10 facts about the animal kingdom

How many species of animals are there? What is the largest animal, and how are we related to rats? Peter Holland, the author of The Animal Kingdom: A Very Short Introduction, tells us 10 things everyone should know about the animal kingdom, and how we humans fit in.

1. An animal must have several characteristics to be an animal: it has a body built of many cells, it gets the energy necessary for life by eating other organisms, it has the ability to move, and can sense its environment.
2. Animals are a natural group, or clade, descended from a shared common ancestor. This clade is called the Animal Kingdom, or Metazoa.
3. Animals originated in the sea, but they have colonized fresh water, land, and air. Some, such as flukes and tapeworms, have invaded the bodies of other animals, while a few, such as dolphins, have returned to sea again.
4. Nobody knows how many species of insects exist; estimate range from a few million to over 30 million. At least 800,000 different species have been described and named formally.
5. The “big four” of the insect orders, accounting for over 80% of prescribed species are the beetles; butterflies and moths; bees, wasps, and ants; and flies.
6. Mostly, animals are divided into two groups: vertebrates and invertebrates. Millions of invertebrate species are described, whereas only around 50,000 different vertebrates are known.
7. Large size, efficient blood circulation, dynamic skeleton, intricate brain, protective skull, and elaborate sense organs are the features that set vertebrates apart.
8. There are only five living species of monotreme, or egg-laying, mammal: the platypus, and four types of spiny anteater. All other mammals are “therians” and have live birth.
9. Humans are opportunist omnivores, just like mice and rats. That means that they eat pretty much anything that is available to them at the time.
10. The largest and heaviest that has ever lived is the blue whale. It weighs 150 tons and can be up to 30 meters long  —  and it is still alive today.

Featured image credit: lion-portrait-animal by Sponchia. Public domain via Pixabay.