In a remarkable paper Allison et al. (2011) gather data on the weight at mid-life from 12 animal populations covering 8 different species all living in human environments. Dividing the sample into male and female they find that in all 24 cases animal weight has increased over the past several decades.
Cats and dogs, for example, both increased in weight. Female cats increased in body weight at a rate of 13.6% per decade and males at 5.7% per decade. Female dogs increased in body weight at a rate of 3% per decade and males at a rate of 2.2% per decade.
One ready, although not necessarily correct explanation, is that fat people feed their cats and dogs more and exercise them less. Thus, the authors also looked at animals not directly under human control such as rats.
…For the 1948–2006 time period, male rats trapped in urban Baltimore experienced a 5.7 per cent increase in body weight per decade from 1948 to 2006 and a nearly 20 per cent increase in the odds of obesity. Similarly, female rats trapped in urban Baltimore experienced a 7.22 per cent per decade increase in body weight, along with a 26 per cent increase in the odds of obesity.
that too has a ready, although not necessarily correct, explanation:
… just as human real wealth and food consumption have increased in the United States, rats which presumably largely feed on our refuse, may also be essentially richer.
To counter both of these objections the authors do something very clever, they gather data on the weight of control mice used in many different experiments over decades.
Among mice in control groups in the National Toxicology Programme (NTP), there was a 11.8 per cent increase in body weight per decade from 1982 to 2003 in females coupled with a nearly twofold increase in the odds of obesity. In males there was a 10.5 per cent increase per decade.
Control mice are typically allowed to feed at will from a controlled diet that has not varied much over the decades, making obvious explanations less plausible. Could mice have gained weight due to better care? Possibly although that is speculative.
More generally, there are specific explanations for the weight gain in each of the animal populations, just as there are for humans. Each explanation looks plausible taken on its own but is it plausible that each population is gaining weight for independent reasons? Could there instead be a unifying explanation for the weight gain in all populations? No one knows what that explanation is: toxins? viruses? epigenetic factors? I am not ready to jump on any of these bandwagons and in some cases the author’s samples are small so I am not yet fully convinced of the underlying facts, nevertheless this is intriguing and important research.