In the 1920s, physicians identified the pregnancy hormone hCG and started to inject women's urine into immature female rats and mice to see if they would go into heat. Adorable rabbits eventually replaced the rodents and scientists started shooting urine into their ear veins to see if she would ovulate within 48 hours.
But the old saying “the rabbit died” is misleading. The rabbit always died because they had to dissect the poor thing to see if it ovulated or not. With rats, they knew much sooner, so they didn't have to kill them. Way to take one for the team, rabbits.