Ectotherm vs. Endotherm Metabolic Rate with Temperature Change

Denise Hart, Bryan McDonald, Kyeonghee Kim, Avery Marzulla

University of Colorado at Boulder, Fall 2003

The experimental goal of this project was to investigate the effects of temperature upon the metabolic rates of endotherms versus ectotherms. Ectotherms include such animals as reptiles, fish, amphibians, or crickets, as used in this experimental model, that utilize environmental energy and behavioral adaptations to regulate body temperature. Endotherms include such animals as mammals, including mice, as used in this procedure, and birds that use metabolic energy and some behavioral adaptations to thermoregulate their bodies between the approximate temperatures of 36 and 38oC, while bird’s body temperatures are slightly higher. Based upon our conclusions in the respiration lab using crickets, and the background information listed above, we hypothesized; if the room temperature at 22oC was equivalent to the thermoneutrality of both the crickets and the mouse, then the slope indicating the rate of change in CO2 , indicating metabolic rate, would be inversely proportional to the change in temperature for endotherms; and directly proportional to the change in temperature for the ectotherm specimens.

To test our hypothesis, 11 crickets and one mouse were used. For an accurate experimental model we controlled for several variables including external disturbances, sensor and container calibration, and temperature consistency between trials. Through the use of a CO2 sensor, heating pad, ice bath, two large glass containers, black jacket, and Logger ProTM the CO2 production, as an indicator of metabolic rate, was collected for both the mouse and crickets at room temperature (22-22.5oC), warm temperature (29oC), and cold temperature (9-9.8oC). The crickets, due to their small mass and smaller tidal volume, were kept in the glass container for 5 min for each trial. The mouse was kept in the container for approximately 2 minutes each trial, or until the CO2 concentration reached 5000 ppm within the container.

Data from this experimental model was not consistent with our hypothesis; we found a significant correlation between temperature and metabolic rate for both the mouse and crickets yielding a P value of 1.99E-09 when room temperature was compared with heat for the mouse, 2.26E-09 when the same comparison was made for the cricket setup, 1.56E-09 when room temperature was compared to cool temperatures in the ice bath for the mouse, and 1.92E-09 when the same comparison was made for the crickets metabolic rates in the same situations. As the temperature decreased the metabolic rate decreased, indicated by a decrease in CO2 production, and as temperature increased, metabolic rate increased indicated by an increase in CO2 production shown by significant correlations indicated by the above P values for both the mouse and the crickets, when it was expected that an inverse relationship would be observed for the mouse, rather than the direct relationship demonstrated. However, the relationship demonstrated experimentally was consistent with the hypothesized relationship for the cricket. Our hypothesis was confirmed by last year’s ninth place CABLE website winner, Black, et al. 2002 Snakes and House Mice: Freezing Their Little Tails Off. Their results were consistent with our hypothesis, observing an increased respiration rate of the endotherms at lower temperatures.

All background research information confirmed our original assumptions stated in the hypothesis, some possible reasons causing a discrepancy in our data are outlined in the following five potential problems. First, transferring the endotherm (mouse) between containers created a high stress environment during the study, possibly causing a higher respiratory rate, skewing our results. Second, the heating pad used, limited the experiment by only allowing an increase of temperature by 7o C from the room temperature baseline on the highest setting, this temperature increase may not have been drastic enough to reach temperatures above the mouse’s thermoneutrality zone, thus having no impact on metabolic rate. Utilizing a warm/hot water bath could correct for the small fluctuation in the warm temperature trial. Thirdly, repeating each of the trials may have accounted for variation in the experimental design, and may have produced more accurate results. Fourth, allowing the mouse to remain in an appropriate container (before measuring CO2 production) for a longer period of time allowing the container to equilibrate to the mouse’s respiratory conditions before adding the stressor (heat or cold) could account for the potential error in our findings. An open-ended chamber could also be more appropriate for measuring CO2 expenditure while the mouse is in a more normal environment. Fifth, utilizing more mice in the experiment with observation could help mimic the behavioral methods used by endotherms, such as movement to increase thermoregulation, grouping together to generate heat, non-shivering thermogenesis and insulation.

Due to the inconsistencies between our hypothesis, supported by background research, and our experimental findings, future studies need to be conducted employing the above corrections to our potential problems. Perhaps an alternative hypothesis could be employed, discussing potential outcomes of a greater endotherm population in an open system, opposed to the closed system we constructed for this experimental procedure.