For years, scientists have been trying to untangle the complicated connections between genes and behavior, knowing that just as DNA gives rise to physical traits, it must also influence how people and animals act. But many of those efforts have been difficult, given that behaviors such as aggression or empathy are complicated, influenced by individuals’ environment and past experiences, as well as evolution.

Now, a team of Harvard University biologists has untangled a connection between genes and one simple behavior using a most unlikely set of tools: a large box filled with a ton-and-a-half of soil and sand, two closely related species of mice, and a polyurethane foam that can expand to fill crevices.