When a male kitten is in the womb, its testicles develop inside the body, near the kidneys. Before birth the developed testicles normally begin a journey from inside the abdomen through the inguinal region into the scrotum. Both testicles should have descended into the scrotum by birth but are small and difficult to feel until about 2 months of age.

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If one or both testicles cannot be found in the scrotum by the age of 2 months (they should be big enough by then to feel) the cat is considered to be a cryptorchid (unilateral or bilateral). Your veterinarian should always check for the presence of testicles in the scrotum during the examination at the time of first vaccination. Sometimes one testicle can be felt in the scrotum but the other has not completed its journey from the abdomen. The retained testicle can be anywhere from inside the abdomen near the kidney, lower towards the inguinal area, or under the skin of the groin, where it can be felt. Sometimes the missing testicle(s) is/are still inside the abdomen and therefore cannot be felt at all but can usually be seen with ultrasound. Retained testicles are a relatively uncommon occurrence in male kittens. If your kitten has one or two retained testicles your veterinarian will recommend an operation eventually to remove both of them (castration).

Cryptorchidism is caused by defective genes, one inherited from both parents. The defective gene may be passed on to future generations. There is no genetic test to screen for the gene in potential mates and no physical evidence in females who carry the gene. For these reasons, animals that are cryptorchid should not be allowed to breed.  Repeat breeding should also be avoided with the mother and father of affected animals as they both carry the gene. Persians, Ragdolls and Himalayan breeds may be over-represented for cryptorchidism.

After birth, testicles descend into the scrotum. Testicles are on the surface of the body, within the scrotum, because this is cooler than the normal body temperature of the animal, a necessity for making sperm.  A retained testicle will still make testosterone, but it cannot make sperm.  If a testicle remains inside the body for several years, it is more likely to develop into a cancer. The cancer of retained testicles is serious and may produce estrogen, causing significant changes such as hair loss, bleeding and anemia. Another possible serious complication of cancer in a retained testicle is torsion, where the testicle twists, due to its increased size. Torsion will limit the blood supply to the testicle causing severe abdominal pain. Ultrasound usually can identify a twisted testicle, and emergency surgery is indicated and expensive. Cats with both testicles retained are sterile (unable to father kittens) but still virile. Those with one scrotal testicle and one retained in the abdomen will still be virile and fertile. Cats with one or both testicles retained in the abdomen will still have testosterone mediated male cat behaviors such as spraying, fighting, roaming and yowling. Interestingly, a cat with an abdominal testicle that had the scrotal testicle removed (partial castration) previously will still have barbs on his penis just like a normal non-neutered cat, indicating testosterone is being made by the internal testicle. This is an important clue to the presence of a retained testicle. Completely neutered male cats have a smooth penis.

To avoid future complications, cats with retained testicles should be completely castrated. Removing retained testicles can be a more difficult operation, but they are located somewhere between the kidney and the inguinal canal or sometimes under the skin just next to the scrotum. Sometimes the internal testicle can be detected with ultrasound. Your veterinarian will need to perform a castration surgery more like a spay in a female, through the abdominal wall, which is more complicated than the normal, scrotal castration. The complete castration is done before the cat reaches middle-age to reduce the risk of the retained testicle becoming cancerous. Typically, the male behaviors caused by the retained testicle making testosterone will prompt surgery asap. The scrotal testicle should be removed as well to make the cat sterile.