Fig. 5. Dry mass of gonads (testes and seminal vesicles) as a function of body mass for male field voles exposed to short day photoperiod (SD, 8 h:16 h L:D, N=38) and long day photoperiod (LD, 16 h:8 h L:D) for 1–28 days (the increase phase, N=20) or 29–70 days (the plateau phase, N=25). The relationships are described by y=–0.097+0.009x for SD, y=–0.146+0.012x for LD increase and y=0.152+0.006x for LD plateau (for statistical details, see Results).