According to Georgia State University, all organisms in the animal kingdom have six common characteristics, including being multicellular, heterotrophic and mobile at some life-cycle stage. They also include sexual reproduction, quick responses to stimuli and the absence of cell walls. Some of these characteristics, for example, sexual reproduction, occur in organisms from other kingdoms, but organisms with all six of these characteristics are animals.
Being heterotrophic, most animals derive their energy indirectly from the Sun. Green plants engage in photosynthesis and convert sunlight into an energy source. When animals eat this food source they can use some of the energy in the plants for survival. However, some animals live near deep geothermal vents and feed on organisms that capture the energy found in chemicals rather than sunlight.
While animals exhibit a similar set of traits, this is not the reason they are all placed in the same kingdom. They are all placed in the same kingdom because all living animal species descend from a common ancestor. While these six characteristics describe most animals, there are a few exceptions. Some lizards and snakes are able to reproduce asexually.
Most animals have bodies composed of muscles, nerves, organs and blood. However, sponges and a few other primitive lineages do not produce different tissues.