In the past few years, new technology has helped to unlock the secrets of the microbiome -- the bacterial communities that thrive around, on, and in all of us. But little has been done to investigate the bird microbiome.
"If you're going to study any kind of bird's microbiome, we figured, vultures would be a great place to start," said study co-author Gary Graves, the curator of birds for the Smithsonian National Museum of Natural History. "It's a microbially rich environment, to say the least."
That's because of the bird's dietary habits: You've probably seen a vulture picking at roadkill at least once in your life. But it gets even more gross than that.
To tear into the rotting carcasses, they usually stick their beaks into the anus of their prey. It's the easiest opening, but exposes them to an extra set of nasty bacteria: In addition to the microbes that thrive in rotting flesh, they also get a taste of whatever bacteria live in the deceased prey's fecal matter.
Graves and his colleagues didn't know what to expect. They were basically going in with a blank slate. Previous studies had taken small samples of bacteria from different species of birds, but no one had ever sat down to profile the entire microbiome of one -- let alone one of the dirtiest birds in the business.
And it turns out that in some ways, vultures have a microbiome in reverse -- in comparison to humans, anyway. Humans have much more bacterial diversity in their stomachs and mouths than they do on the surface of their skin. We carry multitudes of beneficial bacteria inside us, with our own colonies vastly outnumbering any bacteria we might pick up from the outside world.
But vultures have dirty, dirty faces -- and shockingly clean guts. The researchers found an average of 528 kinds of bacteria on the vulture faces they sampled, but only 76 kinds of bacteria in their guts.
"They're sticking their heads into decaying carcasses, so it's not surprising that their faces have so many kinds of bacteria," Graves said. "But when you get to the lower intestine, it's dominated by a small number of very common bacteria. There's a huge reduction from what they actually consumed."
So during their digestive processes, vultures must kill off the vast majority of the microbes they consume. It's possible that most of the bacteria just can't compete with the champion few that survive.
The species that continue to thrive in their guts are ones that cause a lot of problems for humans. Clostridia and Fusobacteria are the winners of the vulture gut showdown. The former includes bacteria that causes botulism, gangrene, and tetanus -- dangerous pathogenic infections for humans. Fusobacteria contribute to gum disease and ulcer formation, and may also be involved in some cases of colon cancer. But vultures, whose guts are teeming with the deadly microbes, don't suffer from bleeding gums or rotting limbs. So if scientists can figure out how vultures manage to tolerate those microbes, they might be able to give humans the same skill.
So it's possible that these gross birds will actually save lives one day. But really, Graves said, they already do.
"People oftentimes don't recognize the enormous ecosystem services that vultures offer to humans," he said. "It's a free, mobile sanitation department. They're discarding and consuming and getting rid of millions of pounds of decaying flesh that could threaten public health." And now we know that they also kill off most of the microbes instead of passing them back into the ecosystem.
Vultures definitely deserve our appreciation -- but it's probably best not to show your thanks with a smooch. At least not one on the mouth.