By Sharon Begley, Newsweek

If it were just kids, we could blame obesity on the cutbacks in phys-ed classes, school vending machines that sell high-calorie junk, and the substitution of videogames for kickball. If it were just adults, we could blame obesity on supersizing, fast-food meals, and pedestrian-unfriendly towns that force everyone into a car. But while 68 percent of American adults qualify as overweight or obese, and 17 percent of children do (compared with 5 percent in 1971), there are other increasingly pudgy populations. Meet some overweight pets, lab animals, and even urban rats.

David Allison, an obesity researcher at the University of Alabama at Birmingham, has long criticized the Big Two explanation for America’s thundering thighs: dwindling physical activity as a result of social changes like fewer sidewalks, and increasing calorie intake as a result of nefarious food-industry marketing. By chance, he came upon a record of marmosets in a Wisconsin research colony: the little primates’ weight had soared over the previous 15 or so years, even though they had not been bred for larger size, switched chow supplier, or undergone any other change that would obviously explain their extra heft. That set Allison looking for weight records of other animals.

With colleagues, he scrutinized the weight histories of 24 populations, from alley rats in Baltimore to lab macaques in California and even control groups of mice in federal toxicology studies. In a paper to be published in Proceedings of the Royal Society B (for biology), they report that in 23 of the 24—eight species, 20,000-plus animals—the percentage of obese individuals has risen since the 1940s (or since the oldest records they found). The odds of that happening by chance are 8 million to 1. And since neither feral rats nor lab chimps nor any of the others have cut back on phys ed or patronized vending machines more, says Allison, we need to look for explanations beyond the Big Two.

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