If a female ferret is not bred she will stay in heat. This will cause an excess of estrogen in her system which will make her anemic eventually causing death.

In the wild, European polecat ferrets and black footed ferrets are solitary animals, and to insure survival of the species, females stay in heat to attract the male for mating. After mating, they go their separate ways and the female alone gives birth and raises the young. The male has no role in the rearing of young ferrets.

Female ferrets (called jills) have a unique reproductive system. Jills are induced ovulators which means that they will stay in heat or estrus until the physical act of mating occurs with either a Hob or Hoblet. An alternative method is a veterinarian can give an injection of hormones called a "Jill jab" to terminiate estrus. The Hoblet is a vasectomized hob (not castrated) that has his tubes cut that transport the sperm to the testicles, and is still able to perform the physical mating process. The physical act of mating induces ovulation in the Jill and brings her out of estrus. She will have a phantom or false pregnancy that will last for the same length of time as a normal pregnancy (40 - 44 days) and will display symptoms of being pregnant. At the end of this time, she might come into estrus again.

If the Jill is not mated or given an injection to end estrus, she remains in that state. The white blood cells will not be produced in adequate numbers, and she will become much more susceptible to an infection. Jills in heat secrete high levels of estrogen and if this hormone stays in the blood for a prolonged period of time, it will cause a progressive depression of bone marrow that causes a severe, life threatening aplastic anemia (abnormal depression of all three elements of blood) which is fatal. The high level of estrogen causes the bone marrow to stop producing red blood cells.