Of course, animals also fatten normally — and healthily — in response to seasonal and life cycles. Remarkably, it is the landscape around an animal that determines whether its weight stays steady or rises.

And nature imposes its own “weight-maintenance plan” on wild animals. Cyclical periods of food scarcity are typical. Threats from predators limit access to food. Weight goes up, but it also comes down. If you want to lose weight the wild animal way, decrease the abundance of food around yourself and interrupt your access to it. And expend lots of energy in the daily hunt for food. In other words: change your environment.

Looking across the species divide and seeing weight gain in a broader context forces us to consider factors beyond the “diet and exercise” dogma. Even without an assist from 32-ounce sodas, the yellow-bellied marmots in the Rockies, blue whales off the coast of California and country rats in Maryland have gotten steadily chubbier in recent years. The explanation might lie in the disruption of circadian rhythms. Of the global dynamics controlling our biological clocks — including temperature, eating, sleeping and even socializing — no “zeitgeber” is more influential than light.

New research suggests that when, and how much, light beams through your eyes may play a quiet and unrecognized role in determining your dress or pants size. And the breaking up of light-dark cycles may be a culprit. Light pollution from suburban sprawl, big-city skyglow, electronic billboards and stadium lights has brightened our planet. A rodent study published in the Proceedings of the National Academy of Sciences showed that mice housed with constant light — whether bright or dim — had higher body mass indexes (B.M.I.’s) and blood sugar levels than mice housed with standard cycles of dark and light.

Another invisible weight driver is housed within our own abdomens: the trillions of microscopic organisms that live in our guts. This world is called the microbiome, and it is colonized by two dominant groups of bacteria: the Firmicutes and the Bacteroidetes. In the mid-2000s, some scientists made an interesting observation. They found that obese humans had a higher proportion of Firmicutes in their intestines. Lean humans had more Bacteroidetes. As the obese humans lost weight over the course of a year, their microbiomes started looking more like those of lean individuals — with Bacteroidetes outnumbering Firmicutes.

When the researchers looked at mice, they found the same thing. Although not all research has replicated those results, if that observation turns out to be true, it means that a booming Firmicute colony might help harvest, say, 100 calories from one person’s apple. That person’s friend may have a dominant Bacteroidete population that would extract only 70 calories from the same apple. This could be one factor in why your co-worker can eat twice as much as everyone else but never seems to gain weight. The power of the microbiome is well known to the veterinarians who oversee the care of animals we make fat on purpose: livestock. Nowadays, it’s common for factory farming operations to administer antibiotics to food animals from 1,500-pound steers to one-ounce baby chicks. The effect of those antibiotics on the living colonies of gut bugs in the animals’ intestines may inform human obesity research.

Antibiotics don’t kill just the bugs that make animals sick. Simply by giving antibiotics, farmers can fatten their animals using less feed. One hypothesis is that by changing the animals’ gut microflora, antibiotics create an intestine dominated by colonies of microbes that are calorie-extraction experts. Anything that alters gut flora, including but not limited to antibiotics, has implications not only for body weight but for other elements of our metabolism, such as glucose intolerance, insulin resistance and abnormal cholesterol.