A scientific exploration of big juicy butts

BUMS. HEINIES. FANNIES. DERRIERES. Few muscles in the human body carry as much cultural clout as the gluteus maximus. “Butts are a bellwether,” writes journalist Heather Radke in her 2022 book Butts: A Backstory. Radke goes on to explain that our feelings about our hindquarters often have more to do with race, gender, and sex than with the actual meat of them. Unlike with a knee or an elbow, Radke argues, when it comes to the tuchus, we’re far more likely to think about form than function—even though it features the largest muscle in the human body. 

For all the scrutiny we spare them (outside of when we’re trying on new jeans) our butts aren’t mere aesthetic flourishes. A booty is, in fact, a unique feat of evolution: Out of any species, humans have the most junk in their trunks. Many other creatures have muscle and fat padding their backsides, and some even have butt cheeks. But none pack anything close to the same proportions as us.

So why did our ancestors develop such a unique cushion? Evolutionary biologists’ best guess is that our shapely rears help us walk upright. The curved pelvic bone that gives the butt its prominence likely developed as our weight moved upward and our muscular needs shifted. Research increasingly suggests that more massive muscles in the vicinity of the buttocks make for faster sprinting and better running endurance too. “The butt is an essential adaptation for the human ability to run steadily, for long distances, and without injury,” Radke writes. 

That said, the gluteus maximus does more than just keep us on our feet. The fat that sits atop it affects how we feel whenever we sit or lie down. The organs nestled behind those cheeks also have a massive influence on our health and wellbeing. Here are a few of the ways our bums factor into scientific understanding, lifesaving medicine, and the future of engineering. 

Digging deep for ancient backsides 

For as long as humans have been making art, they’ve been thinking about bodacious butts. The 30,000-year-old Venus of Willendorf is a famous pocket-size figurine carved by a Western European civilization during the Upper Paleolithic. The statuette, which some archaeologists suspect served as a fertility charm, immortalizes a body too thick to quit.

Scientists also love peeping at the actual posteriors of our early ancestors, which hold a broader archaeological significance in telling the stories of ancient people and their lifestyles. Differences in the pelvis and other sat-upon bones have long been used to determine the sexes of unearthed skeletal remains, though we know now there isn’t as clear-cut a binary as researchers long assumed. In 1972, anthropologist Kenneth Weiss flagged that experts were 12 percent more likely to classify skeletons found at dig sites as men versus women, which he blamed on a bias for marking indeterminate skeletons as male. Recent research bears that out, with anthropologists now designating many more remains as having a mix of pelvic characteristics (or simply being inconclusive) than they did historically. Still, while the distinction isn’t completely black and white, the signs of a body primed for or changed by childbirth are useful in figuring out the age and sex of ancient remains. Butt bones can also tell us about how people lived: This March, archaeologists published the oldest known evidence for human horseback riding in the journal Science Advances. They identified their 5,000-year-old equestrians—members of the Yamnaya culture, which spread from Eurasia throughout much of Europe around that same time—with the help of signs of wear and tear to hip sockets, thigh bones, and pelvises. 

Supporting heinies of all shapes and sizes

As Sharon Sonenblum, a principal research scientist at the School of Mechanical Engineering at Georgia Tech, puts it, “What could be better than studying butts?” The Rehabilitation Engineering and Applied Research Lab that she’s part of is perhaps more aptly referred to by its acronym: REAR. 

Stephen Sprigle, a Georgia Tech professor in industrial design, bioengineering, and physiology, started REARLab with better solutions for wheelchair users in mind. A decade ago, he and Sonenblum saw the potential for an engineering-minded solution to the serious clinical problem of injuries from sitting or lying down for extended periods. Pressure sores and ulcers are a risk whenever soft tissue presses against a surface for a prolonged time, and they become more dangerous in hospital settings—where antibiotic-resistant bacteria often lurk—and in people with conditions that hinder wound healing, like diabetes. 

Sonenblum recalls that they set out to answer a deceptively simple question: What makes one backside different from another? To answer it, they had to put a whole lot of booties into an MRI scanner. Those imaging studies and others (including some done on supine patients) have provided an unprecedented amount of data about butt cheeks and the stuff inside them. 

The big headline, Sprigle says, is that “we’re big bags of water.” “What the skeleton does in that big bag of goo is totally fascinating,” he says. 

The work proved particularly humbling for Sonenblum, who’d intended to spend her career studying how the gluteus maximus affects seating. Instead, she and her colleagues figured out that humans don’t rest on muscle at all—the fat is what really counts. Sonenblum and the rest of the REARLab team are investigating how the natural padding in our rears changes over time, particularly in people who spend a lot of time sitting or supine.

Today, REARLab creates more precise computer models and “phantoms” to help cushion testing—mainly for wheelchair seats, but also for ergonomic chairs of all stripes—better account for real-world bums. Phantoms aren’t quite faux butts; they’re simple and scalable geometric shapes, almost like the convex version of a seat cushion designed for your tuchus to nestle into. They don’t account for bodies’ individual differences either. 

“Phantoms are always a tricky balance between time and representation,” Sonenblum says. “You want to represent the population well, but you can’t have too many or you’ll spend your entire life running tests.”

REARLab’s current approach is to use two shapes—elliptical and trigonometric—to represent a fuller backside and one more likely to pose biomechanical problems when seated, respectively. It would be reasonable to assume the trigonometric butt is the bonier of the two, Sonenblum says, but the reality isn’t so simple. Large individuals with lots of adipose tissue can still lose the round cushioning when they sit. 

“I’ve seen scans of butts that look like this, and when I do, I think, Wow, that’s a high-risk butt,” Sonenblum explains. It comes down to the quality of the tissue, she adds. “If you touch a lot of butts, you’ll find that the tissue changes for people who are at risk [of pressure injuries]. It feels different.”

Sonenblum and Sprigle hope that continued work on backside modeling, cushion-testing standards, and adipose analysis will help wheelchair users and patients confined to their beds for long stretches stay safer and more comfortable. But their work has implications for absolutely anyone who sits down. When asked what folks should take away from their studies, they’re both quick to answer: Move. People with limited mobility may not be able to avoid the loss of structural integrity in their butt tissue, but anyone with the ability to get up often and flex their muscles can keep that natural padding in prime health. 

Finding better bellwethers for bowel cancer

When it comes to protecting your posterior, it’s not just the bodacious bits of the outside that count. One of the biggest backside-related issues scientists are tackling today is the sharp rise in colorectal cancer, which starts with abnormal cell growth in the colon or rectum. It’s already the third-most-common cancer and second leading cause of cancer death, but it represents a mounting threat, especially for millennials. New cases of young-onset colorectal cancer (yoCRC)—defined as a diagnosis before age 50—have gone up by around 50 percent since the mid-1990s. 

Blake Buchalter, a postdoctoral fellow at Cleveland Clinic Lerner Research Institute focused on cancer epidemiology, says that the most troubling thing about this recent uptick in cases is how little we know about what’s causing it. He and his colleagues suspect that 35- to 49-year-olds who die from colorectal cancer may share many of the same demographics and risk factors—higher body weight, lower activity levels, smoking, alcohol use, and diets high in processed and red meats—seen in patients aged 50 and older. But those under the age of 35 don’t follow those patterns as closely as expected. 

“This indicated to us that mortality among the youngest colorectal cancer patients may have different drivers than among older populations,” Buchalter says. “Our future work in this space aims to identify underlying factors that might be driving higher incidence and mortality among certain age groups in particular geographic regions.” 

Buchalter hopes that more granular data will encourage more granular screening guidelines too. While he was heartened to see the US Preventative Services Task Force shift the recommended colon cancer screening age down from 50 to 45 in 2021, it’s clear that some populations are at risk for the disease earlier, he says. Buchalter and his colleagues hope to zero in on who should be getting screened in their 20s and 30s. 

But colonoscopies, the most commonly recommended form of detection, present a major hurdle in themselves. A 2019 study found that only 60 percent of age-eligible US adults were up to date on their colorectal cancer screenings, with others citing fear, embarrassment, and logistical challenges such as transportation to explain their delayed colonoscopies. At-home fecal tests offer a less invasive alternative, but research shows that fear of a bad diagnosis and disgust with the idea of collecting and mailing samples still keep many folks from using them. Blood tests and colon capsule endoscopy (CCE), in which patients swallow a pill-size camera to allow doctors to examine the gastrointestinal tract, both show promise in supplementing, and perhaps someday replacing, the oft-dreaded colonoscopy.

For now, it’s worth going in for the physical screening if you can manage it. While blood and stool tests can accurately detect signs of the cancer, colonoscopies can actually help prevent it. During a standard colonoscopy, gastro­enterologists are able to identify and remove potentially precancerous polyps known as adenomas on the spot. No DIY kit can manage that.  

Tracking microbiomes with futuristic commodes

Meanwhile, other researchers are uncovering health secrets from long-ago water closets. In 2022, archaeologists uncovered what they believe to be the oldest flush toilet ever found, in Xi’an, China. The 2,400-year-old lavatory features a pipe leading to an outdoor pit. Researchers believe the commode, which was located inside a palace, allowed servants to wash waste out of sight with buckets of water. Flush toilets wouldn’t appear in Europe until the 1500s, and wouldn’t become commonplace until the late 19th century. Up until that point, major US cities employed fleets of “night soil men” to dig up and dispose of the contents of household privies and public loos.

As far as we’ve come from the days of night soil, the future of the humble toilet looks even brighter. Sonia Grego, an associate research professor in the Duke University Department of Electrical and Computer Engineering, says she’s “super-excited” to see commodes enter the 21st century. 

“Smart” toilets boast everything from app-controlled heated seats to detailed water-usage trackers, and could grow into a $13.5 billion industry by the end of the decade. But Grego’s team—the Duke Smart Toilet Lab at the Pratt School of Engineering—is focused on turning waste flushed down porcelain bowls into a noninvasive health tool. She envisions a future in which your toilet can warn you of impending flare-ups of gut conditions like irritable bowel syndrome, flag dietary deficiencies, and even screen for signs of cancer. 

“When we first started to work on the smart toilet for stool analysis, laboratory scientists were skeptical that accurate analytical results could be obtained from specimens that had been dropped in a toilet instead of a sterile collection container,” Grego recalls. “The perspective is very different now.”

Drawing inspiration from wild butts 

Humans may be unusually blessed in the butt-cheek department, but that doesn’t mean other animals’ rears hold less scientific appeal. From modeling the evolution of the anus to cracking the code on climate-friendly gut microbes, scientists are keeping close tabs on all sorts of animal bottoms. Some researchers are even hoping to harness the power of butt breathing—yes, actually breathing through your butt—for future applications in human medicine. 

We’ll circle back to backside breathing in a moment. First, let’s consider the wombat. While it’s true enough that everybody poops, these marsupials are the only animals known to drop cubes. For years, no one was quite sure how they managed to get a square peg out of a round hole. Some even assumed the wombat must have an anus designed for squeezing out blocks instead of cylinders. In 2020, mechanical engineers and wildlife ecologists at Georgia Tech teamed up to publish a surprising new explanation for the shape in the aptly named journal Soft Matter. They’d borrowed roadkill from Australia to do the first-ever close examination of a wombat’s intestines. By inflating the digestive tract and comparing it to more familiar pig intestines, they were able to show that the marsupial’s innards have more variation in elasticity: Instead of being fairly uniform throughout, the organs have some inflexible zones. The team’s findings suggest that a few nooks within the digestive system—some stretchy, others stiff—provide a means to shape the refuse into a square. 

Wombat butts themselves, by the by, are veritable buns of steel. Their rumps contain four fused bony plates surrounded by cartilage and fat and can be used to effectively plug up the entrance to a burrow when potential predators come sniffing around. While this has yet to be caught happening live, some scientists think wombats can even use their powerful bums to crush the skulls of intruders like foxes and dingoes who manage to make it inside. 

So now we have more clarity on how wombats poop cubes, but the question of why remains unanswered. Experts have posited that wombats communicate with one another by sniffing out the location of poop cubes, making it advantageous to produce turds less likely to roll out of place. Others argue that the unusual shape is a happy accident: Wombats can spend as long as a week digesting a single meal, with their intestines painstakingly squeezing out every possible drop of moisture to help them survive the arid conditions Down Under. Their entrails, when unwound, stretch some 33 feet—10 feet more than typical human guts—to help facilitate the frugal squeezing. When the species is raised in captivity with loads of food and water, their poops come out moister and rounder. 

Elsewhere in the world of scat science, folks are working to understand the secrets of nonhuman gut microbiomes. Earlier this year, biotechnologists at Washington State University showed that baby kangaroo feces could help make beef more eco-friendly. Joey guts contain microbes that produce acetic acid instead of methane, which cows burp out in such abundance that it significantly worsens climate change. By reseeding a simulated cow stomach with poop from a newborn kangaroo, researchers say they successfully converted the gut to a factory of acetic acid, which doesn’t trap heat in the atmosphere. They hope to try the transfer out in a real bovine sometime soon. 

Going back to the butt breathing, scientists are hoping to suss out how to give humans a superpower already exhibited by catfish and sea cucumbers. In 2021, Japanese researchers reported in the journal Med that they’d been able to keep rodents alive in oxygen-poor conditions by ventilating them through their anuses. Inspired by loaches—freshwater fish that can take in oxygen through their intestines—the scientists are trying to find new ways to help patients who can’t get enough air on their own. They’ve moved on to study pigs, which they say do wonderfully with a shot of perfluorodecalin (a liquid chemical that can carry large amounts of oxygen) up the bum. 

From an evolutionary standpoint, it’s not all that surprising that our outbox can handle the same duties as our inbox. Though it’s still not clear which came first, it’s well established that the anus and the mouth develop out of the same rudimentary cell structures wherever they appear. Some of the most basic animals still use a single opening for all their digestive needs. And one creature—just one, as far as we know—has a “transient anus.”

In 2019, Sidney Tamm of the Marine Biological Laboratory in Woods Hole, Massachusetts, demonstrated that the warty comb jelly creates new anuses as needed. Whenever sufficient waste builds up—which happens as often as every 10 minutes in young jellies—the gut bulges out enough to fuse with the creature’s epidermis, creating an opening for defecation. Then it closes right back up. It’s possible that the world’s first anuses followed the same on-demand model, proving yet again that the butt and its contents are worthy of our awe, curiosity, and respect.  

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