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NDA Biology · Reproduction

Animal and Human Reproduction — Cycles and Contraception

Non-primate female mammals run an oestrus cycle while primates (including humans) run a menstrual cycle; oral contraceptive pills prevent pregnancy by using hormones to inhibit ovulation (the release of the egg).

All Reproduction notesNDA Biology strategy

Why this matters

Two recent MODERATE PYQs (2022, 2026) on applied human/animal reproduction. Both reward understanding a mechanism rather than a single fact: how a contraceptive pill actually works (it stops ovulation, it does not kill anything), and the oestrus-vs-menstrual distinction that separates primates from other mammals.

Concept 1 of 2

How oral contraceptive pills work — inhibiting ovulation

Intuition

An oral contraceptive pill does not attack the egg, sperm, or zygote. It works one step earlier: it carries hormones (oestrogen/progesterone-like) that trick the body into NOT releasing an egg in the first place. No ovulation means no egg for sperm to meet, so no fertilisation. The mechanism is prevention by suppression, not destruction.

Definition

Oral contraceptive pills prevent pregnancy by inhibiting ovulation — the release of the egg from the ovary.

  • The pills contain hormones that suppress FSH and LH (the pituitary hormones that normally trigger the egg's release).
  • Without the LH surge there is no ovulation, so no egg is available for fertilisation.
  • They do NOT 'kill' the egg, sperm, or zygote — they act before any of those would meet.

Worked example

A student claims oral contraceptive pills work by killing the sperm. Explain the actual mechanism.
  1. The pills contain hormones that suppress the pituitary hormones FSH and LH.
  2. Without the LH surge, the ovary does not release an egg — ovulation is inhibited.
  3. With no egg released, there is nothing for sperm to fertilise.
  4. So they prevent pregnancy by stopping ovulation, not by killing sperm.
Answer:Oral contraceptive pills inhibit ovulation (the release of the egg) — they do not kill the egg, sperm, or zygote.
Practice this concept3 quick reps

Practice — Level 1 (3 reps)

Quick reps to lock in the method. Try each, then check.

  1. 1.
    What is the mechanism of action of oral contraceptive pills?
  2. 2.
    Which pituitary hormones do contraceptive pills suppress?
  3. 3.
    Do contraceptive pills kill the sperm or egg?

From the bank · past-year question

Example 1ReproductionMODERATE
Which one of the following is the mechanism of action of oral contraceptive pills?

[Q92 · Apr · 2022]

They inhibit ovulation — they don't 'kill' anything

Distractors say the pill kills the egg, sperm, or zygote. All wrong — the pill's hormones prevent the egg from being released (inhibit ovulation), acting before fertilisation could ever happen.

Concept 2 of 2

Oestrus cycle vs menstrual cycle — primates are different

Intuition

Female mammals cycle their fertility, but in two different styles. Non-primate mammals (cows, rats, tigers, dogs) have an oestrus cycle — they come into 'heat' at fertile times and reabsorb the uterine lining if there is no pregnancy. Primates (monkeys, apes, humans) instead have a menstrual cycle — the lining is shed as menstrual bleeding. So if a question asks where the oestrus cycle is NOT seen, the answer is the primate.

Definition

Two patterns of the female reproductive cycle in mammals:

  • Oestrus cycle — seen in non-primate mammals (cows, rats, tigers, dogs). The female is sexually receptive only during 'heat' (oestrus); if no pregnancy, the uterine lining is reabsorbed (no bleeding).
  • Menstrual cycle — seen in primates (monkeys, apes, humans). If no pregnancy, the uterine lining is shed as menstrual flow.

So among a list of animals, the one with NO oestrus cycle is the primate (e.g. the monkey).

Worked example

Among cow, monkey, rat, and tiger, which does NOT show an oestrus cycle, and what cycle does it have instead?
  1. Cow, rat, and tiger are non-primate mammals — all show an oestrus cycle.
  2. The monkey is a primate.
  3. Primates have a menstrual cycle, not an oestrus cycle.
  4. So the monkey is the one without an oestrus cycle.
Answer:The monkey — being a primate, it has a menstrual cycle, not an oestrus cycle.
Practice this concept3 quick reps

Practice — Level 1 (3 reps)

Quick reps to lock in the method. Try each, then check.

  1. 1.
    Which group of mammals has a menstrual cycle instead of an oestrus cycle?
  2. 2.
    Do cows, rats, and tigers show an oestrus or a menstrual cycle?
  3. 3.
    In the oestrus cycle, what happens to the uterine lining if there is no pregnancy?

From the bank · past-year question

Example 2ReproductionMODERATE
In which one of the following groups of animals, oestrus cycle is not\textbf{\text{not}} observed?

[Q92 · Apr · 2026]

Primates have the MENSTRUAL cycle, not oestrus

If asked which animal lacks an oestrus cycle, pick the primate (monkey). Non-primate mammals (cow, rat, tiger, dog) all have oestrus cycles; primates replace it with the menstrual cycle.

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

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