Mammalian reproduction

Abstract

Reproduction is the process of by which organisms give rise to young ones of their own kind. All living organisms reproduce. Mammals reproduce sexually through internal fertilization. Mammals take care of their young ones through production of milk produced by mammary glands of the females. Although mammals share many common characteristics, there are notable differences among them that necessitate them to be grouped into three: Monotremes, marsupials, and placental mammals. This paper has explained the differences in the reproduction of these groups of mammals, and the importance. Herein, the importance of mammalian reproduction, the process of fertilization, and different ways of mammalian reproduction has been discussed.

Full text

Running head: MAMMALIAN REPRODUCTION
1
Mammalian Reproduction
Micah Asamba
Kenyatta University

MAMMALIAN REPRODUCTION
2
Mammalian Reproduction
Abstract
Reproduction is the process of by which organisms give rise to young ones of their own
kind. All living organisms reproduce. Mammals reproduce sexually through internal fertilization.
Mammals take care of their young ones through production of milk produced by mammary
glands of the females. Although mammals share many common characteristics, there are notable
differences among them that necessitate them to be grouped into three: Monotremes, marsupials,
and placental mammals. This paper has explained the differences in the reproduction of these
groups of mammals, and the importance. Herein, the importance of mammalian reproduction, the
process of fertilization, and different ways of mammalian reproduction has been discussed.
Introduction
Mammals are chordates whose young ones are fed on milk from mammary glands of
their mothers1. Members of this class have other unique features in addition to mammary glands.
They have hair except in whales where it occurs only in fetal stage. Their lower jaw is hinged
into the skull directly. They have a muscular diaphragm which separates the thorax from the
abdomen.2 More than 5,500 species of mammals exist today and they are grouped into 125
families3. Examples of mammals include dogs, cats, mice, monkeys, kangaroo, and humans.
They inhabit different ecological habitats. Some are aquatic, such as the whale; some are
terrestrial, such as humans; and some are arboreal such as the monkeys.
Importance of mammalian reproduction
Reproduction involves a series of processes that result in the production of an offspring.
Sexual reproduction is the union of the egg and the sperm.4 Mammals reproduce sexually. The
main importance of sexual reproduction is that it leads of diversity. The offspring produced
genetically differ from each other and from their parents.5 Diversity increases the organism’s
adaptation. Sexual reproduction has disadvantages too. (1) The exercise of finding a mating
partner and mating itself can be dangerous and time consuming. (2) An individual only
contributes half of the genetic makeup of the offspring. There are two types of fertilization:
external fertilization where the union of sperm and ovum takes place outside the body; and
internal fertilization where the sperm and the ovum fuse within the body of the female. Mammals
undergo internal fertilization. Internal fertilization is important because it increases the chances
of survival of the offspring.
1 Wilson, D. E., Armstrong, D. M., & Jones, K. (2017, Oct 25). Mammal. Retrieved from
Encyclopedia Britanica: https://www.britannica.com/animal/mammal
2 Wilson, D. E., Armstrong, D. M., & Jones, K. (2017). Mammal.
3 Wilson, D. E., Armstrong, D. M., & Jones, K. (2017). Mammal.
4 Alters, S. (2000). Biology: understanding life. Boston: Jones and Barlette Publishers.
5 Alters, S. (2000). Biology: understanding life.

MAMMALIAN REPRODUCTION
3
Another importance of mammalian reproduction is that the mothers provide milk for the
young ones. This is important for the survival of the newborn because the milk contains
important nutrients.6 Development of mammary glands is stimulated by progesterone and
estrogen hormones.7 The hormone prolactin stimulates secretion of milk. Milk is rich in nutrients
and antibodies. The antibodies protect the newborn from infections.
Key concepts in mammalian reproduction
The union of the sperm and the egg is called fertilization. Copulation is the act of placing
sperms in the vagina of the female.8 The release of the egg form an ovary is called ovulation.9 In
primates, including humans, this occurs regularly throughout the year. In other mammals, it
occurs only during the mating season. For such mammals, the period of heat (Oestrus) normally
coincides with ovulation.10 Some mammals undergo a behavior known as courtship. This is a
behavior aimed at attracting mates. After fertilization, most mammals go through a period called
the gestation period. This is the time between conception and birth of the newborn.11
Fertilization
The process of fertilization is preceded by copulation.12 Copulation results in deposition
of sperms in the vagina after ejaculation. Sperms swim past the cervix, into the uterus and to the
fallopian tube where fertilization takes place.13 As they move up the female reproductive system,
sperms complete their maturation and are now ready for fertilization. Most of the sperms die
along the way.14 The sperm attaches to the walls of the ovum (zona pellucid) and release lytic
enzymes from the acrosomes, which dissolve the membrane. One sperm enters the cell causing
the zona pellucid to change and prevent other sperms from entering. The sperm loses its tail, it
then fuses with the egg to form a zygote. After fertilization, the egg starts to move towards the
uterus. As it moves, it divides mitotically to form a mass of cells called morula. Morula
continues to divide and develops into a hollow ball of cells called the blastocyst. The plastocyst
gets to the uterus and attaches to the endometrium in a process called implantation15. At this
stage, the development of the fetus is different depending on the type of the mammal.
6 Wilson, D. E., Armstrong, D. M., & Jones, K. (2017)
7 Wilson, D. E., Armstrong, D. M., & Jones, K. (2017
8 Alters, S. (2000). Biology: understanding life.
9 Ibid.
10 Ibid.
11 Ibid.
12 Okabe, M. (2013). The cell biology of mammalian fertilization. Retrieved from The company
of biologists: http://dev.biologists.org/content/develop/140/22/4471.full.pdf
13 Ibid.
14 Ibid.
15 Ibid.

MAMMALIAN REPRODUCTION
4
A. Diagram showing sperms attaching to zona pellucid16; B. Diagram showing the
different processes after fertilization17.
Kinds of mammalian reproduction
Based on the way they reproduce, mammals are grouped into three: The Monotremes; the
Marsupials, and Placental mammals18.
Reproduction in monotremes
Egg laying mammals are referred to as Monotremes19. The duck-billed platypus and
echidnas are the only surviving species20. They are found in Australia and New Guinea only. The
monotreme female do not have vagina and uterus21. They have cloaca just like in birds. The
opening is used for excretion, for copulation and for laying eggs. Fertilization in monotremes
takes place internally when the sperms are deposited through the cloaca. The eggs are leathery
and remain in the body of the female for a few weeks22. The mother gives nutrients to the eggs.
Echidna eggs are laid in a pouch while platypus eggs are laid in burrows. They are incubated for
about ten days before they hatch through tearing the shell using egg teeth that are temporary.
Monotremes have mammary glands just like all other mammals. They, however, do not have
nipples23. The importance of laying eggs is that the mother is out of risk involved with delivery,
16 [ CITATION Mas13 \l 1033 ]. Mammalian fertilization.
17 Ibid.
18 Behringer, R. R., Eakin, G. S., & Renfree, M. B. (2005). Mammalian diversity: gametes,
embryos and reproduction. Reproduction, Fertility and Development, 18(2), 99-107.
19 Monotreme Project. (2017). Monotremes. Retrieved from Monotreme project:
http://monotremeproject.weebly.com/reproduction-and-fertilization.html
20 Jones, G. N. (2008). Reproductive biology in egg-laying mammals. Reproduction, Fertility
and Development , 2 (3), 115-127.
21 Monotreme Project (2017)
22 Monotreme Project (2017)
23 Jones, G. N. (2008).

MAMMALIAN REPRODUCTION
5
and she doesn’t have to eat more24. One disadvantage of this kind of reproduction is that the
offspring have less chances of survival.
Photo showing monotreme eggs25
Reproduction in Marsupials
Marsupials give birth to young ones that are not yet fully developed26. They have a very
short gestation period. The blastocyst attaches to a shallow depression in the uterus27. The
chorionic membrane is in direct contact with the walls of the uterus that are slightly eroded28. For
this reason, there is no efficient exchange of materials. After birth, the premature newborn gets to
the pouch where it drinks milk and continues to develop. The newborn marsupials are referred to
as Joeys.
Photo showing a Kangaroo with its joey in a pouch29
24 Jones, G. N. (2008).
25 Source: Monotreme Project (2017)
26 Opossum Society of the United States. (2017). Reproduction-life cycle of Marsupials.
Retrieved from http://opossumsocietyus.org: http://opossumsocietyus.org/general-opossum-
information/opossum-reproduction-lifecycle/
27 Hill, M. (2017, Nov 27). Embryology reproductive cycles. Retrieved from Embryology:
https://embryology.med.unsw.edu.au/embryology/index.php/Reproductive_Cycles
28 Ibid.
29 Source: Opossum Society of the United States. (2017).

MAMMALIAN REPRODUCTION
6
According to the Opossum Society of the United States (2017), members of the
Marsupiala include the Kangaroo, koala, Tasmanian devil and the Virginia opossum. The pouch,
also referred to as the marsupium, helps the Joey to develop by providing warmth. After birth,
the newborns climb through the fur on the mother’s abdomen into the pouch.30 Researchers of
opossum reproduction have discovered an interesting feature. The opossum sperms are
ejaculated as pairs. They remain attached within the female reproductive system until when
fertilization is about to take place31. It is postulated that the pairing helps to increase their
motility and protect them as they navigate through the female tract.32
Reproduction in Placental mammals
In Placental mammals, the placenta helps in exchange of nutrients between the mother
and the fetus.33The chorio-allantoic placenta also aids in exchange of respiratory and excretory
materials. After adhering to the uterus, the blastocyst sinks into the endometrium with the help of
enzymes produced by the outer membrane. This increases the absorption surface which allows
for efficient exchange of materials. Eutherian mammals, as placental mammals are also referred
to, give birth to offspring that are well developed. Examples of Placental mammals include:
humans, lions, cats, elephants, dogs among others. Placental reproduction is important because
the fetus becomes mature and large before birth. This is because of long periods of fetal growth
inside the uterus. Hence they have an increased chance of survival. The disadvantage of placental
reproduction is that it is risky to the mother and makes the mother to eat more food. Birth can
result to a mother’s death.
Reproduction cycles in placental mammals
Eutherian mammals have complex reproductive cycles in their females. Primates undergo
the menstrual cycle while non-primates undergo the estrous cycle.34The menstrual cycle is
regular and controlled by hormones. It leads to changes in the ovary and the uterus. It begins at
puberty and terminates at menopause.35The estrous cycle has several forms. Polyestrous animals
have a cycle throughout the year. They include the cattle, mouse and pigs.36 Seasonally
polyestrous animals exhibit seasonal cycles that only occur at specific times of the year; for
30 Opossum Society of the United States. (2017).
31 Ibid.
32 Ibid.
33 Encyclopedia Britannica. (2017). Placental mammals. Retrieved from Brittanica.com:
https://www.britannica.com/animal/placental-mammal
34 Hill (2017)
35 Hill (2017
36 Hill (2017

MAMMALIAN REPRODUCTION
7
instance the horses, goats and cats.37 Dogs, wolves and bears are examples of monostreous
Animals that have one estrous cycle per year.38
General anatomy of mammalian reproductive system
Male reproductive system
It is made up of the testes, glands and ducts. The testes are found in the scrotum. Their
main function is to produce sperms and male hormones. Their location outside the normal body
provides a cool environment required for sperm production. Sperms are produced in
seminiferous tubules and stored in the epididymis where they mature. During ejaculation, the
sperms move through the vas deferens (Sperm duct) to the urethra from where they pass to the
vagina of the female. Several glands produce secretions that are added to the sperms to form
semen. The diagram below shows the main parts of the male reproductive system of a human
being.39
Female reproductive system
It is made up of the ovaries, the oviduct, uterus and the vagina. Organs are located in the
abdominal cavity. The funnel of the oviduct lies close to the ovary. After ovulation, the ovum is
deposited in the oviduct from where it is moved by action of cilia and muscle contractions to the
uterus. The diagram below shows the female reproductive system of humans.40
37 Hill (2017
38 Hill (2017
39 Jain, D. (2017). Human reproductive system: Male and female reproductive system (with
diagrams). Retrieved from Biology discussions:
http://www.biologydiscussion.com/essay/human-reproductive-system-male-and-female-
reproductive-system-with-diagrams/1584
40 Jain, D. (2017). Human reproductive system

MAMMALIAN REPRODUCTION
8
Conclusion
Through sexual reproduction, the male and the female contribute equal number of genetic
material to produce an offspring. Mammals utilize internal fertilization in which the sperms are
deposited into the body of the female. Most mammals use a strategy known as viviparity in
which the embryo develops inside the mother. Food is provided by the mother. Live young ones
are born. Some mammals, however, lay eggs. They are said to be Oviparious. Reproduction in
mammals is the most advanced among all living things. It has numerous advantages. Mammals
provide parental care to their young ones, including providing milk.
.

MAMMALIAN REPRODUCTION
9
References
Alters, S. (2000). Biology : understanding life. Boston: Jones and Barlette Publishers.
Behringer, R. R., Eakin, G. S., & Renfree, M. B. (2005). Mammalian diversity: gametes,
embryos and reproduction. Reproduction, Fertility and Development, 18(2), 99-107.
Encyclopedia Britannica. (2017). Placental mammals. Retrieved from Brittanica.com:
https://www.britannica.com/animal/placental-mammal
Hill, M. (2017, Nov 27). Embryology reproductive cycles. Retrieved from Embryology:
https://embryology.med.unsw.edu.au/embryology/index.php/Reproductive_Cycles
Jain, D. (2017). Human reproductive system: Male and female reproductive system (with
diagrams). Retrieved from Biology discussions:
http://www.biologydiscussion.com/essay/human-reproductive-system-male-and-female-
reproductive-system-with-diagrams/1584
Jones, G. N. (2008). Reproductive biology in egg-laying mammals. Reproduction, Fertility and
Development, 2(3), 115-127.
Monotreme Project. (2017). Monotremes. Retrieved from Monotreme project:
http://monotremeproject.weebly.com/reproduction-and-fertilization.html
Okabe, M. (2013). The cell biology of mammalian fertilization. Retrieved from The company of
biologists: http://dev.biologists.org/content/develop/140/22/4471.full.pdf
Opossum Society of the United States. (2017). Reproduction-life cycle of Marsupials. Retrieved
from http://opossumsocietyus.org: http://opossumsocietyus.org/general-opossum-
information/opossum-reproduction-lifecycle/
Wilson, D. E., Armstrong, D. M., & Jones, K. (2017, Oct 25). Mammal. Retrieved from
Encyclopedia Britanica: https://www.britannica.com/animal/mammal