Animal Reproduction
Nancy G. Morris
Volunteer State Community College
Campbell, Chapter 46
Animal Reproduction:
Asexual reproduction is the creation
of new individuals whose genes all
come from one parent without the
fusion of egg and sperm.
Sexual Reproduction is the creation
of offspring by the fusion of haploid
gametes to form a zygote (fertilized
egg), which is diploid.
Animal Reproduction:
Sexual Reproduction is the creation of
offspring by the fusion of haploid
gametes to form a zygote (fertilized
egg), which is diploid.
 Gametes are formed by meiosis.


Female gamete: egg or ovum
relatively large & nonmotile
Male gamete: spermatozoon
generally, a small motile cell
Diversity in asexual reproduction:
Fission
 Budding
 Gemmules
 Fragmentation
 Regeneration

Advantages of asexual reproduction:
Animals in isolation will not die out
 Rapid – ideal for colonizing a a
habitat quickly


Most advantageous in stable,
favorable environments because it
perpetuates successful genotypes
precisely
Control of Reproductive Cycles:
Hormones
 Environmental cues

(temperature, rainfall, day length)

Lunar cycles
Variation in reproductive patterns:

Parthenogenesis
–

Daphnia, aphids, rotifers, social
insects, whiptail lizards
Hermaphroditism
–
Tapeworms, earthworms, sessile,
burrowing animals
Variation in reproductive patterns:
 Sequential hermaphroditism
– individual reverses its sex during its
lifetime
– either protogynous – female first
– or protandrous – male first
– Reef fishes called wrasse p.915
Mechanisms of
Sexual Reproduction

Fertilization: union of egg and

External fertilization: eggs

Internal fertilization: requires
sperm
shed by the female & fertilized by
the male
cooperative behavior
External Fertilization
Requires environment where
zygotes can develop without heat
or stress
 Many amphibians & fishes use
specific mating behaviors resulting
in one male fertilizing the eggs of
one female

External Fertilization
Courtship behavior is a trigger for
release of gametes. There are
two effects:
 1) the probability of successful
fertilization is increased &
 2) the choice of mates may be
somewhat selective.

Phermones
Chemical signals produced by one organism
that influence the behavior of another
organism of the same species
 Small, volatile or water-soluble molecules
 Active in very small amounts (like hormones)
 Many function to attract mates
 Powerful:Some insects can detect a mile away
 Gypsy moth attracts in concentration as low
as 1 molecule in 107 molecules of other gases
in the air

Survival



All species produce more offspring
than survive to reproduce.
Species with internal fertilization
produce fewer zygotes but provide
more parental protection than species
with external fertilization.
Species with external fertilization
usually produce enormous numbers of
zygotes, but the number that survive
and develop further is small.
Survival
Amniotic eggs have calcium & protein
shells which resist water loss & physical
damage.
 Eggs of amphibians & fishes have only a
gelatinous coat.
 Marsupials retain the embryos rather
than secreting a shells around them.
 Placental mammals develop entirely
within the uterus, nourished by the
mother’s blood supply.

Evolution of complex
reproductive systems

Most complex reproductive system in the animal
kingdom is in the parasitic flatworms
(Platyhelminthes). Figure 46.6

The least complex system is found in the
polychaete worms (Annelida).

In some species, the female possesses a
spermatheca, a sac in which sperm are stored
for a year or more.

In many nonmammaliam vertebrates, excretory,
digestive, and reproductive products are
eliminated through a cloaca.
Figure 46.6
Reproductive
anatomy of a
parasitic worm.
Human Male Reproductive System
• external genitalia
•Scrotum
•Penis
• internal genitalia
•Gonads – testes
•Accessory glands
•Associated ducts
Male Reproductive Anatomy
Figure 46.8
Male Reproductive Anatomy
Figure 46.8
Male Reproductive Process

Testes develop in abdomen &
descend into the scrotum just
before birth.

Sperm cannot develop at normal
body temperature.

Placing the testes outside the
abdominal cavity in the scrotum,
reduces the temperature by 20 C.
Male Reproductive Process
Penis serves as the copulatory organ
 Ejaculatory duct joins the urethra
and opens at tip of the penis.
 Movement of semen through the
urethra results in the sperm being
deposited directly in the female
system (internal fertilization).

Figure 42.8 “Parts”

Testes are comprised of highly coiled
tubes surrounded by layers of
connective tissue.

Seminiferous tubules- the tubes where
the sperm form.

Interstitial cells are scattered
between the tubules & produce
testosterone & androgens.

Sperm pass from the seminiferous
tubules to the epididymis.
Figure 42.8 “Parts”

Epididymis contains coiled tubes where sperm
are stored & mature- gain motility

Sperm are forced through the vas deferens
to the ejaculatory duct.

Ejaculatory duct forms by the joining of two
vas deferens ducts and the duct from the
seminal vesicles. It opens into the urethra.

Penis is composed of 3 cylinders of spongy
erectile tissue that fill with blood during
erection.
3 sets of Accessory Glands

These glands add their
secretions to the semen:

SEMINAL VESICLES

PROSTATE GLAND

BULBOUREHRAL GLAND
3 sets of Accessory Glands

SEMINAL VESICLES- secrete fluid
containing
1) mucus
2) amino acids
– causes semen to coagulate after deposited in
female
3) fructose
– energy
4) prostaglandins
– stimulate uterine contractions to help move
semen to the uterus
3 sets of Accessory Glands
PROSTATE GLAND – secretes a thin,
milky alkaline fluid containing
enzymes to balance acidity in vagina.

BULBOUREHRAL GLAND – secretes
viscous fluid before sperm
ejaculation which may neutralize any
acid urine remaining in urethra.
Female Human Reproductive Anatomy
• More complicated than male
• Structures not only for production of gametes,
but also to house the embryo and fetus.
•INTERNAL
Gonads – Ovaries & associated ducts
•EXTERNAL
Clitoris & two sets of labia that surround it &
and the vaginal opening
Female Human Reproductive Anatomy
• Ovaries located in the abdominal cavity
• Each ovary contains many follicles (one egg
cell surrounded by follicle cells, which nourish
and protect the developing egg.)
•All EGGS are formed prior to birth!
•Follicle cells produce estrogens.
•Starting at puberty and continuing until
menopause, one follicle matures and releases
its egg cell during each menstrual cycle.
Female Human Reproductive Anatomy
• During ovulation, the egg is expelled from
the follicle. The follicular tissue becomes the
corpus luteum, which secretes progesterone
(maintains uterine lining) and additional
estrogen.
• If the egg is not fertilized, the corpus
luteum degenerates.
• The egg is release into the abdominal cavity
near the opening of the oviduct.
•Cilia lining the oviduct draw the egg in and
convey it to the uterus.
Female Human Reproductive Anatomy
•The uterus (or womb) is a thick muscular
organ 7 cm long and 5 cm wide.
• The inner uterine lining, the endometrium,
is richly supplied with blood vessels.
•
Female Human Reproductive Anatomy
•The remaining reproductive structures are:
• Cervix – neck of the uterus opening into
the vagina
•Vagina – thin walled chamber; the repository
for semen during copulation; forms the
birth canal.
•The hymen, a vascularized membrane, usually
covers the vaginal opening from birth
until ruptured by vigorous physical
activity or sexual intercourse.
Female Human Reproductive Anatomy
•Labia minora -slender skin folds bordering the
vestibule
• Labia majora – a pair of thick, fatty ridges
enclosing & protecting the labia minora
• Clitoris – bulb of erectile tissue covered by a
prepuce (small hood of skin)
Female Reproductive Anatomy
Figure 46.9
Female Reproductive Anatomy
Figure 46.9
Human Sexual Response


Although variable, human sexual behavior is based
in a common physiological pattern, the sexual
response cycle.
Physiological reactions which predominate in both
sexes can be divided into two types:
– 1) Vasocongestion – increased blood flow through
the arteries of that tissue
– 2) Myotonia – both smooth & skeletal muscles may
show sustained or rhythmic contractions
Human Sexual Response
There are four phases in the sexual response
cycle in human males & females:

1) Excitement

2) Plateau

3) Orgasm

4) Resolution
Human Sexual Response
Excitement
the penis & vagina are prepared for
coitus (sexual intercourse)

1) Vasocongestion of the penis & clitoris;
enlargement of testes, labia, breasts

2) Vaginal lubrication & myotonia occur
Human Sexual Response
Plateau
breathing & heart rates increase from
stimulation of the autonomic nervous
system

1) vagina depresses to receive the sperm

2) the outer third becomes vasocongested,
the inner third slightly expands & the
uterus elevates
Human Sexual Response
Orgasm
is characterized by rhythmic, involuntary
contractions in the reproductive systems of
both sexes
Male:

Emission – forcing the semen into the urethra due to

Ejaculation - expels the semen due to contraction of
contraction of the glands & ducts of the reproductive
system
the urethra
Female:

Contraction of the uterus & outer vagina
Human Sexual Response
Resolution
reverses responses of earlier phases &
completes the cycle


1) Vasocongested organs return to normalcy
2) Muscles relax
Spermatogenesis
•Spermatogenesis
– production of mature
sperm cells in the adult male (Figure 42.12)
• Continuous process in adult males; 250- 400
million sperm cells per ejaculate
• Occurs in the seminiferous tubules
• Begins with differentiation of germ cells (2N)
into spermatogonia (2N) in the embryonic testes
• Maturation of spermatogonia begins at puberty
and continues until death
Spermatogenesis
•Primary spermatocytes undergo meiosis I to
produce two haploid secondary spermatocytes
• Each secondary spermatocyte undergoes meiosis
II to form two spermatids
• Result is 4 haploid spermatids through meiotic
division
• Each spermatid becomes attached to a Sertoli cell
from which it receives nutrients
•All four spermatids differentiate into mature
spermatozoa
Figure 46.12
Spermatogenesis
Figure 46.12
Spermatogenesis
Spermatozoon
• Thick head contains the haploid
nucleus
• Acrosome at tip contains
enzymes to aid in egg penetration
• Many mitochondria are present
behind the head to provide ATP
for flagellum movement
Figure 46.11 Structure of a Sperm Cell
Oogenesis
• Oogenesis is the development of ova
(mature unfertilized eggs) Figure 42.13
• Begins at puberty and ends at menopause
• Begins in embryo -- germ cell undergoes
mitotic divisions to produce a diploid
oogonium.
•By birth, oogonia exist as primary
oocytes, thus ALL potential ova at present
at the time of birth!
Oogenesis
•
Between birth & puberty, primary oocytes
enlarge & their surrounding follicles grow.
• They replicate their DNA and enter
prophase I & remain there until activated by
hormones – one per month beginning at
puberty.
• After puberty, during each ovarian cycle,
FSH stimulates a follicle to enlarge & the
primary oocyte inside completes meiosis I.
• The result is a haploid secondary oocyte
and the first polar body.
Oogenesis
• Meiosis then stops again
• LH triggers ovulation & the
secondary oocyte is released from the
follicle.
• If a sperm cell penetrates the
secondary oocyte’s membrane, meiosis
II will occur and the second polar
body will separate form the ovum.
• This completes oogenesis.
Figure 46.13 OOGENESIS
Figure 46.13 OOGENESIS
Important Differences:
• 1) In spermatogenesis, all four products of
meiosis become spermatozoa, whereas
•1) In oogenesis, unequal cytokinesis
occurring in meiosis I & II results in most of
the cytoplasm being distributed to a single
daughter cell which will form the ovum. The
other cells (polar bodies) will degenerate.
Important Differences:
• 2) Spermatogenesis is a continuous
process throughout reproductive life of
the male, whereas
• 2) in oogenesis, all potential ova are
present as primary oocytes in the
ovaries at the time of the female’s
birth.
Important Differences:
• 3) Spermatogenesis occurs continuously,
whereas
• 3) in oogenesis, long “resting” periods
occur between the formation of the
initial steps and final production of the
ovum
Mammalian female cycles:
• Estrous cycles occur in non-primate
mammals.
• Ovulation occurs after the endometrium
thickens & is vascularized
• If pregnancy does not occur, the
endometrium is reabsorbed by the uterus.
• Involves pronounced behavioral changes:
seasonal & climate changes effect the estrous
cycle more than the menstrual cycle.
• ESTRUS is the period of sexual activity
surrounding ovulation & is the only time most
mammals will copulate. The length & frequency
varies widely among species.
Mammalian female cycles:
• Menstrual cycles occur in humans
and many other primates.
• Ovulation occurs after the endometrium
thickens & is vascularized
• If pregnancy does not occur, the
endometrium is shed form the uterus through
the cervix & the uterus.
• The term menstrual cycle refers to changes
that occur in the uterus during the
reproductive cycle. (Figure 46.15)
•In humans, cycle is on average 28 days.
Figure 46.15
Reproductive
Cycle of the
human female