We are really fat. Some 36 percent of American adults are obese. Almost 70 percent of adults age 20 years and over are overweight. In the last 15 years the country’s obesity rate has increased every single year.

While America is the worst offender here, the West in general is getting bigger. We’ve actually, over the past few decades, seen “simultaneous increases in obesity in almost all countries,” according to a 2011 paper in The Lancet. And worldwide obesity has almost doubled since 1980.

The world’s growing girth has prompted extensive cultural criticism and efforts at reform, from ThisIsWhyYoureFat, a blog of some of the most ridiculously calorie-laden foods (including the Baco, a taco in a bacon shell, and the Banana Peanut Butter Cup and Marshmallow Grilled Sandwich) to First Lady Michelle Obama’s “Let's Move” public health campaign, designed to encourage exercise and healthier food choices in America’s children.

Even soda companies, which produce products of no nutritional value, are getting involved in combating fat, launching their own anti-obesity campaigns. Cola-Cola recently announced it was giving $3.8 million in grants for “active living” programs. "There is a place for all of our beverages in a healthy lifestyle," said Coca-Cola CEO Muhtar Kent (ignoring the fact that a sincerely healthy lifestyle would probably involve rarely, if ever, consuming soda at all).

The fact that animals (pets, captive, and wild) are getting fatter even without doing or eating the things that are supposed to be making people so overweight indicates that something strange is going on here.

Doctors and pundits don’t see why we’re so fat as much of a mystery. The introduction of high fructose corn syrup into the American diet in the 1970s, coupled with agricultural subsidies for many products used in fast food (beef, wheat, corn) made high-calorie food cheaper, which made America’s most fat-packed foods available to everyone at affordable prices, dramatically increasingly the size of the country’s poor, who were historically thin. Add to this the fact that most people’s jobs are, thanks to industrialization, largely sedentary, and the proliferation of computer and video games, which meant children’s leisure time was less calorie burning, and you’ve got a perfect storm for obesity.

What’s interesting, however, is that it’s not just people who are getting fatter. Animals are getting bigger, too.

In 2007 the U.S. Food and Drug Administration approved the first drug to treat obesity in dogs. A post on PRNewswire notes that “trends in pet insurance are mirroring human health care. In 2007, PetFirst saw a 19 percent increase in claims related to obesity, including conditions such as cardiac arrest, hypertension and hypothyroidism.” (Also, there is such a thing as pet insurance?) Veterinarians report that 19 percent of horses in a large cohort were obese, an increase from previous years.

David Allison, a biostatistician at the University of Alabama-Birmingham, was part of a team that tried to figure out what was going on. In a paper charmingly named “Canaries in the Coal Mine” Allison and his collaborators...

[e]xamined samples collectively consisting of over 20,000 animals from 24 populations of animals representing eight species living with or around humans in industrialized societies. In all populations, the estimated coefficient for the trend of body weight over time was positive (i.e. increasing). Surprisingly, we find that over the past several decades, average mid-life body weights have risen among primates and rodents living in research colonies, as well as among feral rodents and domestic dogs and cats.

Animals are not getting fatter at the same rate as Americans, of course, or even humans living elsewhere on the planet, but something is changing. “When we combine males and females of each species into a single analysis,” the authors write, “we find that in all 12 populations, per cent weight change and odds of obesity time trends were positive....”

But while pets are on some level a reflection of the lives of their owners—they eat our food scraps and also, well, if you’re too lazy to go out and take your dog for a vigorous walk you’re not the only thing that’s going to get fat—zoo animals, whose lives are highly regimented and designed to promote health, are also growing around the middle. Chimpanzees get about the same food and the same level of exercise that they always have. And yet:

Among colonized chimpanzees, males and females, respectively, experienced a 33.2 and 37.2 per cent weight gain per decade, and a nearly 18-fold and 11-fold increase in the odds of obesity. In vervets, for females and males, respectively, there were 9.4 and 2.9 per cent increases in body weight per decade associated with 83 and 834 per cent increases in the odds of obesity. Among marmosets, females experienced a 9.7 per cent increase in body weight per decade, and a 1.73-fold increase in the odds of obesity. Among males, there was a 9.2 per cent increase in body weight per decade, and a 64 per cent increase in the odds of obesity.

And some wild animals are getting bigger, too. Body weight in male rats in Baltimore increased 5.7 percent between 1948 and 2006. Even rural rats have gained weight. Body weight of male rats trapped in the Maryland countryside increased by 4.5 percent.

Animals are getting fatter, just like humans, “even in the absence of those factors that are typically conceived of as the primary determinants of the human obesity epidemic via their influence on diet,” according to the “Canaries in the Coal Mine” study. Animals don’t have access to vending machines and they aren’t playing PlayStation 3, but they’re still starting to look like those fat-people’s-butts-walking-on-city-streets shots that always seem to accompany mainstream news stories about human obesity.

The change here is something of a mystery but it may reflect what we see in human lives, too. Americans, after all, eat poorly, but even people in Japan, where diets are some of the healthiest in the world, are getting fatter. People everywhere, in fact, just weigh more than they used to, despite the fact that they don’t all consume large amounts of high fructose corn syrup and have the sedentary jobs of people living in America or Western Europe.

What’s going on? No one really knows. According to the Allison study:

One set of putative contributors to the human obesity epidemic is the collection of endocrine-disrupting chemicals (endocrine-disruptors), widely present in the environment. Another conceivable explanation is obesity of infectious origin. Infection with adenovirus-36 (AD36) leads to obesity in multiple experimental models and antibodies to AD36 are correlated with obesity in humans. These observations suggest that AD36 and conceivably other infectious agents could be contributing to obesity within populations. Other explanations may include epigenetic-mediated programming of growth and energy-allocation patterns owing to any number of environmental cues such as stressors, resource availability, release from predation or climate change.

Basically, the environment could be to blame. The widespread presence of chemical carcinogens in nature may not only be giving us cancer, it might also be making humans—and animals—obese. Certainly the fact that animals (pets, captive, and wild) are getting fatter even without doing or eating the things that are supposed to be making people so overweight indicates that something strange is going on here.

As David Berrebywrites at Aeon, however, it’s possible there’s actually some mysterious other thing going on here, something unknown and hidden. Humans sweat and shiver to maintain the appropriate body temperature. The things our bodies do naturally to maintain temperature burn calories. Now that we increasingly live and work in climate-controlled environments, our bodies don’t need to burn as many calories to keep the right temperature. It’s reasonable to suppose that pets, zoo animals, and even rats would be impacted by this as well, since they live mostly in climate-controlled, human-built environments.

All of this is not to say that the basic narrative of the Western World’s gut—that we’re becoming overweight because we eat poorly and don’t exercise—is wrong. Certainly if you weigh too much the only good way to shed the pounds is to improve your diet and be more active. But it’s possible that the true story of fat might be more complicated, and not just a function of lifestyle. Look to the animals.