Lead researcher, Prof. David Allison, a biostatistician at the University of Alabama, said:
"It may be something in the air, the water or the food, other than the nutrients."
After analyzing recorded data on over 20,000 animals for at least a decade, the scientists believe that their parallel (to humans') increase in obesity rates cannot be explained away by diet and activity levels alone. The variety of animals in the study, and their diets and lifestyles ranged too far for that conclusion, they added.
The scientists studied cats, dogs, feral rats, laboratory rodents, and four types of laboratory primates, including chimpanzees.
The researchers collected 24 data sets and found that a pattern of weight gain emerged in all of them over time. In 23 of the data sets the proportion of animals classed as obese had gone up.
A study of records from a large veterinary hospital in Northeastern USA revealed that from 1990 to 2002, dogs had put on an extra 3% of bodyweight, and cats nearly 10%.
Pest control records on Baltimore alley rats date back to 1948. Since then they appear to have put on 40% more body weight. It is possible their foraging in dumpsters today results in morsels with more calories and fat. However, their laboratory equivalents, which have been fed on pellets only, with no change in ration sizes over the decades, have also been putting on weight.
Allison accepts that weight gain among pet dogs and cats could be explained by diet and lifestyle factors - perhaps we feed them more calories and they don't run around or go for walks as much as their ancestors did. "However, that does not explain why lab rodents in the National Toxicology Program are getting bigger", he added.
The authors wrote:
There was no single thread running through all 24 data sets that would explain a gain in weight.
Prof. Allison became interested in causes of the obesity explosion after noticing that marmosets at a US research center also appeared to be getting fatter over time, despite eating the same amount of food and with no noticeable changes in physical activity.
The authors wrote:
"The consistency of these findings among animals living in different environments, including some where diet is highly controlled and has been constant for decades, suggests the intriguing possibility that increasing body weight may involve some unidentified or poorly understood factors."
Yann C. Klimentidis, T. Mark Beasley, Hui-Yi Lin, Giulianna Murati,Gregory E. Glass, Marcus Guyton, Wendy Newton, Matthew Jorgensen, Steven B. Heymsfield, Joseph Kemnitz, Lynn Fairbanks and David B. Allison
Proc. R. Soc. B Published online before print November 24, 2010, doi: 10.1098/rspb.2010.1890