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

The reproductive cycle in female placental mammals is categorized into two types: the oestrus cycle and the menstrual cycle. The oestrus cycle is observed in non-primate mammals such as cows, rats, and tigers. In this cycle, the endometrium is reabsorbed by the body if pregnancy does not occur, and the animal is sexually receptive only during a specific phase known as "heat."

On the other hand, the menstrual cycle occurs in primates, including humans, apes, and monkeys. In these animals, the endometrium is shed through menstruation if fertilization does not take place. Therefore, monkeys do not undergo an oestrus cycle; they undergo a menstrual cycle.