Ceratogyrus

[Craig Mackay]

Hi, got a few questions regarding Ceratogyrus spp.

I was just wondering what the role of the horn in Ceratogyrus spp. was and how/why something that unique has evolved. Is it some kind of defence against predators from above or a way to increase the internal surface area for some reason or something completely different? Can it cause any problems during moulting? If anyone has any papers on the subject I'd be interested to read them.

Cheers
Craig

[Grant Brown]

I've heard some keepers mention it could be some sort of food store for lean times, but I am not really that familiar with their ecology to agree with this idea or not.

Not sure if they read this from the literature or not, or whether this is based upon speculation.

[Edit] Here's a reference to do with the horn as a site for muscle attachment.

West, R.C. (1986). Ceratogyrus. Journal of the British Tarantula Society 1(4):79-80

There is a small reference on the wiki article for Ceratogyrus, but the literature seems a little sparse.

http://en.wikipedia.org/wiki/Ceratogyrus

[Phil Rea]

Ceratogyrus spp. aren't the only theraphosids posessing a foveal horn, so it's not unique

It seems to have convergently evolved in the New World too. Sphaerobothria hoffmanni, which is from Costa Rica also has a foveal protrusion

[vincent winyard]

hi phil i just read your reply to craig's letter but couldn't help notice how many t's you have 180 of them must be a full time job. how on earth do you manage to look after so many of them.

[Grant Brown]

Ceratogyrus spp. aren't the only theraphosids posessing a foveal horn, so it's not unique

It seems to have convergently evolved in the New World too. Sphaerobothria hoffmanni, which is from Costa Rica also has a foveal protrusion

Is the foveal horn thought to function in a similar manner in Sphaerobothria? Might be interesting to compare the ecology of the two species and see if they encounter similar hardships, if the "food store" idea is to be believed.

Again, I'm not too familiar with the species, but it's something I did ponder...(perhaps time for some new hobbies...)

[Phil Rea]

hi phil i just read your reply to craig's letter but couldn't help notice how many t's you have 180 of them must be a full time job. how on earth do you manage to look after so many of them.

You get into more or less a set routine with them vincent

Is the foveal horn thought to function in a similar manner in Sphaerobothria? Might be interesting to compare the ecology of the two species and see if they encounter similar hardships, if the "food store" idea is to be believed.

Again, I'm not too familiar with the species, but it's something I did ponder...(perhaps time for some new hobbies...)

I have no idea about function Grant. I've not really thought about it to be honest.

[Craig Mackay]

Ceratogyrus spp. aren't the only theraphosids posessing a foveal horn, so it's not unique

It seems to have convergently evolved in the New World too. Sphaerobothria hoffmanni, which is from Costa Rica also has a foveal protrusion

Interesting Phil, I didn't know that. Thanks. Foveal horns must certainly be efficient at fulfilling their purpose if they can evolve independently elsewhere to.

I've heard some keepers mention it could be some sort of food store for lean times, but I am not really that familiar with their ecology to agree with this idea or not.

Not sure if they read this from the literature or not, or whether this is based upon speculation.

[Edit] Here's a reference to do with the horn as a site for muscle attachment.

West, R.C. (1986). Ceratogyrus. Journal of the British Tarantula Society

There is a small reference on the wiki article for Ceratogyrus, but the literature seems a little sparse.

http://en.wikipedia.org/wiki/Ceratogyrus

Thanks Grant. Did a search for the paper but can't find it. Just snippets of info from it. Does anyone have this paper? Seems it serves two functions: Increased surface area for the dorsal dilator muscle to attach to allow food to be consumed faster. Do the members of this genus have a lot of predators in the wild that would require them to finish their food quickly? And also as a nutrient store, possibly, to survive droughts. Kinda like a camels hump I guess. Although, the species that Phil mentions lives in humid mountain forests so that contradicts that theory.

[James Box]

i've not read that BTS journal (1986! wow!) about Ceratogyrus, but maybe this quote from Sam Marshall in "Tarantulas and Other Arachnids" (published by Barron's, 2001 pp 47-4 is referring to it.
"No one has any concrete ideas as to what possible function this horn has. This horn sits on top of the...the fovea. When you examine the carapace of the shed skin of any tarantula, you can see that what is a dimple on the outside is the base of a point of cuticle that projects into the cephalothorax on the inside. This point is called an apodeme and is the muscle attachment site for the stomach muscles under the carapace. In the African horned tarantulas the internal apodeme is not a point but a ring-shaped projection. So, the appearance of a horn on the carapace may, in fact, relate to internal anatomy. In other words, maybe the African horned tarantulas did not need a horn, but the horn was a by-product of the evolution of the ring. Stranger things have happened in nature!"

of course, maybe the ring structure evolved to support the horn, not the other way around, so who knows
that theory may have been disproved by now for all i know, but it's the only attempt at an explanation i've had the chance to read... not sure that food shortage theory works, i doubt they have much more difficulty getting food than other African species, or other arid species the world over... also, they'd need some kind of way to get the food in and out of the storage area, ie tubes or pipes or extra muscles to move food around, so i'd assume such a thing would be fairly well known and documented, by now...

[Richard Gallon]

To be honest I don't know what the 'horn' in Ceratogyrus is for. But will make some observations which may discount some theories:

In the wild Ceratogyrus darlingi can be found side-by-side with other more successful (in terms of numbers of individuals) theraphosids (without 'horns') within mixed colonies. So it doesn't automatically appear to confer any advantage over non-horned theraphosids.

If it was a storage system for hard times you'd expect the Ceratogyrus species with the largest 'horns' to come from the driest, more-hostile habitats. However, C. sanderi is the species from the driest habitats (desert areas) and that doesn't have a particularly large 'horn'.

Personally I'm not convinced by any of the theories banded about. Remember there doesn't necessarily have to be a reason for its evolved presence. So long as its presence doesn't reduce biological 'fitness' it can exist without an obvious function.

Given that the 'horn's' expression between species (and within them) is extremely variable, it could imply that it doesn't confer any great evolutionary advantage.

Richard

PS. some Acanthoscurria and Crytopholis also have foveal protuberances.

[Craig Mackay]

Thanks for all the replies everyone. Some interesting observations Richard. I look forward to any further research being done.

[Nick L]

I recall having seen the diagram that accompanied West’s BTS article and was struck by how the muscle attachment matched that on the inner surface of the fovea in a “normal” T – the fovea had just been everted. Such a morphological inversion could be achieved by a relatively simple mutation, and probably still be fully functional. If this becomes fixed in a population you’ve then got your ancestral horned baboon with a fully developed horn for natural selection to then act upon ‘refining’ its size and shape in the species that evolved from it. It’s difficult to see that it evolved in any other way – a flat carapace without a fovea would offer relatively weak muscle attachment; any such intermediate would be at a disadvantage.

I’m being lazy and can’t be bothered to look it up but has anybody proposed a phylogeny for Ceratogyrus? Biomechanically a medium to small sized horn would probably be the most efficient so I would predict that larger horned species are probably ancestral to smaller horned species – is there any reality to this?

The occurrence of foveal protuberances in other unrelated genera would also seem to support this idea.

Other explanations seem unconvincing to me. Shoot me down…

[Craig Mackay]

I recall having seen the diagram that accompanied West’s BTS article and was struck by how the muscle attachment matched that on the inner surface of the fovea in a “normal” T – the fovea had just been everted. Such a morphological inversion could be achieved by a relatively simple mutation, and probably still be fully functional. If this becomes fixed in a population you’ve then got your ancestral horned baboon with a fully developed horn for natural selection to then act upon ‘refining’ its size and shape in the species that evolved from it. It’s difficult to see that it evolved in any other way – a flat carapace without a fovea would offer relatively weak muscle attachment; any such intermediate would be at a disadvantage.

I’m being lazy and can’t be bothered to look it up but has anybody proposed a phylogeny for Ceratogyrus? Biomechanically a medium to small sized horn would probably be the most efficient so I would predict that larger horned species are probably ancestral to smaller horned species – is there any reality to this?

The occurrence of foveal protuberances in other unrelated genera would also seem to support this idea.

Other explanations seem unconvincing to me. Shoot me down…

Makes sense, I would imagine a foveal horn that was too large would cause difficulties in flipping over onto it's back to moult.

[Jevon Nie]

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[Craig Mackay]

www.flickr.com/photos/craigmackay/sets
great link. thanks a lot for sharing but still is something wrong. Can you tell me that how to download all
these images at one step. Its seems to be cumbersome.
Just need help, plz

That's random. Well, these are my photos so you shouldn't really be downloading them anyway without permission.

[Damian Storer]

That's random. Well, these are my photo's so you shouldn't really be downloading them anyway without permission.

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