The massive turkey you may have just picked up from the grocery store likely looks quite different from the ones early Americans ate for Thanksgiving dinner. Years of selective breeding and advances in commercial, industrial agricultural practices have created a turkey that’s, on average, nearly triple the size of its predecessor from the early 1900s.  All that added size comes with consequences. These buxom turkeys can no longer fly. Many of them struggle to even walk or balance properly under the weight of their enormous breasts. Our collective desire for ever-bigger turkeys also means birds today can’t even reproduce on their own. In less than 100 years, humans have utterly transformed the quintessentially American bird.  How industry transformed the ‘bird of courage’  Wild turkeys are native to North America. Though the first domesticated turkey is dated to sometime around 800 BC in south-central Mexico, wild turkeys roamed the continent long before that. In the wild, turkeys travel in large flocks (as many as 50 individuals) and spend most of their days foraging. They can use their roughly 5,000 feathers to fly in short bursts, often covering around 100 yards at speeds of up to 55 miles per hour. Benjamin Franklin famously had a soft spot for turkeys and advocated for them to be the country’s national animal over the bald eagle. Franklin allegedly described the turkey as “a bird of courage.”  Get the Popular Science newsletter Breakthroughs, discoveries, and DIY tips sent every weekday. Email address Sign up Thank you! By signing up you agree to our Terms of Service and Privacy Policy. That bird has changed dramatically in form since then. University of California Davis animal science professor Richard Blatchford told Popular Science the jumbo-sized version of the bird really started taking shape around World War II in response to shifting consumer demands for white meat. Texas A&M Poultry Science professor Gregory Archer told Popular Science the change in farming techniques at that time was the result of mass production, increasing global populations (and appetites) as well as the invention of antibiotics and vaccines which allowed more turkeys to be grown in closer quarters than before. That shift reportedly led some farmers to start injecting their flocks with a growth hormone called diethylstilbestrol though that practice quickly ended  after the hormone was found to increase the risk of cancer in humans. Since then, farmers have altered the size of turkeys entirely through selective breeding and more efficient feeding techniques. Archer said it’s currently illegal to inject commercial turkeys with growth hormones.  “It’s all just artificial selection, the same kind of thing you do with cat and dog breeds,” Blatchford said. “You get offspring, you see who has the best traits that you’re looking for and you breed those ones and then you continuously do that until you get that change that you’re looking for.”  Birds with bigger breasts and thighs were deemed more commercially valuable, so farmers would pick those to continue breeding. Over time, traits associated with those properties were passed down and a visual transformation occurred. An average turkey in 1929 reportedly weighed just 13.2 pounds. That figure soared up to 30.3 pounds by 2013, a nearly three-fold increase. All of this happened quickly. Blatchford says the transformation in turkey size would have been noticeable within a couple of decades. Wild turkeys are considerably smaller in size and can still fly. Credit: DepositPhotos “It really shows you how much genetic potential was in those birds to begin with,” Blatchford said. “We just sort of forced the expression of those particular genes through our artificial selection.” Modern commercial turkeys also reach their market weight in half the time of their ancestors. A 2018 paper published in the journal Advances in Poultry Welfare estimates farmers can now breed a 21 KG (46 lbs) turkey in just 19 weeks. The average turkey in 1960 only reached 8 kg (17 lbs) by the time of slaughter according to the paper. “The reason that we’ve made them grow so fast,” Archer said, “is basically so we can get enough food to feed people. We need to feed more people and more people can afford meat now too.”  And it’s not just size that has changed either. Consumer demand for “pink” skin at the supermarket has resulted in a loss of dark-colored pin feathers over generations. Because of that, modern commercial turkeys can look noticeably paler and less vibrant than their wild cousins. Blatchford said most turkeys purchased from supermarkets have been bred to select for white feathers because colored feathers can leave a visible pigment on the turkey’s skin. “One of the reasons we moved away from colored dirties is because people didn’t like the carcass quality,” Blatchford said. “They didn’t like the way it looked.” Obese turkeys can no longer fly or reproduce  This rampant demand for giant turkeys has had startling consequences for the birds’ health. Breasts have gotten so large that commercial turkeys are almost all incapable of flying. Many are unable to even walk or balance properly. The Advances in Poultry Welfare study cites “numerous reports” linking increased turkey growth rates to “skeletal abnormalities” and leg disorders. All that added weight impacts the turkey’s gait and is associated with a greater risk of fractures of the femur and tibia, the study notes. Maybe most shocking of all, the sheer size of the birds’ breasts makes it physically impossible for them to mate on their own. The big turkeys lining the shelves at supermarkets were likely all bred through artificial insemination, which is unique among commercial poultry. That dependence on artificial insemination, which dates back decades, has drawn some criticism from animal welfare experts. “Animal welfare ethics is a composite of three dimensions: basic veterinary health, cognitive affect and species typical behavior,” Michigan State University Agricultural Ethics Professor Emeritus Paul B. Thompson told Popular Science. “Birds that cannot reproduce without human assistance are deficient in this last category.”  [ Related: Is raw milk safe? Science has a clear answer. ] There are other side effects with all that added size as well. Research published in the British Poultry Science has shown genetic selection for rapid growth rate over generations may be “coincidentally accompanied by decreased resistance to disease or reduced immunological response.” In other words, larger birds may get sick easier. That, in turn, could incentivize farmers to treat those birds with antibiotics. Other studies have shown larger larger-bodied turkeys may also react more adversely to stressful situations. Stress, in those contexts, can similarly lead to a worse immune response which can in turn lead to increased susceptibility to infections. Relatively sterile indoor facilities may also prevent birds from building up their natural immune systems.  “This practice will still select for lines with low mortality; however, the ability to resolve chronic or latent infections may be diminished,” researchers noted in the journal Poultry Science.  Demand is growing for smaller, ‘heritage’ turkeys If any of that sounds alarming, there’s some good news. Increased attention to animal welfare and shifting consumer demands has helped create a sizable, though still niche, market for so-called “heritage turkeys.” These smaller breeds are advertised as being much more similar in size and appearance to wild turkeys than their commercial cousins. Aside from being lighter, these heritage birds also generally live longer before they are slaughtered which some say contributes to a more unique taste. These birds may often spend more time foraging outdoors eating bugs, Archer said, which may also affect their taste. Blatchford says these types of turkeys will often have much higher percentages of red or dark meat than their industrial cousins.  [ Related: Why do birds migrate? Scientists have a few major theories. ] They’re also much more expensive. Heritage turkeys can run around $4 per pound compared to just $1 per pound for the average commercially raised store bought option. These still make up only a sliver of the estimated 5.11 billion pounds of turkey meat the USDA expects will be sold this year, but that sliver is growing. But while Heritage turkeys have risen in popularity, unclear regulations mean it’s not always easy to tell whether or not the animal you are buying meets those specifications.  “They [heritage breeds] are often described as distinct from ‘broad-breasted.’” Thompson said  “However, this language is not regulated so one can’t be 100% sure. The organic label do not guarantee a welfare-friendly purchase; many certified organic turkeys are broad-breasted.” This story is part of Popular Science’s Ask Us Anything series, where we answer your most outlandish, mind-burning questions, from the ordinary to the off-the-wall. Have something you’ve always wanted to know? Ask us. The post How did Thanksgiving turkeys get so darn big? appeared first on Popular Science.