Cryptorchidism and sex ratio are associated in dogs and pigs1

The objective of this study was to investigate whether the occurrence of cryptorchidism in a litter is associated with the sex ratio in this litter. The analysis included for dog 1,339 litters of 4 different breeds, of which 12.8% had 1 and 3.1% had 2 cryptorchids, and for pig 119,920 litters of 14 different populations, of which 2.2% had 1 and 0.2% had 2 cryptorchids. The data were analyzed using the GLIMMIX procedure of SAS. In dog, the presence of cryptorchids in a litter does not affect litter size or the stillborn rate. In pig, litter size increases significantly with an increasing number of cryptorchids in a litter. The stillborn rate in pig is larger in litters with cryptorchids than in litters without cryptorchids. To characterize the imbalance of the sexes in a litter, the sex ratio, defined as the number of live males divided by the number of live offspring, and the sex difference, defined as the number of live males minus the number of live females, were used as dependent variables in the analyses. In both dog and pig, the presence of 1 or 2 cryptorchids in a litter clearly goes together with a shift in both the sex ratio and the sex difference in a litter, in favor of male offspring. Our results in dog and pig suggest a common underlying mechanism affecting both cryptorchidism and sex ratio, which may hold true in other mammals as well.