Passage Summary: Because a group of monkeys started using hot springs after watching one member do it, the author concludes they can learn from each other rather than just following instinct.

Conclusion: Macaque monkeys can learn and transmit new social behaviors and are not entirely controlled by their genetics.

Reasoning: After one monkey began using a hot spring in 1963, the behavior spread to the entire troop and became a regular winter habit by 1990, despite never being observed before.

Analysis: The argument relies on the 'Gap' between a behavior spreading and the *cause* of that spread being social learning. The author assumes the monkeys didn't all just start using the spring because of a sudden environmental change or a simultaneous genetic mutation. To be a necessary assumption, the statement must be something the argument needs to stay alive. Ask yourself: 'If this weren't true, would the conclusion fail?' Look for an answer that rules out alternative explanations for the behavior, such as the idea that the monkeys were already genetically predisposed to use the springs and just hadn't had the chance yet.