--- title: "How Animals Reproduce" book: "Primary & Middle School Biology" subject: biology language: en chapter: 23 exercises: 11 source: https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce --- # Chapter 23 — How Animals Reproduce In May the pond is loud with frog song; by June it is black with [tadpoles](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#ex-g2-animals-grow-up-frog). The song, it turns out, was the beginning of the story: reproduction — how [animals](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal) make the next generation — follows rules that hold from frogs to elephants, with marvellous variations in the details. ## 23.1 It takes two **Definition 23.1 (Sexual reproduction).** In [animals](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal), making young usually takes two parents: a *female* and a *male*. The female produces *egg cells*; the male produces *sperm cells*. A young [animal](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal) begins when one sperm cell joins one [egg](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born) cell — that joining is *fertilization*. Reproduction with two parents is called *sexual reproduction*. **Example 23.2 (Why the frogs sing).** The May chorus is the [males](#def-g4-animal-reproduction-sexual) calling the [females](#def-g4-animal-reproduction-sexual) to the pond. Each pair then releases its cells into the water — the [female](#def-g4-animal-reproduction-sexual)’s [eggs](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born), the [male](#def-g4-animal-reproduction-sexual)’s sperm — where [fertilization](#def-g4-animal-reproduction-sexual) happens. The black dots in the jelly a few days later are the young already growing: the song was step one of [Chapter 8](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#ch-g2-animals-grow-up)’s frog cycle. ![A few days after the chorus: frog spawn among the reeds, each jelly sphere a fertilized egg with a young frog beginning inside.](https://one-course.com/images/onecourse/chapters/biology-1/g4-animal-reproduction/img-0b910422a7a0.jpg) *A few days after the chorus: frog spawn among the reeds, each jelly sphere a fertilized [egg](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born) with a young frog beginning inside.* **Remark 23.3 (Two parents, blended luck).** A young [animal](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal) gets something from each parent — which is why a foal can have its mother’s color and its father’s white blaze. What exactly each parent hands over, and how, is one of the great questions ahead in this book; for now, note simply that every sexually produced [animal](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal) has two parents and resembles both. ## 23.2 Eggs laid, or young born alive **Definition 23.4 (Oviparous and viviparous).** [Definition 8.1](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born)’s two ways of being born now get their scientific names: - an [animal](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal) is *oviparous* if the [female](#def-g4-animal-reproduction-sexual) lays *[eggs](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born)* , and the young finish growing inside the [egg](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born) , outside her body: [birds](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups) , most [fish](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups) , most [reptiles](https://one-course.com/books/biology/1/en/chapter/21-classifying-animals#prop-g4-classifying-animals-classes) , [amphibians](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups) , [insects](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups) , snails; - an [animal](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal) is *viviparous* if the young grows *inside the mother’s body* , fed by her, and is *[born alive](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born)* : nearly all [mammals](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups) . **Example 23.5 (Two nurseries).** The hen’s chick spends three warm weeks in a shelled [egg](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born), living off the yolk ([Remark 8.3](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#rem-g2-animals-grow-up-egg)); the foal spends eleven months inside the mare, fed continuously by her body, and is born ready to stand within the hour. Same goal, two nurseries: one packed lunch in a [shell](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering), one room service inside. **Proposition 23.6 (Where fertilization happens).** For water-dwellers like [fish](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups) and frogs, [fertilization](#def-g4-animal-reproduction-sexual) commonly happens *in the water*, outside the [female](#def-g4-animal-reproduction-sexual)’s body — [eggs](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born) and sperm are released together and meet by the thousands. For land [animals](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal) — [birds](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups), [reptiles](https://one-course.com/books/biology/1/en/chapter/21-classifying-animals#prop-g4-classifying-animals-classes), [mammals](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups), [insects](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups) — the cells would dry out in the open, so the [male](#def-g4-animal-reproduction-sexual) places his sperm *inside the [female](#def-g4-animal-reproduction-sexual)’s body* during *mating*, and [fertilization](#def-g4-animal-reproduction-sexual) happens there. **Proof.** *Admitted at this level.* ∎ ## 23.3 Few young and much care, or many and none **Proposition 23.7 (Two strategies for the next generation).** Compare the counts: a codfish releases *millions* of [eggs](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born) a year and cares for none; an elephant has *one* calf every few years and guards it for a decade. Between these extremes, a rule holds across the [animal](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal) world: the more young a species produces, the less care each one gets — and the fewer it produces, the more each is protected and fed. Both strategies succeed: on average, enough young survive to replace the parents. **Proof.** *Admitted at this level.* ∎ **Example 23.8 (Counting the pond).** One frog pair’s clutch holds thousands of [eggs](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born). Herons, sticklebacks and dragonfly larvae (see [Example 19.4](https://one-course.com/books/biology/1/en/chapter/19-food-chains#ex-g3-food-chains-three)) eat [tadpoles](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#ex-g2-animals-grow-up-frog) all spring — and out of the thousands, a handful of froglets climb the bank in July. The enormous clutch is not waste: it is the frog’s answer to the pond’s dangers, with no parental guard at all. **Example 23.9 (The other extreme).** The whale calf is born alone, swims at once beside its mother, drinks her rich [milk](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born) for many months and is defended against every danger. One young at a time — but that one gets everything. [Mammals](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups), with their [milk](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born) ([Definition 8.1](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born)), are the great specialists of the few-and-cared-for strategy. **Method 23.10 (Reading a species’ strategy).** Given a species, ask: 1. How many young at a time — one, a dozen, thousands? 2. Who guards and feeds them — both parents, the mother, nobody? 3. Where do they start — [egg](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born) outside, or grown inside? 4. Check the balance of [Proposition 23.7](#prop-g4-animal-reproduction-strategies) : many young with little care, or few with much. ## 23.4 Exercises **Exercise 23.1 ★.** What is [fertilization](#def-g4-animal-reproduction-sexual)? Which cell comes from each parent? **Solution of Exercise 23.1.** The joining of one [sperm cell](#def-g4-animal-reproduction-sexual) with one [egg cell](#def-g4-animal-reproduction-sexual), which starts a new young [animal](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal). The [egg cell](#def-g4-animal-reproduction-sexual) comes from the [female](#def-g4-animal-reproduction-sexual), the [sperm cell](#def-g4-animal-reproduction-sexual) from the [male](#def-g4-animal-reproduction-sexual). **Exercise 23.2 ★.** Define [oviparous](#def-g4-animal-reproduction-ovivivi) and [viviparous](#def-g4-animal-reproduction-ovivivi), with two [animals](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal) of each kind. **Solution of Exercise 23.2.** [Oviparous](#def-g4-animal-reproduction-ovivivi): the [female](#def-g4-animal-reproduction-sexual) lays [eggs](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born) and the young finishes growing in the [egg](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born), outside her body — the hen, the frog (also trout, lizards, snails, [insects](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups)). [Viviparous](#def-g4-animal-reproduction-ovivivi): the young grows inside the mother, fed by her, and is [born alive](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born) — the mare, the whale (nearly all [mammals](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups)). **Exercise 23.3 ★.** Why were the frogs singing in May? Retell the pond’s story in three steps. **Solution of Exercise 23.3.** The [males](#def-g4-animal-reproduction-sexual) sang to call the [females](#def-g4-animal-reproduction-sexual) to the pond; each pair released its cells into the water, where sperm met [eggs](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born) — [fertilization](#def-g4-animal-reproduction-sexual); the fertilized [eggs](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born) in their jelly grew into the June [tadpoles](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#ex-g2-animals-grow-up-frog). **Exercise 23.4 ★.** Where does [fertilization](#def-g4-animal-reproduction-sexual) happen for the trout, and where for the hen? Why the difference, by [Proposition 23.6](#prop-g4-animal-reproduction-where)? **Solution of Exercise 23.4.** The trout’s happens in the water, outside the [female](#def-g4-animal-reproduction-sexual) — [eggs](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born) and sperm are shed together. The hen’s happens inside her body, during [mating](#prop-g4-animal-reproduction-where). On land the naked cells would dry out; in water they can safely meet outside. **Exercise 23.5 ★.** Compare the chick’s and the foal’s nurseries: where does each grow, and what feeds it? **Solution of Exercise 23.5.** The chick grows in a shelled [egg](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born) outside the hen, living off the yolk packed with it. The foal grows eleven months inside the mare, fed continuously by her body, and is [born alive](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born). **Exercise 23.6 ★.** State the rule linking the number of young to the care they receive. Give the two extreme examples of the chapter. **Solution of Exercise 23.6.** The more young a species produces, the less care each gets; the fewer, the more care. Extremes: the codfish’s millions of uncared-for [eggs](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born), and the elephant’s single calf guarded for a decade. **Exercise 23.7 ★.** The codfish’s millions of [eggs](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born) mostly never become adult cod. Why is the strategy successful anyway? **Solution of Exercise 23.7.** Because success only requires that, on average, enough young survive to replace the parents. Out of millions of [eggs](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born), a few always slip through the sea’s dangers — the huge number *is* the protection. **Exercise 23.8 ★.** Run [Method 23.10](#met-g4-animal-reproduction-read) on the cat: typical litter of about five, nursed and guarded by the mother for weeks, grown inside her. Which strategy side is she nearer to? **Solution of Exercise 23.8.** Few young (five is few compared to thousands), grown inside the mother, nursed and guarded: the cat sits clearly on the few-and-cared-for side — like the [mammals](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups) generally. **Exercise 23.9 ★★.** A foal often resembles both its parents at once. What does this show about what the young receives, by [Remark 23.3](#rem-g4-animal-reproduction-mix)? **Solution of Exercise 23.9.** That it receives something from *each* parent — the young is not a copy of its mother alone. Both parents contribute to what the foal becomes. **Exercise 23.10 ★★.** The stickleback [male](#def-g4-animal-reproduction-sexual) builds a nest, fans the [eggs](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born) and chases off intruders — rare devotion for a [fish](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups). Using [Proposition 23.7](#prop-g4-animal-reproduction-strategies), what would you predict about the size of his clutch, compared with the codfish’s millions? **Solution of Exercise 23.10.** A small clutch — some dozens or hundreds of [eggs](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born), not millions. Care and numbers trade off: a father who guards, fans and defends can bring a small clutch through dangers that would demand millions of unguarded [eggs](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born). **Exercise 23.11 ★★★.** Sea turtles crawl ashore to bury their [eggs](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born) in warm sand, then return to the sea. [Oviparous](#def-g4-animal-reproduction-ovivivi) or [viviparous](#def-g4-animal-reproduction-ovivivi)? Cared for or not? And why must this reptile, a swimmer all its life, still lay on *land*? (Use [Proposition 21.4](https://one-course.com/books/biology/1/en/chapter/21-classifying-animals#prop-g4-classifying-animals-classes)’s reptile features.) **Solution of Exercise 23.11.** [Oviparous](#def-g4-animal-reproduction-ovivivi) — and uncared for: the buried clutch is left to the sand’s warmth, and the hatchlings dash to the sea alone (their large numbers are their strategy). She must lay on land because reptile [eggs](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born) are shelled and breathe air — laid in the sea they would drown. Her class’s features, not her habits, set the rule.