one or more litters. Only those pregnant for the first time showed an appreciable gain during the first two weeks of pregnancy, while rats that had had litters showed practically no gains. The weights of all females, however, increased rapidly during the last 7 to 8 days of pregnancy which is mainly due to the increase in weight of the developing foetuses. Only those rats pregnant for the first time showed an appreciable gain (56 per cent.) after parturition as compared with weight at service. This increase in permanent gain was equal to the gain during the first two weeks of pregnancy. Litter size at birth had no influence on the gains of the females during the first two weeks, hut during the third week the gains of the females increased with increase in litter size. (c) Weight and number of foetuses. Females of the three classes mentioned above were killed at different stages of pregnancy. The pregnant uteri showed an increase in weight only from the 6th or 7th day onward after service. By means of the method we used, foetuses could be weighed only from the 13th day onward, whereas the number of foetuses could be determined macroscopically from the 7th to the 8th day of pregnancy. The three different classes of rats showed large differences in the number of prenatal deaths. In the case of the young females which were about 3 months old and pregnant for the time, there were only two dead foetuses out of 190, average number of living foetuses per litter 9.4. In the case of the old rats 8 to 11 months, pregnant for the first time and of those of more or less the same age but which had already had one or more litters, 38.5 per cent of dead foetuses were found during the last two weeks, in the former class the average being 4 living foetuses per female, in the latter 5. LITTER SIZES AND WEIGHTS AT BIRTH. During the first 24 hours the birth weights remained more or less constant, or could show a slight decrease for young born during the day. Mortality at birth was 1.4 per cent. This figure may be slightly higher since females often eat the stillborn. Average birth weights of males and females born alive were 5.19 g. and 4.96 g. respectively, and of stillborn 4.88 g. and 4.8 g. respectively. Average litter size was 7.4 but for litters in which there were stillbirths average size was 8.8. Sex ratio for all rats born was 94.5 per cent. It decreased from 163 to 77 as litter size increased from 3 to 9, then increased again to 129 as litter size increased to 12. Birth weights decreased with increase in litter size. After a litter size of 9 was reached, birth weights showed a tendency to remain constant. Birth weights increased with age of mothers while litter size decreased. When litter size remained constant then age of mothers did not show any regular effect on birth weights. Females 2 to 3 months old had average litter sizes of 8.4 and those 12 to 13 months old 4.7. This continuous decrease in average litter size from the youngest age class to the oldest, instead of the expected increase to a maximum followed by a decrease, may have been due to the greater prenatal mortality in the older females. Total litter weight decreased from 43.3 g. to 25.0 g. Age of mothers was more important in determining litter size than parity. The weight of the mothers immediately after parturition did not appear to influence the birth weights of the young when litter size remained constant. LITTER SIZES AND WEIGHTS AT WEANING. The average weaning percentage of all litters was 84.0. There was no pronounced drop with increase in litter size as was found in the pig for instance. The average birth weights of young weaned was 5.11 g., but for young that died shortly after birth only 4.87 g. Greatest postnatal mortality occurred during the first week after birth. Litter size at birth showed the same influence on weights at 2, 3 and 4 weeks as at birth, i.e., first a decreasing decline in average weight as litter size increased up to 9, then weights remained more or less constant up to the largest litter size (12). Although litter size at birth and at weaning showed practically a straight hue, the influence of litter size at weaning on the average weight was quite different and peculiar. There was first an increase, then a decrease, another increase and finally a pronounced decrease. The most likely explanation of this appears to be the effect on milk supply of rats of different ages and weights, which were not kept constant in this case. Rats in litters of which all were reared were heavier than rats in litters of which some were destroyed by the females. The young of females 8 to 9 months old were the heaviest at weaning, indicating that the milk supply was the highest at this period. This coincided more or less with the production of the third litter. As mentioned the weight of the mothers did not influence the average birth weights of the young. The weights of the mothers at parturition, however, had a decided influence on the average weight of the young at weaning. As the weight of the mother increased up to 260 g. the average weight of the young decreased, and after that increased again. This is probably due to difference in milk supply. It appeared that only in the case of young females did increase in weight of the mother accompany a decrease in the average weaning weight of the young, whereas in the case of the old females there was a tendency for the young to increase in weight with an increase of the weight of the mother. Young heavy females therefore appear to be poor milkers, probably due to excessive fat, while weight in old females is more likely due to greater muscle and bone development. Although the best females, i.e., those rearing the largest and heaviest litters, were retained, it does not appear that the fertility of the population had been raised during the four years. Selection was probably mainly for good mothers, poor mothers being discarded. This strain of albino rat has been in bred for so many generations that the stock is probably nearly homozygous for the number of eggs shed, showing that variation in litter size is probably only due to factors such as foetal atrophy. SEASONAL INFLUENCE. (1) Length of time females took to become pregnant. (2) Litter sizes and weights at birth and weaning. (3) Sexual maturity of females. (4) Weights and tail lengths. (5) Health. (1) From March to July, autumn and winter, the females remained long with males before they became pregnant, but from August to February it took a shorter time. After February there was a sudden increase which may have been caused by the decrease in length of days. Females appear to have been affected more than males. (2) The average birth weights decreased from 5.19 g. during the first year to 4.96 g. during the fourth year. Our average birth weights were appreciably lower than those obtained at the Wistar Institute. The average litter size was higher during the summer months, November to January (7.8), than during the winter months, May to July (6.8). Birth weights, when effect of litter size was eliminated, did not appear to have been affected. Weaning percentages showed a decided seasonal trend, being highest between May and June, then decreasing until August and remaining low until October, after which there was an increase. During the four years the weights at 2, 3 and 4 weeks have decreased. This may have been caused by the effect on milk production of the mothers as well as by the environmental influence directly. There were decided seasonal fluctuations which appeared to be about the same at 2, 3 and 4 weeks. The highest weights were obtained during the winter, with a decline during the spring and the lowest level coinciding with the warm months, October and November. The weights of the suckling females showed about the same seasonal trends as the weights of the young, so that milk supply may also have been affected. (3) The sexual maturity of the females was affected as follows. During the first year the vaginas opened when the females were 47.2 days old and weighing 100 g . and during the fourth year when they were 42.7 days old and weighing 95 g. (4) When the average weights of our rats are compared with those of the original stock at the Wistar Institute, our males are appreciably heavier at the different ages up to 20 weeks, while the females show practically no difference. There was a marked seasonal effect on the weights. The highest and lowest points shifted about 3 to 4 months as the rats become older - 4 to 16 weeks - indicating the influence of the month of birth, also that an early influence may persist until maturity. The highest weights for all ages were obtained for those born during May to August, then there was a decrease with the lowest weights from about October to February. It appears that after 8 weeks the ultimate weight of the rat is not affected any more, or only to a small extent, by the environment under which it grew up. Tail length was affected less than weight by the environment. Although temperature may have influenced the weights of the rats, it appears more likely that there may have been an effect by a combination of factors, probably temperature and humidity, while a change in the length of the days probably also affected the growth of the animals through the effect on the pituitary. (5) Lung trouble occurred in 12.2 per cent. of the males and 6.8 per cent. of the females. This trouble occurred mostly when rats were l6 to 20 weeks old - later cases were not recorded. The disease was most prevalent during the winter months, 35.1 per cent. of the cases, spring following with 30.5 per cent. and summer and autumn much lower (18.3 and 16.0 per cent. respectively). GROWTH AND FOOD INTAKE. Daily gains of suckling young showed an increase from birth to 13 days, milk supply being then adequate, a standstill or decrease in daily gains from 14 to 18 days, milk supply being the limiting factor, and a continuous increase in daily gains from 19 to 28 days after the young have become accustomed to solid food. As litter size increased from 4 to 7, females required 40 g. additional food for each additional young for the 28 days period and 98 g. for each additional young from 7 to 11. Food utilisation of females was lower than that of males and showed a much greater variability. When the same amount of food was given individually to pairs of males and females of the same weight, then there was no difference in the food utilization. The difference was, therefore due to the more rapid growth of the males when both sexes were fed ad lib.