The reproductive physiology and behaviour of red deer hinds on the Isle of Rhum, Scotland, were studied throughout three breeding seasons. The hinds first came into oestrus in the 2nd week of October as yearlings and when 2 and 3 years old. There was no evidence of a silent oestrus with ovulation preceding the first overt oestrus and, in the wild, it seems that most hinds must conceive at the first ovulation of the season. The mean length of the oestrous cycle was 18·3± 1·7 days. When animals were not allowed to become pregnant, some continued to come into oestrus until March, showing as many as eight cycles. The mean duration of gestation was 231±4·5 days (n = 13), and one animal gave birth to dizygotic twins of opposite sex. The mean birth weight of calves was 7·5±1·0 kg. All hinds gained weight during pregnancy, and lost weight during lactation relative to non-pregnant animals.
Most hinds conceived at the first oestrus in which they were mated, even if this was late in the season. There was a high incidence of maternal agalactia in hinds that calved late, which would normally have resulted in the death of the calf. Most hinds had an accessory ovulation early in pregnancy, which was not associated with oestrous behaviour.
Oestrous behaviour was recorded on more than 120 occasions and was least pronounced at the beginning and end of the mating season. Oestrus seldom lasted more than 24 hr. Matings were equally frequent by day or night; a stag might mount a hind many times before ejaculating, or succeed at the first attempt. Hinds were sometimes served more than once at a given oestrus, and by more than one stag.
The vaginal mucus at oestrus had a strong, penetrating smell and throughout the mating season there was a second odour which was sweet and musty. If a hind had been served by a stag, it was possible to smell his rutting odour on her rump. A hind's position in the social hierarchy did not change at oestrus, but some became much more aggressive towards subordinates and demonstrative towards humans. Homosexual behaviour was sometimes seen if two hinds were in oestrus simultaneously.