Imagine being a leech in one of the lush tropical forests of Southeast Asia (or Madagascar or continental Africa) that you call home Maybe you’ve latched onto the belly of a low-lying plant or buried yourself just below the surface in a patch of moist soil.

Bloodsucking Animals

Dozens of human hikers then begin marching across the terrain, providing countless opportunities for you and your companions to suck on their bootsor descend from the upper trees.

Certainly for leeches, large groups of people walking through their habitat is like stumbling into a forest. But whose blood do segmented parasitic worms suck when we’re not around?

And perhaps more importantly, why on earth would an animal evolve to do something as complex—and potentially dangerous—as suck blood to survive?

Bloodsucking 101

More than 30,000 species of animals feed on blood. The oxpeckers, for example, do not collect ticks and bugs from the backs of large buffaloes and giraffes — soak up blood from the wounds they leave behind.

Meanwhile, kandiru catfish sneak under the gills of larger fish and drink all the blood in their feathery respiratory capillaries. And lampreys—the jawless fish that have existed unchanged for 360 million years—have round mouths with a suction disk and teeth on their tongues to scraping through the skin and accessing blood.

Read more: 20 things you didn’t know about blood

Hematophagy

The practice of feasting on blood is known to science as hematophagy. Because blood is a tasty, nutritious food full of proteins and lipids, scientists have learned that this phenomenon likely evolved multiple times and independently across all branches of evolution in mammals, birds, insects, worms, fish, and others.

Some animals can survive only on blood, like the common bed bugbut others supplement their diet with other snacks – many mosquitoes also nibble on flowers and fruit.

The evolution of animals that drink blood

There is no single point in the evolutionary timeline where blood-sucking animals and insects decided they were going to start sucking blood.

“Evolution is kind of bullshit,” he says William Shuteresearch fellow at the American Museum of Natural History.

Take for example vampire bats, small, blood-feeding bats from Central and South America only mammals that survive entirely on blood and Shute’s research major. What probably happened, Schutt says, is that bats’ ancient ancestors were eating small arboreal vertebrates (such as birds, lizards or small mammals) when they were greeted by a sudden influx of new, larger arboreal creatures , such as primates or even relatives of raccoons.

Read more: First, they newlyweds; Then they share blood – how vampire bats bond with strangers

Since these larger animals no longer served as viable prey, these bat ancestors evolved to favor a mutation—bloodsucking—that still allowed them to feast without having to capture, subdue, and kill their food.

“You can no longer take them down, but you develop a mutation that allows you to easily take a bite out of them,” Schutt says. “And you start to develop an alternative method to get food from them.” With that, he adds, vampire bats now had access to a food source without competing with other vertebrates.

But hematophagy probably goes back even further, as researchers have shown that it arose independently at least six times in arthropods about 145 to 65 million years ago. However, it is important to note that it can be difficult to find accurate fossil records of this type of behavior.

5 Traits of Bloodsucking Creatures

Although its evolutionary roots have resurfaced time and time again, bloodsucking is still a rare dietary choice—an estimated 30,000 creatures isn’t a lot, especially when you consider these estimates of total number of species on Earth range from 3 million to 100 million or more. That’s because feeding blood can be a complicated and stressful endeavor.

“Feeding on blood is a tough way to make a living, but I guess it’s a good job if you can get it,” says Schutt, also the author of Grim Banquet: Blood and the Curious Life of Blood-Feeding Creatures. A certain type of animal is required to get this achievement.

1. Creatures tend to be Sshopping center

The largest animals in this category are the vampire bat, which it still usually weighs up to 30 grams.

This is because the larger the animals, the more blood they need to suck, but blood is not always available. That is why the most gigantic animals on the planet are always herbivores.

2. They should be Hidden

“If you’re going to get a blood meal, you need to be able to sneak up on your potential prey,” says Shute, who calls their behavior “cryptic.”

The vampire bat, for example, will fly into trees at night when birds roost, then climb under their feet and flip a scale off one of their toes before feeding.

“It’s pretty painless if you think about it,” Schutt says. Similarly, mosquitoes can hear you coming from far away.

3. Sharp mouths and teeth

Most blood-sucking insects, such as mosquitoes, have fine, hollow needle-like mouthparts that pierce the skin and capillaries.

“The vampire bat’s teeth are wild because they’re razor sharp,” says Shute. “If you get bitten by one, you don’t even know it, you just start bleeding.”

4. They try to avoid hurting their prey

Among this class of animals, many creatures share a tendency to subdue their prey with some form of anesthesia and anticoagulants, chemicals that ensure that the prey’s blood is thin and liquid enough to be drained smoothly without clotting or injury.

Each of the various bloodsuckers has developed their own different cocktail of chemicals for this task. The anticoagulant in the leech’s saliva, known as hirdwin, is one of the strongest on the market — and is even synthesized in labs for use in the medical world for conditions like thrombosis. Appropriately, vampire bats release an anticoagulant cocktail called Draculin in their saliva instead.

5. Creatures are hardworking

The difficulty with bloodsucking as a way of life isn’t just about getting access to blood in the first place: Digesting blood is also a lot of work.

“If you’re eating 50 percent of your body weight in blood every night, you need to get those nutrients out as quickly as possible,” Schutt says, or it’ll weigh you down.

And although blood provides many tasty nutrients, it also contains a lot of iron, which can be toxic to animals, so bloodsuckers have developed special mechanisms to break down iron molecules during digestion. On the other hand, blood also lacks vitamin B – and bloodsucking animals have evolved to take care of that too by taking microscopic bacteria into their bodies extract vitamin B instead.

“When you look at these creatures, they’re wildly different, you know, the difference between a vampire bat, a tick and a leech, there’s very little connection here,” Shute says. “But there’s a whole list of fascinating similarities, things these animals have in common.”

Read more: Vampire bats that socialize together have a similar microbiome

Bloodsucking Animals