SAMPLE LAB REPORT
A COMPARISON OF METABOLIC RATES OF
POIKILOTHERMIC
TO LARGE AND SMALL HOMEOTHERMIC MAMMALS
Norman E. Garrison
Course: Biology 270 (Human Physiology)
Instructor: Dr. Norman E. Garrison
James Madison University
Department of Biology
December 4, 2004
A COMPARISON OF METABOLIC RATES OF
POIKILOTHERMIC
TO LARGE AND SMALL HOMEOTHERMIC MAMMALS
Introduction
All life as we know it requires the expenditure of energy, and the rate with which this energy is used is defined as the metabolic (Sherwood, 2004). Homeothermic mammals regulate their body temperatures, and hence their energy expenditure, to a relatively constant and fairly high rate (Fox, 1984). In contrast, poikilothermic animals assume the temperature of their surroundings, and their only method of controlling their temperatures is by behavior. For example, a turtle sitting on a log in the sunshine may have a temperature higher than that of a mammal, but would have to dive into the water to cool down (Prosser, 1973). In addition, since heat is lost through the body surface (i.e. the skin), one might expect that small mammals would have a higher metabolic rate than larger ones, because their surface area to volume ratio is greater (Goldstein, 1977). Therefore, the purpose of this laboratory exercise was to test the null hypothesis that metabolic rates of poikilothermic animals are greater than or equal to those of homeothermic animals, and that metabolic rates of large homeotherms are greater than or equal to those of small homeotherms. (Ho: mice ≤ humans ≤ frogs; Ha: mice > humans > frogs.)
Materials and Methods
Ten mice and 10 frogs of known weights were placed in separate metabolism chambers containing a layer of soda lime. The chambers were sealed with a large rubber stopper fitted with a moistened, calibrated glass tube. After a 15 min equilibration period, the tube was sealed with soap bubbles. The time required for the bubbles to transverse a distance corresponding to a particular volume (1.0 or 5.0 mL for the frogs and mice, respectively) was measured in triplicate. The metabolic rates of 10 human subjects were determined with a Benedict-Roth metabolism apparatus (Slabhead, 1983). All gas volumes were corrected to standard conditions of temperature and pressure assuming dryness (STPD). Statistical significance was assessed using Student's "t-test" (Johnson, 1984).
Results
Metabolic rates of mice were statistically significantly higher than those of both frogs (p < .001) and humans (p < .05), and those of humans were significantly higher than those of frogs (p < .01) (Fig. 1). Thus, there was a statistically significant failure of the data to support the null hypothesis, so the alternative hypothesis (Ha: mice > humans > frogs) was supported.
Fig. 1. A comparison of metabolic rates of poikilothermic animals (frogs) and large and small mammals (mice and humans). Values are means of 10 observations ± S.E..
Indeed, the metabolic rates of mice were about 8.7 times greater than those of frogs and 2.3 fold greater than those of humans. However, the difference between mice and humans is considered to be a conservative estimate because there was evidence of leakage in the Benedict-Roth apparatus, which would result in excessive values for the human metabolic rate. In any case, these results were interpreted to mean that there is an inverse relationship between metabolic rate and body size in homeotherms, but not between homeotherms and poikilotherms, since the frogs were smaller .
Discussion
Basal metabolic rate is the minimal wakeful energy expenditure (Fox, 1984). It is influenced by factors such as sex (Jones, 1978), hormones (Swiveltisch, 1981), and diet (Lumpkin, 1980). The normal metabolic rate in humans is about 47 Cal/hr/M2 and averages 87 Cal/hr/M2 in mice (Squire, Smith, Dillingham and Jackson, 1967a). Metabolic rates of the frogs (2.15 Cal/hr/M2) agreed closely with values of 2.08 Cal/hr/M2 reported by Squire, Smith, Dillingham, & Jackson (1967b). The average relative values found in this study are consistent with those above, but the absolute values are much higher, because activity may have influenced the experiment. The frogs were the least active, followed by humans and the relatively active mice. Since any activity requires energy, metabolic rate was probably exaggerated in the mammals. However, these differences were expected since none of the animals were under basal conditions.
According to Prosser (1973), oxygen consumption in exercise may be 15 to 20 times greater in exercise than in the resting state. In addition, the values obtained by Squire et al. (1967a) resulted from both male and female subjects, but males, which typically have higher metabolic rates, were used in this study.
When large and small adults of a species or the same general types of animals are compared, the total metabolism is higher in larger animals, but the metabolic rate is higher in small animals. In general, metabolism is more uniform when expressed as a power function of body size. For example, if M = total oxygen consumed per unit time and W = body weight, then the power function is expressed as
M = KWb
where the constant "b" is obtained from the slope of a plot of the logarithm of oxygen consumption against the logarithm of body weight. "K" is obtained from the intercept. The constant "b" gives the rate at which metabolism changes with size. If metabolism is directly proportional to weight, then b = 1. Actually "b" is usually less than 1. This means that in similar species or during growth, metabolism increases less than does body mass (Prosser, 1973).
As the volume of an object increases as a cube, the surface area increases as a square (Goldstein, 1977). When this principle is applied to homeothermic animals, it is apparent that as the size of the animal decreases, its surface area to volume ratio will also increase and must be accompanied by a corresponding increase in metabolic activity in order that body temperature be maintained in spite of the greater radiational cooling. By the same token, an organism that expends no energy to maintain a constant body temperature would have a much lower metabolic rate.
Conclusions
1. Homeotherms have higher metabolic rates than poikilotherms.
2. Small homeotherms have higher metabolic rates than large ones.
Literature Cited
Fox, S.I. 1984. Human Physiology. William C. Brown Publishers, DuBuque, Iowa. pp. 495-497.
Goldstein, L. 1977. Introductory Comparative Physiology. Holt, Rinehart, and Winston, New York. p. 682.
Johnson, R. 1984. Elementary Statistics. 4th Edition. Duxburg Press, Boston. pp. 304,351,353,360,451.
Jones, J. 1978. Role of sex in mammalian metabolism. Amer. J. Physiol. 48(2): 796-804.
Lumpkin, P. 1980. Diet and human metabolism: A comprehensive study. Amer. J. Nutrit. 66:63-74.
Prosser, C.L. 1973. Comparative Animal Physiology. 3rd Edition. W.B. Saunders Co., Philadelphia. 99 181-186.
Sherwood, Lauralee. 2004. Human Physiology. 5th Edition, Brooks/Cole, Pacific Grove, CA. 766 pp.
Slabhead, A. 1983. Measuring human metabolic rate with a Benedict-Roth Metabolism Apparatus. In Experimental Procedures for Physiology. (Edited by Yamamoto, T. and Harris, F.), pp 208-245. Harper and Row, New York.
Squire, P, B. Smith, H. Dillingham, and R. Jackson. 1967a. A comparison of metabolic rates I. Mammals. J. Comp. Physiol. 68(3):81-97.
____. 1967b. A comparison of metabolic rates II. Amphibians and reptiles. J. Comp. Physiol. 68(3):98-107.
Swiveltisch, S. 1978. Hormones as determining factors in human metabolic rate. Comp. Biochem. Physiol. 39:81-93.
APPENDIX
Sample Calculations
15 ml/243 sec x 744 mm Hg/760 mm Hg x 273 K/300 K x 60 sec/min x 60 min/hr x
4.8 cal/ml oxygen x 1/65 cm2 x
1 Cal/1,000 cal x 10,000 cm2/M2 = 146.2 Cal/hr/M2
