Purpose of Ceratogyrus horns?

[Toxoderidae]

I recently got a C. darlingi, and even as a young sling I can see the horn. My only question now is, why the horn, and why does it only really grow in females? I can only see this thing as a hinder for survival, yet they have it.

 

[Spidermolt]

I have seen many theories over this but overall nobody truly knows why so this is sadly one of those questions that will go on for a very long time.

 

[AphonopelmaTX]

Lets first examine what the horn is before we consider it's purpose or lack thereof. Within the horn of Ceratogyrus species, at least those that have it, the dorso dilator muscle (the muscle responsible for the dilation of the pumping stomach) is attached and also contains an extension of the midgut diverticula (the part of the digestive tract that stores digested food). This contrasts with every other tarantula taxa that doesn't have the horn where the dorso dilator muscle is attached to the apodeme with no space for the midgut diverticula. Given this information, it would make sense to come to the conclusion that the horn gives Ceratogyrus species an advantage in the harsh environment they come from as they are equipped to take in larger amounts of food and water quicker. However, this theory breaks down when you consider the other tarantula taxa that live in the exact same harsh environment and have survived just fine without it along side Ceratogyrus species. I will point out again that not all Ceratogyrus species have the horn.

I'm in the camp that thinks the horn in the Ceratogyrus species that have them are vestigial. In other words, it has no evolutionary advantage and doesn't really have a purpose at all. A fluke of evolution if you will. An interesting fluke at that when you consider Sphaerobotheria hoffmanni also has a horn a world away in Costa Rica. In this case, I believe there was an independent development of the same character at the same time, but didn't serve any real advantage to the organism and died out.

 

[viper69]

Lets first examine what the horn is before we consider it's purpose or lack thereof. Within the horn of Ceratogyrus species, at least those that have it, the dorso dilator muscle (the muscle responsible for the dilation of the pumping stomach) is attached and also contains an extension of the midgut diverticula (the part of the digestive tract that stores digested food). This contrasts with every other tarantula taxa that doesn't have the horn where the dorso dilator muscle is attached to the apodeme with no space for the midgut diverticula. Given this information, it would make sense to come to the conclusion that the horn gives Ceratogyrus species an advantage in the harsh environment they come from as they are equipped to take in larger amounts of food and water quicker. However, this theory breaks down when you consider the other tarantula taxa that live in the exact same harsh environment and have survived just fine without it along side Ceratogyrus species. I will point out again that not all Ceratogyrus species have the horn.

I'm in the camp that thinks the horn in the Ceratogyrus species that have them are vestigial. In other words, it has no evolutionary advantage and doesn't really have a purpose at all. A fluke of evolution if you will. An interesting fluke at that when you consider Sphaerobotheria hoffmanni also has a horn a world away in Costa Rica. In this case, I believe there was an independent development of the same character at the same time, but didn't serve any real advantage to the organism and died out.

Very true, what I find even more interesting is different positions the horns take up, facing rostral, caudal and fairly straight up in the case of C. marshalli.

 

[bryverine]

I think the horn is used as a vortex generator. As they move extremely fast, it helps disturb laminar air flow that would produce drag. Like a golf ball or those little things on airplane wings.
This gives them that extra speed they need to hide in their burrow super fast!

 

[mistertim]

You guys are so dumb. The obvious answer is that the horn is the source of their magic.

 

[Poec54]

I think the horn is used as a vortex generator. As they move extremely fast, it helps disturb laminar air flow that would produce drag. Like a golf ball or those little things on airplane wings.
This gives them that extra speed they need to hide in their burrow super fast!

I don't think they move fast enough for that to take effect. Protruding horns would seem to be a hindrance in tight retreats and other cramped spaces. For them to maintain them, especially the extreme height of marshalli's, it would seem there's a benefit associated with them, or they would have lost them long ago.

Darlingi MM's have elevated horns, marshalli (ironically) don't. Since MM's are focused on finding females, and not eating, if horns increased efficiency in eating, then it's not surprising that some males don't have them. Instead of being an obsolete feature on it's way out, maybe it's the opposite and horns are slowly becoming more common and larger in Ceratogyrus.

Why don't Pterinochilus and Harpactira have horns? That alone isn't proof that they aren't useful in some manner to Ceratogyrus. Why do some snakes have rattles, or make hoods, or have horns over their eyes? There's physical characteristics that some genera or species have evolved, that they would seem to be able to get by without. Maybe we just haven't figured out the benefits to those features yet.

 

[AphonopelmaTX]

Very true, what I find even more interesting is different positions the horns take up, facing rostral, caudal and fairly straight up in the case of C. marshalli.

It is very interesting indeed. Especially Ceratogyrus dolichocephalus where the protuberance is an extension of the caput (cephalic region).

 

[AphonopelmaTX]

Protruding horns would seem to be a hindrance in tight retreats and other cramped spaces. For them to maintain them, especially the extreme height of marshalli's, it would seem there's a benefit associated with them, or they would have lost them long ago.

I would disagree. From what I can tell, even the tallest foveal protuberance of C. marshalli doesn't extend past the femoral leg segment which wouldn't make it a hindrance in tight spaces like their burrows. If the spider can climb the length of a burrow or rest at the entrance with the legs folded upward, then the horn will fit too. Besides, you're thinking in black and white terms when it comes to evolution. A trait can persist if there is a benefit or no benefit at all. It will disappear, by way of an organism dying out, if it is a detriment to survival. Generally speaking anyway. By your words, it sounds like you're saying a trait must have a benefit to the survival of an organism or the trait will be eventually lost.

 

[jiacovazzi]

I would disagree. From what I can tell, even the tallest foveal protuberance of C. marshalli doesn't extend past the femoral leg segment which wouldn't make it a hindrance in tight spaces like their burrows. If the spider can climb the length of a burrow or rest at the entrance with the legs folded upward, then the horn will fit too. Besides, you're thinking in black and white terms when it comes to evolution. A trait can persist if there is a benefit or no benefit at all. It will disappear, by way of an organism dying out, if it is a detriment to survival. Generally speaking anyway. By your words, it sounds like you're saying a trait must have a benefit to the survival of an organism or the trait will be eventually lost.

I agree, I have a large C.marshalli female and her horn doesn't raise higher than the femoral leg segment.

@Poec54 , Evolution is simply not black and white, its a mix of genetic mutations and adaptations that can't be explained through vague speculations.

 

[Envoirment]

I would disagree. From what I can tell, even the tallest foveal protuberance of C. marshalli doesn't extend past the femoral leg segment which wouldn't make it a hindrance in tight spaces like their burrows. If the spider can climb the length of a burrow or rest at the entrance with the legs folded upward, then the horn will fit too. Besides, you're thinking in black and white terms when it comes to evolution. A trait can persist if there is a benefit or no benefit at all. It will disappear, by way of an organism dying out, if it is a detriment to survival. Generally speaking anyway. By your words, it sounds like you're saying a trait must have a benefit to the survival of an organism or the trait will be eventually lost.

It would be quite cool to look into the genes that code for horn formation. You could then look into whether the horns are under positive, negative or neutral selection. I would assume the reason why the horns differ among Ceratogyrus is due to different selection pressures over the course of their evolution.

 

[bryverine]

I don't think they move fast enough for that to take effect.

I think it must have an affect, just not a noticeable one.
I certainly don't actually think that's why it's there seeing as they probably don't run long distances.

 

[Chris LXXIX]

The winner IMO is C.sanderi. They have a sort of "plug" instead of that horn. Who needs these days a "horn", seriously? Surpassed, since the 'Mad Max' movie serie jumped in. Ask Mother Nature one day why they have "the horn" lol.

<3

 

[bbolivier12]

I recently got a C. darlingi, and even as a young sling I can see the horn. My only question now is, why the horn, and why does it only really grow in females? I can only see this thing as a hinder for survival, yet they have it.

Its a spoiler, like for race because of how digustingly fast this specie is.