At birth, the testicles of a puppy are still within his abdomen. As the animal develops, the testicles slowly 'descend' into the scrotum. In mammals, sperm development does not occur correctly at the high temperatures found within the body. The testicles are therefore held outside of the abdomen and within the scrotum to provide a cooler environment. The production of testosterone is usually not influenced by temperature.

Frequently, owners notice that the puppy they just purchased only has one or possibly no testicles within the scrotum. Although different dates are listed in some of the veterinary literature, both testicles are usually within the scrotum by the time the animal is six weeks of age and they should definitely be there by the time the puppy is eight to ten weeks of age. If one or both testicles are not present at that location by twelve weeks of age, they probably never will be and the animal is said to be suffering from cryptorchidism or 'retained testicles.' This is a disorder that may be passed from generation to generation.

    What are the symptoms?

Cryptorchidism is a condition affecting the dog in which one or both testicles do not fully descend into the scrotum. The condition may be presented in two forms:
 

    1)  Unilateral cryptorchidism refers to the normal descent of a singular testis.
    

    2)  Bilateral cryptorchidism results in the retention of both testes.
 

These animals rarely show any abnormalities because of this condition. They have normal activity levels, growth, and behavior. Although fertility may be affected, they will usually show normal breeding behavior and frequently impregnate females, especially when one of the testicles has descended into the scrotum.

    What are the risks?

Some researchers believe that dogs with cryptorchidism may have a higher incidence of other testicular diseases. Specifically, these would be cancer and torsion.

Cryptorchidism is a congenital anomaly which is described as a sex limited (occurs only in males) trait. The incidence of cryptorchidism is greatly higher in that of pure-bred dogs and specifically, in that of heavily line-bred animals. Bilateral retention is reported to be most common in in-bred animals. The dam and sire of the cryptorchid pup may appear to be normal while being carriers for the defect. Given the polygenic nature of cryptorchidism, it is difficult to calculate quantitative results with respect to frequency and distribution. Simple recessives provide statistical expectations as we consider a limited number of possibilities yet polygenic modes of inheritance are far more complicated as a singular gene may be responsible for but one aspect of an entire process in which several genes dictate the completion of the event ie: descent of the testes. With this in mind, it is not difficult to understand how an affected (cryptorchid) animal may produce seemingly normal offspring.