RIB AND VERTEBRAL NUMBERS IN SWINE.: I. VARIATION OBSERVED IN A LARGE POPULATION

Abstract

In a population of 10,683 pigs of two breeds and their crosses the number of rib pairs ranged from 13.5 to 17.0 and the number of pre-sacral vertebrae (excluding cervical) from 20 to 23. The breeds and crosses studied overlapped completely in range of variability but differed in relative frequency of the various rib and vertical classes. The average regression or rib number on vertebral number ranged from.62 for Lacombe females to.77 for Yorkshire females. Bilateral asymmety in rib number was observed in 6.5 per cent of all pigs and 41 per cent of the 1,147 litters, examined contained asymmetrical pigs. Within-litter variation in rib and vertebral number (thoracic and lumbar) was frequent with a maximum observed range of 2.5 ribs and 3 vertebrae. Complete uniformity of littermates in rib number, in vertebral number, and in both traits was observed in 23, 31 and 13 per cent respectively of all litters. Males and females were essentially identical in average rib and vertebral number and in frequency of asymmetry. Variation in these skeletal traits was not associated with viability.