Reproduction in Domestic and Feral Swine. II. Interrelationships Between Fetal Size and Spacing and Litter Size12

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Abstract
Fetal and uterine relationships were evaluated and compared in 17 feral-Ossabaw and 12 Yorkshire primiparous gilts slaughtered at 80, 90, 100 or 110 days of gestation. Feral fetuses were smaller than Yorkshire fetuses at all gestational ages examined (P < 0.01). Within lines, fetuses located at the cervical, middle or oviductal thirds of the uterus were similar in weight (P < 0.1). The mean length of uterine horn per fetus increased linearly from day 80 to day 110 of gestation in the Yorkshire (r = 0.95, P < 0.01) and feral (r = 0.90, P < 0.01) lines. Uterine horns containing more fetuses were generally longer than horns with fewer fetuses. Total weight of fetuses in the uterine horn was related to the number of fetuses in the horn in both the ferals (r = 0.76, P < 0.01) and the Yorkshires (r = 0.57, P < 0.01), but individual fetal weights decreased as the fetus number in the litter increased. In either line there was no consistent relationship between fetal weight and the uterine space available to each fetus, as indicated by distance between adjacent fetuses. Apparently, the small litter size observed in the feral line is not attributable to a limitation of uterine capacity during late gestation. Further, the feral pig uterus may have the capability for greater growth in length during pregnancy than does the Yorkshire pig uterus.