On Isla Nublar, the management enacted several procedures to keep the dinosaurs under their control. Thus whenever they faced a particularly difficult problem, they hedged their bets by placing two procedures, with the other one as backup in case the first failed. One example of these redundant systems was their means of ensuring that the dinosaurs would not breed.According to Dr. Henry Wu, the first procedure was sterilization of the animals combined with irradiation by x-rays. The second control procedure was that all of the Dinosaurs in Jurassic Park were female.
Jurassic Park by Michael Crichton page 108 (First Edition, Thirty Fourth printing)