What are animals?

Animals or metazoans are living beings that make up the animal kingdom (Animalia), one of the four kingdoms of eukaryotic living beings (that is, of organisms that have cells with a nucleus), and that are distinguished from plants, fungi, and microorganisms. Generally, animals are living beings endowed with their autonomous movement, a metabolism based on breathing, and a nervous system that allows them to interact with their environment.

As a whole, the animals are extremely diverse and are adapted to all the planet’s habitats, establishing different types of relationships with each other and with other living beings. Human beings also belong to the animal kingdom., but since it is the only known living being with self-awareness and articulate language, it is generally given 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 exactly the reasons for such a massive and abundant diversification of life that occurred at that time. However, it is known that of the nearly 20 phyla or groups of animal species known so far, 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, from 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 humans. See also: Elephant

General characteristics of animals

animal characteristics
All animals must obtain their nutrients from the organic matter of other living things.

The main general traits of animals are:

  • They are eukaryotic living beings, that is, they have cells equipped with a well-defined cell nucleus, in which the assembled genetic information is found; and also multicellular, that is, whose bodies are made up of more than one cell.
  • Unlike plants and fungi, animal cells do not have a cell wall (but a plasma membrane, more flexible), vacuoles, or plasmodesmata.
  • Their nutrition is heterotrophic, that is, they obtain their nutrients from the organic matter of other living beings, unlike plants (autotrophs). Therefore, animal cells do not have chloroplasts (organelles for photosynthesis), but lysosomes.
  • Their metabolism is aerobic., 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 breathe.
  • They are endowed in the vast majority of cases with autonomous mobility that is, they can change their environment at will, thanks to the presence of motor organs such as cilia, legs, fins, and wings, among others.
  • They are sexual species, that is, 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 made from collagen, a structural protein found in all the fibers of your tissues. The latter, in addition, usually have a very high level of differentiation and specialization.
  • Their bodies have bilateral symmetry.: can be divided with a transverse 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

vertebrate 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 with a spinal column and a skull, are all made of osseous tissue (bone) and designed to protect the nervous system: the spinal cord and the brain. Their bodies, furthermore, are easily divided into three segments: head, trunk, and limbs.

Examples of vertebrate animals are mammals, fish, reptiles, birds, and amphibians. Vertebrate species include humans, dogs, horses, crocodiles, mice, bats, platypuses, birds, and frogs, among many others.

  • Invertebrate animals. Those that have an external skeleton (exoskeleton) as armor made of chitin. Therefore, they have neither vertebrae nor a 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, and starfish, among many others.

Wild animals and domestic animals

domestic wild animals
Wild animals do not know the presence of human beings or are not used to it.

Depending on their relationship with humans and society, animals can be classified into two groups, depending on whether they are domesticated or not, that is, whether they have adapted to peaceful coexistence with humans, or not. 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. The animals that live in nature are all wild.

Examples of wild animals are wolves, lions, blue whales, condors, rhinoceroses, hippos, penguins, polar bears, snakes, and all those animals that exist within human society only. in zoos.

  • Domestic animals. Those that have adapted over time (and the direct intervention of our species) to interact with human beings and even to depend on them, occupy a stable place within civilization. Companion animals, farm animals, and trained animals are all domesticated.

Examples of domestic animals are the cow, dog, cat, chicken, sheep, goat, horse, pigeons, hamsters, some species of fish, and turtles, among others.

Carnivorous animals, herbivorous animals, and omnivorous animals

herbivorous omnivorous carnivorous animals
Carnivorous animals are predators or scavengers.

Animals are heterotrophic beings, which must consume the organic matter of other living beings to survive, that is, they feed on other living beings and organic matter. But not everyone does it 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 (attacking and devouring other animals) or scavengers (feeding on their carcasses once the predators have finished eating). Carnivorous animals often have sharp teeth to tear meat and limbs with sharp claws to help when hunting.

They are examples of carnivorous animals: the lion, the wolf, the tiger, the shark, the snake, and the eagle, among others.

  • Herbivorous animals. Those whose diet consists strictly or almost strictly of plant and fungal substances, that is, of parts of the body of plants and fungi or materials produced by them. Herbivores typically have flat teeth with strong molars, to grind up plant matter, as well as multiple stomachs to digest plant matter more efficiently.

Examples of herbivorous animals are the cow, the horse, the sheep, the giraffe, the ox, the rhinoceros, and the butterflies, among others.

  • Omnivorous animals. Those whose diet is diverse, adaptable to availability or occasion, and therefore combine plant-based foods with animal-based foods. They are animals with changing or multiple diets, which can play different feeding roles at the same time, being herbivores and carnivores at the same time. Omnivores often have a mixed dentition, combining sharp teeth for tearing and blunt molars for crushing.

Examples of omnivorous animals are humans, monkeys, rats, pigs, bears, foxes, turtles, and crows, among others. Continue with: Alligator

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