Abstract
A study conducted on two mice aimed at determining the effect of ambient temperature on the metabolic rate of endotherms. The objectives were to evaluate how an increase in the ambient temperature affects the rate of metabolism, how a decrease in temperatures affect the metabolic rate, and to determine the changes that occur in oxygen uptake as a result of temperature changes. The experiment’s hypothesis was that as the external prevailing temperature decreases, the rate of metabolism increases. Two mice were involved in the study, where one mouse was exposed to cold temperature ranges (6.6 0C – 15.12 0C) and the other mouse exposed to hot temperature ranges (23.84 – 35.01 0C). The mice were exposed to the varying temperature conditions for 20 minutes each. The hot and cold temperature environments were created using hot water baths and ice baths respectively. The measurements of changes in oxygen concentration and oxygen uptake were taken, which together with the weight of the mice were used to calculate the corresponding rates of metabolism at a particular temperature. It was found that the metabolic rate was highest (2.28 J.g.-1min-1) at the lowest measured temperature (6.6 0C) and lowest (1.52 J.g.-1min-1) at the highest tested temperature (35.01 0C). From the results obtained, it was determined that the rate of metabolism and oxygen uptake increased with a decrease in the external temperature, and vice versa. The results from this experiment supported the experiment’s hypothesis.
Introduction
Metabolism is the chemical and physical processes that take place in the living cells, which are necessary for maintaining life. These processes may involve the synthesis of new compounds (anabolism) or the breakdown of existing compound (catabolism). The overall result of metabolic processes may be the consumption of energy or the production of energy. The food consumed, for example, is broken down into a series of intricate processes to produce energy in the form of adenosine triphosphate (ATP), which is vital for other cellular activities. A study by Martin et al (2015) pointed out that some animals also convert the energy generated by metabolic processes to the warmth and thereby regulate their body temperature. The study also highlighted that the rate of metabolism is a function of the calories metabolized per body mass in a minute, i.e. calories per minute per gram (cal/min/gm). The metabolic rate can be determined using carbon dioxide (CO2) and oxygen (O2).
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An animal’s energy metabolized per unit time is dependent on several factors. These factors are diet, age, body state and level of activity, time of the day and year and the temperature of the external environment. According to Haigler et al (2015), ectotherms and endotherms have different ways of adapting to changes in the environmental temperature. Ectotherms adapt to temperature changes by constantly changing their body temperature relative to the prevailing external environment temperature. Endotherms, on the other hand, maintain a relatively constant body temperature with respect to changes in the external temperature. Mice have similar genetic and physiological traits to human beings. This includes similarity in the thermal regulation mechanisms, making it an ideal specimen model to evaluate the effect of temperature changes on the metabolic rate of humans and other endotherms. This paper discusses how changes in environmental temperature affect the metabolic rate in endotherms.
At the thermoneutral zone (TNZ), the body temperature is kept constant without the necessity to alter the metabolic rate. At this temperature range, the animal is in harmony with the ambient temperature. The metabolism measured at the TNZ is the basal metabolic rate. Temperatures on either extreme of the TNZ prompt a change in body activity to compensate for the too cold or too hot temperatures. At the extreme low temperatures, endotherms increase their body activities to generate heat, while at the extreme high temperatures; they initiate heat loss activities e.g. sweating. It can, therefore, be hypothesized that extreme decrease in environmental temperature increases the metabolic rate in endotherms (Gillooly et al. 2016). In this experiment, the temperature which the mice were exposed to was intentionally varied and the corresponding metabolic rate at each temperature was measured using an open circuit respiratory system. The findings were then used to fortify the hypothesis.
Materials and Methods
Materials
Air pump
Ice Bucket
Oxygen Gas sensors
Drierite Column
Ziplog Bags
Ice Bucket
Warm water bath
Mouse Chamber
Experimental Set-Up of Metabolism Measurement
The measurement of metabolic activity was preceded by setting up of the measurement apparatus. The order in which the apparatus was arranged was:
Atmosphere Pump Drying Flow Meter Chamber Trap Water Measure
An interface connected the oxygen and temperature sensor probe to the computer where the data was collected.
Temperature Variations
The weight of the mice was then determined using the scale, after which the animal was allowed to rest for some minutes. The environment for the two different temperature conditions (hot and cold) was set up in two different ways. Hot water obtained from the hot water bath was used to set up the hot environment, in which the mice chamber was subjected to the hot water for 20 minutes. An ice bath was used to set up the cold temperature. Two mice were used, one exposed to the hot temperature and the other to the cold temperature. The temperature of the chamber was controlled using a coil that linked the chamber to either the hot or ice-cold water. The temperature of the chamber and the reference oxygen inflow was first determined before placing the mice in the chamber. The mice were then placed in the chamber sequentially; the data recorded and were taken out of the chamber. The data obtained was the temperature in degrees Celsius (0C) and percentage (%) of oxygen concentration. The process was repeated for all the different temperature conditions.
Taking Air-flow Measurements
The sensor was first calibrated before taking the measurements. This was done by pressing and holding the CAL (20.9%) button approximately four seconds until the reading stabilized at 20.9% and the flashing of the green LED stops. After the calibration, the pump was plugged in and the rate of flow set at 400Ml/min. The air flow in the system was then checked to ascertain that the flow was adequate.
Statistical Analysis
The temperature was the independent variable, whereas the metabolic rate was the dependent variable. The oxygen concentration obtained was used to determine the animal’s metabolic rate. The oxygen concentration values were first multiplied by 10 to convert them to μL. The O2 flow rate was then calculated using the formula below and was adjusted to 400ml/min. The weight of each mouse was divided by the oxygen uptake value, which was then multiplied by the average amount of heat produced for each liter of oxygen consumed. The value obtained was the metabolic rate for the mice.
Result
| Temperature (°C ) |
Δ O2 (µL/mL) |
Metabolic rate (J/g.min) |
| 23.84 | 5.25 | 1.45 |
| 15.12 | 6.5 | 1.77 |
| 6.66 | 8.16 | 2.28 |
| 24.13 | 7.8 | 2.09 |
| 29.99 | 6.33 | 1.66 |
| 35.01 | 5.83 | 1.52 |
Table 1: The average oxygen concentration and metabolic rate of mice at hot and cold temperature ranges.
From table 1 above, the metabolic rate of the mice is inversely proportional to the environmental temperature. The average metabolic rate is higher at the low-temperature range (2.28 J.g.-1min-1) than the average metabolic rate at the high-temperature range (1.52 J.g.-1min-1).
The graphical representation in figure 2 above shows how the metabolic rates relate to the temperature variations. The metabolic rate is dependent on the temperature changes, which is the independent variable of the two. When the graph is extrapolated, the metabolic rate of the mouse at 50C is approximately 2.28 J.g.-1min-1, while that at 30 0C is 1.52 J.g.-1min-1. It can thus be concluded that at lower external temperatures, the metabolic rate of the mice was higher.
Figure 3 above shows how the environmental temperature affects the rate of metabolism, plotted using the results obtained by the six groups in the class. The general trend of the graphs is that the metabolic rate is inversely proportional to the environmental temperature, whereby the rate is high at low environmental temperatures while it is high at low environmental temperatures. The outliers present in some graphs, e.g. in group 2 and 4, are as a result of experimental errors.
Discussion
The aim of this experiment was to investigate the effect of temperature variations on the metabolic rate of endotherms and by extension the effect on the concentration of oxygen uptake. The graphical representation in figure 2 points out that the decrease in external temperature results in a corresponding increase in metabolic rate, while an increase in the external temperature causes a reduced metabolic rate in the experimental mice. These findings supported the experiment’s hypothesis. The study by Gillooly et al, (2016) also concluded that as the environmental temperature decreases, the metabolic rate of endotherms increases to provide more energy required to keep the body warm. According to Fischer et al, (2018), the thermoneutral zone for mice is approximately 30 0C. In the thermoneutral zone, it is observed that there is no significant change in the metabolic rate. When the ambient temperatures fall below the body’s lower critical temperature, the body counters this by increasing the breakdown of compounds to generate more energy required to keep warm. Metabolic processes are facilitated by oxygen molecules. Maximum energy production requires complete metabolism, which occurs in the presence of adequate oxygen concentration. The increased oxygen uptake in cold temperatures is as a result of the increased metabolic processes. The cells require more supply of O2 molecules which prompts the brain respiratory center to trigger increased inhalation. In contrast, exposure to higher external temperature results in a decrease in the metabolic rate. The higher ambient temperatures signify that the body has to lower its respiratory processes and thereby reduce its energy production to cope with the hot environment (Martin et al, 2015). Consequently, the brain respiratory center is prompted to trigger inhibitory signals that result in reduced inhalation and increased exhalation. The oxygen uptake is reduced, which means that the cellular aerobic metabolism is inhibited.
The experiment did not, however, delve into the tolerance of the mice to extremely cold or hot temperatures. Also, the maximum rate of metabolism or concentration of oxygen uptake in extreme temperatures was not evaluated. Further experiments need to be conducted to evaluate these parameters and ascertain the limits beyond which the mice could die. The sample size used in the experiment (two mice) was very low. This subjected the results to more errors. Using one mouse for the cold and hot temperatures each is not sufficient to use the results to form a conclusion on the whole mice species and the endotherms at large. The mouse used in the cold temperatures, for example, might have had other underlying factors that increase the rate of metabolism e.g. increased enzyme activity. Future experiments need to be conducted with a larger sample size to minimize the sources of error. Furthermore, the experiment was only conducted in one run. Conducting the experiment in multiple runs to determine the accuracy and precision of the results will also reduce the sources of experimental errors. Control mice should also be included in future studies. The controls, for example, may be fed prior to the experiment or made to exercise more than the test mice. Food intake and exercise increase the metabolic rate. Feeding and exercising the control will help determine the extent of these activities on the test mice’s metabolic rate.
Endotherms and ectotherms have different ways of adapting to the changes in their external environment temperature. Ectotherms self-adjust to the environmental changes by changing their body temperatures to suit their prevailing ambient temperatures. Endotherms, on the other hand, have more complex mechanisms of adapting to changes in environmental temperatures. These mechanisms include changes in the metabolic rates. In cold temperatures, the metabolic rate of endotherms increases to produce more energy required to keep the body warm. In hot temperatures, the metabolic rate reduces to decrease the energy production. Increase in metabolic rate requires an increased supply of O2 molecules, thereby the increased oxygen uptake during cold temperatures.
References
Fischer, A. W., Cannon, B., & Nedergaard, J. (2018). Optimal housing temperatures for mice to mimic the thermal environment of humans: an experimental study. Molecular metabolism , 7 , 161-170.
Gillooly, J. F., Gomez, J. P., Mavrodiev, E. V., Yue, R., & McLamore, E. S. (2016). Body mass scaling of passive oxygen diffusion in endotherms and ectotherms. Proceedings of the National Academy of Sciences of the United States of America, 113(19), 5340-5345.
Haigler, B., Jarolim, E., McCray, A., & Agan, J. (2015). Temperature and the Effects it has on Metabolic Rate in Endotherms and Ectotherms. Journal of Introductory Biology Investigations , 2 (2).
Martin, R., Kelsey, K. V., Kneedy, S., & Kalum, A. (2015). Ambient temperature and metabolism in endotherms (Mus musculus). Journal of Introductory Biology Investigations , 3 (1).
