According to the biologists Gerald and Teresa Audesirk, all members of the animal kingdom have six characteristics in common. They are as follows:
- All animals are made up of cells that do not have cell walls.
- All animals are multicellular organisms.
- Most animals reproduce sexually. Some are capable of asexual reproduction by fragmentation, budding, or parthenogenesis.
- All animals are capable of self-propelled motion at some point in their lives.
- All animals are heterotrophic and must consume other organisms for energy.
- All animals are capable of responding to external stimuli through the use of nerve, muscle, or contactile tissue.
Some animals are capable of or rely on asexual reproduction, like sea stars and some forms of snakes. In all other respects, these organisms meet the criteria for being members of the animal kingdom.