Depending on how they reproduce, animals can be classified as oviparous, viviparous, or ovoviviparous. The oviparous ones lay eggs, the viviparous ones give birth to live young and in the ovoviviparous the eggs remain inside the mother.
Both viviparous and oviparous and ovoviviparous animals are very common in nature, but we are certainly more familiar with the first two: women give birth to babies (they are viviparous) and birds "lay" eggs, of which then the chicks or chicks emerge (they are oviparous).
From an evolutionary point of view, ovoviviparity is considered to be a more “primitive” character and that viviparity is rather a significant adaptation that characterizes a large number of vertebrate animals, especially mammals such as humans, dogs, and animals. cats, cows, among others.
Let's look at some of the most important characteristics and examples of these three types of animals according to their way of reproduction..
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Oviparous animals are those that are born from eggs, so it is said that the development of the embryo product of sexual reproduction occurs inside a structure called the egg.
Birds and other terrestrial oviparous normally have internal fertilization, that is, the male must deposit his sex cells inside the female for the fusion of the sex cells of both to occur.
However, although it is not a general rule, many aquatic oviparous are externally fertilized, which means that the females produce their sex cells and release them into the water, in a region where a male will later release his; egg development occurs outside the body of both.
Inside the oviparous eggs, as in the previous case, there is enough nutritive material so that the embryos can develop before leaving the egg.
In these animals there is little or no parental care. Some birds, for example, mothers incubate their eggs for warmth and accompany their chicks after hatching, but this does not happen with aquatic oviparous.
Besides birds, there are many other oviparous animals: reptiles, insects, fish, amphibians, and even mammals that hatch from eggs..
- The hens (Gallus gallus domesticus): a type of domesticated flightless bird, extremely common on farms. It lays its eggs and incubates them for a time until the chicks are ready to hatch. Fertilization is internal. Importantly, all birds are oviparous.
- Frogs and toads (from the order Anura): these amphibians, very similar to each other, hatch from eggs. Fertilization is external, as it occurs in water, and neither the eggs nor the tadpoles that emerge from them are usually cared for by their parents.
- The reptiles (snakes, crocodiles, lizards and turtles): all reptiles hatch from eggs. Fertilization is almost always internal and the females often deposit their eggs in one place and abandon them..
- The clown fish (Amphiprion percula): Although there are many exceptions, many fish are oviparous. The clown fish, very popular in aquariums, is an example. Members of this species have external fertilization and usually lay several dozen eggs in the same place..
- The insects: practically all insects hatch from eggs, that is, they are oviparous. Ants and bees, for example, which have important “social” organizations, take care of their eggs and keep the larvae as they grow. Other insects, most of them internally fertilized, lay and leave their eggs, and the larvae are usually able to fend for themselves during metamorphosis..
- The platypus (Ornithorhynchus anatinus): it is a very peculiar semiaquatic poisonous mammal, only found in Australia and the Island of Tasmania. It is characterized by its duck snout, otter legs and beaver tail; besides that it lays eggs.
Viviparous animals are “born alive”, as females retain fertilized eggs inside and give birth to live, relatively independent and well-developed offspring (with their exceptions).
In these animals there is never talk of external fertilization or fertilization, it is always internal, since the zygote and the embryo develop in the reproductive structures of the mother that are destined for this purpose..
Viviparous do not produce eggs, like ovoviviparous ones. Instead, many of them produce a tissue known as the placenta, which nourishes the embryo, protects it, and provides it with a host of important factors for development..
As with humans, who must be cared for by their parents for a long period of time at birth, many viviparous animals present complex parental care behaviors, that is, they take care of their young after giving birth.
The most extreme case is represented by mammals, which continue to feed their young with mother's milk until they have sufficiently developed the digestive system to feed on something else..
There are multiple examples that we can cite of viviparous species, starting with humans and continuing with a large number of animals related to this: horses, cows, dogs, cats, squirrels and other rodents, etc. However, there are other equally important examples in the animal kingdom:
- The Lions (Panthera leo): they take care of their young after giving birth and during their growth. When they can digest meat, females hunt animals and take them away for food.
- The rats (Mus musculus): house mice or common mice belong to a species of small mammals that, like all mammals, are viviparous and feed their young with mother's milk.
- Horses (Equus caballus): horses are also mammals and, although they have parental care, after calving, their young develop quickly with relative independence, being able to get up and gallop and even nibble on some herbs.
- Dogs (Canis lupus familiaris): Like other mammals, dogs are a good example of viviparous animals. Just like humans, when their young are born, dogs take care of them and nurse them for several weeks, until they better develop their senses and can digest other foods.
- Gorillas They are also mammals, viviparous, of course, that have parental care and that protect their young for a long time.
- The guppy fish (Poecilia reticulata) - An extremely popular fish in home and ornamental aquariums. Parent live fry, which previously feed on the mother's placenta.
- The hammerhead shark, the blue shark and the bull shark: are examples of other fish exhibiting viviparity. These are born alive, as the females keep them inside, feed them through the placenta and then "parent" them..
- Boas: Although it is not very common among reptiles (and not among amphibians), some species are viviparous, such as those belonging to the boas group, which have placenta and give birth to their young alive, not laying eggs.
Ovoviviparous animals are those that produce eggs and retain them inside, even after the moment of hatching arrives, which is when the egg breaks and the baby that was growing inside it comes out..
During sexual reproduction, the fertilization of ovoviviparous is internal, that is to say that the male must deposit his sexual cells inside the female, and it is there where the female and male cells fuse and form the zygote (which is the cell which will become an embryo and then a baby).
The embryo develops inside an egg that contains enough food to help it grow during the time of incubation, which occurs inside the mother's body..
When this food runs out, hatching occurs and the young spend additional time inside their mother, outside the egg, where they continue to mature..
Ovoviviparous do not have a placenta or umbilical cord, as is the case of viviparous ones, so they are not able to give more food to their young than what is included in the egg.
The young are released after hatching, which is very important for their survival, as they are better able to feed themselves and better defend themselves in their habitat..
When compared to the juveniles of other animals that reproduce oviparously, the offspring of ovoviviparous animals are larger and more developed, which means that they are less vulnerable and will have a better chance of surviving without their parents..
There are many examples of ovoviviparous animals in nature, especially in regards to aquatic animals, reptiles or the like and invertebrates, here are some:
- The basking shark (Cetorhinus maximus): it is the second largest fish in the sea. They have 2 to 6 young per reproductive cycle, which are produced in eggs within the female, which hatch inside and are released as fully formed young.
- The garter snake (Thamnophis sirtalis): of the most common snakes in North America, which can be found from Canada to the coast of Florida.
- The horses of sea (gender Hippocampus): small marine animals characterized by the fact that the females deposit their eggs in the males, who incubate them until they hatch inside and the developed offspring are released into the water.
- The rough vipers (Vipera aspis): a very common species of viper in Europe (less than a meter long).
- The lution wave "crystal shingles”(Anguis fragilis): a kind of legless lizard that looks like a snake (related to reptiles).
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