To confirm that these microbes are important, Kohl killed them with antibiotics. Afterwards, the woodrats could all still eat normal laboratory chow. But when they were fed with creosote, they couldn’t tolerate the resin and lost a lot of weight. Within two weeks, all of them had lost 10 percent of their weight and were removed from the experiment. When Kohl removed their microbes, the experienced woodrats couldn’t even handle the tiny levels of creosote that their naive cousins can. “[It] effectively removed 17,000 years of ecological and evolutionary experience with creosote compounds,” he wrote.