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You're listening to SHORT WAVE from NPR.

Maddie Sofia here with NPR science correspondent Nell Greenfieldboyce. Hi, Nell.


SOFIA: Today we are going to talk about blood and bleeding, but not the usual way we think about bleeding.

GREENFIELDBOYCE: Absolutely. I mean, I think most people generally think of bleeding as a bad thing...

SOFIA: Right.

GREENFIELDBOYCE: ...Like something you want to stop as soon as possible so that, you know, your life force, your precious bodily fluid doesn't just flow out of you.

SOFIA: And you mean, like, if we're bleeding from a cut or a puncture wound or something - not like menstruation because that's, you know, normal, healthy bleeding.

GREENFIELDBOYCE: Yeah. So I'm not talking about your period or getting a cut or anything like people normally experience in terms of blood and bleeding. What I'm talking about is a completely different kind of bleeding.

SOFIA: (Laughter) OK.

GREENFIELDBOYCE: I am talking about blood coming out of an unexpected body part abruptly and alarmingly and on purpose.

SOFIA: What do you mean on purpose?

GREENFIELDBOYCE: Well, it might not necessarily be a decision. It could be more like a reflex...

SOFIA: Right.

GREENFIELDBOYCE: ...In response to a threat as a defensive maneuver. There are some creatures that just copiously bleed.

SOFIA: (Laughter) Really?

GREENFIELDBOYCE: Blood comes pouring out of their eyes. Here's one scientist I spoke with talking about a snake he found.

SEBASTIAN HOEFER: So the eyes fully flooded with blood that was, you know, something spectacular. I did not expect that at all. And that really sort of blew my mind.

GREENFIELDBOYCE: So that Sebastian Hoefer. He said it looked like the snake was trying to freak him out.

HOEFER: It really does look like it's deliberate and it deliberately is bleeding sort of to put this show on.

SOFIA: It feels like a lot of anthropomorphism for a snake. But go on, Nell. I am sufficiently disturbed.

GREENFIELDBOYCE: Well, disturbed is probably how you are supposed to feel.

SOFIA: (Laughter).

GREENFIELDBOYCE: It seems to be the whole point of this strategy. You know, there's these reptiles that shoot blood out of their eyes and insects that bleed out of places like their knees.

SOFIA: (Laughter).

GREENFIELDBOYCE: And not exotic insects, either. I'm talking, like, ladybugs, fireflies.

SOFIA: OK, this is wild. Is there a name for this type of bleeding?

GREENFIELDBOYCE: It's just generally called autohemorrhage or reflex bleeding.

SOFIA: All right. Well, (laughter) here we go. Today on the show, we take on reflex bleeding, a delightful self-preservation tactic. We'll hear from scientists who study it and ask the question - what in the evolution is going on around here? I'm Maddie Sofia, and you're listening to SHORT WAVE from NPR.


SOFIA: OK. Nell, so why did you get interested in this whole reflex bleeding thing?

GREENFIELDBOYCE: Well, I just saw this report by that guy, Sebastian Hoefer. And I called him up, and he told me that not too long ago he got a job at an island ecosystem research institute in the Bahamas.

HOEFER: And so, you know, I was looking into what's around, what kind of animals there are and just scanning through the literature just to see what sort of questions I could answer.

GREENFIELDBOYCE: And he read about these local snakes called thunder snakes. That sounds very dramatic.

SOFIA: (Laughter).

GREENFIELDBOYCE: You know - thunder, thunder, thunder snakes.

SOFIA: (Laughter).

GREENFIELDBOYCE: But it - really, it's just that they come out when it rains a lot.

HOEFER: And I remember reading this article from 1955 describing this behavior, this autohemorrhaging behavior in these snakes. And I just thought to myself, that's insane.

GREENFIELDBOYCE: So there weren't any photos or any kind of detailed description. It just said the snakes bled from their heads when handled.

SOFIA: So he did what most good scientists would do, I assume. He went out on an expedition to find these snakes.

GREENFIELDBOYCE: Absolutely. You know it.

SOFIA: (Laughter).

GREENFIELDBOYCE: He absolutely did. He and a couple of colleagues went out looking under rocks. They flipped a ton of rocks until they finally found one. And the snake made some defensive behaviors. It kind of rolled into this tight ball, and it started defecating and musking - you know, emitting this pretty bad-smelling liquid.

SOFIA: Understandably.

GREENFIELDBOYCE: And then Sebastian applied a little pressure to its nose, like just gently pinched its nose, to see if they could trigger any bleeding from handling.

SOFIA: All right, Nell. You sent me this video, and I'm watching it. It's like a little nose pinch. Maybe - you know, he pinches it, like, good. And - but then, boom, the snake's eyes fill up with blood, like immediately. There are just two big drops of blood where the eyes used to be. And then, suddenly, the eyes kind of clear up, and then a drop of blood comes out of its mouth.

GREENFIELDBOYCE: Yeah, Sebastian told me it was wild.

HOEFER: Just because I've never seen anything like that. And just the fact of how quickly that eye or the eyes fully flood with blood, and then the blood exudes from the mouth, and then the eyes fully clear up again in just a couple of seconds. It just - I was stunned. I was like, this is mad.

SOFIA: I mean, that is pretty dramatic. And you said other critters do this, too, right?

GREENFIELDBOYCE: Right. Some other snakes. You know, the one that's been studied the most is the horned lizard. That's a lizard that lives in the U.S., in the Southern U.S. And what it does is pretty nutso. It shoots blood out of its eyes, and the blood can fly several feet.

SOFIA: (Laughter) Several feet?

GREENFIELDBOYCE: Yeah. It doesn't happen that often, but when it does, it's so dramatic that it really gets people's attention.

SOFIA: Yeah.

GREENFIELDBOYCE: Indigenous people have known about these lizards for a long time. European scientists wrote about them centuries ago. And for 40 years, these lizards have been studied by Wade Sherbrooke He's director emeritus of the Southwestern Research Station of the American Museum of Natural History in Arizona. And he told me, at the beginning, he was just wondering, like, if this is supposed to be a defensive response, why would blood be a turnoff for what is, after all, a bloodthirsty predator?

SOFIA: (Laughter) Good question.

WADE SHERBROOKE: When you try to say, well, maybe a predator wouldn't like it, well, the predator's going to eat these things. And, you know, that doesn't make sense. So that's where I started.

GREENFIELDBOYCE: So he began watching the lizards and doing experiments. And what he found is that the horned lizard wouldn't always squirt blood from its eyes when threatened by an animal.

SHERBROOKE: And what I found was with roadrunners, with grasshopper mice, with leopard lizards, with rattlesnakes, with other snakes - they never squirt blood. They don't do it then.

SOFIA: So when do they do it? When do they squirt blood?

GREENFIELDBOYCE: He told me it's when the lizard is about to be eaten by something like a coyote or a bobcat.

SHERBROOKE: And the reason is the blood has a distasteful quality to mammalian predators, and it has to be - it has to arrive in the mouth.

SOFIA: OK, so these lizards are not really trying to, like, hit a predator from several feet away by sending streams of blood flying out of their eyes.

GREENFIELDBOYCE: Apparently not.

SOFIA: That's disappointing (laughter).

GREENFIELDBOYCE: It's more like a coyote's mouth clamps down on the lizard - the lizard squirts out foul-tasting blood. Wade told me that when he goes out and handles horned lizards, only a couple times out of 100 will it actually squirt blood from its eyes. You know, he said humans aren't a typical predator, so he thinks the lizard is just kind of confused. It doesn't really know what to do. And he told me he's actually tasted the blood.

SOFIA: No (laughter). No.

SHERBROOKE: For a long time, I thought this basically tastes like my blood.

SOFIA: I wish, Nell, I was surprised that this took a turn to a scientist tasting blood. But I'm not.

GREENFIELDBOYCE: Look - this is science, Maddie. It's science.

SOFIA: (Laughter).

GREENFIELDBOYCE: So then, you know, after a while, though, he started thinking, actually, there is this kind of, you know, aftertaste, this kind of acid aftertaste that lasted for maybe 20 minutes or so. He said it was really minor, you know. But if he squirts the blood into the mouth of a coyote or a bobcat, they have a really strong reaction.

SHERBROOKE: It's disgusting. Immediately they begin to salivate quite a bit. They shake their head. They do all kinds of things like that to - they have different taste buds than I do.

SOFIA: OK, OK. So all of this kind of suggests that for these horned lizards, this is really a strategic, specific defense, that it's not just, like, a random stress response. I mean, you could imagine that if an animal gets scared, like, its blood pressure goes up, blood might just shoot out of a leaky capillary somehow, I guess. But this seems to be a pretty specific defense move aimed at particular predators.

GREENFIELDBOYCE: Right. And like I mentioned before, the horned lizard's autohemorrhage is more studied than any other creature's.

SOFIA: OK, so, like, what beyond reptiles? You said some insects do this, too. Ladybugs?

GREENFIELDBOYCE: Yeah. Actually, quite a lot of insects do this. And if you're poking at a ladybug and some liquid comes out, it might just seem to you like it was urine or feces or something, you know? In ladybugs, it comes out of the legs, kind of underneath, so it's hard to see where it's coming out. It's yellow, you know. So unless you know what it is, you'd have no reason to think, this thing is just bled on me, you know, that...

SOFIA: (Laughter) Right.

GREENFIELDBOYCE: ...It just spontaneously bled on me.

SOFIA: I've had this happen. And now I'm looking back and I'm like, oh, that was blood. Cool (laughter).

GREENFIELDBOYCE: So I talked to Michal Knapp. He studies ladybugs at the Czech University of Life Sciences. And he told me that reflex bleeding is a highly effective way for ladybugs to deter predators because their blood is full of substances that smell and probably taste awful to birds or small mammals. And he told me that, actually, if a ladybug gets attacked by ants, the coagulating blood can act like a kind of glue that glues an ant's mouthparts together during the attack.

SOFIA: I mean, that's pretty amazing, Nell. But here's the thing I don't understand about this. Doesn't a ladybug need its blood? I mean, like, it seems like critters would be at a disadvantage of some sort if they went around bleeding any time they got threatened. So, like, how much blood are we talking about?

GREENFIELDBOYCE: So for ladybugs, he told me it can be a lot, like up to, you know, 15% or 20% of all its blood.

SOFIA: (Laughter) Wild. Wild.

GREENFIELDBOYCE: I mean, for you, that would be like if you lost a liter of blood, like a couple pints of blood.

SOFIA: Right.

GREENFIELDBOYCE: So it is significant. And that's why he's been looking into the consequences of this. I mean, you know, he said bleeding could save a ladybug's life, but...

MICHAL KNAPP: Probably there are also some costs. And we were searching for physiological costs in our research.

GREENFIELDBOYCE: He just published one set of experiments. He and his colleagues forced young ladybugs to reflex bleed repeatedly, like every day, and then they studied them.

SOFIA: Sure.

GREENFIELDBOYCE: And what they saw is that the bugs' immune systems seemed a little weakened, but the number of eggs they produced was the same. Their reproductive success was almost unaffected. You know, maybe just some slight delays in the age at first reproduction, but nothing major.

SOFIA: So what I'm taking from this is that these insects kind of know what they're doing, like evolutionarily speaking. I mean, do they filter the blood in any way to, like, try to preserve the good stuff and leak only just, like, the nasty compounds in it?

GREENFIELDBOYCE: His group has actually looked at this. And he told me that the blood that spontaneously comes out of ladybugs is exactly the same as the blood inside. And when I asked him, you know, exactly, like, what is going on here? Like, is there an opening in the leg?

SOFIA: Right (laughter).

GREENFIELDBOYCE: He said, no, like, somehow the insect is able to kind of injure itself to somehow create an opening...

SOFIA: Whoa.

GREENFIELDBOYCE: ...In its skin or cuticle. But it's not like all the mechanical details of reflex bleeding have been well studied. It's still pretty obscure.

SOFIA: All right, Nell. Well, thank you for this mini tour of the world of autohemorrhage. Until next time, friend.

GREENFIELDBOYCE: Ad astra, Maddie.


SOFIA: This episode was produced by Thomas Lu, edited by Gisele Grayson and fact-checked by Ariela Zebede. I'm Maddie Sofia - full of blood. Thanks for listening to SHORT WAVE from NPR. Transcript provided by NPR, Copyright NPR.