The testicles of the male dog (like the ovaries of the bitch) develop inside the abdomen, but in contrast to the ovaries they normally descend through the inguinal canal to lie in the scrotum – a pouch of skin between the thighs. This is because for the testicles to produce sperm (i.e. for the dog to be fertile) they must be kept at a slightly lower temperature than that of the abdomen.

This descent is very gradual but in the majority of dogs the testicles are in the scrotum either at birth or within the next ten days. In some dogs they descend later, but if descent is going to take place it is always before six months old and usually the testicles are in the scrotum by three and a half months old. However, there are dogs in which up to nine or ten months old one testicle can readily be withdrawn from, and returned to, the scrotum. In early life the small size of the testicles may make detection difficult; however they can usually be felt by two months old, although sometimes not before they begin to enlarge (prior to puberty) at about four months of age.

The failure of one or both testicles to descend is known as cryptorchidism and in the USA probably occurs in about 10% of dogs, especially Yorkshire Terriers, Pomeranians and Poodles. The right testicle is almost twice as likely to be retained as the left. There is no truth in the belief that cryptorchid dogs are more likely to be vicious. An undescended testicle is able to produce male sex hormone (testosterone), though not sperm, and is much more likely (more than twelve times) to develop a tumor in later life. Where neither testicle has ever left the abdominal cavity there has clearly been arrested development nor are many affected animals male pseudohermaphrodites (i.e. their external genital organs resemble, in varying degrees, those of a bitch).

There is no medical treatment which will cause the testicles to descend. Although it is possible to breed from a male with only one retained testicle this is inadvisable since the condition is inherited. For this reason cryptorchids are disqualified from official shows. Surgery to bring an undescended testicle into the scrotum should notbe undertaken because it will not correct the underlying genetic defect; it is far preferable for both testicles to be removed (i.e. for the dog to be castrated) to prevent breeding taking place. The total failure of one, or both testicles, to develop at all is possible, but extremely rare.

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