We explain everything about animals, their characteristics and how they are classified according to their body, their relationship with humans and their diet.
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Animals are extremely diverse and are adapted to all habitats on the planet.
What are animals?
Animals or metazoans are the living beings that make up the animal kingdom ( Animalia ), one of the four kingdoms of eukaryotic living beings (that is, organisms that have cells with a nucleus ), and that are distinguished from plants , fungi and microorganisms . In general, animals are living beings endowed with their own autonomous movement , a metabolism based on respiration , and a nervous system that allows them to interact with their environment.
As a whole, animals are extremely diverse and are adapted to all habitats on the planet, establishing relationships of different types among themselves and with other living beings. The human being also belongs to the animal kingdom , but since it is the only living being endowed with its own consciousness and articulated language that is known, it is generally granted a separate philosophical category within the group of animals. Seen in this way, a human being would be an animal endowed with language.
Animals appeared on the face of the Earth approximately 542 million years ago , during the so-called “Cambrian explosion” or Cambrian evolutionary radiation.
It is unknown what exactly were the reasons for such a massive and abundant diversification of life to occur at that time . However, it is known that of the nearly 20 phylums or groups of animal species known up to now, at least 11 had their origin in that specific time, that is, that the first and oldest multicellular animal species arose there , whose descendants can still be found on the planet.
The word “animal” comes from the Latin animalis , translatable as “being endowed with breath” (that is, anima ), that is, “breathing creature”. The science that studies animals is called zoology , and it is a specific branch of biology , which allows animals to be classified according to different categories, based on traits such as their behavior, anatomy or even their relationship with the human being.
See also: Animal kingdom
General characteristics of the animals
All animals must obtain their nutrients from the organic matter of other living things.
The main general features of the animals are:
- They are eukaryotic living beings, that is, they have cells endowed with a well-defined cell nucleus , in which the gathered genetic information is found; and also multicellular, that is, whose bodies are made up of more than one cell.
- Unlike plants and fungi, animal cellsdo not have a cell wall (but a more flexible plasma membrane), nor vacuoles, nor plasmodesmata.
- Their nutrition is of the heterotrophictype , that is, they obtain their nutrients from the organic matter of other living beings, unlike plants ( autotrophs ). Because of this, animal cells do not have chloroplasts (organelles for photosynthesis ) either, but rather lysosomes .
- Its metabolism is of the aerobictype , that is, it requires a continuous supply of oxygen (since it obtains its energy from the oxidation of organic molecules ). The process of capturing oxygen from the environment (either air or water) is known as respiration , and therefore all animals respire.
- They are endowed in the vast majority of cases with their own and autonomous mobility, that is, they can change their environment at will, thanks to the presence of motor organs such as cilia, legs, fins, wings, among others.
- They are sexual species, that is, with predominantly sexual reproduction . This type of reproduction requires a male and a female to produce a new individual, through the union of their well-differentiated sex cells, or gametes . There are, however, some animal species capable of asexual reproduction under certain circumstances.
- Their bodies are composed of collagen, a structural protein found in all the fibers of their tissues. The latter, moreover, tend to have a very high level of differentiation and specialization.
- Their bodies present bilateral symmetry: they can be divided with a transversal line into two identical and corresponding halves. The exception to this rule is made up of a few primitive phyla such as the porifera and echinoderms.
Vertebrate animals and invertebrate animals
Invertebrate animals do not have vertebrae or a skull.
The kingdom of animals can be differentiated into two large groups, based on the presence in their bodies of an internal skeleton equipped with a skull and a vertebral column. Thus, we can talk about:
- Vertebrate Animals. Those who have an internal skeleton endowed with a vertebral column and a skull, all made up of osseous tissue (bone) and designed to protect the nervous system: the spinal cord and the brain. Their bodies, moreover, are easily divided into three segments: head, trunk, and limbs.
Examples of vertebrate animals are mammals , fish, reptiles , birds, and amphibians . Human beings, dogs, horses , crocodiles , mice, bats , platypus, birds, frogs , among many others, are vertebrate species.
- Invertebrate animals. Those who have an external skeleton (exoskeleton) as armor made of chitin. Therefore, they have neither vertebrae nor skull, and they are mostly oviparous animals (they hatch from eggs). In evolutionary terms, they are more primitive than vertebrates.
Examples of vertebrate animals are insects and arthropods , mollusks, sponges, cnidarians, annelids, and echinoderms. Among the invertebrate species are sea urchins, octopuses , bees , scorpions and spiders , earthworms, starfish, among many others.
Wild animals and domestic animals
Wild animals do not know the presence of humans or are not used to it.
Depending on their relationship with humans and society, animals can be classified into two groups, depending on whether or not they are domesticated, that is, whether or not they have adapted to peaceful coexistence with humans. Thus, we can distinguish between:
- Wild animals. Those who do not know the presence of the human being or who are not at all used to it, and who therefore react to it with aggression or other instinctive behaviors. Animals that inhabit nature are all wild.
Examples of wild animals are: wolves , lions , blue whales , condors, rhinos , hippos , penguins , polar bears , most snakes , and all those animals that exist within human society only in zoos.
- Domestic animals. Those who have adapted over time (and the direct intervention of our species) to interact with human beings and even to depend on them, occupying a stable place within civilization. Companion animals, farm animals , and trained animals are all domestic.
Examples of domestic animals are: cows, dogs, cats, chickens, sheep, goats, horses, pigeons, hamsters, some species of fish and turtles, among others .
Carnivorous Animals, Herbivorous Animals, and Omnivorous Animals
Carnivorous animals are predators or scavengers.
Animals are heterotrophic beings, which must consume the organic matter of other living beings in order to survive, that is, they feed on other living beings and organic matter. But not all of them do it in the same way, and depending on their food preferences, it is possible to classify them into three categories:
- carnivorous animals. Those whose diet consists strictly or almost strictly of meat, that is, the body of other animals. This means that the vast majority are predators (they attack and devour other animals) or scavengers (they feed on their corpses once the predators have finished eating). Carnivorous animals often have sharp teeth for tearing meat and limbs with sharp claws to help them hunt.
Examples of carnivorous animals are: the lion, the wolf, the tiger, the shark , the snake, the eagle, among others.
- herbivorous animals. Those whose diet consists strictly or almost strictly of plant substances and fungi, that is, parts of the body of plants and fungi or materials produced by them. Herbivores typically have flat teeth with strong molars, for grinding up plant matter, as well as various stomachs for more efficient digestion.
Examples of herbivorous animals are: cow, horse, sheep, giraffe , ox, rhinoceros, butterflies , among others.
- Omnivorous animals. Those whose diet is diverse, adaptable to availability or occasion, and therefore combine foods of plant origin with foods of animal origin. They are animals with a variable or multiple diet, which can play different feeding roles at the same time, being herbivores and carnivores at the same time. Omnivores tend to have mixed teeth, combining sharp teeth for tearing and blunt molars for crushing.
Examples of omnivorous animals are: humans, monkeys, rats, pigs, bears, foxes , turtles, crows, among others.
