The main characteristics of animals are sexual reproduction, eukaryotic cells, movement, multicellularity and heterotrophy. Animal cells are differentiated and organized into tissues, except in sponges. Animal tissues serve specific functions, and they include muscle tissue, nervous tissue, connective tissue and epithelial tissue.
Animal cells are complex and have a membrane, nucleus and organelles, such as ribosomes, Golgi apparatus and mitochondria. They also contain DNA, which is organized into chromosomes. The eukaryotic nature of animals is not unique to them, because other organisms, such as plants, protists and fungi, are also eukaryotic.
Animal cells are also diploid in nature, meaning that they contain two sets of genetic material. Animals are also heterotrophs because they ingest other animals and plants to get the carbon they need for survival. They also depend on oxygen for the production of energy in their cells.
Sexual reproduction in animals entails the combination of male and female genetic material, which results in the creation of new offspring. However, sexual reproduction is not unique to animals because bacteria, fungi and flowering plants also undergo it. There are exceptions in movement because some animals, such as annelids, brachiopods, sponges, hydras and corals, stay in a fixed position for most of their life. The speed of movement also varies among animals. Movement is an important characteristic because it enables animals to capture prey, avoid predators and settle in new habitats.