Prediction of fertility by mating latency and photoperiod in nulliparous and primiparous meadow voles (Microtus pennsylvanicus)

Abstract

Mating behaviour and litter production of female meadow voles (Microtus pennsylvanicus) housed in long (14 h light:10 h dark; long day; LD) or short (10 h light:14 h dark; short day; SD) photoperiods were monitored to determine whether the reduced birthrate of SD females resulted from a lack of copulation. All females mated, but fewer SD females gave birth. LD and SD females fell into three distinct groups based on mating latency. The rapid onset group (RO) mated between 7 min and 9 h after pairing, the intermediate onset group (IO) mated between 16-44 h and the late onset group (LO) mated after 58-262 h of male contact. Sixty-seven per cent of LD females were assigned to group RO, 27% to IO, and 6% to LO. In contrast, 30% of SD females were assigned to group RO, 35% to IO and 35% to LO. Fertility was predicted by mating latency. Sixty-nine per cent of RO, 93% of IO and 33% of LO animals gave birth. In a further experiment, a small-mouthed cup was added to the environment to serve as an escape for females wishing to avoid mating. Although females did not use the cup to escape male approaches, mating occurred in only 66% of SD females, but was observed in all LD females. In a final experiment, mating latency and litter production were recorded in primiparous LD and SD females initially observed in the first experiment. Group LO was eliminated in parous females; all primiparous LD and SD females mated within 48 h. Birthrates of LD (82%) and SD (73%) parous females were increased compared with birthrates of nulliparous females (LD=65%; SD=55%). These observations suggest that long day-length and parity increase spontaneous oestrus in meadow voles (50% of nulliparous and 80% of primiparous RO animals mated in less than 1.5 h). Females in the IO and LO groups are probably induced into oestrus, as normally described for arvicoline rodents, by direct male contact. Rapid mating ( < 48 h) predicts greater fertility for both LD and SD females, while delayed mating (>58h) predicts low fertility. Parity decreases mating latency and increases litter production. Short day females produce fewer litters than LD females in equivalent groups.