'Cryptorchidism is a failure of one or both testicles to descend into the scrotum. Testes retained within the abdomen suffer thermal suppression of spermatogenesis—the normal temperature of the scrotum necessary for spermatogenesis is between 1 and 8°F (0.5-4.5°C) below the normal body temperature.Bilateral cryptorchidism results in sterility; unilateral cryptorchidism is more common, and fertility is usually near normal because of normal sperm production from the testicle located in the scrotum. Cryptorchidism is seen in all domestic animals but is most common in stallions and boars. The undescended testicle may be located anywhere from just caudal of the kidney to within the inguinal canal. Abdominal testicles produce male hormones, and cryptorchids have normal secondary sex characteristics and mating behavior. It is reported that cryptorchidism in the horse is inherited as a dominant trait, while in other species it is a recessive trait. Because of the inherited nature of the condition, unilateral cryptorchids should not be used for breeding. Because cryptorchid testicles may become neoplastic, affected animals should be castrated.'
The Merck Veterinary Manual