Small endotherms have higher respiration rates per gram of body mass than large endotherms because

they have higher surface area to volume ratio.

Explanation:

Small endotherms have a higher surface area to volume ratio as compared to the larges endotherms. The higher surface area to volume ratio means that the small body size provides more surface area for the exchange of heat with the surroundings.  

Therefore, the endotherms with small body sizes need the production of more energy to maintain body temperature. The higher need for energy results in higher rates of respiration in these organisms.

On the other hand, the endotherms with large body sizes have less surface area to exchange the heat with surrounding and require less energy to maintain the body temperature.  

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