Imagine you're a leech in one of the lush rainforests of Southeast Asia (or Madagascar or mainland Africa) that you call home. Perhaps you're clinging to the underbelly of a low-lying plant or burrowed just below the surface in a patch of damp soil.
Then, dozens of human tourists start marching through the terrain, providing countless opportunities for you and your companions to suction yourselves onto their boots, or drop down from the trees above.
Surely, for the leeches, large groups of humans trekking through their habitat is akin to stumbling upon a potluck in a forest. But whose blood do the segmented, parasitic worms suck when we’re not around?
And perhaps more importantly, why on Earth would an animal evolve to do something as complicated — and potentially dangerous — as sucking blood to survive?
More than 30,000 species of animals feed on blood. Oxpecker birds, for example, don’t just pick ticks and bugs off the back of big buffalos and giraffes — they lap up blood from the wounds they leave behind.
Meanwhile, candiru catfish sneak under the gills of larger fish and slurp all the blood in their feathery breathing capillaries. And lampreys — the jawless fish that have been around, unchanged, for 360 million years — have round, suction-disk mouths and teeth on their tongues to scrape through skin and access blood.
Read More: 20 Things You Didn't Know About Blood
The practice of feasting on blood is known to science as hematophagy. Since blood is a tasty, nutritious meal replete with proteins and lipids, scientists have learned that this phenomenon has likely evolved repeatedly and independently across all branches of evolution in mammals, birds, insects, worms, fish and more.
Some animals can only survive on blood, like the common bedbug, but others complement their diet with other snacks too — many mosquitoes also nibble away at flowers and fruits.
The Evolution of Animals That Drink Blood
There isn’t one single moment on the evolutionary timeline in which blood-sucking animals and insects decided they were going to start sucking blood.
“Evolution is sort of a crapshoot,” says Bill Schutt, research associate at the American Museum of Natural History.
Take vampire bats as an example — tiny, blood-feeding bats from Central and South America, the only mammals that survive entirely on blood and Schutt’s research specialty. What probably happened, Schutt says, was that ancient ancestors of bats were eating small vertebrates up in the trees (like birds, lizards, or small mammals) when they were met with a sudden influx of new, larger arboreal creatures, like primates or even raccoon relatives.
Since those bigger animals no longer served as viable prey, these bat predecessors evolved to favor a mutation — bloodsucking — that still allowed them to feast without having to catch, subdue and kill their food.
“You can no longer take them down, but you evolve a mutation that lets you easily take a bite out of them,” says Schutt. “And you start to develop an alternative method to get a meal out of them.” With that, he adds, vampire bats were now able to access a food source without competing against fellow vertebrates.
But hematophagy probably goes back even further, as researchers have shown that it arose independently at least six times in arthropods around 145 to 65 million years ago. Still, it's important to note that accurate fossil records for this kind of behavior can be difficult to find.
5 Traits of Bloodsucking Creatures
Even though its evolutionary roots have sprouted time and again, bloodsucking is still a rare dietary choice — the estimated 30,000 creatures aren’t a lot, especially when you consider that estimates for the total number of species on Earth range from 3 million to 100 million or more. That's because blood feeding can be a complicated and strenuous endeavor.
“Blood feeding is a tough way to make a living, but I guess it's a good job if you can get it,” says Schutt, also author of Dark Banquet: Blood and the Curious Lives of Blood-Feeding Creatures. It takes a certain type of animal to pull off this feat.
1. The Creatures Tend to Be Small
The biggest animals in this category are the vampire bat, which still only usually weighs up to 30 grams.
That’s because the bigger animals are, the more blood they’d have to suck, but blood isn’t always largely available. That’s why the most giant animals on the planet are always herbivores.
2. They Need to Be Stealthy
“If you're gonna get a blood meal, you got to be able to sneak up on your potential prey,” says Schutt, who calls their behavior “cryptic”.
The vampire bat, for example, will fly into the trees at night when birds are roosting, then climb under their feet and flip a scale from one of their toes before feeding.
“It's pretty painless if you think about it,” says Schutt. Similarly, mosquitoes can hear you coming from far away.
3. Sharp Mouths and Teeth
Most bloodsucking insects, like mosquitoes, have fine hollow needle-like mouths that prick a hole into the skin and capillaries.
“Vampire bat teeth are wild in that they are razor sharp,” says Schutt. “If you get bit by one, you don’t even know, you just start bleeding.”
4. They Try to Avoid Harming Their Prey
Among this class of animals, many creatures share a penchant for subduing their prey with some sort of anesthesia and anticoagulant, chemicals that ensure the prey’s blood is thin and liquid enough to be sucked out seamlessly without clotting or hurting.
Different bloodsuckers have each evolved their own different cocktail of chemicals for this task. The anticoagulant in leech saliva, known as Hirduin, is one of the strongest on the market — and is even synthesized in labs to be used in the medical world for conditions like thrombosis. Fittingly, vampire bats release an anticoagulant cocktail called Draculin in their saliva instead.
5. The Creatures Are Hard Workers
The difficulty in bloodsucking as a lifestyle doesn’t only lie in accessing blood in the first place: It’s a lot of work to digest blood, too.
“If you're feeding on 50% of your body weight in blood, every night, you've got to get those nutrients out as quick as possible,” says Schutt, or it’ll weigh you down.
And although blood provides a lot of delicious nutrients, it also contains loads of iron, which can be toxic to animals, so bloodsuckers have evolved special mechanisms to break down iron molecules during digestion. On the other hand, blood is also short on vitamin B — and bloodsucking animals have evolved to take care of this, too, by hosting microscopic bacteria inside their bodies to derive vitamin B from instead.
“When you look at these creatures, they are wildly divergent, you know, the difference between a vampire bat, and a tick, and a leech, there’s very little relationship here,” says Schutt. “But there is a laundry list of fascinating similarities, things that these animals have in common."