It is not just human beings that are getting fatter. Animals are, too

IN THEIR attempts to explain the global epidemic of obesity, researchers have often taken to fingering culprits beyond people's direct control. It is now believed that increased levels of stress, climate change and even artificial light at night may contribute to expanding waistlines. However, if such factors affect humans, they ought, in principle, to have similarly nefarious effects on other creatures. This should hold especially true for species that are physiologically similar to people and live in proximity to them. Pet owners have long fretted that this may, indeed, be happening.

Of course, anecdotal evidence carries little weight, so a group of researchers led by Yann Klimentidis, of the University of Alabama, decided to check whether animal obesity rates do in fact mirror the worrying trend among people. They published their findings this week in the Proceedings of the Royal Society.

Dr Klimentidis and his team set about their task by scouring online repositories of scientific papers, contacting fellow researchers and even petitioning pet-food companies for data on changes in animals' bodyweights over the decades. They limited their search to mammals, whose bodies work much like humans' do—and, specifically, to those mammals living with or around people in the rich world.

The trawl threw up information on more than 20,000 animals from 24 distinct populations covering eight species. These included cats, dogs, mice, rats and several types of monkey. Some were bred in highly controlled research environments. Others lived in people's homes or in the wild. None had their food intake artificially limited or, as with livestock, ramped up.

For each population, Dr Klimentidis looked at the animals' weight at an age corresponding to 35 human years. Middle adulthood was chosen to ensure the data were not fudged by the effects of either early development or old-age withering. Any animals that died within a year of this mid-life physical were also excluded.

He then proceeded to calculate each population's average weight, as well as its obesity rate, for every decade of available data. The obesity rates were based on a bespoke indicator akin to the body-mass index that is used to gauge (roughly) whether a person is too rotund. This ploy permitted comparison between species in which weights have different meanings. (Nutritionists employ similar tricks to establish what is a healthy body-mass index for children in different age groups.)

Subsequent number-crunching revealed a statistically significant increase in bodyweight in 11 of the 24 populations. The weights of the other 13 rose too, though not to an extent that was significant for any of the individual groups. Nevertheless, the fact that all of these insignificant changes were upward was, itself, statistically significant. Moreover, the obesity-rate indices followed a similar pattern. Dr Klimentidis reckons the odds of his data having come about by chance are about one in 10m for the weight gain and three in 1m for the rise in obesity.

Most intriguingly, perhaps, the laboratory animals showed more pronounced gains than those living outside a lab. This is strange because the sorts of lab animals the researchers looked at tend to be given lots of food and left to nibble at leisure. This practice has not changed for decades. That the animals put on weight nonetheless suggests the phenomenon cannot be caused solely by pet owners appeasing their Garfields, or feral rats rummaging through refuse composed of ever larger quantities of calorie-rich processed food. Dr Klimentidis is unable to pinpoint any single mechanism that could account for his results. But this does not stop his data from lending exculpatory explanations for fat tummies more credence.