Weight gain is usually blamed on poor diet and a lack of exercise. But the marmosets and macaques living at a Madison, Wis., laboratory have followed the same diet and exercise regimens since 1982. Still, they grew heavier with each passing decade, leading David B. Allison, a biostatistician at the University of Alabama at Birmingham, to believe that environmental factors may be at play. He and his colleagues studied weight changes in 20,000 animals, including primates and rodents used for research, domestic cats and dogs, and urban feral rats. They tracked the animals’ percentage weight gain per decade, as well as their odds of being obese. Both showed a strong upward tendency. Chimpanzees grew 33.6 percent heavier per decade; mice grew 12.46 percent heavier.

Allison speculates that factors such as endocrine-disrupting toxins in the water supply or pathogens affecting mammalian metabolism may be to blame. But some say his data could be explained by diet and ex­ercise changes—caused, perhaps, by an increase in the numbers of lab animals being housed in a single cage. Allison agrees that housing might affect metabolism, but humans, too, live in increasingly crowded conditions. “This is exactly the kind of innovative thinking ... we think our results warrant,” he says. “If density of housing affects weights in animals, maybe density of housing also affects body weight in humans.”