Types of Reproduction in Animals
Most animals reproduce sexually, but some simpler animals also use asexual reproduction:
- Budding (Hydra): a small outgrowth (bud) forms on the parent, grows, and eventually detaches as a new individual.
- Binary fission (Amoeba): the organism splits into two identical daughter cells.
- Fragmentation (Planaria/flatworms): the body breaks into fragments, each regenerating into a complete organism.
Advantages of sexual reproduction: produces genetic variation through the combination of two parents' DNA, enabling populations to adapt to changing environments.
Disadvantage: requires finding a mate; energy is invested in attracting a partner (e.g., courtship displays, bright colouration).
External vs Internal Fertilisation
| Feature | External Fertilisation | Internal Fertilisation |
|---|
| Where it occurs | Outside the body, in water | Inside the female's body |
| Examples | Fish, frogs (amphibians) | Reptiles, birds, mammals |
| Number of eggs | Very many (thousands) — most lost | Few — well protected |
| Parental care | Usually none | Usually present |
| Environment needed | Aquatic — sperm swim to eggs | Any — sperm travel inside body |
The Male Reproductive System
The male reproductive system produces sperm (male gametes) and delivers them during reproduction.
- Testes (singular: testis): two glands located in the scrotum (outside the body cavity) where sperm are produced by a process called spermatogenesis. The lower temperature of the scrotum (~2°C below body temperature) is essential for healthy sperm production.
- Epididymis: a coiled tube where sperm mature and are stored temporarily.
- Vas deferens (sperm duct): carries sperm from the epididymis toward the urethra.
- Seminal vesicles, prostate gland, Cowper's glands: accessory glands that add fluids to form semen — providing nutrients (fructose) and buffering the acidic environment of the vagina.
- Urethra: the tube through which both semen and urine pass out of the body (but not simultaneously).
- Penis: delivers semen into the female reproductive tract during intercourse.
Structure of a Sperm Cell
A sperm cell is highly specialised for its function of reaching and fertilising an egg:
- Head: contains the nucleus with the male's DNA (haploid — 23 chromosomes). The tip of the head has the acrosome, which releases enzymes to penetrate the egg's outer coat.
- Midpiece: packed with mitochondria to provide energy (ATP) for movement via cellular respiration.
- Tail (flagellum): whip-like structure that propels the sperm through fluid. Sperm can swim at ~3 mm/min.
Males produce approximately 200–500 million sperm per ejaculation, continuously from puberty throughout life.
The Female Reproductive System
- Ovaries: two almond-shaped organs that produce eggs (ova) by a process called oogenesis. They also produce the hormones oestrogen and progesterone.
- Fallopian tubes (oviducts): two tubes connecting each ovary to the uterus. Fertilisation usually occurs here. Tiny cilia lining the tube sweep the egg toward the uterus.
- Uterus (womb): a thick-walled, muscular organ where the embryo implants and the foetus develops. Its inner lining is the endometrium.
- Cervix: the narrow lower opening of the uterus; dilates during childbirth.
- Vagina: the muscular tube connecting the cervix to the outside; receives the penis during intercourse and is the birth canal.
Females are born with all their eggs already formed (~1–2 million immature oocytes). By puberty, ~400 000 remain. Only about 400 are ever ovulated in a lifetime.
The Menstrual Cycle
The menstrual cycle averages 28 days and is controlled by hormones. Key events:
- Day 1–5: Menstruation — the endometrium (uterine lining) sheds as a period because no pregnancy occurred in the previous cycle.
- Day 6–13: The endometrium rebuilds under the influence of oestrogen; an egg matures in a follicle in the ovary.
- Day 14: Ovulation — a surge in luteinising hormone (LH) triggers the release of the mature egg from the ovary.
- Day 15–28: The endometrium thickens under progesterone, preparing for possible implantation. If fertilisation does not occur, hormone levels drop and the cycle repeats.
Fertilisation
During sexual intercourse, millions of sperm are deposited in the vagina. They travel through the cervix and uterus and into the fallopian tubes. Of the millions released, only a few hundred reach the egg. Only one sperm penetrates the egg cell — the moment it does, chemical changes in the egg membrane prevent any other sperm from entering.
Sperm + Egg cell → Zygote (fertilised egg, 46 chromosomes)
The zygote contains a full set of 46 chromosomes (23 from each parent) and is genetically unique.
Implantation & Early Development
After fertilisation, the zygote begins to divide by mitosis as it travels along the fallopian tube toward the uterus. By the time it reaches the uterus (~5–7 days later) it has become a ball of cells called a blastocyst. It then implants into the thickened endometrium (uterine wall). Failure to implant results in loss of the embryo.
- 0–8 weeks: called an embryo; all major organs begin to form (organogenesis).
- 9 weeks – birth: called a foetus; organs develop and grow; recognisably human. Gestation period is ~38 weeks (about 9 months).
The Placenta, Umbilical Cord & Amniotic Fluid
These structures support and protect the developing foetus:
- Placenta: a disc-shaped organ embedded in the uterine wall. It has a large surface area of villi in contact with the mother's blood supply. Across this barrier, oxygen and nutrients pass from mother to foetus, and carbon dioxide and waste products pass from foetus to mother. The mother's and foetus's blood do not mix — they are separated by thin membranes. The placenta also produces hormones (hCG, progesterone, oestrogen) to maintain pregnancy.
- Umbilical cord: a flexible cord containing two arteries and one vein that connects the foetus to the placenta, carrying blood between them.
- Amniotic sac & fluid: the foetus floats in amniotic fluid inside the amniotic sac. The fluid cushions the foetus against physical shocks, maintains a stable temperature, and prevents the foetus from drying out.
Key point: The placenta acts as the foetus's lungs, digestive system, and kidneys all in one. It also acts as a barrier against most (but not all) pathogens and harmful substances — some drugs, alcohol, and viruses (e.g., HIV, rubella) can cross the placenta and harm the foetus.
Birth (Parturition)
At the end of gestation (~38 weeks), birth is triggered by a cascade of hormones (oxytocin is key):
- The cervix begins to dilate (widen) to allow the baby to pass through.
- Strong rhythmic uterine contractions (labour) push the baby downward.
- The amniotic sac ruptures ("waters break"), releasing amniotic fluid.
- The baby is born head-first through the vagina (birth canal).
- The umbilical cord is cut — the newborn must now breathe independently.
- The placenta is expelled as the "afterbirth" shortly after.
Sexually Transmitted Infections (STIs)
STIs are infections spread primarily through sexual contact. Two important examples:
- HIV/AIDS: Human Immunodeficiency Virus destroys CD4 T-cells of the immune system, eventually causing Acquired Immunodeficiency Syndrome. Transmitted via unprotected sex, blood contact, and mother-to-child (during birth or breastfeeding). No cure exists, but antiretroviral therapy (ART) allows people to live long, healthy lives.
- Syphilis: caused by the bacterium Treponema pallidum. Progresses in stages. Curable with antibiotics (penicillin) if caught early.
Consistent and correct use of condoms reduces the risk of most STIs and is the only contraceptive method that also prevents STI transmission.
Contraception
| Type | Method | How it works |
|---|
| Barrier | Male/female condom | Physically prevents sperm reaching egg; also protects against STIs |
| Hormonal | Contraceptive pill | Synthetic hormones prevent ovulation and alter uterine lining |
| Hormonal | Injection / implant / patch | Long-acting hormone release prevents ovulation |
| Intrauterine | IUD (coil) | Device in uterus prevents implantation |
| Surgical | Sterilisation (tubal ligation / vasectomy) | Permanent — fallopian tubes or vas deferens cut/tied |
| Abstinence | Not engaging in sexual intercourse | 100% effective when practised consistently |