Fertilization in the animal kingdom is classified as external and internal fertilization.
External fertilization is a process in which the fusion of sperm and egg takes place outside the female body.
Embryo development, in the case of external fertilization, takes place in an external environment.
Internal fertilization is a process in which the fusion of sperm and egg takes place inside the female body, while the embryo development may take place outside or inside the environment.
Based on embryo development in internally fertilized animals, they are classified into oviparous and viviparous animals.
Oviparous and Viviparous Animals
Oviparous animals don’t give birth to their off-springs directly. The development of an embryo takes place outside the female body.
That is internal or external fertilization but external embryonic development in these animals.
All egg-laying animals/mammals are oviparous animals, and this feature is called oviparity.
The yolk present provides all nutrients to the off-springs in an egg.
Eggshells are made up of calcium carbonate.
Some animal’s eggs are fragile (Reptiles), and some develop hard shells. (Birds)
Eggs are vulnerable to predators.
In oviparous animals, like fish, fertilization occurs in floating water, where the female and male gametes meet. Then the metamorphosis takes place.
Example: All birds are egg-laying, egg-laying mammals are Echidna and Platypus, reptiles, amphibians, fishes.
Viviparous animals are the ones that give birth to their off-springs directly. Embryonic development takes place inside the female body, and this feature is called viviparity.
Viviparity comes from a Latin word viviparous in which vivus means living, and pario means give birth to.
In viviparous animals, both fertilization and embryonic development are internal.
Since the development of an embryo is internal, young ones are protected.
Example: All mammals ( Cow, Humans, dog, etc.).
They feed their young ones.
The young ones take nutrients directly from the mother but not the yolk.
If the zygote develops in oviducts, then it is known as histotrophic viviparity.
If the nutrients are provided by the placenta, then it is known as hemotrophic viviparity.
Placental viviparity is the advanced embryo development of animals. It is seen in humans. Embryonic development takes place in the uterus; all nutrients required for the embryo are provided through the umbilical cord.
Reproductive modes in viviparous animals vary in two basic traits: those are, i) stages of development at which the young ones are born. ii) the mode at which the fetus receives nutritional energy for their further development.
Viviparous females have a larger body size, reproductive investment, and high hatching success rate than oviparous animals.
Sex does not determine whether the animals are viviparous or oviparous.
In some reptiles, sex is determined by the temperature, in some snakes, and most of the reptiles genotype sex determination is present.
Metamorphosis in Oviparous and Viviparous Animals
There are two types of embryonic development, direct and indirect development:
Direct development: Direct development of the young ones thus resembling the adult. E.g., Hen, monkey, cow, human.
In indirect development, the young ones don’t resemble the adult. Eg. Silkworm, frog, butterfly.
Metamorphosis is a process in which the young ones transforms into an adult.
Example: Larva transformation into a butterfly. Tadpole to the frog.
Metamorphosis of a Frog
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Frog has four stages during metamorphosis, they are:
Egg-laying, the egg masses are laid in a pond.
Hatching of a tadpole from eggs in the pond.
Froglets formation from tadpoles. Legs form, and body shrinks.
The development of lungs and back legs takes place.
Tail shrinks, the frog is developed.
Metamorphosis in Butterfly
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Butterfly metamorphosis is the process of transformation of the egg to an adult butterfly.
It has four main steps:
Egg: Female adult butterflies lay eggs on plants, which later becomes food for caterpillars.
Caterpillar: This is the second stage of metamorphosis, also called a larval stage. During its development, it sheds skin about four to five times.
Pupa: This is the third stage of metamorphosis, the transition stage. The butterflies’ pupas are called chrysalis. This stage is long and can range from a few weeks to years.
Adult: It is the last stage where the butterfly is fully developed. It is also called the reproductive stage in the butterfly life cycle.
Animals wherein the embryos develop in the eggs and remain in the body of the mother until they are capable of hatching. This mode of reproduction is the same as viviparity, but the only difference being the connection to the placenta of the mother and receiving nourishment. Yolk sac supplies are replaced with secretions from the uterus in some of the species.
The species of animals that exhibit the ovoviviparity come under the classification of ovoviviparous animals. The animals, including some species of fish like manta rays, some amphibians like Surinam toad, and reptiles such as garter snakes are ovoviviparous in nature. By making a delay in birth even after hatching, the young ones become capable of defending and feeding themselves during the time they take birth. The number of offspring that the ovoviviparous mother gives birth to simultaneously is dependent on the species. Some species give birth to two or three, while others like a guppy give birth to around 200 babies at once in the stretch of a few hours.
Oxygen is not provided for the developing eggs of Ovoviviparous insects, however, a brooding chamber is. In the case of lizards, fish, and snakes, oxygen is provided along with nutrition to the progeny in the oviduct. Most nutrition is provided by the yolk that is present in the eggs. The eggshell has a lesser thickness and is reduced in size due to a membrane. Internal fertilization is also a term used in the case of ovoviviparous animals referring to fusion if gametes are within the body of the female, then incubation to the eggs is provided within the oviduct. After the eggs are ready to hatch, the female gives birth to young ones, and they start moving independently and are capable of defending themselves. In some species, the young ones are at the larval stage when they are born, and later metamorphosis happens.
Difference Between Oviparous, Viviparous and Ovoviviparous Animals
|01||Definition||Egg-laying animals||Give birth to young ones||The egg develops inside the female body|
|02||Fertilization||External fertilization||Internal or external fertilization||Internal fertilization|
|03||Embryo development||Indirect development||Direct development||Direct development|
|04||Example||Amphibians, birds, reptiles, fishes.||Humans, cows, dogs, cats, etc.||Snakes, insects and some sharks,|
Oviparous animals are the ones who lay eggs and do not give birth to their off-springs directly.
Oviparous animals are birds, reptiles, amphibians etc.
Viviparous animals are the ones that give live birth and do not lay eggs.
Viviparous animals are humans, cats, dogs, etc.
Echidna and Platypus are two egg-laying mammals.
Ovoviviparous animals are those wherein the embryos develop in the eggs and remain in the body of the mother until they are capable of hatching.
In ovoviviparous animals embryonic development takes place in female oviducts.
Some examples of ovoviviparous are manta rays, Surinam toad, garter snakes, etc.