Changes in the body composition of mice selected for high and low eight week weight.

Body composition was studied in three lines of mice, one selected for high (H) and one for low (L) 8 week weight, and one maintained as an unselected control (C). After 25 generations 8 week weights were 41.2g, 30.6 g and 20.5g for the H, C and L lines. Mice were sampled from the lines and analysed for fat, protein, ash and water at generations 14 and 25. Apart from fat in the H line, there was little alteration due to selection in the relationships between individual body components and total body weight. In the H line, the contribution of fat to body weight gain was considerably increased. Although leaner than the C and L mice at low body weights, H line mice rapidly became fatter with increasing body weight. Selection appeared to reduce the body weight at which fat was deposited at its maximum rate in the H line. The H and C lines were equally fat at body weights of 29.0 g and 21.6 g at generations 14 and 25 respectively. Body weights at points of inflection of the growth curves of the H, C and L lines at generation 25 were 18.3 g, 14.3 g and 12.8 g. The implications of these findings for meat species slaughtered at set weights are discussed.