People in developed nations all over the world are becoming increasingly fatter. But the usual culprits — too much food and too little exercise — might not be entirely to blame for our expanding waistlines, according to a new study.

It turns out we're not alone in our tendency to pack on the pounds. Animals from a variety of species in the care of humans — from lab rats to zoo monkeys — have also experienced weight gain over the past several decades.

The fact that this trend is so consistent, and shows up even in animals on tightly controlled diets, suggests unseen factors may be at play, the researchers said.

"We can have populations that can radically alter their body weight…even in the absence of factors that we conventionally think of as primarily responsible for the human obesity epidemic," said study researcher David Allison, a professor of biostatistics in the University of Alabama at Birmingham School of Public Health.

His findings show that new thinking about the problem is needed, Allison said.

"For policymakers and for clinicians and for research scientists, it does say we need to broaden our thinking about possible contributors to obesity," he said.

The factors contributing to obesity could range from increased exposure to light at nighttime , to infection with certain viruses, to exposure to certain chemicals, the researchers said.

Obesity in other species

Allison became curious about atypical sources of obesity when he reviewed information on a group of 143 marmosets at the Wisconsin Non-Human Primate Center. He saw the small primates had gained weight over time, approximately 15 years, but could not find a reason why.

That led him to search for other studies about animals under the care of humans for more than a decade. He identified 12 such studies, involving more than 20,000 animals.

Some of these studies involved domestic cats and dogs. Others were research animals, including primates, chimpanzees and rodents. Wild rats — more than 6,000 of them, caught on the streets of Baltimore between 1948 and 1986 — also made the list.

The researchers divided the animals into two groups, males and females, for a total of 24 groups. They found that animals in all 24 groups gained weight over time. The probability of this happening by chance is extremely small — the equivalent of tossing a coin 24 times and having it come up heads every time, Allison said.

And 23 of the 24 data sets showed a rise in the percentage of obese animals.

There was no single thread to explain the weight gain in all 24 data sets, Allison said. While some animals might have had richer food, that would not have been the case for all animals. And while some might have been less active, that would still not have been true for all of them, he said.

What's to blame?

As of now, the researchers don't know what's behind the widespread weight gain. And Allison noted, there's no guarantee that one factor will explain all the obesity. But the researchers have some speculations.

One factor could be artificial light. Studies on mice have shown mice exposed to a dim light at nighttime gain 50 percent more weight over an eight-week period than mice not exposed to dim light at night. Exposure to light when you're supposed to be sleeping may disrupt hormones that affect metabolism, and in turn lead to weight gain, researchers have speculated.

It's also possible that viruses could be influencing obesity rates. Studies have shown that infection with the virus called adenovirus-36 is associated with obesity . A recent study on children found that nearly 80 percent of those in study group exposed to the adenovirus-36 were obese.

Even chemical changes to our genes, so called epigenetic changes , brought on by factors in our environment could play a role. A recent study identified 13 of these chemical changes that were linked to body mass index, or BMI, and could influence obesity.

And chemicals in the environment, such as tributyltin, have been shown to cause weight gain when given to mice, Allison said.
More research is needed to determine what these other factors are and whether they are also affecting humans, Allison said.

Then we can determine what to do about them, he said.

The results will be published Nov. 24 in the British journal Proceedings of the Royal Society B.