In which one of the following groups of animals, oestrus cycle is not observed?

Non-primate mammals such as cows, rats, dogs and tigers exhibit an oestrus (heat) cycle, where the female is receptive to mating only during a specific phase and the uterine lining is reabsorbed if pregnancy does not occur. Primates including monkeys, apes and humans show a MENSTRUAL cycle instead, where the uterine lining is shed as menstrual flow and the female may be receptive at any time. Therefore monkeys do not exhibit the oestrus cycle.