As were laboratory macaques, chimpanzees, vervet monkeys and mice, as well as domestic dogs, domestic cats, and domestic and feral rats from both rural and urban areas. In fact, the researchers examined records on those eight species and found that average weight for every one had increased. The marmosets gained an average of nine per cent per decade. Lab mice gained about 11 per cent per decade. Chimps, for some reason, are doing especially badly: their average body weight had risen 35 per cent per decade.

In fact, lab animals’ lives are so precisely watched and measured that the researchers can rule out accidental human influence: records show those creatures gained weight over decades without any significant change in their diet or activities.

http://aeon.co/magazine/health/david-berreby-obesity-era/

(EDIT: Found another article about that here: http://marginalrevolution.com/marginalrevolution/2013/08/the-animals-are-also-getting-fat.html)

The study referenced appears to be from here: http://rspb.royalsocietypublishing.org/content/278/1712/1626.short

Here is one theory on an environmental cause of obesity: https://en.wikipedia.org/wiki/Obesogen

Here is a study that suggests Jet fuel causes obesity. And it's an epigenetic effect: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3587983/

Another interesting link I'd like to save here: http://slatestarcodex.com/2015/08/04/contra-hallquist-on-scientific-rationality/

I sometimes explain this to people with the following metaphor: severe weight gain is a common side effect of psychiatric drug Clozaril. The average Clozaril user gains fifteen pounds, and on high doses fifty or a hundred pounds is not unheard of. Clozaril is otherwise very effective, so there have been a lot of efforts to cut down on this weight gain with clever diet programs. The journal articles about these all find that they fail, or “succeed” in the special social science way where if you dig deep enough you can invent a new endpoint that appears to have gotten 1% better if you squint. This Clozaril-related weight gain isn’t magic – it still happens because people eat more calories – but it’s not something you can just wish away either.

Imagine that some weird conspiracy is secretly dumping whole bottles of Clozaril into orange soda. Since most Americans drink orange soda, we find that overnight most Americans gain fifty pounds and become very obese.

EDIT: More links. Haven't gone through them thoroughly yet though. Putting this stuff here more for future reference:

https://www.reddit.com/r/science/comments/3xyu5r/fat_but_fit_may_be_a_myth_researchers_say_the/cy9b52l

the study of identical twins that could not show any effect on longevity from exercise?

http://www.nature.com/articles/srep18259

It concludes that high intrinsic fitness (high base VO2 max for example) is correlated with lower mortality, but identical twins with different exercise habits did not have different mortality rates.

https://www.reddit.com/r/science/comments/3xyu5r/fat_but_fit_may_be_a_myth_researchers_say_the/cy91nu5

A previous study found that overweight people who were 'metabolically fit' (no insulin resistance, no diabetes, no high triglycerides or high blood pressure and good cholesterol levels) were not at a higher risk of mortality compared to people who weren't overweight.

This study had participants pedal on a bicycle until they were tired (they used this as a proxy for 'aerobic fitness'). They found that men (the study only looked at men) in the 'normal' bodyweight range had lower mortality rates than men who were overweight regardless of 'aerobic fitness'. Moreover, the study reports that the benefits of aerobic fitness are decreased in overweight men.

Looking for any relevant research or articles on the causes of obesity, or effectiveness of interventions.

another link to dump for now: https://www.reddit.com/r/science/comments/4zupkq/new_study_finds_that_the_bmi_of_adopted_children/