Sexual Cannibalism and the Genetics of the Redback Spider

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The video explores the fascinating and deadly mating ritual of the redback spider, a relative of the black widow, where males perform a deliberate somersault to place themselves directly under the female's fangs during copulation. This act of sexual cannibalism is often interpreted as an evolutionary strategy to prolong mating time; by positioning himself for death, the male ensures he remains attached to the female long enough to potentially father more offspring before being consumed. Interestingly, some redback males have evolved a specific abdominal narrowing that makes it slightly harder for the female to bite them immediately, allowing them to survive long enough to attempt a second mating event with the same partner before eventually succumbing to their fate. The study delves deeper into the genetic mechanisms behind these behaviors by examining hybrids created between redback spiders and the katipo spider, a related species that does not exhibit cannibalistic tendencies or the sacrificial somersault. Researchers discovered that despite the complex nature of the somersault maneuver, which involves coordinated leg movements triggered by visual stimuli, it is controlled by a single gene region on the X chromosome. This finding was surprising because such a sophisticated neural behavior was expected to require multiple genes, whereas the physiological change of abdominal narrowing proved to be genetically more complex and did not follow a simple inheritance pattern. Furthermore, the study revealed that these two traits are genetically separate, as some hybrid males inherited only one characteristic while lacking the other. These genetic findings challenge traditional definitions of species and highlight the fluidity of evolution in action. Since redback males will still perform their sacrificial somersault even when mating with katipo females who do not eat them, the behavior is clearly hardwired genetically rather than being a learned response to immediate threat. The fact that these two distinct-looking groups can interbreed and produce viable offspring suggests they may actually be one species with different phenotypes rather than two completely separate species. The divergence between the cannibalistic redbacks and the non-cannibalistic katipos likely occurred relatively recently, around 100,000 years ago, possibly triggered when katipo females stopped eating their mates, rendering the male's sacrifice unnecessary for that lineage while remaining a fixed genetic trait in the redback population.

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Um, hey, you know how it is. You're a male redback spider, relative of the black widow. >> [snorts] >> Oh yeah. >> You uh you see uh a nice sexy female redback and you're like, "That's >> Awooga. >> That's the mother of my future baby spiders, my little spiderlings." And I'm like, "I know what I'm going to do. I'm going to whoop somersault directly so that my abdomen is underneath that female's fangs while we're mating." >> What? >> Which means it'll be all the easier for her to eat me when we're done. >> Oh my gosh. Why would you do that? >> Yeah. >> [laughter] >> Well, uh there's there's theories that um this will allow them to mate longer for some reason. I guess cuz like while she's eating, I don't know. But it allows them to mate a little bit longer and therefore father more offspring. So even if they get eaten, of course that means they were evolutionarily successful because they have fathered the most offspring. >> That's right. You can go into the void knowing that if you fathered offspring biologically, you are successful. >> Yeah, exactly. Now, uh we've known about this. We've discussed it before. Um, but this particular study is not about the somersault into death >> It's not? >> that the redbacks perform at the at the the fangs of their lover. But it is when an when a a male decides to do it or not do it when he is hybridized with another spider and what that might mean for genetics and evolution. Okay. So, let me set the scene. So, aside from this behavior that exists, some redback spiders who do this behavior have also exhibited abdominal narrowing over evolutionary time, which appears to make it more difficult for the female to bite them during mating, which doesn't keep them from getting eaten. >> Oh, but it it surely helps. >> It delays death long enough for the male to do a second mating event. >> How good is the female How How good is the female redback spider's vision close-up? Are they like cats where they can't see in front of their nose? >> Probably not cuz they have eight eyes, but yeah. Um but yes, so they they are they somehow are able to do a second mating event with that female before they get eaten because they have this abdominal narrowing. >> [snorts] >> Um now, this is all in New Zealand. A relative of the redback, the katipo spider, does not do the self-sacrifice. The females do not eat the males. They have no cannibalistic mating behavior. But, these two species can mate and produce hybrids in the wild. >> What does that do? >> Yes, exactly. Um and this is only possible with katipo females and redback males since the redback females will just eat the males. They won't mate with them at all. >> Okay. >> not. So, um >> So, it has to be the katipo female and the redback >> eat the male. Yes. So, this is these are the only successful pairings that allow for hybrids. >> What are the genetics of these spiders and how do What What forward, Blair? >> Yes. Yes. What's crazy is those male redback spiders, of course, do the somersault And and they get into the position of like, go ahead, eat me. Even with the female caught up females. >> the male behavior. >> Yes. And so that means that the somersaults and the cannibalistic behavior have genetic basis because they're doing it regardless of the the the the new stimuli of a different female. They're just like, this is the only way to be. >> Does the caught up female Did you say the caught up female eats the males, too, or no? >> No, they do not. So that's why this is so crazy that they do that. So they think that like, all right, we think this is genetic origins. We do not think this is behavioral. We think this is a genetic thing. Okay. So, um they in previous studies, there have been um kind of genetic investigations on on mating traits. And usually there's a small number of genes or linked gene clusters. And when you do hybrid studies, you can kind of see how those genes get selected or not and what influence they have on their behavior in mating. So, they found that first generation hybrid males did not somersault. So it's most likely a recessive trait. So then, in order to figure out the genetic basis for these behaviors, they did a backcrossed hybrid. So they did females with males from either uh hybrid females with males from either species. Then they filmed mating trials with 104 male offspring to determine whether each male did a somersault, developed abdominal narrowing, or both because they also assumed that the abdominal narrowing was related because it was a mating tactic. So they were like, these have to be similar genes or similar gene loci or like something related. According to their analysis, the mating somersault is consistent with inheritance from a single gene region on the X chromosome. This is way more simple of an explanation than they thought because a somersault is a complex coordinated behavior. It's not just like >> process. It's not It's >> Yeah. It's not like the abdominal narrowing where it's just like this is a physiological change. Like no, this is a hardwiring of an entire behavior. >> Okay, so we have genes that have led to certain reflexes, right? So we have genetic basis for innate reflexes. And so, you know, things like hit hitting your knee with the hammer to see whether you have that reflex. Your tendon has that reflex there in the tendon. It's not even really a nervous thing, right? It's It's within the tendon. >> Right, but this is so crazy cuz this is like visual stimuli. There's a female. Or it could be, you know, it could be hormonal, who knows, but there's some sort of stimuli related to the female. >> to do a flying through the air maneuver that puts me on my back under her fangs. >> Yes. Yes. Yes. They also found contrary to what they expected that the somersault and the abdominal narrowing not inherited together. They were all over the map. About half of the backcross males had only one of the two traits. The traits are not are in fact genetically separate. Um >> [sighs] >> and unlike the somersault behavior, abdominal narrowing did not fit a simple X-linked or single gene pattern. It's much more complex, which you would expect the exact opposite. Like this is just a phenotypic change. This should be simple. But that's the complex one. The somersault is the is the simple one. >> Yeah. The somersault and it's not simple. >> Yeah. >> That's a multi-step process to make the legs move in a certain way to create this movement. I mean, that is a complex neural behavior behavior. How How does one gene And spiders do not have a large brain, right? They've got a little tiny ganglia, right? >> they just have like a ball of nerve cells, yeah. >> Ball of nerve cells. So, there's one gene that's going jump >> Right. >> across all the nerve cells because of a certain visual stimulus. >> Which So, that is contrary to expectation based on what we're talking about. However, the fact that these two species are close enough related, have similar enough genomes that they can hybridize, and have offspring, they're true hybrids. >> But >> That means that evolutionarily, they are very closely related, and they do not share these behaviors. So, it had to happen fast and simply in a short number of mutations. >> Yep. >> They think that they only diverge about 100,000 years ago. So, evolutionarily speaking, that's pretty recent. >> So, are they still found in over like overlapping regions? Could this even go further to create a uh a hybridized subpopulation that exists between two separate populations? So, are they >> weird. >> So, this is cuz they're not sexually separated, which is the definition of a species, right? >> Right. So, this is where we get into the messy "Are these actually two distinct species, or is this one species with different phenotypes?" >> Yes. Yeah, that I mean >> And behavioral mating strategies. Like >> Yes. >> It's ah >> I mean, the fact that they are together that they're together often enough and mating and creating hybrids often enough that researchers noticed it suggests that there's a real blending happening and they are not sexually that they are not behaviorally separated. >> Yeah. >> That males and females are still going yep, you work. So >> and I so this is also what's crazy is that >> Wow, this is so fascinating. [laughter] Gosh. >> Okay, so X chromosomes are thought to evolve faster because of their smaller effective population size in this particular, you know, like spider. But they for some reason and I don't understand why I'd have to look into this further. They think that the sacri- sacrificial phenotype was lost. That that's the base behavior whether or not there's a somersault involved, but the sacrifice is the base behavior that then was evolved out in the in the Katipo spiders. >> Because the female stopped eating them. The female >> Right. >> The fe- if the female's not going to eat the male, why does the male need to sacrifice anymore? >> Exactly, yes. And so that appears to be what happened 100,000 years ago is that the female Katipo spider is stopped eating its mates. >> So maybe there always was that and then you know how we talk about beta males, but there's beta females. What about just different behavioral traits that they just got to be big enough populations that we're like yeah, they're not the same. But you have a you have a Katipo male who's like I'll flip for you. >> Yeah. I mean, what is what is a species? We talk about this enough. Like I will bring it up one more time. Like what is a species? >> questions. >> It's it's complicated and there's no clear answer. And in this case, if you can have hybridization and you can back hybridize and you can have um sexually viable offspring over and over and over through hybridization processes, then you most likely have one species. So, honestly, that's the part of the study that I find the most interesting um is that I I am stuck to wonder. Like, is this is this really two distinct species or is it two distinct genotypes within one species that results in phenotypic and behavioral differences? But, I don't know. I don't know. >> I love this. I love that somebody did this. I want to know more about these. I mean, this is just these questions get at the basis of what drives speciation, what that question, what is a species? What are we talking about? >> Yep. >> [laughter] >> That's right, guys. Make sure you keep that somersault trait. Might keep you from being eaten. >> That's right. Wait. No, it'll facilitate you being eaten, actually. >> Oh, yeah. >> Yeah. Then you Yeah, we're Well, we won't go any further. >> No.