What is an animal? It's a simple enough question, but the answer is complex and requires an understanding of some rather hefty scientific concepts and terms. Here we'll explore the basic characteristics that make an organism an animal and attempt to decipher the scientific jargon that surrounds these concepts.
It's easy to say, for instance, that a giraffe, a whale, or a dog is an animal. But when it comes down to making a list of animal characteristics—identifying specific traits shared by all animals—it gets complicated.
The list that follows describes characteristics shared by all animals, from snails and zebras to mongooses and sea stars. These characteristics help us to declare that, for instance, corals are animals, not plants. They help us trace the evolution of animals and build a framework for placing animals into subcategories that are more familiar to us such as birds, mammals, reptiles or amphibians.
All animals are eukaryotes.
Prokaryotes and eukaryotes differ from one another in many fundamental ways but the differences are difficult to see. Cells are tiny and you must use a microscope to view their internal structure. But once magnified, the differences between prokaryotes and eukaryotes becomes more apparent. The internal structure of a prokaryote cell is less complex than that of a eukaryote. Prokaryotes have no nucleus, eukaryotes have a nucleus. In eukaryotes, the nucleus holds the cell's genetic material within a membrane. In prokaryotes, genetic material is located in a central region of the cell and is called a nucleoid, it is not enclosed in a membrane.
And the differences do not stop at the nucleus. There are numerous other differences between prokaryotes and eukaryotes:
- Eukaryotes have linear strands of genetic material (DNA); prokaryotes have circular DNA.
- Eukaryotic DNA is attached to proteins (called histones) and is organized into chromosomes; prokaryotic DNA is not associated with proteins nor is it organized in chromosomes.
- Eukaryotic cells have numerous membrane-bound structures within the cell called organelles. These organelles perform various functions within the cell. Prokaryotes lack any such membrane-bound structures.
- Eukaryotic cells are more complex, structured, and larger than prokaryotic cells.
Despite these differences, eukaryotes and prokaryotes share many characteristics and are therefore believed to be evolutionarily related. Scientists believe that the more complex eukaryotes evolved from the earlier prokaryotes.
So all animals are eukaryotes. That is, they are made up of complex cell that have membrane-bound nuclei and organelles. Their DNA is linear and organized into chromosomes. Keep in mind though, that the reverse is not true—not all eukaryotes are animals. Plants, fungi, and protists—in addition to animals—are eukaryotes.
All animals are heterotrophs.
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:
- An organism can assimilate carbon from their environment in the form of carbon dioxide (from the atmosphere) or inorganic compounds. They can ingest other organisms and thus obtain carbon from the organic materials that make up the other organism. The term autotroph describes such organisms
- An organism can obtain the carbon they need by getting it from other living organisms that are made up of carbon-containing organic molecules. The term heterotroph describes this type of organism, one that uses organic materials as a source of energy for growth and development.
Green plants are autotrophs. They 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. In contrast, all animals are heterotrophs. They ingest plants and other organisms as a way to get their carbon the energy they need to live.
Animals are multicellular. Most animals (with the exception of sponges) have bodies that are differentiated into tissues.
Animals' bodies are made up of multiple cells. In most animals, those 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 with similar specializations that form a common function are referred to as a tissue. One exception to this is sponges (Phylum Porifera) which have do not posses tissues (they don't have muscles or nerves).
Most animals are capable of movement.
Unlike plants, which are fixed to the substrate in which they grow, most animals are motile (capable of movement). One exception is again sponges, which are sedentary organisms.
Most animals undergo sexual reproduction although a small number are capable of asexual reproduction.
Most animals reproduce sexually via egg and sperm. This means genetic information is crossed between individuals and variability between parents and offspring is maintained. There are a few animals that are capable of asexual (non sexual) reproduction. These include .
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.
Animals do not possess rigid cell walls (plants do have rigid cell walls).
In plants, the cell wall provides structural support and protection for the cell. Some bacteria also possess cell walls. Animals do not have cell walls.