Here's why:
* Endothermy: Mammals are endothermic, meaning they regulate their own body temperature internally. This requires a lot of energy and therefore a high metabolic rate.
* Active lifestyle: Many mammals are active and mobile, which also requires a lot of energy.
* Brain size: Mammals generally have larger brains relative to their body size, which are metabolically expensive organs.
* Hair/fur: Mammals need to maintain a stable body temperature, and their fur/hair helps with insulation. This requires energy to maintain.
Exceptions:
There are some exceptions to this rule. Some small mammals, like shrews, have extremely high metabolic rates. Other mammals, like sloths, have slower metabolisms due to their sedentary lifestyle and lower body temperature.
Overall, though, mammals are characterized by their relatively high metabolic rates.