Define Allen's rule.

Explanation

### Step-by-Step Solution:

1. **Understanding the Context**:
- Before defining Allen's rule, it's important to understand the concept of conformers. Conformers are organisms that cannot maintain a constant internal environment, meaning their body temperature and other physiological parameters fluctuate with the external environment.

2. **Thermoregulation**:
- Thermoregulation is the process by which organisms maintain their body temperature within certain boundaries, even when the surrounding temperature is different. This process can be energetically expensive, making it challenging for many organisms to maintain a constant body temperature.

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