Prediction of carcass fat from body measurements made on live rats differing in age, sex and strain.

I . Individual body fat and body measurements such as lengths, girths and selected skinfold thicknesses were determined in our laboratory strain (BHE), a highly inbred strain (IN) of BHE rats and in a strain of Wistar rats. Measurements were made on unconscious rats in less than 5 min per rat just before autopsy; body fat content was determined in individual rats of both sexes at 50, 100 and 300 days of age. 2. Among the strains, mean total body fat was highest in BHE rats at each age; IN rats of similar average body size were leanest. Statistically significant differences in body fat among the strains were due primarily to differences among the male rats at 300 days. Total protein and skeletal mdSS increased with increases in age and body size, as did body fat in rats after maturity. IN rats had the largest fat-free weights. Although significant age differences in body fat and body measurements occurred, they were in part dependent upon changes in bodyweight, sex and strain of the animals. Female rats had more fat per unit body-weight than males at each age studied. Females, though fatter than males, had smaller skinfold thicknesses, indicating that female fat increases are primarily in visceral fat. 3. Large variation in fat among individuals within strains of the same body-weight and age suggests a genetic influence in fat potential in rats not associated with age or body-weight. 4. Results from multiple regression analyses showed differences among adjusted means and partial regression coefficients due to strain and sex of the animals. Although final body-weight was the predictor common to all equations, body length, abdominal girth and subscapular skinfold were common to all but one set of equations. Chest girth, tibia length, triceps and abdominal skinfolds decreased in frequency of appearance in that order. Some of the measurements were more effective predictors at one age than at another. It was clearly necessary to take into account body dimensions other than weight to obtain an optimal prediction of body fat. 5. Differences in characteristics of the rats support the concept of genetic influences in fat deposition in individuals and indicate the complex nature of these influences.