Fig. 2. Food intake (A) and metabolizable energy intake (B) as a function of body mass for 12 male field voles prior to exposure to long day photoperiod (LD, 16 h:8 h L:D), during the increase phase (days 1–28 of LD exposure) and during the plateau phase (days 29–56 of LD exposure). Prior to exposure to LD, the animals were kept in short day photoperiod (SD, 8 h:16 h L:D). The measurements of food intake were repeated three times during the pre-exposure phase and four times during the increase and plateau phases. After adjusting for differences in body mass, photoperiod had no effect on food intake, but it did have a significant effect on MEI (for statistical details, see Results). The relationships are described by y=27.7+2.0x for pre-exposure, y=26.0+2.4x for LD increase and y=30.4+2.2x for LD plateau.