Animal Reproduction

Introduction 

Reproduction, the conception, development
and bringing forth of young offspring, is one of the characteristics of living
things.  It may be denoted as sexual, in
which case it necessarily involves the participation of a male and a female of
the same species. It may also be denoted as asexual, in which case it involves
only one member of a species.  Both
sexual and asexual reproductions occur in various categories of plants and
animals.

          Four- footed livestock belong to the group of
animals called mammals of the biological class Mammalian.  Mammals give birth to living young offspring which are
the replica of their parents, posseses certain features like hair on their bodies and feed their young offerings
with; milk.  All mammals reproduce
sexually.  For each mammalian species,
the process of reproduction, although broadly similar, is still sufficiently distinct.  Through reproduction, livestock species are
maintained on earth and their populations greatly multiplied to increase the much needed protein in human diets.

     Poultry belong to the biological class
known s Aves and this class includes all types of birds.  Birds reproduce by laying eggs which later
hatch into young ones outside the bodies of their parents.  Reproduction in birds, though sexual, is
completely different from that of mammals. 
This chapter now continues with a consideration of the basic aspects o
reproduction, processes of egg formation in poultry and the main reproduction
hormones as well as their functions.

 Basic Aspects of
Animal Reproduction 

 Reproduction in animals is defined as the
process of producing young ones through the sexual union of mature male and
female animals of the same species.  The
following are associated with reproduction.     

Mating (copulation)

Mating is the coming together
of sexually mature male and female animals of the same species in a sexual
relationship. When mating takes place during the heat period, it usually
results into fertilization. 
Fertilization is the union of the egg cell from the female animal with
the sperm cell from the male animal to form zygote.   The
zygote begins to develop into an embryo and this later develops into a young
animal.

     Mating can be classified into two types
namely: natural mating and artificial mating. Natural mating involves the
physical coming together of the matured male and females animals and contacts
at their reproductive organs.  The entrance
to the vagina of a female mammal which has never mated is covered by a fold of
skin called hymen.  This hymen is usually
broken at the first time of mating. During mating, seminal fluid passes from
the male animal into the female animals. 
 Artificial mating (more commonly
called artificial insemination) is the introduction of the semen from a male
animal into the reproductive tract of a female animals of the same species by
the use of instruments. Genetically virile male animals are carefully selected for
use in mating.  Under natural mating, a
good ratio of male to females is maintained. 
In rabbits, for example, one male is usually used to mate with ten females

2.  Heat
period

     Heat period is the period of time within
which the female mammalian livestock is most ready to accept the male for
mating.  This is also the period of time
when fertilization is most likely to take place.  In livestock, a female can readily accept a male
during the heat period.  In cattle, the
heat period lasts from 1 to 2 days in length. An animal which is having its
heat period is said to be ‘on heat’.  A
female mammal which is on heat may exhibit any combination of the following
signs:

The sexually mature animal is a
good detector of a female animal undergoing her heat period.  It is important to house male and female
animals together in order to facilitate natural mating.   On
farms where artificial insemination is practiced, the farmer or farm attendants
should be well trained to recognize female animals oh heat so that they can be
separated in time to be inseminated.

3. Oestrus Cycle

For most kinds of farm
animals, the heat period lasts fro 1 to 2 days at ten most, after which several
days will pas and another hear period will begin. This reasonably appearance,
disappearance and re-appearance of the heat period in sexually mature female
mammals constitute what is called Oestrus cycle. Oestrus cycle is defined as
the cyclic period lasting from the appearance of the signs of heat to the next
period of heat in sexually matured female mammals. The oestrus cycle starts in
the female at puberty. Puberty signifies the one set of sexual maturity. Sexual
maturity means that the female mammals is capable of becoming pregnant,
sustaining the pregnancy through the required time period and giving birth to
the fully-developed, living young animals.

          Generally, the age at which puberty occurs
is dependent on the livestock species, the type of bred, the quantity and
quality of feed given the animals , as well as the general health of the
animal.  The length of the oestrus cycle
varies form one species of livestock to the other.

At a point in time during the
heat period, ovulation occurs.  Ovulation
is the shedding of tan egg cell called ovum from the ovary into the fallopian
funnel. The ovary is the main female reproductive organs.  It is a natural store for egg cells before
they are progressively shed during the reproductive life of the animals.  Usually, a pair of ovaries is present. The
fallopian funnel (or infundibulum)  leads
directly into the fallopian tube or oviduct. 
The egg cell which is shed travels along the oviduct.  If mating occurs around the time of
ovulation, there is high likelihood that sperm cells will travel along the
oviduct, and one of them will meet and fertilize the egg cell.  Given the right conditions, the fertilized
cell will develop into an embryo, which later develops into a young one.

     Ovulation is influenced by the action of luteinizing
hormone.  Animals can be grouped into two
based on the type of ovulation exhibited. 
These are spontaneous ovulators and induced ovulators.

The period of pregnancy in
female animals is known as the gestation period. 

Pregnancy occurs only in
mammals.  The gestation period begins at
the time fertilization occurs, covers the time of the formation of the embryo
as well ass the development young one in the mother’s womb and ends at the time
when birth occurs. Each mammalian species has a reasonably constant and
distinct gestation period, although for the female member of any species, there
may be slight variation in the period of time a particular pregnancy
lasts.  The gestation periods for cattle,
sheep, goats, pigs and rabbits are stated in the table below.  

Since human beings are
mammals, the student should take note that the human female also has a
gestation period, the length of which is 40 weeks or 289 days.

     All livestock need to be well-fed
qualitatively and quantitatively, more or during pregnancy. Adequate nutrients
are needed by the foetus for good growth and development and also by the mother
for sustenance and maintenance of overall good health.   Predators, male animals and other sources of
disturbance should be kept away from the pregnant animal and she should be made
as comfortable as possible.  With the knowledge
of the of the gestation period, the expected date of birth can be determined to
a high degree of accuracy by merely counting from the date of mating. Thus,
accurate records of mating should be kept on farms.  Adequate preparations should be made for the
young which is being expected as well as its mother.

6.  Parturition

Parturition is the general
term refers to the act of giving birth to young ones in mammalian livestock.  The specific terms for the act of giving
birth in individual livestock areas shown in the above table presented earlier
in this chapter.  Some signs exhibited by
animals as the time of parturition draws near include the following:

Parturition is influenced by
the actions of hormones for example, oestrogen, osytoxin and relating. These
hormones and their activities will be studied later in the course.

     The preparations that should be made for
animals about to give birth include the provision of clean water, adequate feed
and clean housing with straw as bedding materials.  Kindling box should be provided for the
rabbit doe.   The mammary glands of livestock are fully
developed by the time of parturition and are in readiness to produce milk to
feed the young ones.

The occurrence of milk in the
mammary glands and its secretion or letdown through the teats is known as
lactation.  Lactation occurs in nursing
female mammals that have very recently given birth to young ones.  The hormone which is involved in the milk let
down is called prolactin.  The quantity
of milk produced as well as the period over which lactation lasts depend on the
livestock species, type of breed, the quantity and quality of feed consumed and
the general health of the lactating animal. 
Environment also affects lactation to some extent.  For example, high ambient temperatures can
cause reduced milk yield in lactating animals. 
In order for the nursing animals to produce enough milk to feed her
young ones, she should be adequately fed with nutrients rich concentrates and
forages.

     8  Colostrum

Colostrum  is the liquid produced in and secreted from
the mammary glands of the lactating animals in the first of 3 to 4 days after
parturition.  It is especially rich in
vitamins, anti bodies and some nutrients. 
The young ones feed on colostrums in the first few days of their lives
and the antibodies thus consumed give them immunity against many diseases.  Inability of the young ones to feed on
colostrum for whatever will substantially reduce their chances of
survival. 

       After the first few days since parturition,
liquid emanating from the mammary glands changes from colostrums to milk.  Mammalian milk is characteristically white in
colour and rich in food nutrients as well as vitamins.  Young one continued to feed on milk until the
time of wearing.

Newly born young ones cannot
immediately consume grasses, concentrates and other solid feeds partly because
their stomach compartments are not yet developed and are gradually prepared for
solid food. The process by which the young animals are made accustomed to
consuming less and less milk and more solid food is called weaning.

     The time at which weaning is started
differs with livestock species as well as husbandry practices of the
farmer.  However for most livestock, wearing
is done not earlier than 6 weeks and usually not later than 36 weeks after
parturition. Weaning should not be done abruptly.  Rather, it should be carried out over a
period of 4 to 6 days in order to avoid upsetting the digestive system of the
animals.  A little amount of solid food
is introduced in the beginning and this gradually increased while the milk
content of the animal’s diet is steadily decreased.  By the time weaning is completed, the animal
should be entirely dependent on solid food and water.

Processes of Egg Formation in Poultry

The word poultry refers to
fowls, turkeys, ducks, guinea fowls, or other domestic birds.  All birds, whether or not, domestic, wild,
flying, or flightless belong to the biological class Aves.  The main distinguishing feature or birds is
that they have feathers on their bodies. Birds and mammals are the only biological
classes of animals which are warm-blooded, which is to say they maintain
constant body temperatures irrespective of ambient air temperature.

     As regards reproduction, birds differ
remarkably from mammals in the unlike mammals which give birth to living young
birds lay eggs which hitch into young ones. 
The egg-laying ability to also manifested in fishes, amphibians and
reptiles.  Apart from mating, most other
aspects of animal reproduction occurring in mammalian livestock do not occur in
birds.

     Egg formation takes place in the female
reproductive tract of mature poultry bird. 
The female reproductive tract of poultry bird essentially consists of
ovary and oviduct  whose parts are
infundibulum, magnum, inthmus, shell gland for uterus) and cloaca (or
vent).  In fowl, egg formation begins when
the female bird (hen) is about 18 weeks old. 
The process initiated by light entering into the eyes of the hen.  This causes a signal to pass to the pituitary
gland it the brain, from where a signal is sent to the ovary to begin the
formation of egg.  With increase in day
length through the use of artificial lighting, poultry birds produce more
eggs.  Hormones influence various stages
of the process of egg formation. These stages occur in various parts of the
reproductive tract as follows:

      1.   Ovary

 The yolk is formed in the ovary. The yoke is
the round yellow part of the egg.  The
yolk heats the female gamete.  The yolk
is released into the oviduct through the infundibulum.  A female into the oviduct through the
infundibulum. A female bird has only one functional ovary, situated on the left
side of its body. Another atrophied and non-functional ovary and oviduct exists
on its right side.

 The infundibulum is the first part of the
oviduct.  It is shaped like a funnel. No
part of the egg is formed here.  However,
fertilization of the female gamete on the yolk by a male gamete can take place
in the infundibulum if mating with a mature male fowl  (called cock) 
occurs around this time. The yolk spends 15 to 18 minutes here after
which it is moved to the magnum.

While the York is the magnum,
part oft eh albumen (or albumin)_is formed, enveloping the yolk, The albumen is
the colour less and slimy substance in the egg. 
When the egg is boiled, the albumen becomes hard and white.  The Yolk is in the magnum for about 3 hours
after which it is passed into the isthmus together with the albumen.

 The shell membrane is formed in the isthmus to
envelope the albumen.   The shell membrane is the thin material which
in boiled egg appears as a white, smooth –textured sheet covering the albumen.
The yolk with its covering of albumen spends about 1 hour to 15 minutes into
the isthmus after which it moves to the shell gland.

In the shell gland (or uterus),
the formation of the albumen continues. Also, shell, which is the hand out
covering of an egg, formed here.  The
egg’s shell is largely make up of calcium and phosphorus. The eggs spends about
19to 21 hours in the uterus after which it is expelled (laid) through the
(cloaca) or vent.  Overall, reckoning
from the time the yolk passes from the ovary; till the fully formed egg is laid
takes about 26 hours in the common fowl. This means that a well-fed and
well-takes care of layer can produce roughly an egg per day.

The parts of a fowl’s egg are shown
in the table below. The structure is such that distinct parts of it are arranged
in a concentric fashion.   The innermost portion is the yolk which is
rich in protein and fat.  Somewhere on
the is the germinal spot for blastoderm) the germinal spot is the place where
fertilization occurs and from where a young one develop. Next to the yolk and
they connect the air space which is situated next to the shell membrane.  A hard outer shell covers the whole egg.       

    

The table below shows the summary
of egg formation. The time periods given are those of chicken

Summary of the egg formation

 Egg laying,
Incubation and Hatching       

As soon as soon as the egg is
fully formed s the hen expels it through its cloaca  (also called vent)  This is referred to as   laying of eggs.   The
student should note that a matured, well-fed and healthy hen will lay eggs with
some degree of regularity, whether or not mating has taken place.  If mating does not take place between the hen
and a mature male bird (called cock) the eggs laid will be infertile and as
such cannot hatch into young ones. 
Despite this, such infertile eggs are sill edible and good for human
consumption.  

This is the basis for large
scale commercial egg production.  In such
a case, large population of layers are kept in battery; cages and
scientifically fed and managed for optimal egg production.  Some highly intensive large scale poultry
farms produce millions of eggs per day.        

  

      However, if mating takes place between a
hen and a cock about the time of egg formation, fertilization can occur and
eggs thus laid will be capable of hatching into young ones.  For hatching to take place, eggs laid by hens
which have mated need to be incubated. 
Incubation is the provision of heat for fertile eggs for a specifics
time period for the purpose of hatching into young ones.  

        Incubation can be done in any of two ways
namely, naturally by the hen, or artificially through the use of an incubator.

      Natural incubation involves the hen sitting
over and covering the eggs with its body for a period of at least 21 days
usually.  Of course during this time, the
hen occasionally gets up to feed, excrete and exercise.

      During incubation, the chicks develop in the
eggs and at the end of the period; they hatch out of the eggs.  By this time, all that remains of the eggs
are the hard shells the materials previously contained in the eggs would have
been used in forming the chicks. 
Artificial incubation takes place in an incubator and this is basically equipment
having an interior space for keeping the eggs and means by which the
temperature and relative humidity of this space can be controlled. 

Eggs are kept in the incubator
for 18 days under a constant temperature and relative humidity of 37.8Oc
and 55% respectively, after which they are transferred to a hatcher, A
temperature of 36,7Oc and relative humidity of 60 % are maintained
in the hatcher.  With in a period of 3
days in the hatcher, the chicks hatch out of the eggs.

     The building where eggs are hatched into
chicks is called hatchery. In turkey, duck and guinea fowl, incubator period is
28 days.

Conclusion
      1. Animal reproduction involves the fusion of the male and female gamers giving rise to a new offspring.
      2. Reproduction can be sexual or asexual in nature.