Brown fat is a specialised adipose tissue with abundant mitochondria and rich blood supply. Brown fat

Options

A. insulates animals that are acclimatised to cold
B. is the major source of heat production of birds.
C. provides energy to muscles.
D. produces heat without producing ATP.

Explanation

**Step-by-Step Solution:**

1. **Understanding Brown Fat:**
- Brown fat is a specialized type of adipose tissue that is rich in mitochondria and has a high blood supply. It is different from white fat, which primarily serves as an energy reserve.

2. **Function of Brown Fat:**
- The primary function of brown fat is thermogenesis, which is the production of heat. This is particularly important for maintaining body temperature in cold environments.

3. **Comparison with White Fat:**
- Unlike white fat, which is mainly involved in energy storage and insulation, brown fat is more active metabolically. It generates heat by burning calories, which is essential for thermoregulation, especially in newborns and hibernating animals.

4. **Mitochondria Role:**
- The abundant mitochondria in brown fat cells contain uncoupling protein 1 (UCP1), which allows for the dissipation of energy as heat rather than storing it as ATP (adenosine triphosphate). This is a key characteristic of brown fat.

5. **Correct Answer Identification:**
- Among the given options, the statement that best describes brown fat is that it produces heat without producing ATP. This is because the primary role of brown fat is to generate heat for thermoregulation rather than to store energy in the form of ATP.

6. **Conclusion:**
- Therefore, the correct answer is that brown fat produces heat without producing ATP.