08:09 13-01-2012

Jan

The "mowers" in the picture from the article seem to show unattached hind legs quite well.

01:26 13-01-2012

Drás Researcher

Is it just me or does Bogoria look like Sweden/Norway and Tendaguru looks like Alaska?

16:27 08-01-2012

SN

Evan, I forgot: you indeed cannot see well how hexapod limbs are attached in any of the published paintings. Still, it's there.

16:20 08-01-2012

SN

About sight and Barlowe's expedition: I reread my copy (first printing 1990), and it said that there was a 'planet-wise lack of true eyes'. Also: 'optical sensory organs are absent, ...supplanted... by a battery of sonar and infrared faculties.' There is also a lateral line system of pressure receptors and numerous tiny infrared receptor pits. Furthermore: '...biolights, heat-radiating bioluminous spots that appear quite vivid to infrared sensors.' As for the likelihood that sighted animals would overcome the blind ones quickly, the text states that all these senses evolved to become so good that the sighted ones did not have an advantage in 'Darwin IV's thick primordial mists. Now the mists are gone- but so are the optically-sighted animals'. I am not convinced, and wonder whether eyes might still outperform infrared senses in a thick mist.

Leg suspension: I do not think the causes and consequences of how a limb skeleton is attached to the body skeleton are well known. In fact, I could not find any papers on the matter, which is why I turned to McNeill Alexander. I thought that suspension by way of muscles would be costly in energetic terms, and the only gain I saw would be that the system would act as a shock absorber. If so, you do not want propulsion forces to be absorbed as well, so you would not expect to see muscle suspension in limbs with a primary propulsive purpose. Then again, perhaps the muscle suspension of mammal front legs is simply part of the body plan.
I assumed that shock absorption might be a good thing, so I kept that. The next choice was which limbs should be so equipped. I doubt that there is a imperative need to equip hind limbs with more propulsive force than other pairs; but again some sort of papers are hard to find. Cars seem to do work well with regardless of whether the propulsion comes from hind or from front wheels.

15:04 06-01-2012

Jan

There is a notice on "Walking with..." that the horse would fall on its face during bounding. This could be the explanation for hexapod run - it would be essentialy like gallop-bounding with six legs. Maybe an animation of antelope-like hexapod in run would help.
For less agile or arboreal animals (like marshwallow) six legs is probably better solution than four, imho. Lizzards seems to cry for six legs with every step.

Btw, there is also said that no large furahan animals walk like kangaroo. It is ok, but the efficiency of kangaroo walk call for the attention of speculative biology. There seems to be two big problems with it, moving during relative rest (the kangaroo way is, well, odd) and the ability of the joints and tendons to withstand the strain of hopping. http://en.wikipedia.org/wiki/Macropodidae Some ideas?

22:58 05-01-2012

Anthony Docimo

Jan - I'm not sure I understand you: how is the gyrosprinter like a cheetah? (also, wasn't the gyrosprinter a monopod?)

Evan - given that we're looking at Furahan fauna with their armor/fur/other on, it may be obscured to our eyes.

14:53 04-01-2012

Evan Black

I for one would be interested in seeing more about this subject, because 1) I don't fully understand the biomechanical ramifications of freely suspended limbs and 2) I have a hard time seeing the visual evidence for how furahan fauna have a different limb attachment than terrestrial life. Are there any existing blog posts that can help my ignorance?

09:51 04-01-2012

Jan

Yes, but look at the cheetah. It is more like gyrosprinter from Darwin IV, another type of biped. Maybe this is the reason for the movements of Avatar beasts, although their solution was quite uninventive.

18:09 03-01-2012

SN

Jan, as regards the 'necessity' of freely suspended pairs of limbs, even that 'necessity' is guesswork. Have a look at many quadruped dinosaurs, such as ceratopsians. Their front legs are connected to the sternum/ribcage and hence to the vertebral column. Could they run fast? many people think so...

As for how fast you can run with four or six legs is a matter of dispute. For very fast gaits there are phases in which no leg at all touches the ground, so these phases represent jumps. Increase the number of legs, and it will become more difficult to design gaits in which such jumps occur. One gait always remains, and that is to kick off with all legs at once. But that does not seem exactly useful for a millipede.
It is true that there is a tendency for animals to use fewer legs when running fast. Cockroaches and crabs come to mind: in both cases some run with two legs only, even though they have more. In larger animals this behavioural adaptation does not seem to play a large role. There are morphological adaptations in that some animals run on two legs, but it does not seem a universal trend. Gerbils, hares and kangaroos seem to lie on different points of a hopping trend. The absence of tetrapodal running species evolving towards bipedalism again suggests that the trend is weak. I thought it weak enough to mean that a need for speed does not readily result in the loss of a pair of limbs in hexapods.

17:21 03-01-2012

Jan

SN: I do not know the reason for the unattached front legs either. Flying vertebrates have very strong shoulder girdle for obvious reason, while fast runners lack the clavicle to allow greater mobility. But still, there is something upsetting about this design.
My original question about functional bipedalism or quadrupedalism is aimed at those hexapods that use all six legs and still seem to be fast runners. I like it. But is it realistic? Kangaroos, cheetah, or ostriches use very different ways to achieve high speeds, but furahan animals seem to be completely different. Even insects seems to be chosing bipedalism fot top speeds http://en.wikipedia.org/wiki/Facultative_biped
This is not mean to criticize the jingos design. Furaha without hexapods would not be Furaha.

20:20 02-01-2012

SN

And some more (the message was too long)

William: Evan made a good point, I think. I also think it is very unlikely for animals to evolve without sight. It is just so useful and so very effective. Hearing is good as well, but if something does not produce sound it cannot be heard... As for 'radar'/'sonar', these are clearly excellent when there is little light, as holds for bats and whales, but you have to give yourself way by sending out an impulse. One Earth eyes developed not once but many times, in a staggering variety of designs. You may counter that by saying that all these animals came equipped with the same basic genetic setup for vision, and you would be right. But all you need on another world are two things: light, and a photosensitive pigment that responds to light. Once you've got a nervous system to do something with the resulting system, I would think you would have instant runaway evolution of vision. Any animals lagging behind will be eaten. Quickly. I could do a blog post on it, but am waiting for a few new books on animal vision to come out.

Mexanik and everyone else: a happy new year to you too!

Anthony: your suggestion of the bioluminescent spots on Darwin IV functioning as heat sources is very inventive. Then again, on Earth the really wonderful thing about bioluminescence is that it produces light almost without heat, in contrast to most man-made sources of light. If you want to shine in the infra-red, you do not need to send out visible light as well. I have not read Barlowe's book for several ears; is there really no mention of any animal using vision? I guess not, as otherwise you would have the sighted animals outcompeting the rest very quickly (particularly if you give the new ones toes; that Barlowe did not give his animals toe analogues is a pet peeve of mine).

20:18 02-01-2012

SN

Happy new year everyone. I was too busy to get back here recently, but I'm back now. I will have to go back to Zerraspace's question of the 18th. Oops.

Zerraspace: I like your idea of rings in tentacles that will touch one another when subjected to pressure. In this way you could still have a tentacle that would be able to elongate, but would be able to withstand compression. It is a bit like one of my Mark X tentacles, but with a difference: in my case the noncompressible bits always touched one another, so the tentacle could no longer change its length. The only part of your scheme that worries me a bit is how you fill in the volume between the rings when they come apart. Whatever you use, it needs to move aside when the rings come together again.

As for rusps, I see them as omnivores, with the larger ones mostly herbivoral. Their metabolism isn't that high, so they do not need vast volumes of food as mammals do. They are not very good at ambushing their prey: the larger ones simply are too large too sneak up on anything, so whatever meat they take is likely to be in the way of carrion. As for smaller ones, well, your brother might be on to something. I can see some of them stinging or snaring prey, albeit of the fairly small and cuddly variety.

Jan, Anthony, Evan: in many hexapods the middle pair indeed is the one that carries the largest weight. I am aware that in mammals the hind legs provide most propulsive force, so I am departing from that idea. My reason for doing so was based on limb anatomy: as you know, mammalian hind legs are attached to the vertebral column through bones, providing a neat way to transfer forces. I always wondered why this was no so for front legs, suspended by limbs and muscles. For large animals such as elephants this seemed wasteful. I wondered whether there was a reason for this arrangement, such as a need to absorb shocks. I asked McNeill Alexander, but he thought it might simply be an expression of the mammal Bauplan. With little evidence to go on, I decided to design large hexapods to have the middle legs attached to their vertebral column analogues and to have both the front and hind pairs suspended. Evan is quite right about the predatory hexapods though: they are functional tetrapods. And tetrapterates walk on two legs, when not flying...

19:56 31-12-2011

Anthony

Mr. Stephens - the visible biolights of Darwin IV could be a side effect, with the primary purpose of the biolights might be to provide bright spots of heat, or to disrupt an otherwise smooth outline to any sonar-using hunters nearby.

11:02 31-12-2011

Mexanik

Happy New Year! =)

08:42 31-12-2011

Evan Black

William, I know you asked Sigmund this question, but I think I'll put my two cents in any way...

I've always reckoned the possibility of widespread sightless life to be rather low. There will always be the plausibility of there being INSTANCES of sightless life forms (even Earth has complex life forms without eyes) but the benefit of a visual sense combined with the relative ease of its evolution point toward a strong likelihood of animals-- generally speaking-- having sight.

The argument is always made (and I've pondered the position myself on several occasions) of some other sense being more useful than sight and supplanting it as the primary sense for most life on a planet. What I always run into with this line of thought is paradox after paradox. For example, if it's so dark that light is a scarce sensory signal then the planet itself may be too cold to develop complex life; even single-celled organisms take advantage of a sliver of light. Or consider the possibility of sonar/echolocation dominating: why would life forms opt for the more neurologically complex senses when a simpler, more effective sense can be evolved.

Another strong argument is that of a conceptual Europa: a water world that is trapped between a warming inner core and an outer shell blocking out all sunlight. Why would life evolve where there is no sunlight to use for vision? Well, light is a particular wavelength of electromagnetic radiation, a form of radiation that is also responsible for heat radiation. Life forms on Europa and its kin would do well to utilize that infra-red radiation in a form of vision, IMO.

Any way, like I said that's just my two cents. I'd be interested in hearing what anyone else has to add.

Berichten: 226 t/m 240 van de 930.
Aantal pagina's: 62
Nieuwer13 14 15 [16] 17 18 19Ouder

Free guestbook of MyGB.nl