In a certain wildlife park, park rangers are able to track the movements of many rhinoceroses because those animals wear radio collars  When, as often happens, a collar slips off, it is put back on  Putting a collar on a rhinoceros involves immobilizing the animal by shooting it with a tranquilizer dart  Female rhinoceroses that have been frequently recollared have significantly lower fertility rates than uncollared females
Probably, ttherefore, some substance in the tranquilizer inhibits fertility
How often park rangers need to use tranquilizer darts to immobilize rhinoceroses for reasons other than attaching radio collars