The Effect of Predation Risk on Metabolic Rate in Males and Females

The purpose of this discussion is to evaluate how the authors set up an experiment to test their idea that male metabolic rate might respond differently to the threat of a predator than female metabolic rate does. Pay careful attention to which variables the authors keep the same for all the animals and which variable(s) they manipulate to test their idea. Look at the authors' results and decide what you think the answer is to their question. Discussion Questions Introduction 1. What is the authors' research question? Does predation risk affect the metabolic rate of animals? Does this affect differ between males and females? 2. What hypothesis are the authors testing? The metabolic rate of an animal will increase after the addition of a predatory cue. In addition, the increase in metabolic rate will be higher in females than males. 3. What specific predictions do the authors make from their hypothesis? Due to differences in life-history between sexes, predation risk and the change in metabolic rate that results may also differ. Since the pressure of predation is stronger on females, the increase in metabolic rate will be higher in females than males. 4. Knowing the authors' hypothesis, state the null hypothesis. (It is not stated in the article, only implied.) The addition of a predatory cue will have no effect on the metabolic rate of an animal and therefore there is no difference in metabolic rate change in response to predation risk between sexes. 5. What specific predictions can you make from the null hypothesis? When a Black field cricket is introduced to a chemical cue from an Eastern water skink, a sympatric predator of the cricket, it’s metabolic rate will not change regardless of its sex. Materials and Methods 6. What were the living conditions of the crickets before the experiment? The crickets were adults of the 8th and 9th generation of a laboratory-reared population. They were exposed to 12 hours of light and 12 hours of dark per day and were able to have food and water as desired. 7. What dependent variable do the authors measure? The authors are measuring the production of CO2 to evaluate the change in metabolic rate. 8. What independent variable do the authors manipulate? Treatment(control and predator) and sex(males and females) combinations 9. What other variables do the authors take into consideration that might influence the independent variable? The size of the chamber was small in order to restrict the movement of the crickets, preventing any change in CO2 production to be caused by physical activity. Temperature is also a possible factor that can affect metabolic rate, so both males and females were randomly exposed to every range of higher and lower temperatures. Visual disturbances were also taken into account and a grey piece of fabric was used to cover the chamber during all measurements. Habituation 10. A treatment is the range of conditions for the independent variable. What are the treatments in this experiment? Control and predator Predator, control 2, eucalyptus scent 11. Describe the control. A cylindrical chamber(6cm length and 3cm diameter) containing a clean piece of paper(2x2cm approx.) was prepared. The respirometer was set to take a measurement every 5 seconds with a precision of 1 mL CO2/mol. The measurements were taken between 11am and 4pm, between October 2013 and February 2014. Both sexes were randomly exposed to temperatures ranging between 18.9 and 29.3ðC(mean ñ SD = 23.4 ñ 3.8ðC) and throughout the measurements, the chamber was covered with a gray piece of fabric. 12. How was the procedure different between the control treatment and the predator treatment? The control treatment chamber contained a clean piece of paper(2x2cm approx.) while the predator treatment contained a piece of paper(2x2cm approx.) with fresh feces from an Easter water skink, a sympatric predator of the cricket. When placed in the control chamber, the crickets were allowed to habituate for 10min prior to the measurement of CO2 production. When the crickets were placed in the predator chamber, measurement of CO2 production was recorded right away. 13. What do the authors do to find out if the procedural difference matters? To test for the possible effect of habituation to the experimental setup, the authors performed a second set of experiments where they replaced the predator treatment with a second control treatment. In addition, the authors also tested for the possibility that the change in respiration may have been due to the detection of a new odor rather than the detection of a predatory cue. In a third set of experiments, they replaced the predator cue with a solution of five drops of 100% pure eucalyptus oil on 0.5mL of water. 14. How many animals were tested in total? Were equal numbers of males and females tested? Do you think it matters? In total, 156 crickets were tested. More females were tested than males. The unequal number of males and females tested does not matter. Even though less males were tested, the sample pool was still sufficient to exhibit the difference in increase of metabolic rate between the sexes. Results: Figure 2. 15. What do the bars show? What do the error bars represent? The bars show the average logarithm of volume of CO2 produced in mL/g*day by the crickets under control conditions and after a predator’s chemical cue was introduced. The error bars represent ñ 1 standard error. 16. Data in Figure 2 were collected from how many animals? Data from 66 crickets was collected, 43 females and 23 males. 17. Does CO2 production change in response to the smell of a predator? Yes, a significant increase in CO2 production after introduction of the chemical cue from a predator can be seen via the graph. Results: Figure 3. 18. Fig. 3A. What was the test procedure for Control 1? For Control 1 data, the cricket was first introduced to the control chamber and then moved to the predator chamber. Cricket was introduced to control chamber and allowed to habituate for 10 min 19. Fig. 3A. What is the logic behind the test procedure used for Control 2? For Control 2 data, the cricket was first introduced to the control chamber and then moved to a second control chamber. This experimental procedure was carried out in order to test for the possible effect of habituation to the experimental setup on metabolic rate. 20. Fig. 3B. What is the logic behind testing the response to eucalyptus scent? The authors replaced the predator cue with eucalyptus scent to verify that the increase in metabolic rate was due to the detection of a predatory cue and not simply because of the detection of a new smell. Results: Figure 4. 21. What is missing from Figure 4 that you should always include in graphs? The y-axis label with units is missing in Figure 4. 22. Is CO2 production different between males and females in the control treatment? No, CO2 production did not differ between males and females under control treatment. 23. What effect does the smell of a predator have on CO2 production? The smell of a predator causes an increase in CO2 production in both females and males. 24. Is the effect different between males and females? The smell of a predator caused a greater increase in CO2 production in males than females. Inference 25. Are males more scared of predators than females? Give evidence from the results to support your answer. Yes, males are more scared of predators than females because the increase in metabolic rate after the introduction of a predatory cue was higher in males than in females. In figure 4 it can be seen that while under control treatment, no difference in CO2 production was seen between males and females. However, under predatory treatment males had a higher production of CO 2 of approx. 2.3 mL/g*day while females had a CO2 production of approx. 2.1 mL/g*day.