Mammals are said to be homoiothermic because their body temperature is

Step 1: Analyze the question The question asks why mammals are called homoiothermic. We need the option that best explains the term.

Step 2: Explain the concept Break the word down:

• Homoio- means "similar" or "the same"
• -thermic means "heat" or "temperature"

Homoiothermic animals (also called endotherms, or "warm-blooded") keep their internal body temperature relatively constant, even when the outside temperature changes. Mammals and birds do this through:

• Sweating and panting to cool down
• Shivering and raising metabolic rate to warm up
• Vasodilation and vasoconstriction (widening or narrowing of blood vessels in the skin)
• Insulation from fur, feathers, or fat
• The hypothalamus acting as the body's thermostat

In contrast, poikilothermic (cold-blooded) animals such as lizards, frogs, and fish have body temperatures that change with their surroundings.

Step 3: Evaluate the options

• A. Always higher than the environment: Wrong. On a very hot day, the environment can be hotter than a mammal's body.
• B. Always lower than the environment: Wrong, for the same reason.
• C. Always the same as the environment: Wrong. This describes poikilotherms.
• D. Relatively constant irrespective of the environment: Correct. This is the definition of homoiothermy.
• E. Remains at 37 °C all the time: Wrong. Different mammals have different body temperatures, and even in humans the temperature fluctuates slightly. It is "relatively constant", not fixed at exactly 37 °C.

Step 4: Answer The correct option is D.

Exam tip: Remember "Homo = Same". Homoiotherms keep the same temperature inside. Whenever you see "relatively constant irrespective of the environment", think homoiothermic. Watch out for words like "always" and "all the time" in options, because they are often traps in biology questions.