Sex ratio in silver foxes: effects of domestication and the star gene

The course of changes in secondary sex ratio (proportion of males at birth) in silver foxes bred at the fur farm of this Institute was analyzed. Data collected over several years of breeding of a domesticated (experimental) population selected for amenability to domestication and of a commercial (control) were compared. A tendency to increase in secondary sex ratio was demonstrated in both populations. However, the proportion of males at birth was higher in domestic foxes. This proportion, calculated from the combined data for 1978-1993, was 0.538±0.005 and 0.511±0.007 in the selected and commercial populations, respectively. The minimal departure of the observed sex ratio from 0.5 was demonstrated for litters with five pups, which is close to the average litter size in fox populations. The proportion of males increases with both increasing and decreasing litter size. An analysis of secondary sex ratio with respect to maternal age revealed a minimal departure of sex ratio from the expected in offspring from foxes of optimal reproductive age (2-4 years). An effect of the autosomal semidominant coat color mutation star on male excess at birth was also found: secondary sex ratio was higher (0.583±0.015) in offspring of mothers heterozygous for the star mutation than from standard types of the domesticated population. The increase in secondary sex ratio in the analyzed fox populations is viewed as a correlated response to selection for domestication. The hormonal mechanisms mediating the effects of both this selection and the star mutation on sex ratio at birth are discussed.