Neurobiologist Sapolsky from Stanford University used a blowgun to tranquilize baboons in the Kenyan savanna. Once immobile, he could attach radio collars and track the baboon's movement. In one study, whenever a baboon's radio collar slipped off he would put it back on. This would involve another session of immobilizing the baboon with a tranquilizing dart. He noticed that female baboons that were frequently recollared had significantly lower fertility rates than uncollared females. Probably, therefore, some substance in the tranquilizer inhibited fertility.
Which of the following is an assumption on which the argument depends?