In which of the following mammals is an estrous cycle absent because the females instead show a menstrual cycle similar to that of humans?

Introduction / Context:
Female mammals show cyclic changes in their reproductive systems that prepare the body for possible pregnancy. In many mammals, these changes are called the estrous cycle. In a smaller group, including humans and some monkeys and apes, the pattern is called the menstrual cycle. Knowing which animals have an estrous cycle and which have a menstrual cycle is a common question in reproductive biology and general knowledge.

Given Data / Assumptions:

• The animals listed are rabbit, horse, cow, and monkey.
• The question asks where the estrous cycle is absent because the animal shows a menstrual cycle instead.
• We assume standard zoology classification of reproductive cycles.
• We focus on whether the animal is typically described as having an estrous or menstrual cycle in textbooks.

Concept / Approach:
In animals with an estrous cycle, the reproductive tract is prepared for mating and pregnancy, and if pregnancy does not occur, the uterine lining is reabsorbed rather than shed. Behavioural changes called heat or estrus are prominent. In animals with a menstrual cycle, such as humans and many primates, the uterine lining is shed through menstruation if pregnancy does not occur. Rabbits, horses, and cows are commonly cited as examples of mammals with estrous cycles. Monkey females, like humans, are primates and are described as having menstrual cycles. Therefore, among the options, monkey is the mammal in which the estrous cycle is absent and replaced by a menstrual cycle.

Step-by-Step Solution:
Step 1: Recall that domestic mammals such as cows and horses are classically described as having estrous cycles. Step 2: Rabbits are also discussed in terms of estrous behaviour and induced ovulation rather than menstrual bleeding. Step 3: Primates, including humans and many monkeys, show menstrual cycles where the endometrium is shed if pregnancy does not occur. Step 4: Recognise that, in exam oriented zoology, animals that menstruate are said not to have an estrous cycle, but a menstrual cycle instead. Step 5: Match this information with the options and select monkey as the correct answer.

Verification / Alternative check:
Reproductive biology chapters often separate mammalian reproductive cycles into two types: estrous cycles and menstrual cycles. Examples given for menstrual cycles include humans, apes, and some monkeys, while many domestic animals like cows, horses, dogs, and rodents are cited under estrous cycles. The presence of visible menstrual bleeding is a key feature in primates. Since monkey belongs to this primate group, textbooks associate it with menstrual cycles rather than estrous cycles, confirming that monkey is the correct choice.

Why Other Options Are Wrong:
Rabbit is wrong because it is commonly described as having an estrous cycle and is often used as a model organism in reproductive studies focusing on induced ovulation, not menstruation. Horse is incorrect, as mares show an estrous cycle with distinct heat periods. Cow is also wrong because cows very clearly exhibit an estrous cycle, used in animal husbandry and breeding management. None of these animals show a typical menstrual cycle with cyclical shedding of the uterine lining as seen in primates.

Common Pitfalls:
A common pitfall is to assume that all mammals have the same type of reproductive cycle or to confuse estrous with general heat behaviour. Another mistake is not knowing that the term menstrual cycle is mainly associated with primates, including humans and certain monkeys. To avoid such errors, remember the simple rule: many domestic animals (cow, horse, dog, sheep) have estrous cycles, while humans and other higher primates (including monkeys and apes) have menstrual cycles. This rule directly leads you to select monkey as the correct answer.

Final Answer:
The correct option is Monkey, because in monkeys and other primates the estrous cycle is absent and is replaced by a menstrual cycle similar to that in humans.