Speaker 1: Hey, you welcome to Stuff to Blow your Mind. My
Speaker 1: name is Robert Lamb.
Speaker 2: And I'm Joe McCormick, and it's Saturday. We are heading
Speaker 2: down down into the vault for an older episode of
Speaker 2: the show. This one originally published on July seventh, twenty
Speaker 2: twenty two, and it's called The Lesser of Two Crab Claws,
Speaker 2: Part three. This is part three of the series we've
Speaker 2: been running over the past couple of Saturdays. This is
Speaker 2: about asymmetry in nature.
Speaker 3: Welcome to Stuff to Blow Your Mind, production of iHeartRadio.
Speaker 1: Hey, welcome to Stuff to Blow your Mind. My name
Speaker 1: is Robert Lamb.
Speaker 4: And I'm Joe McCormick, and we're back with part three
Speaker 4: in our series on asymmetry in animals. When the left
Speaker 4: and the right side do not match. If you have
Speaker 4: not listened to parts one and two yet, why not
Speaker 4: go back and do that first, then you'll be all
Speaker 4: caught up. But in the last episode of this series,
Speaker 4: we were talking about asymmetrical development in the clause of
Speaker 4: the American lobster or homarusamericanas and how during larval development,
Speaker 4: the lobster's previously symmetrical claus differentiate into one real thick boy.
Speaker 4: It's a crusher claw. It's gigantic, it's got like molar
Speaker 4: like teeth on it, and it's got slow muscle fiber.
Speaker 4: And then the other one turns into a sharper cutter
Speaker 4: or pincher claw that can close very fast because of
Speaker 4: its fast muscle fiber. And we talked about attempts to
Speaker 4: pin down exactly why and how this happens at these
Speaker 4: early stages in lobster development. But there are other crustaceans
Speaker 4: where the mismatch between left and right is much more
Speaker 4: extreme than it is even in the American lobster. And
Speaker 4: I think we should begin this episode by considering the
Speaker 4: male of the many species of phil crabs, so fiddler
Speaker 4: crabs comprise. It's not just one species of animal, its
Speaker 4: many species within a family of crustaceans known as oc
Speaker 4: pot day And once again, I think we keep saying
Speaker 4: it during the series, but this is one you should
Speaker 4: go and look up pictures of because you need to
Speaker 4: have it in your head. In some species, male fiddler
Speaker 4: crabs have one claw that grows not just bigger than
Speaker 4: the other one, not even just significantly bigger than the
Speaker 4: other one, but hilariously bigger than the other one.
Speaker 1: Yeah, going back to lobsters for a second, you'd be
Speaker 1: forgiven if you were just not aware of the fact
Speaker 1: that lobsters had this different, this difference between one claw
Speaker 1: and the next. Certainly, many illustrations I'm thinking, like restaurant
Speaker 1: logos and whatnot, may not even bother to make one
Speaker 1: claw look different from the other. But with a fiddler crab,
Speaker 1: it is very pronounced. It is absurdly different one side
Speaker 1: from the other.
Speaker 4: Yeah. And for a bit of expert summary on what
Speaker 4: life is like for your average fiddler crab, I wanted
Speaker 4: to quote from a New York Times interview I was
Speaker 4: reading with a researcher named Sophie L. Moles, who is
Speaker 4: a scientist at Anglioruscan University in Cambridge and has done
Speaker 4: some research on fiddler crabs, including the use of a
Speaker 4: robotic fiddler crab claw that I want to come back
Speaker 4: to later in this episode, but just in summarizing fiddler
Speaker 4: crab life, she says, they live in burrows and you
Speaker 4: only see them at low tide. At high tide, they
Speaker 4: go back into the burrow and they seal it up.
Speaker 4: They feed on mudflats by sifting the sediment through their
Speaker 4: mouthparts and eating micro organisms. That's the buffet of life.
Speaker 4: You just sift the mud in your mouth and get
Speaker 4: the microbes out, but moles goes on. The female has
Speaker 4: two little claws, two normal size claws for her which
Speaker 4: she uses to help that feeding, to help pass the
Speaker 4: sediment up to her mouth. The male has one that
Speaker 4: it uses for feeding, and the other is huge. It's
Speaker 4: greatly enlarged, to the point that it can be approximately
Speaker 4: half of his body weight. It's often really brightly colored
Speaker 4: as well. Now, what the males do is they wave
Speaker 4: this claw in a species specific pattern. So each species
Speaker 4: of fiddler crab has its own kind of wave, and
Speaker 4: they do this to maintain a territory but also to
Speaker 4: attract a female. So for a rough analogy on the
Speaker 4: size and appearance of a fiddler crab claw, just imagine
Speaker 4: an adult human that had one normal sized right hand,
Speaker 4: but then a left hand with a digit span of
Speaker 4: about four feet, and that hand weighs eighty pounds.
Speaker 1: Yeah. One example of this was that it was brought
Speaker 1: in a book I'm going to reference in a bit
Speaker 1: Animal Weapons by Douglas j Emmlin. He says, if you're Basically,
Speaker 1: if you're at the store, go pick up the largest
Speaker 1: bag of dried dog food that you you can find
Speaker 1: and start carrying it around, and that will give you
Speaker 1: approximately what we might think of as the male fiddler
Speaker 1: crab experience.
Speaker 4: So yeah, so imagine the largest sized bag of dog food.
Speaker 4: Not just you're carrying it around, but that is one
Speaker 4: of your hands, yes, and again the other one is
Speaker 4: regular sized. So what is going on with having a
Speaker 4: crab claw that big? Well, it turns out that the
Speaker 4: main theory explaining this asymmetric size difference in fiddler crab
Speaker 4: claus is much like the main theory for explaining the
Speaker 4: narwhal tusk, that this hugely asymmetrically exaggerated feature found in
Speaker 4: males is probably primarily a sexually selected trait, meaning it's
Speaker 4: more important for maximizing reproductive success than it is for
Speaker 4: direct survival value, though it may be in part reproductively
Speaker 4: attractive attractive to mates because of some value it has
Speaker 4: in helping maximize like burrowing. So, for example, things that
Speaker 4: have been cited are that a male that has a
Speaker 4: very big claw can also probably dig a very big burrow,
Speaker 4: which is better for a female to go into to
Speaker 4: incubate her eggs and also, like crustaceans, tend to just
Speaker 4: keep getting bigger they as they grow as they grow older.
Speaker 4: So a bigger crab with a bigger asymmetric claw is
Speaker 4: also probably an older male, which is good in crab
Speaker 4: mating terms, because that probably means he has survived more
Speaker 4: seasons of life and is just generally fitter, better able
Speaker 4: to survive, and has good genes.
Speaker 1: Yeah. Yeah, And I mean part of the obvious display
Speaker 1: here too is like, look at this thing I have grown.
Speaker 1: It is so big, and yet I am still alive.
Speaker 1: I am able to sustain myself. Plus this massive claw,
Speaker 1: it's like the sports car of the crab anatomy.
Speaker 4: Yeah, that's another thing that has often been put up.
Speaker 4: There's sort of a theory in some sexually selected traits
Speaker 4: in biology that says, well, they may operate on the
Speaker 4: basis of essentially advertising a handicap. They offer a good
Speaker 4: faith display that even by working at a disadvantage, you're
Speaker 4: still fit enough to do well within your environment by
Speaker 4: having this ridiculous thing attached to you. So a male
Speaker 4: fiddler crab is running around also basically advertising I mean,
Speaker 4: this would be true of all the males with the
Speaker 4: big claw that they have basically have to their capacity
Speaker 4: to eat.
Speaker 3: You know.
Speaker 4: So these crabs eat by shoving mud and debris into
Speaker 4: their mouths, and that's true for males and females, but
Speaker 4: of course you can't do that with that gigantic claw.
Speaker 4: So essentially the male fiddler crabs they got them one
Speaker 4: good eat in hand, whereas females have two.
Speaker 1: Yes, absolutely, and this is something that Emlin gets into
Speaker 1: an animal weapons. Basically you're getting into just the energy
Speaker 1: costs of having this gigantic claw. So some of this
Speaker 1: is going to be a repeatd what we just said,
Speaker 1: but it all kind of builds together. So, first of all,
Speaker 1: male fiddlers, he says, burn a lot of energy just
Speaker 1: to have these, just to develop them and carry them
Speaker 1: around resting. Metabolic rates of males with big claws are
Speaker 1: almost twenty percent higher than females due to the cost
Speaker 1: of the claw. And then of course, on top of this,
Speaker 1: you're going to have to scamper around. We see everyone
Speaker 1: out there, I think it's probably seen crabs about doing
Speaker 1: their business on the beach. You've got to scamper around,
Speaker 1: you've got to run with that giant claw and so
Speaker 1: this is going to be energetically demanding as well.
Speaker 4: Yeah, I can how fast can you run holding that
Speaker 4: dog food bag?
Speaker 1: Exactly? Yeah, this is where he originally brought up the
Speaker 1: dog food bag, but he also cites a study. This
Speaker 1: one was really fun. This is a two thousand and
Speaker 1: seven study that was published in Functional Ecology by Alan
Speaker 1: and Levinton, and they were testing this out by putting
Speaker 1: male fiddler crabs on treadmills, little treadmills inside of air
Speaker 1: tight box. Now, sadly, I could not find photos of
Speaker 1: this that I brought up the original paper and there
Speaker 1: were no photos or illustrations, so I only have my
Speaker 1: imagination to go on here.
Speaker 4: This is the shrimp shrimp on a treadmill paper that
Speaker 4: you promised in previous.
Speaker 1: Parts, right right, Yeah, shrimp on a treadmill For anyone
Speaker 1: not familiar, that frequently brought up as there was an
Speaker 1: actual shrimp on a treadmill study, and it was used
Speaker 1: as an outrageous example of like, look at what the
Speaker 1: scientists are doing. They won't cure a cancer, but they'll
Speaker 1: put a shrimp on a treadmill. And as we've discussed
Speaker 1: in the show before, that that's kind of ridiculous. I mean,
Speaker 1: these are not the scientists that would be developing the
Speaker 1: cure for cancer. These are the ones that would be studying, say,
Speaker 1: the metabolic rates of shrimp or in this case, crabs.
Speaker 4: Right, they're not mutually exclusive pursuits to begin with. But
Speaker 4: then also sometimes you don't even know what benefits that
Speaker 4: new knowledge about animal life could lead to down the road.
Speaker 1: Yeah. Absolutely, And so the study put the crabs on
Speaker 1: the treadmills inside of the air tight boxes. And you
Speaker 1: might be one, why the air tight box. This sounds
Speaker 1: like something from a Saw movie. No, it's because it's
Speaker 1: the crabs exert themselves. They burn through oxygen and they
Speaker 1: produce CO two. And so the researchers are then able
Speaker 1: to measure the gas concentrations inside of the little boxes,
Speaker 1: and they use these readings to calculate the exact metabolic
Speaker 1: costs of running. As one might expect, the males with
Speaker 1: big claws burned more energy to run compared to smaller males,
Speaker 1: males with the smaller claw, or females that of course
Speaker 1: just have two regularly sized claws. And these big males
Speaker 1: with the big claws also tired out more quickly. And
Speaker 1: then there's the impact on feeding, which we have already
Speaker 1: alluded to. We've all seen crab eat I know, we've
Speaker 1: talked about it on the show. Crabs disassemble their food.
Speaker 1: Their claws and mouth bits work very hard to break
Speaker 1: everything down, or in the case of fiddler crabs, they
Speaker 1: are often just sifting through and finding those little tiny
Speaker 1: pieces to eat. Anyway, it's what describes as quote delicate
Speaker 1: and tedious, and with the females it means. It often
Speaker 1: means the feeding claws are just working incessantly.
Speaker 4: Yeah, you can see video of this. So there's just
Speaker 4: like a conveyor belt. They're just machines kind of shoveling
Speaker 4: the sediment into the mouth.
Speaker 1: But the male, on the other hand, like we've said,
Speaker 1: only has the one claw that's suitable to eat with anymore.
Speaker 1: He's got that big claw just setting there, and then
Speaker 1: the other claw, the normal sized claw, is the one
Speaker 1: that he's using to eat. So this cuts their energy
Speaker 1: intake in half, just as lugging the giant claw around
Speaker 1: increases their energy output. So they generally have to feed
Speaker 1: faster and or more often in order to make up
Speaker 1: the difference.
Speaker 4: Right again, because you got it, they've divided their body
Speaker 4: into eating hand and handsome hand, right.
Speaker 1: And this complicates things for the crabs even more because
Speaker 1: remember this is not an apex predator we're talking about here.
Speaker 1: The crab, the filler crab especially, they have to concern
Speaker 1: themselves with predators, especially of the avian variety. So if
Speaker 1: this crab with this big claw that's having to do
Speaker 1: extra feeding, that means extra exposure to potential predation. In fact,
Speaker 1: studies have proven out that these males are picked off
Speaker 1: by birds at an enhanced rate.
Speaker 4: Right, So you're saying, because it eats slower because it
Speaker 4: can only eat with one of its claws, it has
Speaker 4: to spend more time outside the burrow, and that's got
Speaker 4: to target on its back exactly.
Speaker 1: Yeah, it's more time out in the open, more time
Speaker 1: exposed to predators, and the predators in many cases they
Speaker 1: have advanced tactics for dealing with these These these either
Speaker 1: tired or distracted or essentially one clawed crab at this
Speaker 1: point when it comes to the feeding process Inland points
Speaker 1: to a study from Christy Blackwell and Coga regarding fiddler
Speaker 1: crabs in Panama getting basically taken out by grap fed
Speaker 1: on by grackles, who as a type of bird that
Speaker 1: have devised a diagonal feint attack where they they kind
Speaker 1: of they come in, they kind of fake the crab out,
Speaker 1: and apparently this is even more effective on the mail crabs.
Speaker 4: Now you might think, well, but wait a minute. Having
Speaker 4: a bigger claw surely also means that that crab can
Speaker 4: pinch with greater force, which you would think could make
Speaker 4: it able to defend itself better.
Speaker 1: Right, yeah, I mean you you might you might think that,
Speaker 1: but but then you know, as we'll get into like
Speaker 1: these claws, this big claw anyway, it doesn't seem to
Speaker 1: be that useful when you're dealing with something like a
Speaker 1: hungry grackle that's sweeping in at you. So like the
Speaker 1: end result is that the big clawed mails they're easier
Speaker 1: to find, they're easier to pick off, they're potentially more tired,
Speaker 1: and also they're a better there. They're a better kill
Speaker 1: for the predator because that big old claw has big
Speaker 1: old meat in it. So there's every reason in the
Speaker 1: world to kill them and eat them if you were
Speaker 1: a grackle or some other hungry bird.
Speaker 4: Now, from what I've read about fiddler crab clause, it
Speaker 4: seems like what they are most of the time used
Speaker 4: for is probably visual signaling, but they are on occasion
Speaker 4: actually used for fighting or actually used as a weapon.
Speaker 1: Yeah. Yeah. Emlin gets into this in the book as well,
Speaker 1: highlighting particularly the work of John Christy. I believe he
Speaker 1: was in the second study that was cited there that
Speaker 1: we decided. So basically, yeah, they wave them around to
Speaker 1: communicate their reproductive fitness. They do fight other male fiddler
Speaker 1: crabs with them, so they do serve as actual weapons
Speaker 1: in contests for those burrows that we were talking about.
Speaker 1: But Emlin writes that quote, for every few minutes of
Speaker 1: outright fighting, male spend dozens of hours waving and other
Speaker 1: words communicating, showing off that claw, saying look, you know,
Speaker 1: look at this mighty claw. I imagine what I can
Speaker 1: do with it. That happened. That's what's going on most
Speaker 1: of the time. A very small amount of the time
Speaker 1: they're actually using it. So it's ultimately more of a
Speaker 1: deterrence than anything. And this is evolutionarily sound because fighting
Speaker 1: is dangerous. The battle itself is dangerous and can certainly
Speaker 1: be fatal to an organism, but fights can also just
Speaker 1: wound you, making you more susceptible to predation, it may
Speaker 1: distract you and allow the grackle or some other creature
Speaker 1: to come in and take you out. So even though
Speaker 1: there are hard disadvantages to developing such a deterrence, and
Speaker 1: he compares this to other animals as well, like anytime
Speaker 1: you see something that you might label an elaborate weapon
Speaker 1: in some sort of an animal's anatomy, there's a huge
Speaker 1: payoff there. Nothing is free, nothing is cheap. When it
Speaker 1: comes to the development of these things. There's an energy
Speaker 1: cost involved. So even though there are all these disadvantages
Speaker 1: to growing, say a giant crab claw, there are also
Speaker 1: strong benefits in not having to actually fight all of
Speaker 1: the time.
Speaker 4: Yeah, I mean, I think this goes against our intuition
Speaker 4: because we think of fighting in terms of winning and losing.
Speaker 4: So like fight it fight has a winner who wins
Speaker 4: and thus they come out good the effect for them
Speaker 4: is positive, and a loser who loses, and of course
Speaker 4: the effect for them is negative. But in fact, in nature,
Speaker 4: I would argue that most fighting is probably actually lose
Speaker 4: lose because even the winner is probably somewhat injured or
Speaker 4: tired out by the fight, putting them at a later
Speaker 4: disadvantage for survival even if they come out on top
Speaker 4: in that particular struggle.
Speaker 1: Yeah, yeah, And all of this makes even more sense
Speaker 1: when we start looking at the closer to the scenario
Speaker 1: of these fighting and protecting these burrows and trying to
Speaker 1: move females. Again, the male setup shop in front of
Speaker 1: key burrows that are offered as brooding burrows to perspective females,
Speaker 1: and this is where they make their sh and this
Speaker 1: is where they fight if it comes to that. And
Speaker 1: the numbers here are apparently great. They're just tons of
Speaker 1: crabs out there, and they're just face off after face off.
Speaker 1: And again, most of these face offs are not going
Speaker 1: to result in a big, drag out fight. A lot
Speaker 1: of them are just going to be displays. But still
Speaker 1: lifting that crab claw in the air and to signal
Speaker 1: with it, that's going to have an energy cost, and
Speaker 1: so this is ultimately exhausting too many crabs. Crabs will
Speaker 1: eventually have to bow out and work their way back
Speaker 1: up to good burrows. So there's like this whole system
Speaker 1: of communication. Most of these face offs don't rise to
Speaker 1: the level of a full intensity battle, and the display
Speaker 1: of the claw allows the male crabs to easily determine
Speaker 1: who they have a chance against, so they're able to
Speaker 1: size each other up, like, Okay, this is a battle
Speaker 1: that I definitely can win, and he knows I can
Speaker 1: win it, So we're done. This is a display only situation. Okay,
Speaker 1: he's a crab that can definitely beat me, so I'm
Speaker 1: not going to mess with him. We're just going to
Speaker 1: carry on our ways. This one, however, we're going to
Speaker 1: have to communicate a little bit and we might have
Speaker 1: to fight because we seem to be evenly matched.
Speaker 4: Yeah. That's actually the most dangerous situation is when it's
Speaker 4: not clear which one is stronger. All right, Well, I
Speaker 4: wanted to come back to something which was earlier. I
Speaker 4: mentioned that a New York Times interview with the researcher
Speaker 4: Sophie Moles, who was one of the authors of a
Speaker 4: paper published in twenty eighteen in Biology Letters called Robotic Crabs,
Speaker 4: revealed that female fiddler crabs are sensitive to changes in
Speaker 4: male display rate. The other authors here were Michael D.
Speaker 4: Jinions and Patricia ur Why Backwell.
Speaker 1: Yes, I might have referred to her earlier as Blackwell,
Speaker 1: my apologies.
Speaker 4: Oh I didn't catch that.
Speaker 1: You should apologize, Well, I just want to make sure
Speaker 1: I get the names right. Yeah, had a type in
Speaker 1: my notes.
Speaker 4: There, so, But from the crabs perspective, I think it
Speaker 4: is important to realize that this really is a study
Speaker 4: that involved creating crab sex robots, like this is creating
Speaker 4: the It was an attempt to create the hunkiest male
Speaker 4: robot crab claws that have ever been put together with
Speaker 4: the explicit purpose of attracting female fiddler crabs. So I'm
Speaker 4: just going to read directly from their abstract. They write quote,
Speaker 4: Males often produce dynamic, repetitive courtship displays that can be
Speaker 4: demanding to perform and might advertise male quality to females.
Speaker 4: A key feature of demanding displays is that they can
Speaker 4: change in intensity, escalating as a male increases his signaling effort,
Speaker 4: but de escalating as the signaler becomes fatigued. Here we
Speaker 4: investigated whether female fiddler crabs of the species Uka miyoburgi
Speaker 4: are sensitive to changes in male courtship wave rate how
Speaker 4: fast the arm is waving. We performed playback experiments using
Speaker 4: robotic male crabs that had the same mean wave rate
Speaker 4: but either escalated, de escalated, or remained constant females demonstrated
Speaker 4: a strong preference for escalating robots, but showed mixed responses
Speaker 4: to robots that de escalated fast to slow compared to
Speaker 4: those that waved at a constant medium rate. These findings
Speaker 4: demonstrate that females can discern changes in male display rate
Speaker 4: and prefer males that escalate, but that females are also
Speaker 4: sensitive to pass display rates indicative of prior vigor. So,
Speaker 4: if you are a male fiddler crab, it's not just
Speaker 4: important to have a big claw, but it apparently, at
Speaker 4: least with this species, is more attractive to females if
Speaker 4: you start waving it faster and faster as the female
Speaker 4: comes close to you. And from this New York Times
Speaker 4: interview with the lead author, their moles, it was there
Speaker 4: was the question where the females terribly disappointed when they
Speaker 4: realized they'd been tricked. You know what happened once they
Speaker 4: finally got up to the waving robot arm that they
Speaker 4: were so interested in, Well, Mole says quote, once they
Speaker 4: got to the robot, they would touch the base plate
Speaker 4: of it and realize there's something wrong here, it's not real,
Speaker 4: and they would usually at that point stop moving or
Speaker 4: run away. Some of them actually responded as if he
Speaker 4: were a real male crab, which is by tickling him.
Speaker 4: What the females do is go up to the male
Speaker 4: and use their legs on one side of their body
Speaker 4: to tickle him. This communicates to him that she's interested
Speaker 4: in him as a mate and not just trying to
Speaker 4: steal his home. So this study did indeed implicate female
Speaker 4: fiddler crabs tickling metal base plates because they thought it
Speaker 4: just that claw is so huge, it's swinging so fast,
Speaker 4: I've got to believe it might be a real crab.
Speaker 1: This sounds like something that could be factored into, I
Speaker 1: don't know, Battlestar Galactica sort of situation.
Speaker 4: Yeah, I go the.
Speaker 1: Replicants, the robots. They look just like us, they behave
Speaker 1: just like us, except tickling them will reveal their true nature.
Speaker 4: Anyway, in the spirit of our enthusiasm for the shrimp
Speaker 4: on the treadmill, the crab on the treadmill, I want
Speaker 4: more studies with robot crab hunks. We have to build
Speaker 4: the most attractive male crab that has ever been that
Speaker 4: has ever existed on Earth, and but I guess we
Speaker 4: have to be careful with it because we don't want
Speaker 4: to drive crabs to extinction by like now now the
Speaker 4: real crabs only desire the robot.
Speaker 1: It's weird how this does line up with the sort
Speaker 1: of trope of the muscle man on the beach attracting
Speaker 1: the women and and the and the nerd that's that's
Speaker 1: also inevitably on the beach as well, and may get
Speaker 1: sand kicked in his face or whatnot. But it also
Speaker 1: does bring to mind like even with humans, there's sort
Speaker 1: of there's there's fitness, and there's like visible fitness, but
Speaker 1: then there's also like fitness to the level where it's
Speaker 1: no longer purely functional anymore. Like there's like, for instance,
Speaker 1: there's the muscle that might aid in the delivery of
Speaker 1: a punch, and then there's like than the muscle build
Speaker 1: up that say, makes it harder to move around or
Speaker 1: makes it more difficult to say, touch portions of your back,
Speaker 1: that sort of thing.
Speaker 4: I totally know what you're saying, though, I also that
Speaker 4: reminds me. I always want to caution people, you know,
Speaker 4: just don't try to extrapolate too much from animal sex
Speaker 4: and attractiveness studies to humans, because you know, crabs and
Speaker 4: humans are pretty different.
Speaker 1: Right, right, and certainly the reasons that humans do things
Speaker 1: and the way they react to things, or generally there's
Speaker 1: a lot more going on. There's this whole level of
Speaker 1: human complication that's taking place on the surface of whatever
Speaker 1: else is going on.
Speaker 4: All Right, after we've talked about all of these examples
Speaker 4: of animals that broadly have bilateral symmetry but then some
Speaker 4: major deviation from it, I've been thinking about how symmetry
Speaker 4: and asymmetry come about at the cellular level, because you know,
Speaker 4: you can imagine why it would be genetically efficient to
Speaker 4: have bilateral symmetry. You just basically need half of a
Speaker 4: body plan and then you just copy it over on
Speaker 4: the other side. But within that broadly symmetrical framework, you know,
Speaker 4: we get these deviations major and minor. And it's not
Speaker 4: all narwhal tusks and fiddler crab claws things that are
Speaker 4: like huge and noticeable. There are plenty of forms of
Speaker 4: asymmetry that are common but harder to spot, such as
Speaker 4: the orientation of internal organs. You know, your digestive tract
Speaker 4: and its associated organs and your heart and circulatory system
Speaker 4: are all asymmetrical. They have different organs and pathways situated
Speaker 4: on the left and right of the body cavity, and
Speaker 4: there are also minor more invisible variations at the cellular
Speaker 4: level within mostly symmetrical creatures like us. So how do
Speaker 4: these deviations from perfect symmetry come about at the level
Speaker 4: of cell division, which is actually, you know, actively building
Speaker 4: your body's tissues. How do the cells know which side
Speaker 4: is which and how to do something different on the
Speaker 4: left than what they're doing on the right. Well, one
Speaker 4: jumping off point here is I came across an interesting
Speaker 4: article about this in Quantum Magazine from January twenty seventeen
Speaker 4: by Tim Verneman called how Life Turns Asymmetric, which is
Speaker 4: worth a read in its entirety, but I just wanted
Speaker 4: to summarize and jump off from a few things I
Speaker 4: learned from it. And one of the big takeaways is
Speaker 4: that I think we have some good answers about at
Speaker 4: least some strong factors for like mammalian or vertebrate symmetry
Speaker 4: and symmetry breaking something. We know some things about the
Speaker 4: genetic and cellular basis for a symmetry in the body,
Speaker 4: but we still don't know everything yet, and so one
Speaker 4: of the ideas that gets brought up in this article
Speaker 4: is the nodal lefty genetic connection, and it goes like this.
Speaker 4: Since the nineteen nineties, scientists have been studying a gene
Speaker 4: called nodal in ODL, which appears specifically on the left
Speaker 4: side of the developing embryo of At the time this
Speaker 4: article was written, they said every vertebrate animal yet studied
Speaker 4: and associated with this gene is a somewhat confusingly named
Speaker 4: gene called lefty, which appears to work specifically to suppress
Speaker 4: the nodal gene's activity on the right side of the
Speaker 4: vertebrate embryo. So the purpose of lefty, if I understand correctly,
Speaker 4: appears to be something like telling the right side of
Speaker 4: the body not to do left side of the body stuff.
Speaker 4: According to the Harvard biologist Cliff Tabin, the nodal lefty
Speaker 4: gene combination seems to be the main genetic factor guiding
Speaker 4: asymmetry in animals, or at least invertebrates. So how does
Speaker 4: this difference get expressed? Well, another biologist named Nobotaka Hirokawa
Speaker 4: has offered an explanation that has to do with sillia.
Speaker 4: Cillia are little hair like or thread like projections. Technically,
Speaker 4: they're a type of organelle which stick up from cell
Speaker 4: membranes within the cells or of eukaryotes, and they serve
Speaker 4: various functions like gathering sensory information four cells or facilitating
Speaker 4: the movement of cells through fluid. So you might read
Speaker 4: about as sillia motility. These things often move back and forth,
Speaker 4: though actually they're divided into motile and nonmotile cilia. So
Speaker 4: how would tiny hairs sticking up off of cell membranes
Speaker 4: have anything to do with the body of a vertebrate
Speaker 4: splitting from perfect symmetry into a differentiated left and right half. Well.
Speaker 4: One fascinating clue came in the form of a rare
Speaker 4: genetic disorder found in humans known as Cartagener's syndrome. Actually,
Speaker 4: I'm not sure if I'm saying that right, but it's
Speaker 4: spelled k A R T A G E N E
Speaker 4: R Cartagener syndrome, which presents most often in patients as
Speaker 4: patients with continued respiratory problems such as recurrent lung infections
Speaker 4: and sinus problems, and also sometimes infertility. It turns out
Speaker 4: this condition is caused by a congenital defect that prevents
Speaker 4: the body's scillia from functioning as needed, so these little
Speaker 4: hair like projections on cells don't function as they normally would. Now,
Speaker 4: why would that affect respiration? Well, of course, the inside
Speaker 4: of our breathing passages are lined with cilia, and the
Speaker 4: cilia need to move in synchronization for I think multiple purposes,
Speaker 4: but one of them is to help clear breathing passages
Speaker 4: of mucus. And this disorder causes the cilia to have
Speaker 4: trouble again with motility, with movement, and so they can't
Speaker 4: really synchronize. They can't really work together to get the
Speaker 4: mucus out of the lungs and out to the throat
Speaker 4: to prevent infections. Now, strangely, this disorder affecting cilia also
Speaker 4: frequently coincides with a seemingly totally unrelated issue. About half
Speaker 4: of people diagnosed with cartagen or syndrome also have their
Speaker 4: internal organs flipped. Their body is a mirror image of
Speaker 4: what a thoracic surgeon would expect to see if they
Speaker 4: open you up. So you know, the heart on the
Speaker 4: right and the liver on the left and so forth.
Speaker 1: That's right. Yeah, if we're looking to spy literature, of course,
Speaker 1: if we look at Ian Fleming's doctor know we might
Speaker 1: remember doctor No has this where his heart is on
Speaker 1: the other side of his body. Oh, it survives. I
Speaker 1: think he survives an assassination attempt at some point because
Speaker 1: of this anatomical quirk.
Speaker 4: Oh, I say, somebody shoots him on the wrong side. Yeah,
Speaker 4: that's a good twist. Well, anyway, so you have that
Speaker 4: association people who have this, who have this congenital condition
Speaker 4: affecting proteins that in turn affect sillia. They also half
Speaker 4: of the time their organs are flipped opposite of what
Speaker 4: you normally see. On top of that, there was a
Speaker 4: twenty fifteen paper in Nature by Lee at All, which
Speaker 4: Vernemon in the article points to called global Genetic Analysis
Speaker 4: in mice unveils central role for ccilia in congenital heart disease.
Speaker 4: And this paper apparently found multiple instances of genes where
Speaker 4: if the gene was defective, the mouse presented with some
Speaker 4: kind of unusual issue related to symmetry and asymmetry in
Speaker 4: the body, some issue with the haves left and right,
Speaker 4: And in those instances the gene was somehow also related
Speaker 4: to scillia. So these clues indicate that somehow scillia may
Speaker 4: play a role in symmetry breaking during mammalian development. So
Speaker 4: how could this be Well, A leading explanation has to
Speaker 4: do with something called dorsal flow and a little patch
Speaker 4: on the surface of mammalian embryos called the ventral node. So,
Speaker 4: if you're looking at like a mammalian embryo, the ventral
Speaker 4: node is a little pit or depression on the underside
Speaker 4: or the bottom surface, and the pit is ciliated, meaning
Speaker 4: it's covered in cilia, these little hair like or thread
Speaker 4: like projections. And the explanation goes that the waving of
Speaker 4: cilia in this little pit create a consistent direction of
Speaker 4: flow in the fluid around the ventral node. So the
Speaker 4: cilia rotate to get the fluid moving, and then they
Speaker 4: keep it moving in a consistent direction. The fluid is
Speaker 4: always moving to the left along with the with the
Speaker 4: way the cilia are waving, and the direction of this
Speaker 4: flow seems to cause a chain reaction that results in
Speaker 4: changes in gene expression, specifically in the asymmetry genes coming
Speaker 4: back to again no and lefty. So apparently, if the
Speaker 4: cilia are having trouble with motility, the unidirectional leftward current
Speaker 4: of fluid is not established and the symmetry breaking genes
Speaker 4: aren't expressed as they would normally be, which can lead
Speaker 4: to deviations from the type of mammalian asymmetry we would
Speaker 4: see in most members of that species, such as creating
Speaker 4: a condition where the body fifty percent of the time
Speaker 4: can have its internal organs flipped. However, this can't be
Speaker 4: the only factor leading to standard symmetry breaking in animal bodies.
Speaker 4: Vernemon's article also cites a Tufts University biologist named Michael
Speaker 4: Levin who points out that some animals, even some mammals,
Speaker 4: don't have that ciliated dorsal node we were just talking about,
Speaker 4: and Levin believes there's some involvement of a factor called
Speaker 4: the cellular skeleton or the cytoskeleton. Did you know that
Speaker 4: your cells have a skeleton of their own? I don't know.
Speaker 4: I guess i'd heard the word cytoskeleton, but I hadn't
Speaker 4: quite put it together. It's not exactly like your bigger skeleton.
Speaker 4: I mean, it's not like bones. The cytoskeleton is a
Speaker 4: system of protein filaments that are, at least in a
Speaker 4: metaphorical sense, sort of like the bones of a cell.
Speaker 4: To describe them, I own a quote from a twenty
Speaker 4: ten review in nature by Fletcher and Mullins quote. The
Speaker 4: ability of a eukaryotic cell to resist deformation, to transport
Speaker 4: intracellular cargo, and to change shape during movement depends on
Speaker 4: the cytoskeleton, an interconnected network of filamentous polymers and regulatory proteins.
Speaker 4: Recent work has demonstrated that both internal and external physical
Speaker 4: forces can act through the cytoskeleton to affect local mechanical
Speaker 4: properties and cellular behavior. Attention is now focused on how
Speaker 4: cytoskeletal networks generate, transmit, and respond to mechanical cellar over
Speaker 4: both short and long time scales. An important insight emerging
Speaker 4: from this work is that long lived cytoskeletal structures may
Speaker 4: act as epigenetic determinants of cell shape, function, and fate.
Speaker 4: And it's exactly this last comment that I think is
Speaker 4: most relevant here, because in the case of symmetry breaking,
Speaker 4: it may be that features of this cellular skeleton, this
Speaker 4: system of sort of strands of polymers and proteins that
Speaker 4: help give a cell its shape and help it resist
Speaker 4: deformation when it's under pressure and things like that, that
Speaker 4: this system may ultimately epigenetically determine the development of cells
Speaker 4: and ultimately the handedness or asymmetry of the whole body.
Speaker 4: Oh wow, this was so This next part was also
Speaker 4: a surprise to me. I don't think I knew this.
Speaker 4: Apparently cells themselves have a kind of handedness or asymmetry.
Speaker 4: Some cells are sort of left oriented and some are
Speaker 4: right oriented, and you can and see this in their
Speaker 4: behavior when they're moving through fluid and they come up
Speaker 4: against an obstacle. So you can have experiments where you
Speaker 4: show that cells are flowing along in a controlled environment
Speaker 4: and then they bump up against something, bump a surface.
Speaker 4: When that happens, the cell will tend to turn in
Speaker 4: one direction or the other, and that preference for a
Speaker 4: particular way of turning tends to remain consistent for each cell.
Speaker 4: You have sort of left turning cells and right turning cells,
Speaker 4: and experiments in fruit flies demonstrate that these small differences
Speaker 4: at the cellular level can snowball into major morphological differences
Speaker 4: at the body level. Vernamon's article mentions researchers named Leo
Speaker 4: Ian and Kinji Matsuno, who each identify proteins within the
Speaker 4: cellular skeleton, specifically the actin and myosins, as having an
Speaker 4: influence on whether a cell becomes left handed or right handed.
Speaker 4: And there may also be some interplay between proteins in
Speaker 4: the sidoskeleton and the asymmetrical expression of the nodle gene,
Speaker 4: each playing a role. But then there's one more thing
Speaker 4: that gets mentioned toward the end of this article, the
Speaker 4: Quanta article that I thought was interesting, which is that
Speaker 4: other factors leading to asymmetry. Of course, there might be
Speaker 4: some that haven't been discovered yet, but one candidate has
Speaker 4: to do with communication between cells, for instance, based on
Speaker 4: the relative prevalence of proteins on a cell surface, which
Speaker 4: would in turn determine how cells trade electrical charges back
Speaker 4: and forth between each other. And the Quanta article cites
Speaker 4: Michael Levin again saying quote, if we block the communication channels,
Speaker 4: asymmetrical development always goes awry. And by manipulating this system,
Speaker 4: we've been able to guide development in surprising butt predictable directions,
Speaker 4: creating six legged frogs, four headed worms, or froglets with
Speaker 4: an eye for a gut without changing their genomes at all.
Speaker 4: And in a final twist, bringing this back to medicine,
Speaker 4: it's interesting that all of this knowledge might one day
Speaker 4: be useful in finding treatments for pathological growth and development
Speaker 4: patterns and somatic cells by sort of harnessing these systems,
Speaker 4: by harnessing the bodies existing mechanisms for detecting and directing
Speaker 4: its own shape, you know, the way the cells come
Speaker 4: together to form larger structures that might be harnessed for
Speaker 4: treating cases where cell development is going wrong.
Speaker 1: Yeah, that's fascinating, you know, coming back to what you
Speaker 1: said earlier, I don't know if prior to this, if
Speaker 1: someone had just stopped me on the street, like a
Speaker 1: man on the street reporter situation asked me if cells
Speaker 1: have a skeleton, if I would have been able to
Speaker 1: correctly answer regarding the site of skeleton. Here, this is
Speaker 1: pretty fascinating. And then to get into its sort of
Speaker 1: the ramifications of that and how that ends up being
Speaker 1: reflect elected in the left handedness of the overall system
Speaker 1: or the right handedness, whichever the case may be. You know,
Speaker 1: I have to think back once more to the cock
Speaker 1: eyed squid Histiotoothus that we discussed in Oh, this was
Speaker 1: the first episode. Yeah, one of the reasons to marvel
Speaker 1: this amazing creature is that its eyes have evolved to
Speaker 1: look in different directions to different realms of the ocean.
Speaker 1: Light and dark, and this seems understandably strange and alien
Speaker 1: to us, but perhaps less so when we remember things
Speaker 1: like left handed and right handedness. When we think about
Speaker 1: neural asymmetry that defines us on the inside, and of
Speaker 1: course it's not just us. Asymmetries between left and right
Speaker 1: side of the nervous system are present throughout the animal kingdom,
Speaker 1: from invertebrates to mammals. And as one source I was
Speaker 1: looking at this is by Conca, Bianco and Wilson in
Speaker 1: Encoding Asymmetry within Neural Circuits, published in twenty twelve in
Speaker 1: Nature of Views Neuroscience. The theoretical advantages of brain asymmetry
Speaker 1: and include the capacity for parallel processing, the specialization of
Speaker 1: left and right sides for distinct computations, and the restriction
Speaker 1: of information processing within local circuits with short, fast connections.
Speaker 1: But while there are obvious advantages to brain asymmetry, are
Speaker 1: there advantages to brain symmetry. So I was looking into
Speaker 1: this a little bit and I read some thoughts from
Speaker 1: a Marco Data of the University of Padua who experimented
Speaker 1: with the often lateralized fish species the gold belly top
Speaker 1: mino in two thousand and nine. Basically, what this experiment
Speaker 1: consisted of was dividing these top minnows, these gold belly
Speaker 1: top minnows, into groups of left lateralized, right lateralized, and
Speaker 1: non lateralized specimens. So the seeming advantage to the non
Speaker 1: lateralized came when judging stimuli to either side of the
Speaker 1: creature through either eye. Experiments involved judging advantageous shoals of
Speaker 1: fish to join on either side. Remember, you're a small
Speaker 1: fish in the ocean. There's a lot of survival advantage
Speaker 1: of being able to determine which shoal of fish you
Speaker 1: should take refuge in their strength in numbers, and so
Speaker 1: it seems that in these fish having a lateral tendency,
Speaker 1: most often they just joined the shoal that they saw
Speaker 1: with their dominant eye. So I guess the take home
Speaker 1: here is that in some cases, yeah, it's going to
Speaker 1: it may come down to your dominant side is just
Speaker 1: going to be the tendency that you go in, and
Speaker 1: it's maybe going to potentially get in the way of
Speaker 1: properly evaluating in this case, two different shoals of fish.
Speaker 4: I don't have evidence of this in front of me,
Speaker 4: but it makes me think that surely things like this
Speaker 4: must also be true even with you know, brains we
Speaker 4: would think of as more complex. I mean, I'm sure
Speaker 4: even with humans, handedness probably plays a role in like
Speaker 4: directional reactions to fast you know, something pops up and
Speaker 4: scares you, which direction do you bolt in? I would
Speaker 4: be surprised if there is not some kind of tendency
Speaker 4: there that's not purely dictated by where the stimulus is,
Speaker 4: but also has to do with like body side dominance.
Speaker 1: Yeah, and body side dominance and left handedness and right
Speaker 1: handedness and human beings. This is something I think we'll
Speaker 1: have to come back to in a future episode. There's
Speaker 1: a lot of great research out there, particularly when again
Speaker 1: it comes back to what we were saying earlier about
Speaker 1: how you have whatever's going on on the animal level
Speaker 1: and then you have the human complications involved there. Because yeah,
Speaker 1: when you start getting into whole situations of okay, you
Speaker 1: have a right handed dominant society and then you have
Speaker 1: left handed individuals within that society, you know, what is
Speaker 1: the impact? And of course there's a lot of There
Speaker 1: have been a number of interesting studies over the years
Speaker 1: that have looked at this, how it plays into sports,
Speaker 1: how it plays to conflict and combat. How it just
Speaker 1: plays into thinking about the world around you. So that
Speaker 1: would be a fun one to come back and do.
Speaker 1: And I know that the lefties especially will love it.
Speaker 4: Oh yeah, but.
Speaker 1: Righty's your most of our audience, so don't worry. You'll
Speaker 1: like it too, You're just less special. Sorry, Okay, should
Speaker 1: we wrap it up there? I suppose we should. So
Speaker 1: we hope that you've enjoyed this partial journey through asymmetry.
Speaker 1: Like we said, there are plenty of other examples of
Speaker 1: asymmetry in the animal world. We tried to focus on
Speaker 1: some of the examples that illustrated the topic the best.
Speaker 1: But perhaps you're thinking of something we didn't mention that
Speaker 1: bears mentioning, right in, let us know. Let us know
Speaker 1: if you're interested in an episode in the future about
Speaker 1: left handedness and right handedness in humans, anyway you look
Speaker 1: at it, just right in. We'd love to hear from
Speaker 1: you past episodes, future episodes, and episodes. It's all fair game.
Speaker 1: We read those on Mondays on listener Mail and the
Speaker 1: Stuff to Blow your Mind podcast feed. We have our
Speaker 1: core episodes on Tuesdays and Thursdays. Short Form Artifact or
Speaker 1: Monster Fact episode on Wednesdays and on Fridays, we put
Speaker 1: aside most serious concerns and we just watch a weird
Speaker 1: film on Weird House Cinema.
Speaker 4: Huge thanks, as always to our excellent audio producer Seth
Speaker 4: Nicholas Johnson. If you would like to get in touch
Speaker 4: with us with feedback on this episode or any other,
Speaker 4: to suggest a topic for the future, or just to
Speaker 4: say hello, you can email us at contact at stuff
Speaker 4: to Blow your Mind dot com.
Speaker 3: Stuff to Blow Your Mind is production of iHeartRadio. For
Speaker 3: more podcasts from my heart Radio, visit the iHeartRadio app,
Speaker 3: Apple Podcasts, or wherever you listen to your favorite shows
Speaker 1: At the U