Hummingbirds are birds that comprise the family Trochilidae. They are among the smallest of birds, most species measuring in the 7.5–13 cm (3–5 in) range. Indeed, the smallest extant bird species is a hummingbird, the 5-cm Bee Hummingbird. They hover in mid-air by rapidly flapping their wings 12–80 times per second (depending on the species). They are known as hummingbirds because of the humming sound created by their beating wings, which sometimes sounds like bees or other insects. To conserve energy while they sleep or when food is scarce, they have the ability to go into a hibernation-like state (torpor) where their metabolic rate is slowed to 1/15th of its normal rate. When the nights get colder, their body temperature can drop significantly and thus slow down their heart and breathing rate, thus burning much less energy overnight. As the day heats back up, the hummingbird's body temperature will come back up and they resume their normal activity. They are also the only group of birds with the ability to fly backward. They can fly at speeds exceeding 15 m/s (54 km/h; 34 mph). Individuals from some species of hummingbirds weigh less than a penny.
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and independently. All animals are also heterotrophs, meaning they must ingest other organisms or their products for sustenance.
Most known animal phyla appeared in the fossil record as marine species during the Cambrian explosion, about 542 million years ago. Animals are divided into various sub-groups, including birds, mammals, reptiles, fish and insects.