DIVERGENT SELECTION FOR HEAT-LOSS IN MICE .2. CORRELATED RESPONSES INFEED-INTAKE, BODY-MASS, BODY-COMPOSITION, AND NUMBER BORN THROUGH 15 GENERATIONS

Divergent selection for heat loss (kcal . kg(-.75). d(-1)), measured i n 9- to 11-wk-old male mice, was conducted for 15 generations. Selecti on for high (MH) and low (ML) heat loss and unselected control (MC) oc curred in each of three replicates for a total of nine unique lines. F eed intake in males was measured during Generations 9 through 15. Body mass at commencement of mating in females and at time of measurement of heat loss in males was recorded. Body fat percentage at 12 wk for a nimals of Generations 6, 10, and 14 was predicted as a function of ele ctrical conductivity and body mass. Litter size was recorded for all g enerations, and components of litter size were evaluated at Generation 11 in one replicate and Generation 12 in the other two replicates. Fe ed intake changed in the same direction as heat loss for the MH and ML selections; at Generation 15, the difference between MH and ML (P <.0 02) was 20.6% of the MC mean. Body mass did not change with selection for heat loss. Differences in body fat percentage were not significant in earlier generations, but at Generation 14, MH and ML were signific antly (P <.01) different with MH mice having the lowest fat percentage ; MC was intermediate. Selection had a significant (MH vs ML; P <.01) effect on litter size, causing an increase in MH and a decrease in ML. This difference was explained by a difference (P <.01) in ovulation r ate. There was no asymmetry of response in feed intake, fatness, litte r size, or number of ovulations.