Biology • Year 12 • Module 5 • Lesson 4
Mammalian Reproduction — Fertilisation, Implantation, Pregnancy and Birth
Build HSC Band 5–6 extended-response technique by synthesising real reproductive data with the lesson's hormonal and developmental framework.
1. Extended response — evaluate the hormonal control of pregnancy (Band 5–6)
8 marks Band 5–6
Scenario. In a published study of singleton pregnancies in the Royal Hospital for Women, Sydney (n = 142, modelled on Csapo, Pulkkinen & Wiest, 1973, American Journal of Obstetrics & Gynecology 115: 759–765), researchers measured maternal progesterone and oxytocin concentrations from week 6 through week 40. Pregnancies in which corpus-luteum-derived progesterone was experimentally removed before week 7 invariably miscarried, while pregnancies in which the placenta had taken over progesterone production by week 8 continued normally to term. At term, oxytocin rose sharply in a positive feedback loop with cervical stretch, driving labour.
Figure adapted from Csapo, Pulkkinen & Wiest (1973), American Journal of Obstetrics & Gynecology 115, 759–765 — luteal vs placental progesterone in human pregnancy; oxytocin profile from Fuchs et al. (1991), American Journal of Obstetrics & Gynecology 165: 1515–1523.
Q1. Analyse the data above and evaluate the claim that "hormonal control is at least as important as fertilisation itself in successful mammalian reproduction." In your response you must:
- Define implantation and explain why hormonal support is required for the pregnancy to continue past the first trimester.
- Use specific values from the figure to compare the roles of progesterone and oxytocin across pregnancy.
- Refer to the Csapo (1973) finding (corpus-luteum removal experiment) to argue why progesterone is essential up to ~week 8 and why this dependency shifts to the placenta thereafter.
- Use at least one named hormone other than progesterone or oxytocin from the lesson (hCG, oestrogen or prolactin) to extend the argument beyond birth.
- Reach an evidence-based judgement — is hormonal control more important, equally important, or less important than fertilisation?
Plan your response using the lined space below.
2. Extended response — compare placental vs marsupial reproductive strategies (Band 5–6)
7 marks Band 5–6
Scenario. Researchers at the Australian Museum and CSIRO compared early-life mortality and offspring per reproductive event in two Australian mammals: the placental dingo (Canis familiaris dingo) and the marsupial eastern grey kangaroo (Macropus giganteus). Selected data adapted from Tyndale-Biscoe & Renfree (1987), Reproductive Physiology of Marsupials, Cambridge University Press, and Corbett (1995), The Dingo in Australia and Asia, UNSW Press.
| Trait | Dingo (placental) | Eastern grey kangaroo (marsupial) |
|---|---|---|
| Mean gestation length | 63 days | 36 days (followed by ~11 months in the pouch) |
| Placenta type during gestation | Long-lived chorioallantoic placenta; full exchange of O₂, nutrients, wastes | Short-lived yolk-sac placenta; limited exchange before birth |
| State of newborn at birth | Eyes closed but body fully formed (~250 g) | "Joey" the size of a jellybean (~0.8 g), incompletely developed |
| Litter size (mean per event) | 5 | 1 |
| Newborn-to-weaning mortality | ~30% | ~12% |
| Maternal energetic investment per surviving offspring | Moderate (~63 d gestation + ~8 wk lactation) | High (~36 d gestation + ~12 months pouch + lactation) |
Data adapted from Tyndale-Biscoe & Renfree (1987) and Corbett (1995).
Q2. Using the table above and lesson content on the placenta and hormonal maintenance of pregnancy, compare and evaluate the placental and marsupial reproductive strategies as mechanisms for continuity of species. In your response you must:
- Identify two structural or physiological differences between the two strategies, using the lesson's framework (gestation, placenta, hormonal support).
- Use at least three rows of the data table to support your comparison.
- Explain the reproductive trade-off (per-event offspring number vs per-offspring survival).
- Reach an environment-dependent judgement — neither strategy is universally superior, but each is well suited to a particular environmental context.
Use the lined space below.
Q1 — Hormonal control vs fertilisation (8 marks, Band 5–6)
Sample response. Fertilisation in the oviduct produces the diploid zygote, but on its own it cannot establish or maintain a pregnancy. Implantation — the embedding of the blastocyst into the endometrium — is the step that physically connects the embryo to maternal tissue and starts the placental exchange of gases, nutrients and wastes that allow long internal development. From week 6 the figure shows progesterone rising steadily from ~30 ng mL⁻¹ to ~200 ng mL⁻¹ at term, providing continuous support for the endometrium so that it does not break down. The Csapo (1973) corpus-luteum removal experiment is decisive evidence here: when luteal progesterone was removed before week 7, every pregnancy miscarried, but if removal was delayed until after week 8 — once the placenta had taken over progesterone production — pregnancies continued normally. This proves that progesterone maintenance, not fertilisation, is what keeps the pregnancy alive. The handover from corpus luteum to placenta is itself coordinated by hCG, which the embryo begins releasing immediately after implantation and which keeps the corpus luteum producing progesterone until the placenta is ready. Near term, the figure shows oxytocin staying near baseline (~5 pg mL⁻¹) for ~36 weeks then surging into a positive feedback loop that drives the rhythmic uterine contractions of labour. After birth, prolactin and oxytocin together sustain lactation, supporting offspring survival to reproductive age. Fertilisation is therefore necessary but is a single instant; hormonal control is required across roughly 40 weeks of pregnancy plus weeks of lactation. The judgement is that hormonal control is at least as important as fertilisation — without it, a successfully fertilised zygote cannot continue to a viable, surviving offspring.
Marking criteria.
- 1 mark — Defines implantation as the embedding of the blastocyst into the endometrium.
- 1 mark — Explains that hormonal support (progesterone-driven maintenance of the endometrium) is required for the pregnancy to continue past the first trimester.
- 1 mark — Uses specific progesterone values from the figure (e.g. rise from ~30 to ~200 ng mL⁻¹) to characterise its role across pregnancy.
- 1 mark — Uses specific oxytocin values/timing from the figure (flat ~5 pg mL⁻¹ until ~week 36, then surge) and links to its labour role.
- 1 mark — Correctly interprets the Csapo (1973) corpus-luteum removal result: removal before week 7 causes miscarriage; later removal does not, because the placenta has taken over progesterone production.
- 1 mark — Identifies one additional named hormone (hCG, oestrogen or prolactin) and gives its correct role (e.g. hCG maintains corpus-luteal progesterone, prolactin drives lactation).
- 1 mark — Explicitly compares "fertilisation = single event" with "hormonal control = sustained over weeks/months".
- 1 mark — Reaches a defensible evidence-based judgement (e.g. "hormonal control is at least as important" or "fertilisation is necessary but not sufficient"), with reasoning grounded in the data and lesson framework rather than restating both sides.
Band descriptors. Band 6 = 7–8 marks with quantitative use of the figure, correct interpretation of the Csapo experiment, and a sharp judgement. Band 5 = 5–6 marks with solid biology but only qualitative use of the data. Band 4 = 3–4 marks with mostly descriptive content and limited evaluation. Band 3 = 1–2 marks with recall only and no use of the figure.
Q2 — Placental vs marsupial strategies (7 marks, Band 5–6)
Sample response. The two strategies differ structurally in their placenta and gestation. The placental dingo develops a long-lived chorioallantoic placenta that supports ~63 days of full internal exchange of O₂, nutrients and wastes, so its newborns emerge ~250 g and fully formed. The marsupial kangaroo, by contrast, relies on a short-lived yolk-sac placenta giving only ~36 days of limited internal exchange, so its joey is born at ~0.8 g and incompletely developed and must then complete development externally in the pouch under hormonal control of lactation. This produces a clear reproductive trade-off: the dingo invests heavily in internal development and produces a litter of 5 per event but loses ~30% before weaning, while the kangaroo produces 1 offspring per event yet loses only ~12% before weaning because the pouch and prolonged lactation allow maternal control of the offspring's environment after birth. Per-event output is therefore not the same as per-offspring survival — the kangaroo's strategy gives much higher per-offspring survival despite producing fewer young, and the dingo's strategy gives higher per-event output at the cost of greater newborn mortality. Neither strategy is universally better. The placental strategy suits stable, resource-rich environments where rapid population growth pays off and where the mother can afford the energetic cost of carrying a fully formed fetus to term. The marsupial strategy suits the highly variable and often resource-poor Australian environment because the mother can pause development by holding a young joey in the pouch, and she can abandon a pouch young at low energetic cost if drought arrives. Both strategies successfully maintain continuity of species in their respective environments.
Marking criteria.
- 1 mark — Identifies a structural/physiological difference in the placenta (chorioallantoic vs short-lived yolk-sac) using the lesson's exchange framework.
- 1 mark — Identifies a second difference — typically gestation length or state of newborn at birth — and links it to the lesson.
- 1 mark — Uses at least three rows of the table (e.g. gestation, newborn state, litter size, mortality, energetic investment) with specific values.
- 1 mark — Explicitly states the per-event vs per-offspring trade-off (5 young / 30% mortality vs 1 young / 12% mortality).
- 1 mark — Names both species correctly and uses them consistently as the placental/marsupial exemplars.
- 1 mark — Provides an environment-dependent judgement (stable/resource-rich vs variable/arid) rather than ranking one strategy as universally superior.
- 1 mark — Links the comparison back to "continuity of species", recognising that both strategies achieve this in different ways.
Band descriptors. Band 6 = 6–7 marks with full use of the table, clear environment-dependent judgement and species-specific biology. Band 5 = 4–5 marks with solid comparison but a weaker judgement. Band 4 = 2–3 marks with description rather than evaluation. Band 3 = 1 mark with recall only.