Data gathered from over 20,000 animals of eight different species indicated that increasing body weight is a unanimous problem. Domestic cats and domestic dogs were included in the study, which also focused on laboratory macaques, chimpanzees, vervet monkeys, marmosets, mice and both domestic and feral rats. Female cats, according to the research, grew by 13.6% per decade, while male cats grew by 5.7%. Dogs' body weight increased by 2% to 3% per decade. "It just highlights how little we understand about what's happening in terms of why we see this rise in body weight in our population," said Jennifer Kuk, an obesity researcher at York University in Toronto. "Perhaps this problem isn't as simple as just energy intake and energy expenditure, which has been the prevailing message over the last 10 years."
Pathogens, artificial environments, air conditioning and central heat—they all present themselves as possible causes for the obesity trend in both humans and animals, said lead study researcher David Allison of the University of Alabama, Birmingham. "In the winter, you're not expending as much energy, because the room is kept warmer," said Allison. "In the summer, it doesn't get so hot, and we know that heat drives food intake down." However, he said, more research is needed to bring all the factors that might be causing domestic animals to grow obese alongside their humans out of the realm of speculation.