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Field notes · chapter 51

The Reproductive System

pp. 1133–1154 · Raven Part VII

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51.1

Animal Reproductive Strategies

pp. 1134–1136
  • Sexual reproduction uses meiosis to make haploid gametes — sperm and egg (ovum). Fertilization fuses them, restoring the diploid number in a zygote, which then develops by mitosis. Watch Asexual and Sexual Reproduction on YouTube, from Amoeba Sisters
  • Asexual reproduction makes genetically identical offspring by mitosis from a single parent. Standard in bacteria, archaea, and protists, and used by some animals such as cnidarians and tunicates. Watch Asexual and Sexual Reproduction on YouTube, from Amoeba Sisters
  • Fission: a single-celled organism splits into two separate, identical organisms. Watch Asexual and Sexual Reproduction on YouTube, from Amoeba Sisters
  • Budding: part of the parent body separates off and differentiates into a new individual. It may detach completely or stay attached and form a colony. Common in cnidarians. Watch Asexual and Sexual Reproduction on YouTube, from Amoeba Sisters
  • Parthenogenesis: females produce offspring from unfertilized eggs. Widespread in arthropods. Some species are permanently parthenogenic and all-female; others alternate between sexual and parthenogenic reproduction, producing both diploid and haploid offspring. Watch Sexual & Asexual Reproduction: How Animals Do It: Crash Course Biology #47 on YouTube, from CrashCourse
  • Honeybee example: the queen mates once, stores the sperm, and controls its release. Unfertilized eggs become haploid drones (males, 16 chromosomes) by parthenogenesis. Fertilized eggs become diploid females (32 chromosomes) — workers, or queens if given the right hormone exposure. Watch Sex Determination: More Complicated Than You Thought on YouTube, from TED-Ed
  • That is the haploid-diploid sex-determination system of many bees and ants: no sex chromosomes at all — males are haploid, females diploid. Watch Sex Determination: More Complicated Than You Thought on YouTube, from TED-Ed
  • Parthenogenesis exists in vertebrates too. In 1958 a Russian biologist found all-female Lacerta lizard populations laying viable unfertilized eggs; it was later found in other lizards, fish, and salamanders. Some normally sexual species — certain crustaceans, sharks, snakes — can do it occasionally.
  • Hermaphroditism: one individual has both testes and ovaries and can make both sperm and eggs. Tapeworms are hermaphroditic and can self-fertilize, which matters because they rarely meet another tapeworm. Watch Sexual & Asexual Reproduction: How Animals Do It: Crash Course Biology #47 on YouTube, from CrashCourse
  • Most hermaphrodites still need a partner. Earthworms copulate with each partner acting as both male and female, and both leave with fertilized eggs.
  • Sequential hermaphroditism is a sex change over a lifetime. Protogyny ("first female") is female-to-male; protandry ("first male") is male-to-female. Watch Sexual & Asexual Reproduction: How Animals Do It: Crash Course Biology #47 on YouTube, from CrashCourse
  • Protogyny example: coral reef fish living in schools where only one or a few dominant males breed. Remove the dominant male and the largest female rapidly converts into the new dominant male — sex change under social control.
  • Sex determination is either genetic or environmental. Temperature-sensitive determination occurs in many fish and reptiles, and the pattern varies: in some species cold gives males and warm gives females (or the reverse); in others both high and low temperatures give males with intermediate temperatures giving females. Watch Sex Determination: More Complicated Than You Thought on YouTube, from TED-Ed
  • Temperature-dependent sex determination evolved independently many times from genetically determined ancestors. Its adaptive value is unclear — one unconfirmed hypothesis is that it lets a mother control offspring sex by choosing a nest site. Watch Sex Determination: More Complicated Than You Thought on YouTube, from TED-Ed
  • Genetic sex determination occurs in birds, mammals, and many other vertebrates. In mammals, XY = male and XX = female, so the male is the heterogametic sex. In birds it is reversed: the female is heterogametic. Watch Sex Determination: More Complicated Than You Thought on YouTube, from TED-Ed
  • Human sexual differentiation: male and female systems are indistinguishable for the first ~40 days after conception. Primordial germ cells migrate from the yolk sac into indifferent gonads that can become either testes or ovaries.
  • The SRY gene — sex-determining region of the Y — triggers the indifferent gonads to become testes. With no Y and therefore no SRY, they default to ovaries. Female is the default developmental pathway.
  • Once testes form they secrete testosterone and other hormones that build the male external genitalia and accessory organs.
  • Two lines of evidence for SRY. First, translocating an SRY-bearing Y fragment onto an X can make an XX individual develop as male. Second, androgen insensitivity disorders show what happens when the downstream hormone signal cannot be received.
  • Scientific Thinking test of the hypothesis. Prediction 1: no Y chromosome → female. Unusual karyotypes confirm it — Turner syndrome (XO) develops female, Klinefelter syndrome (XXY) develops male. So maleness tracks the presence of Y, not the count of X.
  • Prediction 2: SRY itself is the trigger. Transgenic XY mice carrying a mutated SRY develop as females — direct causal confirmation.
  • Timing difference in the two pathways: seminiferous tubules and Leydig cells develop early in the embryonic testis, while ovarian follicles do not develop until the third trimester.
51.2

Vertebrate Fertilization and Development

pp. 1136–1140
  • Vertebrate reproduction began in the ocean. Most marine bony fish females release batches of eggs into the water while males release sperm nearby — external fertilization. Watch Sexual & Asexual Reproduction: How Animals Do It: Crash Course Biology #47 on YouTube, from CrashCourse
  • Seawater does not harm gametes but disperses them fast, so release must be tightly synchronized. Many species spawn only in short periodic windows, some just once a year.
  • The ocean lacks strong seasonal cues, so many marine species time spawning to the lunar cycle — the moon comes closest to Earth about every four weeks, raising the tides.
  • On land, small naked gametes desiccate. That risk drove strong selection for internal fertilization in terrestrial vertebrates, keeping fertilization in a protected wet environment. Watch Sexual & Asexual Reproduction: How Animals Do It: Crash Course Biology #47 on YouTube, from CrashCourse
  • Oviparity: internally fertilized eggs are laid and finish developing outside the mother. Seen in some bony fish, most reptiles, some cartilaginous fish, some amphibians, a few mammals, and all birds. Watch The three different ways mammals give birth - Kate Slabosky on YouTube, from TED-Ed
  • Ovoviviparity: fertilized eggs are retained inside the mother but the embryo is still nourished by egg yolk, not maternal blood. Young are fully developed at release. Mollies, guppies, mosquitofish, some sharks and rays, many reptiles. Watch The three different ways mammals give birth - Kate Slabosky on YouTube, from TED-Ed
  • Viviparity: young develop inside the mother and are nourished directly from her blood through a placenta. Most cartilaginous fish, some amphibians, a few reptiles, and almost all mammals. The placenta evolved once in mammals and separately several times in fish and lizards. Watch The three different ways mammals give birth - Kate Slabosky on YouTube, from TED-Ed
  • Live birth (viviparity or ovoviviparity) evolved many times — once in mammals, and independently many times in fish, amphibians, and reptiles. Once it evolves it is essentially never lost, a one-way evolutionary trend. Watch The three different ways mammals give birth - Kate Slabosky on YouTube, from TED-Ed
  • Live birth requires internal fertilization. Internal fertilization arose once in the amniote lineage (reptiles, birds, mammals) but many separate times in fish and amphibians. Watch Sexual & Asexual Reproduction: How Animals Do It: Crash Course Biology #47 on YouTube, from CrashCourse
  • Sperm-transfer methods vary. Salamanders deposit a spermatophore, a gelatinous packet the female picks up with her cloaca. Intromittent organs evolved repeatedly: a modified pelvic fin in cartilaginous fish, a modified cloaca in some frogs and caecilians, a penis derived independently in turtles, crocodiles, and mammals, and paired hemipenes in snakes and lizards. Birds and tuatara lost intromittent organs and fertilize by pressing cloacae together.
  • Bony fish (teleosts) mostly fertilize externally. Eggs carry limited yolk, the embryo must feed itself soon after hatching, development is fast, and very few of the enormous number of eggs reach maturity.
  • Cartilaginous fish (sharks, rays) mostly use internal fertilization and viviparity — few offspring, but each well developed. The opposite strategy from bony fish.
  • Amphibians stay tied to water and mostly fertilize externally. In frogs and toads the male clasps the female — amplexus — and releases sperm over the eggs as she extrudes them. Watch Sexual & Asexual Reproduction: How Animals Do It: Crash Course Biology #47 on YouTube, from CrashCourse
  • Amphibian exceptions: some frogs carry eggs on their backs, Surinam toads brood froglets in back pouches, and Darwin’s frog males brood tadpoles in the vocal sac, from which the young exit through the mouth.
  • About 80% of reptile species are oviparous. Eggs gain a leathery shell in the oviduct and most are then abandoned. Others are ovoviviparous, and some fully viviparous.
  • All birds fertilize internally. Most males lack a penis, though swans, geese, and ostriches use a modified cloacal wall as an intromittent organ. As the egg passes down the oviduct, glands add albumin (egg white) and a hard calcareous shell — unlike most reptile eggs.
  • Birds are endothermic, so most must incubate their eggs, and chicks hatch underdeveloped and require feeding.
  • The amniotic egg is the key land adaptation of reptiles and birds. The embryo develops in a fluid-filled cavity inside the amnion, with three more extraembryonic membranes — chorion (lining the shell), yolk sac, and allantois — plus the shell, making the egg desiccation-resistant.
  • Fish and amphibian eggs have only one extraembryonic membrane, the yolk sac, so they must be laid in water. Viviparous mammals also have extraembryonic membranes.
  • Ovulation is the periodic release of a mature ovum from the ovary; that periodicity defines the female reproductive cycle. Males stay reproductively constant while females cycle.
  • Estrus is the period of sexual receptivity around ovulation in most mammals ("in heat"), and the cycle built around it is the estrous cycle. Cycling continues until pregnancy. FSH and LH changes drive egg development and ovarian hormone secretion in both estrous and menstrual cycles.
  • Humans and apes have menstrual cycles, hormonally similar but different in two ways: they shed the uterine lining with bleeding rather than reabsorbing it, and females may copulate at any point in the cycle, not just around ovulation. Watch Reproductive System, Part 1 - Female Reproductive System: Crash Course Anatomy & Physiology #40 on YouTube, from CrashCourse
  • Induced ovulators — rabbits and cats — break the pattern entirely: they ovulate only after copulation, through a reflex release of LH, not on a fixed schedule.
  • Three mammal groups. Monotremes (platypus, echidna only) are oviparous like their reptile ancestors, incubate eggs, and their hatchlings lick milk off the mother’s skin because there are no nipples. Watch The three different ways mammals give birth - Kate Slabosky on YouTube, from TED-Ed
  • Marsupials (opossums, kangaroos) are viviparous but give birth to tiny, fetus-like young that finish developing attached to a nipple inside the pouch. Watch The three different ways mammals give birth - Kate Slabosky on YouTube, from TED-Ed
  • Placental mammals retain the fetus far longer in the uterus. The placenta forms from an extraembryonic membrane (the chorion) plus the mother’s uterine lining, bringing fetal and maternal vessels close so nutrients diffuse from maternal blood. Watch The three different ways mammals give birth - Kate Slabosky on YouTube, from TED-Ed Watch Reproductive System, Part 4 - Pregnancy & Development: Crash Course Anatomy & Physiology #43 on YouTube, from CrashCourse
51.3

Structure and Function of the Human Male Reproductive System

pp. 1140–1143
51.4

Structure and Function of the Human Female Reproductive System

pp. 1144–1148
51.5

Contraception and Infertility Treatments

pp. 1148–1151
  • In most vertebrates copulation is strictly tied to reproduction and female receptivity is limited to fertile periods. Humans and a few apes differ — receptivity spans the whole cycle, serving a second function: reinforcing pair-bonding.
  • Birth control is the general term for pregnancy prevention. Physiologically pregnancy begins not at fertilization but about a week later at implantation, so methods acting before implantation are called contraception. Watch How do contraceptives work? - NWHunter on YouTube, from TED-Ed
  • Abstinence is the most certain method but the hardest to sustain, and failures happen despite intent.
  • Condoms encase the penis and catch semen. Simple in principle, but real-world failure runs 3–20% per year (Table 51.2 lists 18) from inconsistent or incorrect use. Condoms are the most common US method, and they also reduce STD and AIDS transmission — over a billion are sold yearly in the US. Watch How do contraceptives work? - NWHunter on YouTube, from TED-Ed
  • Diaphragm and cervical cap physically cover the cervix and hold spermicide. Both need individual fitting by a physician because cervix size varies. The diaphragm goes in before intercourse and fails about 12% per year; the cap can stay in for days and fails at a similar or slightly lower rate. Watch How do contraceptives work? - NWHunter on YouTube, from TED-Ed
  • Douching — rinsing the vagina right after intercourse — is unreliable and can backfire, with roughly 40% annual failure, because it can push sperm further into the uterus.
  • Spermicidal jellies and foams applied before intercourse kill or block sperm; failure 10–28% per year. Combining a spermicide with a condom or diaphragm beats either alone. They must be applied 5–10 minutes before each act. Watch How do contraceptives work? - NWHunter on YouTube, from TED-Ed
  • Oral contraceptives prevent ovulation by suppressing FSH and LH with progesterone analogs (with or without estrogen) — deliberately mimicking the luteal-phase negative feedback that normally blocks follicle development. Watch How do contraceptives work? - NWHunter on YouTube, from TED-Ed Watch Female Reproductive System - Menstrual Cycle, Hormones and Regulation on YouTube, from Armando Hasudungan
  • Pill regimen: hormone pills for three weeks, then a fourth hormone-free week that lets levels drop and triggers menstruation. Perfect use ~1% failure per year, typical use ~9% because of missed pills.
  • A hormone implant placed under the skin is the same idea with a failure rate below 1% (0.05 in Table 51.2). Injectable contraceptives given every 3 months block ovulation with a failure rate of 6. Watch How do contraceptives work? - NWHunter on YouTube, from TED-Ed
  • Pill side effects can include blood clotting and nausea; newer generations with less estrogen and different progesterone analogs have reduced these. Newer pills lower the risk of endometrial cancer, ovarian cancer, cardiovascular disease, and osteoporosis in older women, while evidence on breast and cervical cancer risk is not clear-cut.
  • Pill risks rise notably in smokers and greatly in women over 35 who smoke. Consensus: for many women the benefits outweigh the risks, but a physician should weigh the individual case.
  • IUD (intrauterine device): a small coil or object placed in the uterus that prevents implantation by induced irritation; copper and hormone-releasing types exist. Failure about 1%, partly because once placed it can be forgotten. About a third of users get cramps, pain, or bleeding and must discontinue, and insertion carries a risk of uterine infection. Watch How do contraceptives work? - NWHunter on YouTube, from TED-Ed
  • Morning-after pill: emergency contraception containing about 50 times the estrogen of a regular pill. It stops ovum development, prevents fertilization, or blocks implantation. Failure 1–10% per use. Not for routine use because of the hormone dose and possible severe side effects. Watch How do contraceptives work? - NWHunter on YouTube, from TED-Ed
  • Sterilization surgically removes portions of the gamete-transport tubes and is almost 100% effective. Vasectomy cuts and ties the vas deferens from each testis so sperm cannot enter semen. Tubal ligation removes and ties off a section of each Fallopian tube so the ovulated oocyte cannot reach the uterus. Watch How do contraceptives work? - NWHunter on YouTube, from TED-Ed
  • Tubes very rarely reconnect on their own — more often after vasectomy than tubal ligation — which is why neither is quoted at exactly 100%. Both can also be surgically reversed on purpose, though vasectomy reversal is expensive and often unsuccessful.
  • Infertility = failure to conceive after 12 months of contraception-free intercourse. About 40% of cases trace to the male, about 45% to the female, and about 15% are unexplained.
  • Female infertility can fail at any stage from oocyte production through implantation; the most common causes are failure to ovulate and mechanical blockage.
  • Pelvic inflammatory disease (PID) is the leading worldwide cause — bacterial infection blocks the Fallopian tubes, stopping both sperm passage and transfer of the fertilized egg.
  • Endometriosis — endometrial tissue growing outside the uterus — works the same way: the body walls it off with scar tissue that can block egg transfer.
  • Premature ovarian failure (POF): fertility drops with age and chromosome nondisjunction rises; a woman under 40 with a diminished egg supply is considered diagnostic.
  • Hypogonadotropic hypogonadism: infertility from decreased GnRH, caused by hypothalamic or pituitary damage or any disorder shifting hypothalamic hormones (diabetes, thyroid disease, excess adrenal androgens). Excessive exercise and anorexia also lower GnRH.
  • Luteal-phase imbalance: inadequate progesterone leaves the uterine wall too thin, which can block implantation or raise the chance of spontaneous abortion.
  • Male infertility usually means reduced sperm number, viability, or motility — from infection, hormonal imbalance, autoimmunity against sperm, gland abnormalities, or damage to the vas deferens or seminiferous tubules. Male testing is easier because collection is noninvasive.
  • Up to about 5% of infertile men are unexplained, at a rate consistent worldwide despite differing environments, suggesting a genetic contribution. Drosophila studies estimate up to 1500 recessive genes may affect male fertility.
  • Hormone treatment for ovulatory defects aims to produce a single high surge of FSH and LH. The usual drug is clomiphene (Clomid), a competitive estrogen-receptor inhibitor that blocks the normal negative feedback of estradiol, so FSH and LH rise. If that fails, gonadotropins can be injected directly.
  • Assisted reproductive technology (ART): artificial insemination places sperm in the female tract; GIFT (gametic intrafallopian transfer) places both sperm and egg into the Fallopian tube; IVF (in vitro fertilization) fertilizes outside the body, followed by embryo transfer (ET); ICSI injects a single sperm directly into an egg when sperm cannot fertilize on their own.
  • The first "test tube baby" was born in 1978. The main downside of ART is multiple births, from transferring more than one embryo to raise the odds of implantation; better embryo monitoring now allows selecting the best one or two.
  • Freezing sperm, eggs, and whole embryos avoids repeated invasive procedures like oocyte harvesting. Live births have come from every combination of frozen and fresh gametes and embryos, which makes single-embryo transfer practical with the rest frozen for later.