Differences in body compositions, growth and food intakes between mice which have been selected for a small or large body size.

1. Q-strain mice selected for high-line (QLF) or low-line (QSC) body-weights at 6 weeks of age were culled to litters of two or eight (QLF-2, QLF-8, QSC-2, QSC-8) at birth and were suckled in these groups until 19 d of age. 2. Body-weights were measured daily for all groups and body compositions compared at birth and 19 and 42 d of age. Food intakes and urinary and faecal nitrogen were measured during metabolism trials between 19 and 42 d. 3. QLF-2 and QSC-2 mice grew faster than the corresponding groups of eight until 19 d of age. They also deposited more fat as a percentage of total gain. 4. In the period 19-42 d the influence of genetic selection reappeared and was manifest in a slowing of growth rates of QLF-2 and QSC-2 animals so that by 42 d of age there were no differences in body-weight between the groups within a line. 5. During the period 19-42 d the total food intakes of each group within a line did not differ although, on the basis of food intake per unit metabolic body-weight (g/kg body-weight 0.75 per d) QLF-2 and QSC-2 ate less food than QLF-8 and QSC-8 respectively. 6. The differences in body-weight at 19 d between groups were largely overcome by the increased contribution of protein and water to the weight gain of the groups of eight during the post-weaning period.