Females preparing to ovulate produce an abundance of the hormone estrogen which causes them to be receptive to the advances of the male. In the mare, the peak time of receptivity usually coincides with the time of ovulation. Then, approximately 24 to 48 hours later she goes out of heat. Nature has a back-up plan. In the animal kingdom there are two types of ovulations.
Some animals have spontaneous ovulations.
Others have ovulations that are induced, and, perhaps, there are even certain species where a bit of both types of ovulations can occur. To explain further, an induced ovulator is an animal that ovulates when stimulated by sexual intercourse-the most common example being the rabbit. A doe rabbit releases her eggs when she is stimulated by the act of being bred to a buck rabbit. The reproductive efficiency of this system is exemplified by the number and ease with which rabbits seem to multiply given the least opportunity. On the other hand, a cow is a spontaneous ovulator.