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