Capuchin monkeys in Venezuela often rub a certain type of millipede into their fur. Secretions of these millipedes have been shown to contain two chemicals that are potent mosquito repellents, and mosquitoes carry parasites that debilitate the capuchins. The rubbing behavior is rare except during the rainy season, when mosquito populations are at their peak. Therefore, the monkeys probably rub the millipedes into their fur because doing so helps protect them against mosquitoes.
Which of the following would it be most useful to determine in order to evaluate the argument?