T H E RATE OF GROWTH OF T H E ALBINO RAT’ EDNA L. FERRY From the Laboratory of the Connecticut Agricultural Experiment Station and the Shefield Laboratory of Physiological Chemistry in Yale University, New Haven, Connecticut EIGHT CHARTS Growth is commonly measured by the successive changes in stature or in weight, or by the combination of both of these measurements. Although each of these methods shows accurately how the animal is growing, none of them is capable of furnishing a direct measure of the rate of growth. This, as Minot2 has pointed out, may be measured by the percentage increase, based on the initial weight, during a definite length of time, not by the absolute increments during the same length of time. Therefore an animal of 50 grams initial weight which gains 5 grams in a given time, has a much higher rate of growth than the animal of 150 grams which gains 10 grams in the same length of time. Minot3 has published curves showing the rate of growth for man, guinea-pigs, rabbits, and chickens, which agree in exhibiting during the first few days of life a very large daily percentage increment, which rapidly falls toward the zero line. These curves indicate that the rate of growth is a variable function of age, and that the period of life which is ordinarily associated with the most rapid growth is, in reality, the period of most rapid decline in the power of growth. 1 The expenses of this investigation were shared by t h e Connecticut Agricultural Experiment Station and the Carnegie Institution of Washington, D. C., in connection with t h e nutrition investigations of Dr. Osborne and Dr. Mendel. Journ. of Physiol., vol. 12, pp. 972 Minot, Senescence and rejuvenation. 153, 1891. 3 Minot., The problem of age, growth and death. 1908. Chap. 3. 433 434 EDNA L. FERRY This view that the rate of growth is dependent upon age is supported by P f a ~ n d l e r . ~In a set of curves which he gives of two infants, one of which was prematurely born, the rate of growth of the latter was much higher than that of the normal infant with which it was compared, although its actual increase in weight during that time was much less. If, however, the curve is shoved over three months, so as to make it agree with the age of the normal infant, counting from the time of conception (‘Konzeptionsalter’ as opposed to ‘Geburtsalter’) the two curves are almost identical. Since the intra-uterine rate of growth is much higher than the extra-uterine, the abnormally high rate of growth manifested by this infant at first was due to the carrying over of the higher intra-uterine rate of growth, corresponding to a greater ‘Lebenspotential’ or ‘Lebenskraft’ than is normal a t birth. That the cessation of growth is due to age is a necessary corollary to these facts. D a ~ e n p o r ton , ~ the other hand, does not quite agree with this theory. He says “the reason why the animal ceases a t length to grow is-not because there is a necessary limit to growth force at a certain distance from impregnation but because it is in the nature of the species that the individual should cease to grow at this point. The indefinite growth of this part, the limited growth of that, are as much group characters as any structural quality.” The present paper aims to give briefly some data regarding the rate of growth of the albino rat. The method used is essentially that employed by iMinot in his study of guinea-pigs. The average growth curve for the albino rat was determined from the zctual curves of a large number of animals, and from this was calculated the daily percentage increments for ten-day periods. Rats raised under identical external conditions exhibit wide variations in weight a t any given age, so that it is difficult t o get a good average; but as the majority of these animals tended to 4 Pfaundler, Ueber die Behandlung angeborener Lebensschwache. Med. Kchs., Bd. 54, p. 1417, 1907 (July 16). 5 Davenport, Experimental morphology. Part 11, Chap. 10, 1899. Munch 435 RATE OF GROWTH O F ALBINO RAT I n all the charts the abscissae represent days and the ordinates percentage increments. I n charts 1, 2, 3, 4 and 5 t h e initial percentage is calculated from the weight at birth as given by Donaldson: (A comparison of the white r a t with man in respect t o the growth of the entire body. Boas Memorial Volume, 1906). 0 Chart 1 Average rate of growth for t h e male albino rat group themselves about this average curve, it is probably sufficiently accurate for the purpose. Charts 1 and 2 show the average rate of growth for male and female albino rats. These are of the same general form as those of Minot’s for other animals. The very high initial values, 17.7 436 EDNA L. FERRY Chart 2 Average rate of growth for the female albino r a t per cent for the males, and 15.0 per cent for the females, are due to the fact that rats, like rabbits, are born in a very immature state; blind, without teeth or hair, and incapable of any muscular coordination. In fact, a rat three weeks old is no more mature than a guinea-pig at birth, from which time on, the curves for the two animals show more or less agreement. The irregularities in the latter part of the curves are probably caused by an insufficient number of observations. Chart 3 shows the rate of growth of a very large rat, whose curve of growth varied greatly from the average, and chart 4 shows that of a very small rat. These curves have the same gen- 437 RATE O F GROWTH OF ALBINO RAT 0 Days 50 100 150 200 250 300 Chart 3 Rate of growth of a n unusually large male rat era1 features-as those for the normal, varying only in the rapidity of the decline. Charts 5 and 6 show the rate of growth of two rats which were stunted from the time of weaning until they were 89 and 314 days old respectively. At this time the food was changed, and although both of them had reached an age a t which a normal animal grows very slowly, or not at all, they immediately began growing a t a rate normal for their size, or even a t a slightly higher 438 EDNA L. FERRY Chart 4 Rate of growth of an unusually small male rat one, apparently in an endeavor to reach the size which they would have attained had they not been subjected to a preliminary period of stunting. Charts 7 and 8 show interesting curves of repair, followed by late growth. These animals grew normally for a time, then declined seriously and were brought back to a weight normal for their age, at a rate somewhat above the normal. These experiments prove conclusively that the power of growth is not lost so early in life as one would be led to suppose from a study of the curves of normal animals only. Whether or not it is ever lost is a question which can only be answered by ex- 14% 12% 10 % 8% 6% 4% 2% 0 Chart 5 Shows the rate of growth of a female r a t whose growth period was interrupted b y a period of stunting lasting from 30 t o 89 days of age. When the. r a t resumed growing, at a n age when growth is normally very slow, i t weighed 37 grams, t h e average weight of a female 30 days old, b u t its rate of growth was somewhat higher t h a n t h a t of a normal 30-day old female rat. Chart 6 Shows t h e rate of growth of a female rat which was stunted from time of weaning until i t was 314 days old. When.the rat resumed growing, a t a n age at which growth has normally ceased, it weighed 72 grams, the average weight of a female 50 days old, but its rate of growth was slightly higher than t h a t of a normal 50 day old rat. 439 440 EDNA L. FERRY tensive series of experiments in which the animals are stunted until a very late age before being placed under conditions suitable for growth. The rate of growth is not, however, a function of age, but of size. Every animal apparently has a certain normal size which it endeavors to reach in spite of vicissitudes during its early life. To what degree it is successful in this endeavor is probably dependent upon the extent of the injury inflicted upon it by its early experiences, and the favorableness of the external conditions during its period of growth. 4% a J L 2I J I I (0 a J 8 noc -Late Growth-+ Decline Repair Growth 2 6 Days 300. 350 400 8 Chart 7 Shows the rate male rat, at an age at which Chart 8 Shows the rate male rat, at a n age at which of decline and repair, followed by late growth, for a growth has normally ceased. of decline and repair, followed by late growth, for a growth has normally ceased. While this method of measurement is interesting as emphasizing some of the similarities and dissimilarities in the rate of growth of different types of animals when studied by means of comparable curves, for ordinary laboratory work it is far inferior to the more commonly used method of determining merely the successive changes in weight, because even large variations in weight are entirely concealed. At the age of 260 days the rat whose rate of growth is shown in chart 4 weighed little more than one-half as much as the one whose rate of growth is shown in chart 3-a fact which these curves fail to emphasize. More- RATE O F GROWTH O F ALBINO RAT 44 1 over, the actual amount of weight regained by the rats whose curves of repair are shown in charts 7 and 8 was very much larger than these curves would indicate, because both were large rats and the percentage increase in weight is therefore much less striking than the absolute increase. SUMMARY 1. Charts are given showing the rate of growth of albino rats under normal conditions, and after periods of stunting or of decline. 2. The curves for normal animals are very similar to comparable ones for other types of animals. 3. After prolonged periods of stunting rats are capable of growing a t a rate normal for their size rather than for their age, showing that the rate of growth is a function of size rather than of age as has been commonly supposed. 4. Although this method of representing growth is interesting for comparative study, for ordinary laboratory work it is inferior to the usual method of plotting absolute increments, because even large variations in weight are thereby concealed.