[Variation in the number of nipples in Wistar-Imamichi rats (author's transl(]

In order to determine heritability of accessory nipples, variations in the number of nipples were investigated in 13-week old female Wistar-Imamichi rats, maintained as a closed colony in the Institute for Animal Reproduction. Most of the females examined (93.32%) had six pairs of nipples; three in the pectoral and three in the abdomino-inguinal regions. Whereas, 229 out of 3606 rats had one accessory nipple each, 9 animals had 2 accessories, and 3 females were deficient of the pectoral nipple. Most of the accessory nipples were found either in the caudal and cranial regions of the third pectoral nipples or in the regions between the second and third abdomino-inguinal nipples. The frequencies for appearance of the accessory nipples were higher in the pectoral regions than those in the abdomino-inguinal ones, and also in the left side than in the right (p < 0.01), respectively. In 208 out of 791 litters examined, 177 litters had only one sister-mate with accessory nipple, 29 litters had 2 sister-mates with accessories, and 2 litters had 3 sister-mates with accessories, respectively. The localization of accessory nipples of the sister mates within the litter was similar in most of the cases. Heritability of the appearance of accessory nipples was estimated to be 0.8074 by using the formula described by Falconer.