quarta-feira, 19 de junho de 2013
Vampire Bat: Dracula’s Children
Millions of years ago, some bats gave up their old habits of hunting for insects and tried something new: drinking blood. These creatures evolved into today’s vampire bats, and it’s mind-boggling to explore all the ways that they evolved to make the most of their sanguine meal.
A lot of the adaptations are easy enough to see with the naked eye. Vampire bats have Dracula-style teeth, for example, which they use to puncture the tough hide of cows. When they open up a crater-shaped wound, they dip in their long tongue, which contains two straw-shaped ducts that take up the blood.
Finding these prey has led to another remarkable adaptation that you can see–at least if you’re a scientist who studies how vampire bats move. Like other bats, they can fly, but on top of that, they can also walk and, yes, even gallop. Here is a video of a running vampire bat made by Dan Riskin (see this Loom post for details). Of the 1200 or so species of bats, vampire bats are among the very few that can move quickly on the ground.
But vampire bats have many other adaptations for drinking blood that are invisible. They use their combined senses–long-range vision, a sharp sense of smell, acute hearing, and echolocation–to find their victims. In their noses, they even have heat-sensitive pits that detect the heat of warm-blooded animals. Once they land on an animal, they apply those pits to the skin to locate capillaries full of hot blood close to the surface.
When vampire bats dip their tongue into a wound, they don’t just draw out blood. They also put their saliva into their victim. And in this liquid are still more invisible adaptations for a blood-feeding life. Vampire bats, you see, are venomous.
This may sound odd. That’s because we usually think of venom as a chemical an animal sticks in your body to cause you pain or death. But biologists define venom more broadly than that: it’s a secretion produced in a specialized gland in an animal, which is delivered to another animal by inflicting a wound, where it can disrupt its victim’s physiology.
Snake venom, the sort we’re all most familiar with, can disrupt physiology to the point of death. And it does so in several ways–jamming neurons, for example, or causing tissue to rot. But other animals that don’t set out to kill their victims also produce venom. Vampire bats, for example, don’t want eat a whole cow. They just want to take a sip.
Unfortunately, drinking blood has some drawbacks. Vertebrates come equipped with lots of molecules and cells that plug up wounds. As soon as they sense even a tiny tear in a blood vessel, they start making clots to staunch the flow.
Vampire bats use venom to keep the blood flowing. In a new paper with a title worth quoting in full–“Dracula’s Children: Molecular Evolution of Vampire Bat Venom”–an international team of scientists explore the molecules that vampire bats use to subvert blood’s defenses.
What’s most striking about vampire bat venom is how it goes after its victim from so many directions. Blood clots develop through a series of reactions that involve a chain of enzymes. Vampire bats produce different proteins to go after different enzymes in that chain. Platelets, which are cell fragments, also clump around wounds to help heal wounds. Vampire bats make separate compounds that attacks platelets.
To make their venom cocktail, vampire bats have repurposed old molecules for new jobs. When any vertebrate formed a blood clot to stop a wound, it needs to break that clot down once the wound is healed. An enzyme called plasminogen activator creates a supply of molecules called plasminogen, which chops up the clots. Vampire bats produce plasminogen activators in their blood for this job. But they also produce an extra supply in their mouth glands. When the plasminogen activators get into a wound, they use the victim’s own plasminogen to keep the blood flowing.
Once bats borrowed plasminogen activators to use in their venom, the molecules became better adapted to that new job. Normal plasminogen activators get cleared from the blood stream by other enzymes. That’s important for our survival, because otherwise they would hang around and make it hard to form new clots. Vampire bat plasminogen activators have a slightly different shape that shields them from their victim’s enzymes.
Together, these molecules are so effective that a cow will keep bleeding long after a vampire has flown away. While scientists have been studying vampire bat venom for decades, they’re still finding new molecules in the cocktail. The authors of “Dracula’s Children” applied a new method to the search. They caught two vampire bats and cataloged all the genes that were highly active in their mouth glands. The scientists then identified the genes and studied the properties of the proteins they encoded. They discovered dozens of new proteins. Some of them kill microbes, keeping the bat’s food supply clean. Some expand blood vessels, increasing the flow into the wound.
When a cow gets attacked by a vampire bat, it’s not entirely helpless. Ranchers have noticed that when bats feed over and over again on their herds, the cows bleed for a shorter period of time. Scientists have foundthat this happens because the immune systems of the animals learn to recognize some of the venom molecules and attack them. In the new study, the researchers found venom molecules that can ward off the immune system. But the venom itself is evolving to escape the immune system’s recognition, taking on new shapes that may allow them to go unnoticed.
Reading “Dracula’s Children” gives me a potent sense of deja vu. I recently wrote a feature about ticks for Outside, and in the research for the piece I learned all about how ticks produce saliva loaded with proteins that, among other things, open blood vessels, use our own molecules to break up clots, and do many of the things that vampire bat venom does.
Vampire bats are what you get if you turn a mammal into a tick. And I mean that as the highest compliment.