I saw a fat pigeon the other day. There it was, doddering about on a Bowery doorstep, looking all smug and content. The fat pigeon had a fat pigeon ass and a fat pigeon belly that spanned the width of two normal birds, and I tried to imagine how the fat pigeon got to be so big. I pictured the fat pigeon tearing into a Big Mac, tufts of all-beef patty and sesame bun flying into the air with every peck, until the fat pigeon’s face was smeared with special sauce. I imagined the fat pigeon annihilating a holster of spilled French fries, and to be honest the thought of it got me pissed. Fries are for people, not pigeons. I stared at the fat pigeon long and hard, and the fat pigeon stared at me.
I’m not the only one who’s been seeing fat pigeons. In 2007, fat pigeons swarmed into Liverpool, England, pecking and pooping and plodding about, and so upset the locals that the City Council brought in 10 robotic peregrine falcons to scare off the zaftig varmints. A high-fat, garbage-based junk food diet had given the pigeons "a scruffy, unhealthy appearance,” explained one city official. Everyone agreed the fat pigeons had to go.
Of course, fat pigeons may be only the beginning. If one embarks on a tour of animal corpulence, spiraling out from captivity to the wild, a seemingly growing army of rotund beasts rounds into view. At home the fat animals are well known: They are fat husbands and fat children, fat dogs, fat cats, and fat pet macaws. Zoos and aquariums keep their own set of fat creatures in captivity. In 2008, for example, the dolphins at Japan’s Kinosaki Marine World grew too plump to jump, and were placed on a reduced-mackerel diet. A similar difficulty arose in a Shanghai aquarium when the sharks got so tubby that keepers gave them fish stuffed with cabbage to help them slim down.
Out in the country, some free-range fatties have sauntered onto the scene. Horse obesity is a growing concern on farms. In Arizona, wild burros have ballooned into genuine fat asses thanks to carrot-proffering tourists. The Lake Tahoe area’s garbage-eating black bears are unusually paunchy. And then there are the rats. Are the rats fat? One scientific study in 2010 found that feral rats collected near Baltimore had grown heavier over time. According to conventional wisdom, “wild” animals do not get fat. But is that truism breaking down for the commensal animals—like pizza-scarfing squirrels—that live in and around our fat- and sugar-laden “toxic” food environment? Has the obesity epidemic escaped from captivity? Is the Age of Fat Pigeons nigh?
Obesity researcher David Allison was on the team that identified the chunky urban rats. He and his colleagues also found signs of weight gain among captive populations of macaques, chimpanzees, vervets, marmosets, dogs, cats, and mice. For their study of vermin in the field, they reanalyzed weight data from rats captured in Baltimore alleyways as well as from rural rats trapped on parklands and farms between 1948 and 2006. The city rats had porked up more than the country rats, but both groups had probably needed to widen their burrows. I interviewed other rat experts who agreed that city rats tend to be huskier than their wild siblings, which subsist on bugs, slugs, acorns, and one another instead of half-eaten bagels and chicken wings. (Laboratory rats, with no room to exercise and all-you-can-eat feeding schedules, are bigger still.)
Allison says he doesn’t know why Baltimore’s rats have grown so large. (He doesn’t know for sure they have gotten fatter, as opposed to bigger overall, since his study only looked at weight.) It could be the result of the well-documented increase in food garbage, the selective killing of small rats by predators such as cats, changes in light exposure, improvements in rat health (perhaps facilitated by the food supply), or some environmental toxin that is disrupting the rats’ hormonal systems. Increasing temperatures might be another cause of rodent corpulence. A rat that spends less time shivering in the cold expends less energy. Scientists in Colorado report that shorter winters have given yellow-bellied marmots more time to forage and fill up their yellow bellies. (Climate change can work in the other direction, too: Sheep in the Scottish isles are thought to be getting smaller in the heat.) What about pigeons? Did Allison think the Age of Fat Pigeons was upon us? “[I]t would not at all surprise me if pigeons are more obese or fat than they used to be,” he said.
Different animals respond to calorie surpluses in different ways. Some species experience what biologists call “indeterminate growth” until late in their lives: As the animal takes in more food, it just gets bigger and bigger rather than storing excess calories as fat. Indeterminate growth is common among invertebrates such as earthworms and silverfish, but some mammals do it, too. A study from last year on crop-raiding African elephants showed that the well-nourished bulls got taller and longer and stronger and heavier than their pachyderm peers. All that extra food turned into pure tusker beefcake.
Humans, of course, stop growing in adulthood, so excess calories are converted to love handles under the right conditions. Baboons are the same way. Duke biologist Susan Alberts studied a group that lived near a tourist lodge in Kenya, and got most of its food by plundering a garbage dump instead of wandering the African plains. Not only did some of the animals get flabby as a result, she says, but they got dental cavities, too, and developed the sorts of insulin and cholesterol problems you’d see in a human case of metabolic syndrome. The scientists determined that the baboons weren’t getting sick from excess food—they took in the same number of calories as their wild-roaming brethren, despite a diet of discarded cake and pineapples—but rather they suffered from a lack of exercise. They did start having tons of healthy babies, though; increased fertility is the typical biological response to a surplus of food. The fat baboons are no longer being closely monitored, because the area where they hang out is patrolled by too many cantankerous wild buffaloes.