Chapter 46 Animal Reproduction
Overview: Doubling Up for Sexual Reproduction
Concept 46.1 Both asexual and sexual reproduction occur in the
• Asexual reproduction involves the formation of individuals whose
genes come from a single parent.
• There is no fusion of sperm and egg.
• Sexual reproduction is the formation of offspring by the fusion of
haploid gametes to form a diploid zygote.
• The female gamete, the unfertilized egg, or ovum, is usually
large and nonmotile.
• The male gamete is the sperm, which is usually small and
• Sexual reproduction increases genetic variation among
offspring by generating unique combinations of genes inherited
from two parents.
Diverse mechanisms of asexual reproduction enable animals to
produce identical offspring rapidly.
• Many invertebrates can reproduce asexually by fission, in which a
parent separates into two or more approximately equal-sized
• Budding is also common among invertebrates. This is a form of
asexual reproduction in which new individuals split off from
• In fragmentation, the body breaks into several pieces, some or
all of which develop into complete adults.
• Reproducing in this way requires regeneration of lost
body parts. • Many animals can also replace new appendages by
• Asexual reproduction has a number of advantages.
• It allows isolated animals to reproduce without needing to find a
• It can create numerous offspring in a short period of time.
• In stable environments, it allows for the perpetuation of
Reproductive cycles and patterns vary extensively among mammals.
• Most animals exhibit cycles in reproductive activity, usually related to
• This allows animals to conserve resources and reproduce when
more energy is available and when environmental conditions
favor the survival of offspring.
• Reproductive cycles are controlled by a combination of environmental
and hormonal cues.
• Environmental cues may include seasonal temperature, rainfall,
day length, and lunar cycles.
• Animals may reproduce exclusively asexually or sexually or they may
alternate between the two modes, depending on environmental
• Daphnia reproduce by parthenogenesis under favorable
conditions and sexually during times of environmental stress.
• Parthenogenesis is the process by which an unfertilized egg develops
without being fertilized.
• Parthenogenesis plays a role in the social organization of some
bees, wasps, and ants.
• Male honeybees (drones) are haploid, and female
honeybees (queens and workers) are diploid. • Several genera of fishes, amphibians, and lizards reproduce by
a form of parthenogenesis that produces diploid “zygotes.”
• Fifteen species of whiptail lizards reproduce exclusively
• There are no males in this species, but the lizards imitate
courtship and mating behavior typical of sexual species of
the same genus.
• Sexual reproduction presents a problem for sessile or burrowing
animals or parasites that may have difficulty encountering a member
of the opposite sex.
• One solution is hermaphroditism, in which one individual
functions as both a male and a female.
• Some hermaphrodites can self-fertilize, but most mate
with another member of the same species.
• In such matings, each individual receives and
• This results in twice as many offspring as would be
produced if only one set of eggs were fertilized.
• In sequential hermaphroditism, an individual reverses its sex
during its lifetime.
• In some species, the sequential hermaphrodite is female
• In other species, the sequential hermaphrodite is male
Concept 46.2 Fertilization depends on mechanisms that help sperm
meet eggs of the same species
• The mechanisms of fertilization, the union of sperm and egg, play an
important part in sexual reproduction.
• In external fertilization, eggs are released by the female into a
wet environment, where they are fertilized by the male. • In species with internal fertilization, sperm are deposited in or
near the female reproductive tract, and fertilization occurs
within the tract.
• A moist habitat is almost always required for external fertilization,
both to prevent gametes from drying out and to allow the sperm to
swim to the eggs.
• In species with external fertilization, timing is crucial to ensure that
mature sperm encounter ripe eggs.
• Environmental cues such as temperature or day length may
cause gamete release by the whole population.
• Individuals may engage in courtship behavior that leads to
fertilization of the eggs of one female by one male.
• Internal fertilization is an adaptation to terrestrial life that enables
sperm to reach an egg in a dry environment.
• Internal fertilization requires sophisticated reproductive
systems, including copulatory organs that deliver sperm and
receptacles for their storage and transport to ripe eggs.
• Mating animals may use pheromones, chemical signals released by
one organism that influence the behavior or physiology of other
individuals of the same species.
• Pheromones are small, volatile, or water-soluble molecules that
disperse into the environment.
• Like hormones, pheromones are active in minute amounts.
• Many pheromones act as male attractants.
• All species produce more offspring than can survive to reproduce.
• Internal fertilization usually involves the production of fewer zygotes
than does external fertilization.
• However, the survival rate is higher for internal fertilization. • Major types of protection include tough eggshells, development
of the embryo within the reproductive tract of the mother, and
parental care of the eggs and offspring.
• Marsupial mammals retain their embryos for only a short period in the
• The embryos crawl out and complete fetal development
attached to a mammary gland in the mother’s pouch.
• The embryos of eutherian mammals develop entirely within the
uterus, nourished through the placenta.
• Parental care of offspring can occur regardless of whether fertilization
is external or internal.
Reproductive systems produce gametes and make them available to
gametes of the opposite sex.
• The least complex reproductive systems lack gonads, the organs that
produce gametes in most animals.
• Polychaete worms lack gonads. Eggs and sperm develop from
undifferentiated cells lining the coelom.
• As the gametes mature, they are released from the body wall
and fill the coelom.
• In some species, the body splits open to release the gametes,
killing the parent.
• Some reproductive systems, such as those of parasitic flatworms, are
• Most insects have separate sexes with complex reproductive
• In many species, the female reproductive system includes a
spermatheca, a sac in which sperm may be stored for a year or
• The basic plan of all vertebrate reproductive systems is very similar.
• However, there are variations. • In many nonmammalian vertebrates, the digestive,
excretory, and reproductive systems share a common
opening to the outside, the cloaca.
• Mammals have separate openings for the digestive and
• Female mammals also have separate openings for
the excretory and reproductive systems.
• The uterus of most vertebrates is partly or completely divided
into two chambers.
• Male reproductive systems differ mainly in copulatory organs.
• Many mammalian vertebrates do not have a well-
developed penis and simply turn the cloaca inside out to
Concept 46.3 Reproductive organs produce and transport gametes:
focus on humans
Human reproduction involves intricate anatomy and complex
• The reproductive anatomy of the human female includes external and
internal reproductive structures.
• External reproductive structures consist of two sets of labia
surrounding the clitoris and vaginal opening.
• Internal reproductive organs consist of a pair of gonads and a
system of ducts and chambers.
• The role of the ducts and chambers is to conduct the
gametes and house the embryo and fetus.
• The ovaries, the female gonads, lie in the abdominal cavity, attached
to the uterus by a mesentery.
• Each ovary is enclosed in a tough protective capsule and
contains many follicles. • Each follicle consists of one egg cell surrounded by one or
more layers of follicle cells.
• A woman is born with about 400,000 follicles.
• Only several hundred of these will release eggs
during a female’s reproductive years.
• Follicles produce the primary female sex hormones,
• Usually one follicle matures and releases its egg during each
menstrual cycle in the process of ovulation.
• After ovulation, the remaining follicular tissue develops into the
• The corpus luteum secretes additional estrogens and
progesterone, hormones that help maintain the uterine lining
• If pregnancy does not occur, the corpus luteum disintegrates
and a new follicle matures during the next cycle.
• At ovulation, the egg is released into the abdominal cavity near the
opening of the oviduct.
• The cilia-lined funnel-like opening of the oviduct draws in the
• Cilia convey the egg through the oviduct to the uterus.
• The highly vascularized inner lining of the uterus is called the
• The neck of the uterus, the cervix, opens into the vagina.
• The vagina is a thin-walled chamber that forms the birth canal
and is the repository for sperm during copulation.
• It opens to the outside at the vulva, the collective term for the
external female genitalia.
• The vaginal opening is partially covered by a thin sheet of tissue
called the hymen. • The vaginal and urethral openings are located within a recess
called the vestibule.
• The vestibule is surrounded by a pair of slender folds
called the labia minora.
• The labia majora enclose and protect the labia minora
• The clitoris is found at the front edge of the vestibule.
• During sexual arousal, the clitoris, vagina, and labia engorge with
blood and enlarge.
• During sexual arousal, Bartholin’s glands secrete mucus into
the vestibule, providing lubrication and facilitating intercourse.
• Mammary glands are present in both males and females but normally
function only in females.
• They are not a component of the human reproductive system
but are important to mammalian reproduction.
• Within the glands, small sacs of epithelial tissue secrete milk,
which drains into a series of ducts opening at the nipple.
• Adipose tissue forms the main mass of the mammary gland of a
• The low estrogen level in males prevents the development of the
sensory apparatus and fat deposits, so that male breasts remain
small, with nipples unconnected to the ducts.
• The male’s external reproductive organs consist of the scrotum and
• The internal reproductive organs consist of gonads that produce
sperm and hormones, accessory glands that secrete products
essential to sperm movement, and ducts to carry the sperm and
• The male gonads, or testes, consist of highly coiled tubes
surrounded by layers of connective tissue. • The tubes are seminiferous tubules, where sperm are
• Leydig cells scattered between the seminiferous tubules
produce testosterone and other androgens.
• The scrotum, a fold in the body wall, holds the testes outside
the body cavity at a temperature about 2°C below that of the
• This keeps testicular temperature cooler than that in the
• The testes develop in the body cavity and descend into the
scrotum just before birth.
• From the seminiferous tubules of the testes, the sperm pass through
the coiled tubules of the epididymis.
• As they pass through this duct, sperm become motile and gain
the ability to fertilize an egg.
• Ejaculation propels sperm from the epididymis to the vas deferens.
• The vas deferens run from the scrotum and behind the urinary
• Each vas deferens joins with a duct from the seminal vesicle to
form an ejaculatory duct.
• The ejaculatory ducts open into the urethra.
• The urethra drains both the excretory and reproductive
• Accessory sex glands add secretions to semen.
• A pair of seminal vesicles contributes about 60% of total semen
• Seminal fluid is thick, yellowish, and alkaline.
• It contains mucus, fructose, a coagulating enzyme,
ascorbic acid, and prostaglandins.
• The prostate gland secretes directly into the urethra. • Prostatic fluid is thin and milky.
• This fluid contains anticoagulant enzymes and citrate.
• Prostate problems are common in males older than 40.
• Benign prostate enlargement occurs in virtually all males older
• Prostate cancer is one of the most common cancers in men.
• The bulbourethral glands are a pair of small glands along the urethra
below the prostate.
• Prior to ejaculation, they secrete clear mucus that neutralizes
any acidic urine remaining in the urethra.
• Bulbourethral fluid also carries some sperm released before
• This is one of the reasons the withdrawal method of birth
control has a high failure rate.
• A male usually ejaculates about 2–5 mL of semen, with each milliliter
containing about 50–130 million sperm.
• Once in the female reproductive tract, prostaglandins in semen thin
the mucus at the opening of the uterus and stimulate uterine
contractions that help move the semen.
• When ejaculated, semen coagulates, making it easier for
uterine contractions to move it along.
• Anticoagulants then liquefy the semen, and the sperm
• The alkalinity of semen helps neutralize the acidic environment
of the vagina, protecting the sperm and increasing their motility.
• The human penis is composed of three layers of spongy erectile
• During sexual arousal, the erectile tissue fills with blood from
arteries. • The resultant increased pressure seals off the veins that
drain the penis, causing it to engorge with blood.
• The engorgement of the penis with blood causes an
erection, which is essential for the insertion of the
penis into the vagina.
• The penis of some mammals possesses a baculum, a bone that
helps stiffen the penis.
• Temporary impotence can result from the consumption of alcohol or
other drugs, and from emotional problems.
• Irreversible impotence due to nervous system or circulatory problems
can be treated with drugs and penile implant devices.
• The oral drug Viagra acts by promoting the action of nitric
oxide, enhancing relaxation of smooth muscles in the blood
vessels of the penis.
• This allows blood to enter the erectile tissue and sustain
• The main shaft of the penis is covered by relatively thick skin.
• The sensitive head, or glans penis, is covered by thinner skin.
• The glans is covered by the foreskin, or prepuce, which may be
removed by circumcision.
• There is no verifiable health benefit to circumcision, which
arose from religious tradition.
Human sexual response is very complex.
• Human arousal involves a variety of psychological and physical
• Human sexual response is characterized by a common physiological
• Two types of physiological reaction predominate in both sexes:
1. Vasocongestion, filling of tissue with blood, is caused by
increased blood flow. 2. Myotonia is increased muscle tension.
• Both smooth and skeletal muscle may show
sustained or rhythmic contractions.
• The sexual response can be divided into four phases: excitement,
plateau, orgasm, and resolution.
• Excitement prepares the vagina and penis for coitus.
• Vasocongestion is evident in the erection of the penis and
clitoris; the enlargement of the testes, labia, and breasts; and
• Myotonia may result in nipple erection or tension in the arms
• In the plateau phase, these responses continue.
• Stimulation by the autonomic nervous system increases
breathing and heart rate.
• In females, plateau includes vasocongestion of the outer third
of the vagina, expansion of the inner two-thirds of the vagina,
and elevation of the uterus to form a depression that receives
sperm at the back of the vagina.
• Orgasm is the shortest phase of the sexual response cycle.
• It is characterized by rhythmic, involuntary contractions of the
reproductive structures in both sexes.
• In male orgasm, emission is the contraction of the glands and
ducts of the reproductive tract, which forces semen into the
• Ejaculation occurs with the contraction of the urethra and
expulsion of semen.
• In female orgasm, the uterus and outer vagina contract.
• Resolution completes the cycle and reverses the responses of earlier
stages. • Vasocongested organs return to their normal sizes and colors;
Concept 46.4 In humans and other mammals, a complex interplay of
hormones regulates gametogenesis
Spermatogenesis and oogenesis both involve meiosis but differ in
three significant ways.
• Gametogenesis is based on meiosis.
• Spermatogenesis is the production of mature sperm cells from
• Spermatogenesis is a continuous and prolific process in the
• Each ejaculation contains 100–650 million sperm.
• Spermatogenesis occurs in seminiferous tubules.
• Primordial germ cells of the embryonic testes differentiate into
spermatogonia, the stem cells that give rise to sperm.
• As spermatogonia differentiate into spermatocytes and then
into spermatids, meiosis reduces the chromosome number from
diploid to haploid.
• As spermatogenesis progresses, the developing sperm cells
move from the wall to the lumen of a seminiferous tubule and
then to the epididymis, where they become motile.
• The structure of sperm fits its function.
• A head containing the haploid nucleus is tipped with an
acrosome, which contains enzymes that help the sperm
penetrate to the egg.
• Behind the head are a large number of mitochondria (or a
single large one) that provide ATP to power the flagellum.
• Oogenesis is the production of ova from oogonia.
• Oogenesis differs from spermatogenesis in three major ways. 1. At birth an ovary may contain all