Passage Summary: Mammals are smart partly because they can control their body heat, which keeps the chemical reactions in their brains running at the right speed.

Conclusion: The ability of mammals to regulate their internal temperature contributes to their brain development and intelligence.

Reasoning: The brain is a chemical machine with temperature-dependent reactions, and internal temperature control ensures these reactions occur at the proper temperatures.

Analysis: The argument provides a mechanical explanation for how temperature control helps the brain function, but it leaves a gap between 'proper chemical reactions' and 'intelligence.' It assumes that the ability to ensure these reactions happen at the right temperature is actually a requirement for, or a cause of, higher intelligence. To find the necessary assumption, look for a statement that connects the brain's chemical efficiency directly to the development of intelligence. If those chemical reactions didn't matter for intelligence, the whole argument would fall apart.