You may have heard the term 'rig' or 'ridgling' when someone talks about a horse that acts like a stallion, but is is thought to be a gelding. This behavior may be caused by cryptorchidism. Cryptorchidism means that quite often, although a horse cannot reproduce it still produces testosterone and for that reason, the horse will act like a stallion. The best advice for anyone buying a first horse may be to avoid a ridgling because its stallion-like behavior may require handling that is beyond the capabilities of a beginner rider or owner.
During a foal's prenatal growth, the developing testes descend from inside the abdominal cavity through the 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 inguinal canal is restricted as the vaginal rings at the base close (usually about two weeks after birth). Although the blood supply can still reach the testes or single testicle, and one or both testes 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. While it’s difficult to tell why each incidence of cryptorchidism occurs, but for some reason, the timing is off during the colt’s development. There are a number of theories as to why this can happen. Genes, hormones, the mare's health and environmental factors may play a role in cryptorchidism.
A rig or ridgling will exhibit stallion-like behavior. This makes them unsuitable for beginners and sometimes 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 temperamental 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 any diagnoses. Usually, all that is required is palpation externally of the scrotum and internally through the rectum. The horse may have to be tranquilized for this very invasive internal examination. Occasionally, a ridgling with two undescended testes may be mistaken for a gelding and the testes cannot be located by palpitation. 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 purposes. There is a lot of controversy about this because it's felt if there is a genetic component to the condition, it should not be handed down to future offspring. There are of course exceptions to every rule, but it’s unlikely that a rig will make a good first-time horse.
Surgery to remove or 'geld' a ridgling can be costly, as it involves procedures more involved than gelding. Rather than costing around $250 to have your horse gelded, costs can exceed $1000 depending on where the testes are retained in the abdominal cavity. If the testes are hidden high up in the abdomen, the examination and surgery will be quite costly. Laparoscopy has been the most common type of surgery. A tiny camera attached to fiber optics is inserted into the abdomen, and the surgical instruments are put in through the same small incision. While the surgery itself is not overly invasive, it does require specialized equipment and training. Veterinarian clinics offering this type of service may not be readily available to everyone. This is the reason many rigs are not gelded, or only half gelded if they have only one descended testicle.
There is little that can be done to prevent cryptorchidism. Because many people feel—but so far there is no solid proof—that it's genetic And many 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.