We spend a lot of time talking about how fat one type of animal has gotten in recent years: humans. But what about all the other animals living near people — have they gained weight, too?

That's the question a group of American researchers asked in a fascinating, first-of-its-kind study I recently discovered in the Proceedings of the Royal Society B.

The researchers, led by David Allison of the University of Alabama, looked at more than 20,000 animals from eight species that live near or with humans in industrialized places. This included several species of monkeys, chimpanzees, wild rats, lab mice and rats, and domestic cats and dogs.

In every population, the trend was clear: The animals gained weight. In 11 out of 24 populations, the change was statistically significant, meaning it was big enough to be more than pure chance. "Viewed as an ensemble," the researchers wrote, "the positive direction for both weight gain and for the odds of obesity is overwhelmingly statistically significant."

Maybe domestic pets and feral rats fattened up because the availability of food has increased over the years. But how does that explain why lab animals in controlled environments got heavier, too?

The researchers suggest the trend could be driven by "any number of environmental cues such as stressors, resource availability, release from predation or climate change." Perhaps it's the endocrine-disrupting chemicals that are now ubiquitous in commercial products. Or there could be a viral cause of obesity. (Other research has already explored this theory.)

I emailed David Allison, the lead author of the study, to see if his team has done any further research on the changing rates of obesity in populations of animals since this 2011 study. They haven't, and I could not find other follow-up.



To complicate matters, it turns out Allison has faced conflict-of-interest allegations for having received funding from the food industry. So it's possible the study is biased toward attempting to shift the blame for obesity on environmental factors instead of commercial ones.

For now, this is just one observational study that raises more questions than it answers. This is a shame. Even if environmental cues can't explain everything about obesity, it's clearly a terribly complex problem, and studying other species could tell us something about the less obvious contributors to the human epidemic.