Animals, like plants, can be commonly classified in many ways: according to what they eat, according to where they live, according to how they reproduce and even according to whether they have a skeleton or not..
The classification of living things is not something new. In the 18th century, a man named Carlos Linnaeus devised a system to organize and classify living beings, beginning with plants and then continuing with all forms of life that we know..
Today we know that taxonomy is the discipline that is responsible for classifying and naming animals, distributing them among the 7 levels proposed by Linnaeus in his time: Kingdom, Phylum, Class, Order, Family, Genus and Species.
There are 4 main ways to classify animals:
In the animal kingdom, three different types of reproduction have been described:
Oviparous animals are those that are born from eggs, where embryonic development also occurs.
The term oviparous literally means "egg" and "birth" and is used to describe all animals that when they reproduce sexually produce a zygote covered by a protective shell that forms after fertilization.
These animals can be terrestrial, aerial or aquatic and the way in which they lay their eggs varies greatly from one species to another..
Generally, the eggs of oviparous animals contain enough food and space for the embryo inside them to develop before hatching. They are “controlled” spaces that isolate the embryo from the environment, helping it to resist different conditions during its development..
Fertilization in these animals (the fusion of the sex cells, ovum and sperm) can be of two types:
Examples of oviparous animals: all birds are oviparous and there are a large number of fish, amphibians and reptiles that are also oviparous.
Viviparous animals, such as humans, mammals and others, are those that give birth to their live young.
In these animals, the embryos develop inside specialized structures of the female, where they can carry out gas exchange, receive constant food and eliminate their waste.
The tissue that these animals feed on is known as the placenta, which is formed in the first stage of embryogenesis..
They differ from many oviparous animals in parental care, as many animals protect their young for months and even years after birth. Mammals are a good example of viviparous animals.
All viviparous animals are strictly internally fertilized, so they usually have well-developed and more “complex” reproductive structures that seek to ensure contact between the sex cells or gametes of a male and a female organism.
Examples of viviparous animals: Besides mammals, many other animals are viviparous, including some fish, many reptiles, and even amphibians and insects.
Ovoviviparous animals differ from oviparous in that they do not shed their eggs. Embryos develop from the nutritive material within the eggs and not directly from the mother.
Inside the mother, the eggs have physical protection against any environmental danger they may suffer, even if they do not directly receive their nutritional support..
Fertilization in these animals, as in oviparous ones, can be internal or external (more commonly internal) and has been observed in many fish (including sharks or guppies), manta rays, in reptiles and in a great variety of animals without vertebrae.
We can also classify animals with respect to where they prefer to live and where they do best: in the air, in the water or on the ground..
Aerial animals are those that can move through the air thanks to special structures called wings. However, even though they spend much of their time in the air, they usually need to nest on trees, rocks, or mountains..
Not only do birds belong to this group, as there are countless flying insects and we must not forget bats, which are the flying mammals par excellence..
Animals that live in water are known as aquatic animals. These have specialized organs for underwater life, especially when it comes to breathing..
Some animals can live both in and out of water, either for a period of their life cycle or alternating between land and water depending on their physiological needs. These include amphibians, some insects, and others.
Fish are all aquatic, but in the seas and oceans of the world there are also cetaceans that, although they look a lot like fish, are actually gigantic aquatic mammals.
Aquatic is also the platypus. Beavers are semi-aquatic animals, as they can move and stay underwater for a long time, but they live on land. Among the aquatic birds we can mention the penguins and other birds that can dive into the sea to “catch” their prey..
Finally we have land animals. They are those who spend most of their lives on land and who do not have specialized structures for underwater life or adapted for flight through the air..
Terrestrial animals have good senses of sight, hearing, smell and touch, which allow them to function easily in the environments where they live..
Being terrestrial implies being able to live on or among the trees, in wide savannas, in the mountains, in the hottest deserts and even in the coldest tundras.
Terrestrial we are human beings, so are almost all mammals, so are many insects and reptiles, and so are some flightless birds such as chickens, ostriches and rheas, for example.
According to their skeleton, animals can be classified into vertebrates and invertebrates. Vertebrates have a backbone and bone system, while invertebrates lack them..
Vertebrates are very diverse animals, including reptiles, birds, mammals, and amphibians. The largest animals in the world belong to the group of vertebrates, such as elephants, whales, hippos, giraffes, etc..
All are characterized by having a well-defined head, a trunk and a caudal portion or “tail”. The internal skeleton that characterizes them is what allows them to reach large sizes and even walk upright, like us humans.
In addition, vertebrates can be terrestrial, aquatic and aerial; viviparous, oviparous or ovoviviparous; carnivores, herbivores or omnivores.
Invertebrates are the most abundant and diverse group of animals on the planet. These are animals without vertebrae, so they cannot reach as large sizes as vertebrates.
Invertebrates include insects, mollusks, starfish and jellyfish, earthworms, and other worms that are parasitic on man and other animals and plants..
Invertebrates represent about 90% of the animals on earth and their diversity is impressive, so much so that man continues to describe new species every day..
For this reason, invertebrates play a fundamental role in most of the earth's ecosystems, especially if we consider agricultural ecosystems that, although artificial, completely depend on insects that pollinate plants to produce fruits..
Based on what they eat, animals can be classified as herbivores, carnivores, or omnivores.
Herbivorous animals eat plants. Due to this, these animals are physiologically and anatomically adapted for the consumption of plant tissues..
Among such adaptations we can highlight the chewing devices designed to crush leaves, stems, flowers, fruits and roots. They often have flattened and not sharp teeth.
Herbivores can be considered as a basic link in the food chain, as they serve as food for animals such as carnivores and omnivores..
In addition, we can subclassify herbivores according to the "type" or "part" of the plant they feed on: frugivores (of fruits), nectarivores (of nectar), florivores (of flowers), granivores (of grains). ), folivorous (of leaves), etc..
Omnivores are many insects, large mammals such as cows, sheep, horses, and giraffes. There are herbivorous fish, as well as many birds.
Carnivorous animals, as we could infer from their name, feed on the meat of other animals. The general rule is that carnivores must kill their prey to feed on them, being able to feed on herbivores, other carnivorous animals and also omnivores.
These animals are present in all ecosystems and, like herbivores, they are also physically and physiologically adapted to hunt and eat meat. Lions, tigers, cheetahs, cougars, wolves, sharks, and other large animals are strictly carnivores.
Omnivores are those animals that eat anything, that is, they do not necessarily have strictly carnivorous or strictly herbivorous diets: they can eat meat, leaves, flowers and fruits without any impediment. Their stomachs are adapted to digest both types of food.
Some omnivores can hunt the animals they feed on, while others can feed on carrion, the eggs and young of other animals, etc..
The vegetable diet of omnivores is not the same as that of herbivorous animals, since many times they cannot digest the cellulose of some tissues or the substances in grains (they usually feed on fruits and vegetables).
Human beings are an excellent example of omnivorous animals, but chimpanzees, orangutans, bears, raccoons, rats and other rodents, pigs and chickens also stand out; some insects such as flies, cockroaches, among others are also included.