Cats and dogs in the US consume as many calories every year as the population of France - and it's taking a huge toll on the environment.
A recent study published in journal PLOS One shows how the meat-heavy diet consumed by companion animals is having an impact equivalent to driving 13.6 million cars for a year.
And it is set to get worse according to study author UCLA geographer Gregory Okin, who writes that: "Dietary choices have considerable impacts on environmental sustainability.
"Compared to a plant-based diet, a meat-based diet requires more energy, land, and water and has greater environmental consequences in terms of erosion, pesticides, and waste."
According to the research: "Americans are the largest pet owners in the world, but the tradition of pet ownership in the U.S. has considerable costs.
"As pet ownership increases in some developing countries, especially China, and trends continue in pet food toward higher content and quality of meat, globally, pet ownership will compound the environmental impacts of human dietary choices."
Okin started his research by looking at the number of companion cats and dogs in the US (around 163 million - 77.8 million dogs and 85.6 million cats), taking their likely calorie needs, and analyzing the ingredients in pet food, adding up those derived from animals.
He discovered that the caloric consumption of these animals clocked in at a staggering 19 per cent of the humans' intake.
But in terms of calories derived from meat, it was 33 per cent of humans' intake - on account of their diets tending to be more meat-heavy.
When he added in the feces produced by the animals (about a third of those produced by people), he came up with some numbers.
Okin concluded that the American cats and dogs are accountable for around 64 million tons of methane and nitrous oxide (two greenhouse gases) through their diet - this is equivalent to driving 13.6 million cars for a year.
While Okin says he doesn't want to tell people to give up their companion animals, he does want his work to inform them of the environmental consequences, so they can work to reduce it.
He wrote: "This analysis does not mean to imply that dog and cat ownership should be curtailed for environmental reasons, but neither should we view it as an unalloyed good.
"It is clear that a transition to pets that eat less meat, and therefore have less environmental impact, would reduce the overall US consumption of meat."
Change in how people feed companion animals is needed, he claims, saying: "Without large-scale reduction in their number and changes to the food system that drastically reduces the per-capita animal product consumption, the environmental and energetic impact of these animals will remain significant."
He believes that as natural omnivores, dogs at least, should be able to consume more of their protein from plant-based sources, and would like to see producers offer more options for human companions to buy for their animals.
He says: "I certainly hope these kinds of numbers will encourage the market to consider adding those as market choices, and I also think that individuals can make choices."