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This could also be a possible explanation for this: http://www.livescience.com/10277-obesity-rise-animals.html http://www.livescience.com/10277-obesity-rise-animals.html >Allison first stumbled across evidence of overweight animals while looking at data on marmosets from the Wisconsin National Primate Research Center. The average weight of the monkeys had gone up over the decades, he noticed, and there seemed to be no plausible explanation. Allison queried primate center researcher Joseph Kemnitz as to what the cause might be: Were the marmosets from a different supplier? Had they been bred to be larger? The answers were "no" and "no." >But the monkeys' diets had been changed over the years, a switch that was well-documented by the lab. So Allison tried running the numbers again, this time controlling for the diet change. >"It only made the results stronger," he said. With the diet change, the animals should have lost weight, if anything.