What is an animal? It's a simple enough question, but the answer requires an understanding of some of the more obscure characteristics of organisms—characteristics such as multicellularity, heterotrophy, diploidy, and eukaryotic cells. Although it's easy to say, for instance, that a giraffe, a whale, or a dog is an animal, it's more challenging to explain precisely why they are animals.
In this article, we'll explore the basic characteristics that are shared by all animals—from snails and zebras, to mongooses and sea stars. These characteristics help us to declare that, for example, sea anemones are animals, not plants (even though sea anemones superficially resemble flowers).
To begin our exploration of animal characteristics, I've listed the basic characteristics shared by most (or, in many cases, all) animals. Then, in the sections that follow, you can find additional information about each of these characteristics and how they help to define what, exactly, an animal is.
The following are the key characteristics that animals share:
- cells are organised into tissues
- eukaryotic cells
- sexual reproduction
All animals are multicellular, which means that their bodies are made up of multiple cells. In this way, animals differ from single-celled or unicellular organisms (such as bacteria, archaea, protozoa, single-celled algae, and single-celled fungi). Animals are not the only organisms that are multicellular, land plants and some species of algae and fungi also are multicellular. So although multicellularity is a characteristic shared by all animals, it is not a characteristic unique to animals.
In most animals, cells are organized into different tissues that perform different functions. During the development of an animal, cells differentiate so they can perform specific functions. Groups of cells that work together to serve a common function are called a tissue. There are four basic types of tissue that animals have. These include nervous tissue, epithelial tissue, connective tissue, and muscle tissue. One exception to this characteristic is the sponges. Although sponges are animals, their cells are not organised into true tissues.
All animals are eukaryotes which means they are made up of complex cell that have membrane-bound nuclei and organelles. Animals are eukaryotes, this means animals are made up of cells that have a nucleus and membrane-bound organelles such as mitochondria, Golgi apparatus, and ribosomes. The DNA contained in a eukaryotic cell is linear and is organized into chromosomes. Although all animals are eukaryotes, not all eukaryotes are animals. Plants, fungi, and protists—organisms that are not animals—are nonetheless eukaryotes.
Most animals undergo sexual reproduction. Sexual reproduction is the process by which an organism creates new offspring. The process involves the combination of the genetic material from two individuals. Sexual reproduction is not a characteristic that is unique to animals—other organisms such as flowering plants, fungi and bacteria also undergo sexual reproduction. Additionally, not all animals undergo sexual reproduction. A small number of animals produce offspring by a process known as asexual reproduction. Since sexual reproduction involves the combination of genetic material of two individuals, it results in genetic variation between parents and offspring. This genetic variation is an important factor in the process of natural selection.
Most animals are capable of movement. Although there are some exceptions, most animals are motile, or capable of movement. There are some animals—such as sponges, some annelids, brachiopods, bryozoans, tunicates, corals, and hydras—that are sessile, or fixed in place, throughout much of their life cycle. But even these animals are motile during some stages in their development. Animals move using a variety of methods—fish swim, birds fly, wolves run, snails slide, and snakes slither. Some animals, such as sea stars and slugs, move very slowly while others, such as cheetahs and sailfish move at an impressive clip. Movement enables animals to avoid predators, capture prey, and colonize new habitats.
Most animals are diploid. The terms diploid and haploid are used to describe how many copies of genetic material are contained within a cell. In diploid cells, there are two full sets of the cell's genetic material, in haploid cells, there is only one full set of the cell's genetic material. Most animals are made up of diploid cells.
All animals are heterotrophs which means they ingest plants and other organisms as a way to get their carbon the energy they need to live. All living things need carbon to support the basic processes of life such as growth, development, and reproduction. There are two ways an organism can get the carbon they need: by obtaining it from their environment (in the form of carbon dioxide) or by feeding on other organisms (and obtaining carbon from the organic materials that make up the other organism). Organisms that obtain carbon from their environment are called autotrophs; green plants are examples of autotrophs. Green plants take energy from the sun and use it along with carbon available in the atmosphere in the form of carbon dioxide to produce sugar, a simple organic compound. Organisms, such as animals, that obtain carbon by ingesting other living organisms are called heterotrophs; animals are examples of heterotrophs. All animals must ingest other organisms (such as plants or animals) to obtain the carbon they need to live.
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