The two chemicals that repel mosquitoes also repel several other varieties of insects. The capuchins rarely rub their bodies with the millipedes except during the rainy season, when mosquito populations are at their peak. Although the capuchins eat several species of insects, they do not eat the type of millipede they use to rub their bodies. The two insect-repelling chemicals in the secretions of the millipedes are carcinogenic for humans but do not appear to be carcinogenic for capuchins.
Capuchin monkeys often rub their bodies with a certain type of millipede. Laboratory tests show that secretions from the bodies of these millipedes are rich in two chemicals that are potent mosquito repellents, and mosquitoes carry parasites that debilitate capuchins. Some scientists hypothesize that the monkeys rub their bodies with the millipedes because doing so helps protect them from mosquitoes.
Which of the following, if true, provides the most support for the scientists' hypothesis?