Caloric efficiency and locomotor activity of H3^+/+ and H3^–/– mice. (a) Body weight and 3-hour food intake were measured after 21 hours of food restriction in 20- to 27-week-old male mice. H3^+/+ mice are indicated by filled squares (food intake) and open squares (body weight) and H3^–/– mice by filled triangles (food intake) and open triangles (body weight) (n = 10, *P < 0.05). (b) Basal and total oxygen consumption rate in 16- to 17-week-old female mice. Data are normalized to lean body mass. H3^+/+ mice are indicated by white bars and H3^–/– mice by black bars (n = 3 or 4, *P < 0.05; **P < 0.01). (c) Light-phase and dark-phase locomotor activity in 16- to 21-week-old female mice. H3^+/+ mice are indicated by squares and H3^–/– mice by triangles (n = 9–11). Measurements were taken for 4 consecutive days, and data from the last 3 days are shown. (d) Light-phase and dark-phase total locomotor activity in 16- to 21-week-old female mice. H3^+/+ mice are indicated by white bars and H3^–/– mice by black bars (n = 9–11, *P < 0.05). Measurements are averages for the last 3 days.