What Is an Animal?
Animals are one of the most diverse groups of living beings on Earth. But what sets them apart from other organisms like plants?
Main Characteristics of Animals
Animals share specific biological traits:
- Energy acquisition: Animals do not rely on photosynthesis. They obtain energy from other animal or plant organisms.
- Respiration: Animals require oxygen, whereas most plants produce oxygen.
- Central nervous system: Animals possess sensory perceptions (including pain).
- Sensory organs: Most animals have the ability to see, hear, touch, taste, and smell.
- Movement: Nearly all animals can move actively.
- Reproduction: Animals reproduce to ensure the survival of their species.
Which Animals Are There?
Animals are categorized into large groups known as animal classes. This classification is referred to as taxonomy. Biologically, humans are also included in this classification – we are mammals.
Vertebrates (Vertebrata)
- Mammals: Vertebrates with hair, typically possessing four legs.
- Birds: Vertebrates with feathers and beaks that lay eggs.
- Amphibians: Cold-blooded animals that live both on land and in water.
- Reptiles: Cold-blooded vertebrates that primarily move by crawling.
- Fish: Vertebrates that live in water, breathe through gills, and lay eggs.
Invertebrates (Invertebrata)
- Insects: Animals with an exoskeleton made of chitin.
- Arachnids: Invertebrates with eight legs.
- Invertebrates: Mollusks, crustaceans, sponges, and worms.
Animal Family Tree
Evolution
The simplest of all living creatures are single-cell organisms that are able to breathe, grow, and breed. Animals have developed from single-cell organisms over a period of millions of years. Just like human beings, they consist of many different cells, which perform a broad variety of tasks, such as the formation of bones and muscles. The family tree shows how animals have developed from single-cell organisms. It also gives you a rough idea of which animals are more closely related to each other.
When Did Animals First Emerge?
| Years | Animal |
|---|---|
| ~ 4 billions of years ago | Single-cell organisms |
| ~ 550 millions of years ago | Crabs, mollusks, vertebrates |
| ~ 450 millions of years ago | Fishes |
| ~ 350 millions of years ago | Amphibians, reptiles, insects |
| ~ 250 millions of years ago | Dinosaurs |
| ~ 200 millions of years ago | Mammals |
| ~ 150 millions of years ago | Birds |
Habitats: Where Animals Call Home
Animals have inhabited nearly every corner of the Earth. Their living environment is referred to as a habitat in biology:
- In water: Oceans, rivers, and lakes (home to fish, whales, corals).
- On land: Forests, deserts, steppes, and mountains (home to mammals, reptiles).
- In the air: The layer of air primarily used by birds and insects for movement and hunting.
Each animal is perfectly adapted to the conditions of its habitat (temperature, food supply, humidity) through evolution.
How Do Animals and Plants Differ?
The distinction seems clear at first glance, but there are fascinating borderline cases in biology.
Movement: Animals can move, but plants cannot.
This statement holds true in most cases, but not all. Corals are one of the exceptions. They are "sessile animals". In contrast, there are flowers that can move (!): The tropical "Mimosa" folds all its leaves in within seconds when touched).
Nutrition: Animals eat other animals and plants. However, plants do not eat animals.
Correct! Animals can be carnivores and/or herbivores. Plants, on the other hand, absorb nutrients from the soil through their roots and have a metabolism that operates via photosynthesis (light and oxygen). However, there are a few exceptions among plants: carnivorous plants like the Venus flytrap. It enjoys consuming flies, ants, and spiders.
How Do Animals Differ From Humans?
From a biological perspective, humans are classified as animals – specifically, as mammals within the primate order.
- Similarities: We share with other mammals the basic skeletal structure, warm-bloodedness, lung respiration, and the nurturing of our young. Our DNA is over 98% identical to that of chimpanzees.
- Differences: Humans are distinguished from animals by their highly developed brains. This allows for language, tool use, and the creation of culture. Humans can plan for the long term and reflect on their actions. However, scientists are increasingly finding that animals also possess the same or at least similar abilities.
Systematic Classification (Taxonomy)
For a better understanding, animals are sorted into different categories such as animal species, animal families, and animal genera. The categories explain the genealogy of the animals and help to identify them. Please have a look at the dromedary to find out more about the terms and meanings of the categories.
Example: Dromedary
1. Class (classis): Mammals (such as monkeys, kangaroos, rabbits, horses, elephants, or dogs)
2. Order (ordo): Even-toed ungulates (related to giraffes, hippos, deer, goats, and antelopes)
3. Family (familia): Camels (divided into New World camels and Old World camels)
4. Genus (genus): Old World camels (includes dromedaries and Bactrian camels)
5. Species (species): Dromedary
Conservation: Why We Must Preserve Diversity
Biodiversity (species diversity) is the foundation of a functioning ecosystem. Each animal species plays a role: insects pollinate plants, carnivores regulate populations, and decomposers (like worms) enrich the soil.
Today, many animal species are threatened by habitat loss, climate change, and pollution. Conservation is not just about saving individual animals, but about preserving entire ecosystems to maintain the natural cycle of life.
Animal Classes
The most important animal classes are:
| Insects: | ca. 1,000,000 |
| Mollusks: | ca. 130,000 |
| Arachnids: | ca. 100,000 |
| Crustaceans: | ca. 67,000 |
| Worms: | ca. 50,000 |
| Fish: | ca. 32,500 |
| Cnidarians: | ca. 11,000 |
| Birds: | ca. 9,000 |
| Reptiles: | ca. 9,770 |
| Amphibians: | ca. 7,000 |
| Mammals: | ca. 6,600 |
| Echinoderms: | ca. 6,300 |
Conservation of Species: Why We Must Preserve Diversity
Biodiversity (the variety of species) is the foundation of a functioning ecosystem. Each animal species plays a role: insects pollinate plants, carnivores regulate populations, and decomposers (like worms) enrich the soil.
Today, many animal species are threatened by habitat loss, climate change, and pollution. Conservation is not just about saving individual animals; it’s about protecting entire ecosystems to maintain the natural cycle of life.
