Question

A desert lizard (an ectotherm) and a mouse (an endotherm) are placed inside a chamber at C and their body temperature [ T(L) for lizard and T(M) for mouse] and metabolic rates [ T(M) for lizard and M(M) for mouse] are monitored. Which of the following are correct?
A. T(L) and M(L) will fall while T(M) and M(M) will increase.
B. T(L) and M(L) will increase while T(M) and M(M) will fall
C. T(L) and M(L) will fall, T(M) will remain the same and M(M) will increase
D. T(L) and M(L) will remain the same while T(M) and M(M) will decrease.

Answer 1

There are two types of animals depending on whether they regulate their body temperatures. Animals that can regulate their body temperature are called endotherms and animals that cannot regulate their body temperature are called ectotherms. The regulation of body temperatures is closely associated with metabolic rate of the animals.

Complete answer:
Animals are classified into two groups based on thermoregulation or temperature regulation of their bodies. Endothermic animals have the capability to maintain a constant body temperature, while ectothermic animals are unable to, thus, they change their body temperature according to the environmental temperatures.
Endotherms generate and regulate the heat produced from metabolic processes to maintain their temperatures, while ectotherms generally depend on external heat sources like sunlight to raise their body temperatures. It is observed that an endothermic animal has a higher metabolic rate than an ectothermic animal of the same size.
Metabolic rates also depend on the size of animals. Animals lose heat through their body surfaces. Therefore, smaller animals lose heat faster than bigger ones. Therefore, smaller animals need higher metabolic rates to maintain a constant body temperature.
A desert lizard is an ectothermic animal, that is they cannot maintain a constant body temperature – it changes with the environmental temperature. The surrounding temperature in a desert is much higher than $15^\circ$C. Therefore, the desert lizard will decrease its body temperature to adjust to the surrounding temperature of $15^\circ$C. The metabolic rate of the lizard will also decrease with the decrease in temperature.
The mouse, on the other hand, is an endothermic animal, in that it maintains a constant internal temperature of the body. The internal body temperature of a mouse is around $37^\circ$C. Moreover, the mouse, being a small animal, tends to lose heat faster to the environment. If the surrounding temperature is lower than the internal temperature, the mouse has to work very hard in maintaining a constant internal temperature. This is achieved by increasing the metabolic rate of the body.
Therefore, to conclude, we can say that at $15^\circ$C, a desert lizard will lower both its body temperature and metabolic rate, while a mouse will maintain a constant body temperature but increase its metabolic rate.

Hence, option (C) is correct.

Note: Endotherms can maintain a constant internal temperature while the ectotherms cannot. The temperature of endotherms is maintained by generation of heat during metabolic processes. They are also called warm blooded animals. Ectothermic animals change their body temperatures with the surrounding temperatures. They are called cold blooded animals, and depend on environmental sources to raise their body temperatures. Metabolic rate of the body is higher in endothermic animals than in ectotherms.