By Carol Beuchat PhD
A dog with one testicle can kiss a show career goodbye because it fails to meet the minimum threshold of quality for a male dog. Presumably this is because not having a matched pair reveals some deficiency of maleness, despite the fact that the dog can nevertheless be fertile. Most breeders send the cryptorchids on a trip to the vet that removes them from the gene pool before they are sent off to meet the family that will be their wonderful forever home.
If being cryptorchid was really all that bad, we would expect that there would be some detrimental effect to the dog. If not surgically removed, the undescended testicle can develop tumorous tissue. On the other hand, could the cryptorchid dog have some advantage over a normal one?
A dog with one testicle can kiss a show career goodbye because it fails to meet the minimum threshold of quality for a male dog. Presumably this is because not having a matched pair reveals some deficiency of maleness, despite the fact that the dog can nevertheless be fertile. Most breeders send the cryptorchids on a trip to the vet that removes them from the gene pool before they are sent off to meet the family that will be their wonderful forever home.
If being cryptorchid was really all that bad, we would expect that there would be some detrimental effect to the dog. If not surgically removed, the undescended testicle can develop tumorous tissue. On the other hand, could the cryptorchid dog have some advantage over a normal one?
A group in the Netherlands decided to look into this, and they pulled together data for 11,230 litters in 12 purebred dog breeds. The Dutch studbook registers all pups by about 8 seeks of age, so the data they used are for puppies that reached the age of weaning. Cryptorchid animals were identified at 12 months or older. Animals that had produced a cryptorchid offspring were designated as "carriers"; all other dogs were designated as non-carriers (we will call them "normal").
They looked at the number and sex ratio of the offspring that were produced by three kinds of matings: carrier-to-carrier (CxC), normal-to-normal (NxN), and C females to N males and vice versa (CxN). The data from the CxN crosses were not significantly different from the NxN matings, so we'll just summarize the CxC and NxN data here.
The first surprise was that, in most of the breeds studied, carriers bred to carriers produced significantly larger litters than NxN.
They looked at the number and sex ratio of the offspring that were produced by three kinds of matings: carrier-to-carrier (CxC), normal-to-normal (NxN), and C females to N males and vice versa (CxN). The data from the CxN crosses were not significantly different from the NxN matings, so we'll just summarize the CxC and NxN data here.
The first surprise was that, in most of the breeds studied, carriers bred to carriers produced significantly larger litters than NxN.
Litter size of NxN and CxC
Furthermore, the CxC litters had a biased sex ratio, producing significantly more males than females, and slightly fewer females than NxN litters. In CxC, an average of 24% of males were cryptorchid, in a pattern that is compatible with a simple autosomal recessive inheritance, with effects of modifier genes, gene-gene interactions, and environmental effects affecting the actual phenotypic expression in the cryptorchid animals. They did not observe any sort of "intermediate" condition in the NxC crosses.
Number of male and female offspring in CxC litters.
Number of male and female offspring in NxN litters.
The authors ruled out the possible effect of reporting bias, but could only speculate on what might cause the larger, male-biased litters in the CxC dogs. They concluded that there can be a selective advantage to cryptorchidism through an increase reproduce success. Cryptorchidism occurs in both wild and domestic animals, and results similar to these for dogs have been reported in pigs. In fact, cryptorchidism might persist in animal populations because CxC matings produce larger litters and therefore more carriers. Selection for larger litter sizes in dogs might complicate efforts to eliminate cryptorchidism.
- Gubbels EJ, J Scholten, L Janss, & J Rothuizen. 2009. Relationship of cryptorchidism with sex ratios and litter sizes in 12 dog breeds. Anim. Reprod. Sci. 113: 187-195.
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...the science of animal breeding
