Effects of sex composition of the litter on anogenital distance in California mice (Peromyscus californicus)

Anogenital distance (AGD) is longer in male than in female neonatal rodents , but can be altered by the prenatal environment. The aim of this experimen t was to examine the effects of the sex Composition of the litter on AGD in a species with a small. lifter size. We found that the AGD distributions o f male and female California mice, Peromyscus californicus, overlap before weaning (33 days of age), but after weaning, males have a larger AGD than f emales. Because AGD is significantly correlated with body mass in both male s and females, we analyzed the effect of the sex composition of the litter on AGD, using ANCOVA on logarithmically transformed data, with the logarith m of body mass as the covariate. We showed that the sex composition of the litter does not affect AGD in males but has an effect on the AGD in females at birth and this effect is significant at 33 days of age. Females from li tters composed of more than 75% males had a longer AGD than those born in l itters composed principally of females. This study shows that intrauterine litter composition affects AGD in females of a species characterized by sma ll litter sizes, as has been shown in rodent species with large litter size s. Thus, AGD can be used as a predictor of masculinization of females due t o intrauterine position.