PREDICTION OF FERTILITY BY MATING LATENCY AND PHOTOPERIOD IN NULLIPAROUS AND PRIMIPAROUS MEADOW VOLES (MICROTUS-PENNSYLVANICUS)

Mating behaviour and litter production of female meadow voles (Microtu s 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 monit ored to determine whether the reduced birthrate of SD females resulted from a lack of copulation. All females mated, but fewer SD females ga ve birth. LD and SD females fell into three distinct groups based on m ating latency. The rapid onset group (RO) mated between 7 min and 9 h after pairing, the intermediate onset group (10) mated between 16-44 h and the late onset group (LO) mated after 58-262 h of male contact. S ixty-seven per cent of LD females were assigned to group RO, 27% to 10 , 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 10 and 33% of LO animals gave birt h. In a further experiment, a small-mouthed cup was added to the envir onment to serve as an escape for females wishing to avoid mating. Alth ough females did not use the cup to escape male approaches, mating occ urred in only 66% of SD females, but was observed in all LD females. I n a final experiment, mating latency and litter production were record ed in primiparous LD and SD females initially observed in the first ex periment. Group LO was eliminated in parous females; all primiparous L D and SD females mated within 48 h. Birthrates of LD (82%) and SD (73% ) parous females were increased compared with birthrates of nulliparou s females (LD=65%; SD=55%). These observations suggest that long day-l ength 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 fe males, while delayed mating (> 5 8 h) predicts low fertility. Parity d ecreases mating latency and increases litter production. Short day fem ales produce fewer litters than LD females in equivalent groups.