Pets have a considerable environmental impact, especially in countries where they are common or held in high densities. For instance, the 163 million dogs and cats kept in the United States consume about 20% of the amount of dietary energy that humans do and an estimated 33% of the animal-derived energy.
They produce about 30% ± 13%, by mass, as much feces as Americans, and through their diet, constitute about 25–30% of the environmental impacts from animal production in terms of the use of land, water, fossil fuel, phosphate, and biocides.
Dog and cat animal product consumption is responsible for the release of up to 64 ± 16 million tons CO2-equivalent methane and nitrous oxide, two powerful greenhouse gasses. Americans are the largest pet owners in the world, but pet ownership in the US has considerable environmental costs.
While many people have kept many different species of animals in captivity over the course of human history, only a relative few have been kept long enough to be considered domesticated.
Other types of animals, notably monkeys, have never been domesticated but are still sold and kept as pets. There are also inanimate objects that have been kept as "pets", either as a form of a game or humorously (e.g. the Pet Rock or Chia Pet).