KINGDOM ANIMALIA: CHARACTERISTICS, TYPES, REPRODUCTION, NUTRITION - BIOLOGY - 2025

The animal kingdom is the group of living beings that can move (with few exceptions), they are heterotrophic, multicellular, eukaryotic, they reproduce sexually and have an embryonic development. The species found in this kingdom of nature are characterized by a wide diversity in terms of their morphology and behavior.

Animals are classified into invertebrates (they do not have a backbone) and vertebrates (they have a backbone). Vertebrates are classified into reptiles, birds, mammals, amphibians, and fish. Invertebrates are classified into more than 20 phyla, highlighting: arthropods, mollusks, porifers, cnidarians, echinoderms, plateminths, nematodes, and annelids.

There are 9 to 10 million species of animals, and 800,000 have been identified. Since the era of the Cambrian explosion, 540 million years ago, fossils of the first species have been found, which would have evolved by natural selection. On the other hand, animals share the fundamental characteristics of living beings.

The word "animal" is derived from the Latin word "animalis" which means "to have breath."

Main characteristics of the animalia kingdom

They are multicellular

Animals do not have a rigid cell wall, but are made up of many microscopic cells. Cells are found in tissues, which in turn make up the most important organs like the heart and brain.

Most animals form their bodies in the early stages of their development. However, some undergo forceful transformations through a process of metamorphosis.

Such is the case of butterflies, which when they hatch from the egg begin as a caterpillar, a species of worm or a larva. Then they come out of the chrysalis and that is when they transform into the butterfly.

Heterotrophs

Animals cannot make their own food with their organic substances, so they feed on other organisms.

Most animals have a mouth to feed, either by holding or chewing their food. Almost all eat actively, which is when they move to reach their food.

However, some do it passively. This means that they feed on the particles that are suspended in the environment; They take them when they pass by and thus take advantage of them.

Another way is through leaks, although very few animals do. An example of this type of animal is the whale, which swims and filters the water to capture small organisms.

Respiration: gas exchange

Gas exchange can take place in different ways: some do it through the lungs, gills, or branched tube systems.

Animals need to breathe to live, and this produces an exchange of gases between the inside and the outside caused by the cells. The types of respiration in animals can be:

- Skin respiration: it is the least complex type of animal respiration, since the organisms that practice it do not require any specialized organ to practice it. The exchange of oxygen and carbon dioxide occurs directly through the skin.

–Tracheal respiration: it is practiced by arthropods. It is characterized by the appearance of tubes, called tracheas, that connect to each other and to the outside. These tracheas are responsible for transporting oxygen to the cells of the animal.

–Gill respiration: it is the respiratory system used by aquatic animals. These types of organisms carry out the exchange of oxygen and carbon dioxide through organs called gills, which are capable of filtering the O2 that is dissolved in the water.

–Lung respiration: it is the most complex form of animal respiration, and is characteristic of mammals, reptiles and birds. The most remarkable feature of this type of breathing is the appearance of specialized organs called lungs, which are responsible for the exchange of gases with the outside.

Sensory system

Animals maintain a receptor structure that reacts to external stimuli. This structure detects changes in the environment and responds to these stimuli.

This is because animals have networks of nerve cells through which they react. This applies to all animals, with the exception of jellyfish. Almost all animals have their sense organs in their heads.

They move

Without exception, all animals can perform movements, be it gliding, running, flying or swimming.

Classification: types of animals

There are two types of animals: vertebrates and invertebrates.

- Vertebrate animals

Vertebrates are animals that have a backbone, which is a rigid structure that supports the body. In this type of animals there are five groups:

Fishes

They are animals that only exist in water, they breathe through the gills and move with their fins. There are two types of fish: cartilaginous and bony.

Mammals

Mammals are characterized by being warm-blooded. They feed on breast milk early in their lives, give birth to live young, and their habitats are varied.

Birds

They are oviparous animals. Most have the ability to fly; however, not all birds have this skill.

Examples of birds that cannot fly are the hen and ostriches. On the other hand, some birds can dive and even swim.

This group of animals inhabit almost the entire world except in regions with extremely cold temperatures, such as the polar regions.

Reptiles

They are characterized by being cold-blooded animals with skin of dry scales and hardness. Some can regulate their temperature.

They were the first to survive out of the water since they could hatch their eggs on land.

Amphibians

Amphibians are also cold-blooded. Their skin is smooth, they spawn in fresh waters and their habitat is terrestrial.

- Invertebrate animals

These animals do not have a bone skeleton, they reproduce sexually or asexually and some of these have both types of sexual organs; that is to say, feminine and masculine.

Ways of reproduction

Depending on the type of animal, environmental conditions and structure, they can present two types of reproduction: asexual and sexual.

Although asexual reproduction is more common, asexual reproduction has been observed in animals such as the hammerhead shark and the blacktip reef shark in captivity. It has also been observed in armadillos.

- Sexual reproduction

This type of reproduction is better known. The beings that reproduce in this way produce haploid sex cells or gametes, better known as sperm and eggs.

The ovum is the one produced by the female and the sperm is produced by the male. These join through a process of fertilization to create the zygote, which is done through mating.

- Asexual reproduction

In this type of reproduction there is only one parent. It is not necessary that the couple exist; only one member of the species.

This occurs mainly in invertebrate animals. Each organism is capable of producing genetically identical copies of itself when it becomes an adult.

This type of reproduction is highly efficient because it does not require mating, but it does not generate genetic diversity.

The main mechanisms of asexual reproduction are budding, cleavage or fragmentation, regeneration, sporulation, bipartition, and parthenogenesis.

Excision or fragmentation

It is when the parent's body is separated or divided into several fragments and each one originates a new individual, as is the case with the starfish.

There are animals that present a special fragmentation called polyembryony, which is a fragmentation of two phases: sexual, which is to form the zygote; and asexual, which is the division of the zygote into two or more segments from which the embryo is formed.

Gemmation

It refers to when a bulge or bud appears in the parent that is formed. Then this structure can separate and give way to a new animal. By this method of reproduction corals are born.

Sporulation

In this type of reproduction, animals produce a structure similar to cysts with a very resistant cover.

This structure fills with water and the cysts germinate; as soon as they open, the new animal develops.

Regeneration

It consists of a defense mechanism and of regenerating parts of the body. This method does not give way to an entire individual, but to parts of the body. An example of this are lizards.

Parthenogenesis

This form of reproduction is in the development of female sex cells. It is the development of the egg, whether it has been fertilized or not.

This is believed to be due to hormonal, biological, environmental or chemical factors.

Parthenogenesis can occur naturally in flatworms, tardigrades, rotifers, amphibians, insects, some tropical fish, and reptiles.

In the case of mammals, it has not occurred naturally; however, it has been totally or partially induced in rabbits and mice.

Cloning

It consists of obtaining identical copies of a species already developed by an artificial process or assisted reproduction in an asexual way.

Nutrition

All animals are heterotrophs, which means that they feed directly or indirectly on other living things.

The diet of animals is different depending on the species, and tends to vary a lot: they can eat from plants to other animal species. According to their diet, animals are classified into carnivores, herbivores and omnivores.

Carnivores

Carnivores are animals that only eat meat. Sometimes they hunt their prey and then eat it. Such is the case of lions, the wolf and the shark, among others.

There are also carnivorous animals that feed on dead animals. These are also known as scavengers.

Herbivores

Herbivores feed on plants and vegetables. Some herbivorous animals eat animal protein such as eggs. Herbivores include the cow, the giraffe, the horse, the rabbit and the zebra.

Omnivores

Omnivores feed on both animals and plants. They have a mixed diet: they consume both foods.

Examples of animals

Mammals

Whale, dolphin, horse, cat, dog, bat, cow, sheep, mouse, kangaroo, hyena, lion, gorilla, rhinoceros, elephant, among others.

Birds

Parrot, ostrich, penguin, condor, eagle, chicken, duck, vulture, crow, toucan, turkey, macaw, pelican, owl, among others.

Fishes

Salmon, shark, swordfish, eel, tuna, cod, piranha, toadfish, among others.

Reptiles

Crocodile, turtle, snake, lizard, iguana, viper, chameleon, among others.

Amphibians

Toad, frog, salamander, gallipat, newt, gallipats, among others.

References

1. C. Linnaeus (1735). "Systemae Naturae, sive regna tria naturae, systematics proposita per classes, ordines, genera & species".
2. Cavalier-Smith, T. (2004), "Only six kingdoms of life" (PDF), Proceedings of the Royal Society B: Biological Sciences, 271: 1251–62.
3. The World Conservation Union. 2014. IUCN Red List of Threatened Species, 2014.3. Summary Statistics for Globally Threatened Species. Table 1: Numbers of threatened species by major groups of organisms (1996–2014).
4. Slack, Jonathan MW (2013). Essential Developmental Biology. Oxford: Wiley-Blackwell.
5. Shen, Xing-Xing; Hittinger, Chris Todd; Rokas, Antonis (2017-04-10). "Contentious relationships in phylogenomic studies can be driven by a handful of genes". Nature Ecology & Evolution. 1 (5): 0126. doi: 10.1038 / s41559-017-0126. ISSN 2397-334X.