Animals, They're Just Like Us: Fat - OZY | A Modern Media Company
Lazy sleeping fat cat!
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When the human environment appears to fatten up all the animals we keep in it, you have to wonder: Have we created our own fat-inducing cages?

By Libby Longino

It’s not just us: Animals across a range of species and environments are gaining weight — and increasing their risk of obesity. We’re not talking about chubby lions lazing on the Serengeti or overweight wild boars in the underbrush: the thing that unites the animals gaining weight is their proximity to humans.

It’s perhaps not surprising that the weights of domestic dogs and cats are on the rise, or even that rats in American cities, living off of human scraps, are packing on extra ounces as human consumption increases. What’s more perplexing is why animals living in controlled environments are gaining weight. Take the control rats and mice kept by the National Toxicology Program for studies from 1982 to 2005, for example. In a meta-analysis of 106 rat and 93 mouse studies, researchers found an 11.8 percent increase in body weight per decade in female mice and 10.5 percent for male mice. Among female rats, there was a 0.2 percent increase in body weight per decade, while among males there was a 6 percent increase.

These control animals had a constant supply of food available to them and were permitted to eat as they pleased, but the composition of their food changed negligibly, if at all, over time. If their weight gain comes from increased consumption of feed, why are the control mice and rats eating more now than they did a decade or two ago? The head of the study, University of Alabama at Birmingham obesity researcher David B. Allison, Ph.D., told Discovery magazine the animals may be ”changing how they metabolize [food].” 


Allison’s research on our primate cousins had similar findings: chimpanzees living in human-controlled environments saw a nearly 18-fold (males) and 11-fold (females) increase in the odds of obesity per decade from 1985 to 2005. In fact, among all the 20,000 animals from 24 populations, representing eight species living with or around humans in industrialized societies that were studied, all saw a trend of increasing body weight.

What does all this mean? For starters, it suggests that obesity is a far more complicated problem than the familiar, simple narrative that people eat too much or too poorly. Allison says that underlying causes could be poorly understood factors like viral pathogens or epigenetics. As Allison explained to Discovery, certain endocrine-disrupting chemicals like tributyltin can make mice fatter, and several types of animals gain weight when injected with a virus known as adenovirus-36. Hog farmers use temperature control to fatten pigs, which may mimic the neutral temperatures of human homes.

  • 24 out of 24 animal groups demonstrated weight gain over time.
  • Feral male rats in urban Baltimore: nearly 20 percent increase, 1948 to 2006
  • Domestic dogs: males 7 percent increase; females 13 percent increase, 1990 to 2002
  • Domestic cats: females 84 percent increase; males, a slight (not statistically significant) decrease, 1989 to 2001
  • The largest weight gains came from female chimpanzees born and raised on a primate research center: 37.2 percent weight gain and an 11-fold increase in odds of obesity, 1985 to 2005

Proceedings of the Royal Society B

Others propose that industrial chemicals and even environmental aspects of modern life like electric lighting and air-conditioning could be the culprit. All these theories mean that if we’re serious about tackling what has become a global problem (even poor countries now have more obese than undernourished people), we may need to be open-minded about the strategies we use. In the meantime, though, it’s probably still a good idea to go on that run. 

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