What is Cryptorchidism?:
You may have heard the term 'rig' or 'ridgling' as someone talks about a horse acts like a stallion, but isn't. This behavior may be caused by cryptorchidism. Cryptorchidism means that quite often, although a horse can not reproduce it still produces testosterone and for that reason the horse will act like a stallion. My advice to anyone buying a first horse is to avoid a ridgling.
During a foal prenatal growth, the developing testes descend from inside the abdominal cavity, through inguinal canal—a passage in the abdominal wall through which blood vessels, tubes and ligaments pass, and into the scrotum. Normally, both testes descend and all is well, but occasionally, one or both do not descend. The inginual canal is restricted as the vaginal rings at their base close (usually about two weeks after birth), so that although blood supply can still reach the testes or single testicle, they or it can not descend into the scrotum as normal. The testes may stay high up in the abdominal cavity or may stay within the inguinal canal. For some reason, the timing is off, and there are a number of theories and reasons this can happen. Genes, hormones, the mare's health and environmental factors may play in a role in cryptorchidism.
A rig or ridgling will exhibit stallion like behavior. This makes them unsuitable for beginners and a challenge for experienced riders. Some people feel that because descended testes don't function in the same way as normal testes do, that the horse may be far more moody than a stallion, alternately docile and then aggressive.
Often cryptorchidism is not noticed until it is time to geld a colt. Because the testes can be mistaken for other structures within the scrotum, it may require a veterinarian to confirm a diagnoses. Usually all that is required is palpataion, both of the scrotum and rectally. The horse may have to be tranquilized for this very invasive examination. Occasionally, a ridgling with two un-descended testes may be mistaken for a gelding and the testes can not be located by palpat. If this is the case, a blood test can reveal whether there is testosterone present.
Cryptorchidism causes a horse to have stallion-like behavior. Rarely do the undescended testes produce, viable sperm, but they do continue to produce testosterone. Moody, stallion-like behavior makes some ridglings difficult to deal with. If you are looking for a pleasure horse, it may be best to pass a ridgling by. Some ridglings are able to reproduce, and are kept for breeding purpose. There is a lot of controversy about this because it's felt if there is a genetic component to the condition, it should not handed down to future offspring.
There is little that can be done to prevent cryptorchidism. Because many people feel—but so far no solid proof—that it's genetic, they believe cryptorchids that can reproduce should not be used for breeding purposes. However, the argument is, because there are a number of reasons why cryptorchidism occurs, until there is evidence that it is truly genetic, it's reasonable to use them as breeding stock.