Everyone knows Americans are fat and getting fatter, and everyone thinks they know why: more eating and less moving.

But the “big two” factors may not be the whole story. Consider this: Animals have been getting fatter too. The National Pet Obesity Survey recently reported that more than 50 percent of cats and dogs — that’s more than 80 million pets — are overweight or obese. Pets have gotten so plump that there’s now a National Pet Obesity Awareness Day. (It was Wednesday.) Lap dogs and comatose cats aren’t alone in the fat animal kingdom. Animals in strictly controlled research laboratories that have enforced the same diet and lifestyle for decades are also ballooning.

In 2010, an international team of scientists published findings that two dozen animal populations — all cared for by or living near humans — had been rapidly fattening in recent decades. “Canaries in the Coal Mine,” they titled the paper, and the “canaries” most closely genetically related to humans — chimps — showed the most troubling trend. Between 1985 and 2005, the male and female chimps studied experienced 33.2 and 37.2 percent weight gains, respectively. Their odds of obesity increased more than 10-fold.

To be sure, some of the chimp obesity crisis may be caused by the big two. According to Joseph Kemnitz, director of the Wisconsin National Primate Research Center, animal welfare laws passed in recent decades have led caretakers to strive to make animals happier, often employing a method known to any parent of a toddler: plying them with sugary food. “All animals love to eat, and you can make them happy by giving them food,” Kemnitz said. “We have to be careful how much of that kind of enrichment we give them. They might be happier, but not healthier.”

And because they don’t have to forage for the food, non-human primates get less exercise. Orangutans, who Kemnitz says are rather indolent even in their native habitats in Borneo and Sumatra, have in captivity developed the physique of spreading batter.

Still, in “Canaries in the Coal Mine,” the scientists write that, more recently, the chimps studied were “living in highly controlled environments with nearly constant living conditions and diets,” so their continued fattening in stable circumstances was a surprise. The same goes for lab rats, which have been living and eating the same way for thirty years.

The potential causes of animal obesity are legion: ranging from increased rates of certain infections to stress from captivity. Antibiotics might increase obesity by killing off beneficial bacteria. “Some bacteria in our intestines are associated with weight gain,” Kemnitz said. “Others might provide a protective effect.”

But feral rats studied around Baltimore have gotten fatter, and they don’t suffer the stress of captivity, nor have they received antibiotics. Increasingly, scientists are turning their attention toward factors that humans and the wild and captive animals that live around them have in common: air, soil, and water, and the hormone-altering chemicals that pollute them.

Hormones are the body’s chemical messengers, released by a particular gland or organ but capable of affecting cells all over the body. While hormones such as testosterone and estrogen help make men masculine and women feminine, they and other hormones are involved in a vast array of functions. Altering or impeding hormones can cause systemic effects, such as weight gain.

More than a decade ago, Paula Baille-Hamilton, a visiting fellow at Stirling University in Scotland who studies toxicology and human metabolism, started perusing scientific literature for chemicals that might promote obesity. She turned up so many papers containing evidence of chemical-induced obesity in animals (often, she says, passed off by study authors as a fluke in their work) that it took her three years to organize evidence for the aptly titled 2002 review paper: “Chemical Toxins: A Hypothesis to Explain the Global Obesity Epidemic.” “I found evidence of chemicals that affect every aspect of our metabolism,” Baille-Hamilton said. Carbamates, which are used in insecticides and fungicides, can suppress the level of physical activity in mice. Phthalates are used to give flexibility to plastics and are found in a wide array of scented products, from perfume to shampoo. In people, they alter metabolism and have been found in higher concentrations in heavier men and women.

In men, phthalates interfere with the normal action of testosterone, an important hormone for maintaining healthy body composition. Phthalate exposure in males has been associated with a suite of traits symptomatic of low testosterone, from lower sperm count to greater heft. (Interference with testosterone may also explain why baby boys of mothers with higher phthalate levels have shorter anogenital distances, that is, the distance between the rectum and the scrotum. Call it what you want, fellas, but if you have a ruler handy and find that your AGD is shorter than two inches, you probably have a smaller penis volume and a markedly higher risk of infertility.)

Baille-Hamilton’s work highlights evidence that weight gain can be influenced by endocrine disruptors, chemicals that mimic and can interfere with the natural hormone system.

A variety of flame retardants have been implicated in endocrine disruption, and one chemical originally developed as a flame retardant — brominated vegetable oil, or BVO — is banned in Europe and Japan but is prevalent in citrusy soft drinks in the U.S. Earlier this year, Gatorade ditched BVO, but it’s still in Mountain Dew and other drinks made by Gatorade’s parent company, PepsiCo. (Many doctors would argue that for weight gain, the sugar in those drinks is the primary concern.) PepsiCo did not respond to a request for comment, but shortly after the Gatorade decision was made a company spokeswoman said it was because “some consumers have a negative perception of BVO in Gatorade.”

While PepsiCo has taken brominated vegetable oil out of Gatorade, it's still in Mountain Dew and other drinks made by the company. (Krista Kjellman Schmidt/ProPublica)

And then there are the newly found zombie chemicals, which share a nasty habit — rising from the dead at night — with their eponymous horror flick villains. The anabolic steroid trenbolone acetate is used as a growth promoter in cattle in the U.S., and its endocrine disrupting metabolites — which wind up in agricultural run-off water — were thought to degrade quickly upon exposure to sunlight. Until last month, when researchers published results in Science showing that the metabolites reconstitute themselves in the dark.

Says Emily Dhurandhar, an obesity researcher at the University of Alabama-Birmingham: “Obesity really is more complex than couch potatoes and gluttons.”