Three legged aliens?

[bombus]

Are there any scenarios that would favour the evolution of three legged aliens?

Are there any scenarios that could favour three (or more) sexes involved in reproduction?

 

These are two things that I have read in sci-fi that I don't think are likely...

[Martin]

Are there any scenarios that would favour the evolution of three legged aliens?

Are there any scenarios that could favour three (or more) sexes involved in reproduction?

 

These are two things that I have read in sci-fi that I don't think are likely...

 

 

Have you read Arthur Clark Rendezvous with Rama

and Larry Niven Ringworld?

 

those books are by smart guys esp Clark

 

even on Earth with mostly bilateral symmetry

we have evolved some 5-fold symmetry animals like starfish

 

so why not? if 3-fold sym gets a good start and some accident favors it, then it might win the evolution race

 

there are clear advantages----you can have stereo depth vision and still know what is behind you-----------360 degree coverage

 

you can scratch your own back

 

it is easier to learn to walk because you don't fall over so much

 

you can eat and shave at the same time

 

it is easier to fix things around the house because one hand holds the soldering iron or screwdriver and the other two hands hold the parts together

 

obviously if trilateral ever got going it would work fine

 

===============

 

about 3+ sexes it is not so clear

there are some Earth organisms that have evolved numerous sexes, some molds have 8 or 9 sexes

 

with mold sex the only rule is you cant mate with your OWN sex, but you have 7 or 8 other sexes that you can have fun with.

 

it is not clear why they like it that way, or why it evolved.

it could be that molds are very sedentary and don't get around so much so they had to evolve some form of entertainment to spice up their life

 

sex is a mystery---people THINK they know why it evolved but it is still possible that the scientific consensus will be overturned and somebody will come up with better explanations

 

I think mitochondria are at the basis-------sex is definitely a EUcaryotic thing, it is something PROcaryots dont get to enjoy.

and the female is defined as the one whose mitochondria win----transfer to the offspring

 

the basic difference between an egg and a sperm is that the eggs mitochondria are going to win and the sperms mitochondria will selfdestruct.

 

Logically they are otherwise the same.

 

so the molds just have a more complex logical rule about whose mitochondria survive. A beats B, and B beats C,.....

 

probably if some aliens had 8 or 9 different sexes they would make really great pornographic movies and their soap operas would be totally bizarre. Families would be extremely complicated. Tolstoy would never have finished writing War and Peace.

[thedarkshade]

there are some Earth organisms that have evolved numerous sexes, some molds have 8 or 9 sexes

But the fertilization is basically the same for all, right????

I mean spermatozoon + ovray = "incoming organism"

[Paralith]

But the fertilization is basically the same for all, right????

I mean spermatozoon + ovray = "incoming organism"

 

I'm assuming you mean sperm and egg, cuz the ovary is the organ that makes the eggs. Anyway, it's more like gamete A + gamete B or C or D or E or F = incoming organism, but yes.

 

As Martin said, different forms of body symmetry are really not that much of a stretch. And like he said, there are several different theories for why sex evolved in the first place, but one of the ideas is that it increases genetic diversity. So, if a species existed in an environment such that a great deal of genetic diversity is highly advantageous, then perhaps having more than two sexes would be adaptive. Until we're more sure on why sex evolved, though, we can only guess.

[thedarkshade]

I'm assuming you mean sperm and egg, cuz the ovary is the organ that makes the eggs. Anyway, it's more like gamete A + gamete B or C or D or E or F = incoming organism, but yes.

Yes, that's what I meant.

But if the fertilization process is the same, then why 8 or 9 sexes? Diversity? 2 sexes can give enough diversity, look at humans. You can't find two people that are 100% the same. Isn't that enough? It seems meaningless, but I'm sure there must be a good reason after it!

Thnx for reply!

 

Cheers,

Shade

[Mr Skeptic]

The thing about sex is that it increases genetic diversity and allows sharing of genes. This is incredibly important because it increases adaptability, which when all is said and done, is the most important atribute for survival. It is certainly plausible that a different method of gene transfer/mixing win out.

 

As Martin mentioned, there are already critters with multiple sexes, but they are rather similar. If you meant some kind of triploid creature that requires three sex partners to produce offspring, that might be possible if there were sufficient population density to be able to find two mates rather than just one, which already gives enough trouble as it is, and wouldn't have that much more benefit. So I don't think it is likely, but it should be possible.

 

A three legged creature seems to me like it would be inefficient if it had animal-like legs but 3-fold radial symmetry, but would be much likelier than a similar creature with 4 legs. That might work if it had to change direction constantly but not travel far in a straightish line. If the legs were aligned in the same direction, eg two and than one behind or ahead of the two, that seems more plausible especially for our own planet. That might happen if the two front or back legs of an animal fused, kind of like the dolphin's tail. However, I think it would still have a slight disadvantage over four legs because you would have unstable equilibrium if you lifted one leg. But it certainly is possible.

[Zaire]

i dont think anyone can speculate on evolution of other organisms that are billions of light years away. there are hundreds, thousands, of factors that could influence said evolution. all organisms on earth are adapted to an oxygen environment and the earth environment, and hence universally have a loose parallel structure. on another planet, you might as well let your mind run abound.

[CharonY]

The thing about sex is that it increases genetic diversity and allows sharing of genes. This is incredibly important because it increases adaptability, which when all is said and done, is the most important atribute for survival. It is certainly plausible that a different method of gene transfer/mixing win out.

 

Actually the evolution of sex is far more complicated (mot to mention heavily discussed since the 80s). It has been stated that sexual reproduction would increase the fixation of beneficial mutations at different loci. On the other hand, there is little data that heritable variance of fitness is really increased by sex. In fact, recombination can lead to the breakup of favourable sets of genes and thus reduce fitness. Also, if all things being equal, an asexually reproducing female will have double the production rate than that of a sexual female. The latter is also the reason why tri-sexual mating is rather unlikely as it would increase the cost of sex even more.

 

Overall to date there is no definite explanation why there is sex at all. More specifically it is unclear how sex initially evolved (as on its onset the additional costs should lead to its demise pretty fast) and how it is maintained.

 

It is clear that it is a successful mode of reproduction, although quite a large number of animals and plants (not to mention prokaryotes) still reproduce asexually.

 

A more molecular explanation stems from the similarity of the DNA repair and the recombination machinery. More recent evidence indicate that chromosomal recombination is in fact a by-product of DNA repair. As such ecological selection might play a smaller role than initially thought and chromosome maintenance selection might play an additional role.

Spinning this further it has been proposed that sex originated from a form of genomic parasitism. Transposons, for instance rely on sexual recombination to spread and it has been hypothesized that sex is established to foster the spread of these mobile elements (selfish genes in action).

[Edtharan]

Are there any scenarios that could favour three (or more) sexes involved in reproduction?

Here is one:

Sex A fertilizes Sex B, and Sex C can gestate the offspiring after being transfered from Sex B. Or Sex C recieves the Sperm from Sex A and the Egg from Sex B and allows them to combine.

 

There are "similar" scenarios here on Earth, although with only 2 sexes. Seahorses have a male and female sexes. However, they fertilise the eggs outside their bodies, but then the male then sticks the egs onto him and allows them to develop on him.

 

If instead of having them stick to the male, a 3rd but otherwise sterile Sex could replace the male in this.

 

Also, in the Gastric Brooding frog, the male takes up the fertilised eggs and allows them to develop and hatch in his mouth.

 

If the environment was quite hazerdous for developing eggs, then it could be possible that a 3rd Sex develops to act as an incubator for the offspring.

 

So, yes I think it would be possible for a 3 sex organism to evolve, but it would unlikely evolve from the reproduction systems that exist in animals (and plants) here on Earth.

[bombus]

Thanks for the messages above.

 

My own thoughts are...

 

1. Three-legged locomotion although possible (as is tri-radial symmetry etc) is probably not as efficient as having two, four, six, eight, etc legs. i.e., it is likely that there must be some selection factor that favours an even number of legs. However, as mentioned above, it could have been pure chance, and if starfish had colonized land first maybe we'd have something different.

 

Despite this, I can't imagine how three legs could be an advantage over two...

 

2. I was indeed referring to three sexes being involved in reproduction. I would guess that the advantages of mixing up genes (improved chances of resistance to pathogens etc) is fine with two sexes as each individual still passes on 50% of its genes to the next generation. If one moves to three sexes, any individual only passes on 33.3% of genes. The advantage of resistance is probably outweighed by the disadvantage of passing less genes on, so two sexes is the best.

 

Thus, I guess that three sexes (or more) involved in reproduction is probably unecessary anywhere in the universe. Nature doesn't like waste afterall !

 

Here's another one for you (still keep commenting on the first two though!)

 

An individual of an alien race, once 50 years old, loses its ability to reproduce and becomes a pure warrior to defend the colony (it develops a hardened carapace, huge size, immense strength etc).

 

I don't think this can evolve if it occurs in the post reproductive phase. Am I correct?

[Reaper]

2. I was indeed referring to three sexes being involved in reproduction. I would guess that the advantages of mixing up genes (improved chances of resistance to pathogens etc) is fine with two sexes as each individual still passes on 50% of its genes to the next generation. If one moves to three sexes, any individual only passes on 33.3% of genes. The advantage of resistance is probably outweighed by the disadvantage of passing less genes on, so two sexes is the best.

 

Can you show us how this would be the case?

 

Thus, I guess that three sexes (or more) involved in reproduction is probably unecessary anywhere in the universe. Nature doesn't like waste afterall !

 

Why though? I'm pretty certain alien life is probably nothing like Earth life, or evolved anything like Earth-life in most things (except mabye in being carbon based, but that's about it). Hell, about as recently as 400 million years ago (when the first land creatures made an appearance), the atmosphere on Earth was toxic to humans...

 

And, 8-9 sexes seem to work for molds, and other unusual anatomies can be seen right here on Earth; so why not intelligent, technology wielding creatures on other worlds?

 

 

An individual of an alien race, once 50 years old, loses its ability to reproduce and becomes a pure warrior to defend the colony (it develops a hardened carapace, huge size, immense strength etc).

 

I don't think this can evolve if it occurs in the post reproductive phase. Am I correct?

 

I would have to disagree with you there. Many genetic-based diseases in humans (i.e. Alzheimer's) usually kick in long after their reproductive stage. I'm pretty sure that anything beneficial could also have evolved to take place long after reproductive age (if anyone would like to show examples of Earth life that does exactly this, please do so).

 

===========================

 

In fact, I do have a question for most of you here; Why would life on another planet, intelligent or otherwise, have to be anything like we see here on Earth? Hell, even Earth life is very diverse and bears little resemblance to each other between specific ecosystems and geographical locations.

[Sisyphus]

My feeling is a three-legged animal might exist, but it would probably still be bilaterally symmetrical along the direction of movement. Like, one of the legs would be the "back leg." (Like a Pierson's Puppeteer.) Trilateral symmetry might also be possible in some very specific environments, or in organisms that don't move very much. The environment I have in mind specifically is from another Larry Niven novel, the Integral Trees, where essentially there is an atmosphere without a planet, and everything is in freefall all the time...

[bombus]

Can you show us how this would be the case?

 

Sexual reproduction passes on 50% of the genes from each sex (50% from the father, 50% from the mother). This is true in hermaphrodites as well. Three sexes would cut this down to 33.3% each, four sexes 25% each etc...

 

Why though? I'm pretty certain alien life is probably nothing like Earth life, or evolved anything like Earth-life in most things (except mabye in being carbon based, but that's about it). Hell, about as recently as 400 million years ago (when the first land creatures made an appearance), the atmosphere on Earth was toxic to humans...

 

I think the issue is that they all ultimately have to follow the laws of Physics, which biology is based on.

 

And, 8-9 sexes seem to work for molds, and other unusual anatomies can be seen right here on Earth; so why not intelligent, technology wielding creatures on other worlds?

 

But in moulds only two sexes are actually involved in any single reproductive act.

 

 

 

 

I would have to disagree with you there. Many genetic-based diseases in humans (i.e. Alzheimer's) usually kick in long after their reproductive stage. I'm pretty sure that anything beneficial could also have evolved to take place long after reproductive age (if anyone would like to show examples of Earth life that does exactly this, please do so).

 

Mmm. I think the fact that Alzheimers still exists as a disease may be because it cannot be 'weeded out' by evolutionary processes as it in general occurs after the reproductive phase.

 

===========================

 

In fact, I do have a question for most of you here; Why would life on another planet, intelligent or otherwise, have to be anything like we see here on Earth? Hell, even Earth life is very diverse and bears little resemblance to each other between specific ecosystems and geographical locations.

 

Again, probably due to the laws of physics, and the fact that nature likes simple solutions - even if they create complex systems when they interact together.

[Phi for All]

I met a farmer once who raised three-legged chickens. You could see them speeding around the yard like roadrunners; they were so *fast*. The farmer figured he'd get more money for them because of the extra drumstick. Unfortunately, he could never catch one to see how it tasted.

[bombus]

My feeling is a three-legged animal might exist, but it would probably still be bilaterally symmetrical along the direction of movement. Like, one of the legs would be the "back leg." (Like a Pierson's Puppeteer.) Trilateral symmetry might also be possible in some very specific environments, or in organisms that don't move very much. The environment I have in mind specifically is from another Larry Niven novel, the Integral Trees, where essentially there is an atmosphere without a planet, and everything is in freefall all the time...

 

Good answer. Yes, maybe even in four limbed creatures a third leg could start off as a tail and become progressively more limb like?

 

I like the idea of the Integral trees. Three limbs could be very useful in that sort of '3D' environment. Orangutans have in effect four arms, rather than two arms and two legs, so presumably three arms in a 3 D environment may be better than two, but then maybe four is even better - how about five!. Reptiles, birds and mammals have a pentadactyl limbs, and two arms, two legs and a head = five, so there are probably very old genetic codes for five 'things' within Earth creatures...

 

I met a farmer once who raised three-legged chickens. You could see them speeding around the yard like roadrunners; they were so *fast*. The farmer figured he'd get more money for them because of the extra drumstick. Unfortunately, he could never catch one to see how it tasted.

 

nyuk nyuk:-)

[thedarkshade]

Thus, I guess that three sexes (or more) involved in reproduction is probably unecessary anywhere in the universe. Nature doesn't like waste afterall !

I share the same opinion too! There is enough diversity with two sexes too!

 

An individual of an alien race, once 50 years old, loses its ability to reproduce and becomes a pure warrior to defend the colony (it develops a hardened carapace, huge size, immense strength etc).

In what way he becomes like that? If you don't mind explaining...

[bombus]

I share the same opinion too! There is enough diversity with two sexes too!

 

 

In what way he becomes like that? If you don't mind explaining...

 

Well, these were ideas in sci-fi books (Ian M Banks novels) and it didn't explain. It just happens after a certain point in the life of Idirans, which are 9 ft tall three legged aliens:

 

Physique

Full-grown Idirans stand about three meters tall on a tripod of legs and have two arms. There is some hint of fully trilateral symmetry in their ancestry, as a third, vestigial, arm has evolved into a chest-flap which the Idirans use to create loud, booming warning signals. They have a saddle-shaped head with two eyes at each end of the saddle.

 

Idirans are biologically immortal and are very resilient to physical damage as they are protected by a natural keratinous body-armour and can withstand catastrophic damage and even remain conscious, though they do not naturally regenerate. They are dual hermaphrodites, each half of a couple impregnating the other. After one or two pregnancies Idirans lose their fertility and develop into the warrior stage, reaching greater size and weight, the armour hardening fully. Idiran warriors are capable of taking enormous amounts of damage and can survive massive trauma that would kill a human being instantly-for example, losing a large fraction of their head.

 

The biological immortality was a result of their evolution as the 'top monster on a planet full of monsters', where strong natural selection pressure and a strong background radiation (causing mutations) prevented the biological immortality from stifling the evolution of the species

[Mr Skeptic]

An individual of an alien race, once 50 years old, loses its ability to reproduce and becomes a pure warrior to defend the colony (it develops a hardened carapace, huge size, immense strength etc).

 

I don't think this can evolve if it occurs in the post reproductive phase. Am I correct?

 

No. Even when not reproducing, he can affect his relatives, either positively or negatively. If the colony had more need of the warriors than it had of the food to feed them, then they would be positively affecting their relatives in the colony; hence those who were better fighters past their reproductive years would be likelier to pass on their genes than if not.

[thedarkshade]

Well, these were ideas in sci-fi books (Ian M Banks novels) and it didn't explain. It just happens after a certain point in the life of Idirans, which are 9 ft tall three legged aliens:

 

Physique

Full-grown Idirans stand about three meters tall on a tripod of legs and have two arms. There is some hint of fully trilateral symmetry in their ancestry, as a third, vestigial, arm has evolved into a chest-flap which the Idirans use to create loud, booming warning signals. They have a saddle-shaped head with two eyes at each end of the saddle.

 

Idirans are biologically immortal and are very resilient to physical damage as they are protected by a natural keratinous body-armour and can withstand catastrophic damage and even remain conscious, though they do not naturally regenerate. They are dual hermaphrodites, each half of a couple impregnating the other. After one or two pregnancies Idirans lose their fertility and develop into the warrior stage, reaching greater size and weight, the armour hardening fully. Idiran warriors are capable of taking enormous amounts of damage and can survive massive trauma that would kill a human being instantly-for example, losing a large fraction of their head.

Ah, I got it! Pure imagination, typically sci-fi!

 

The biological immortality was a result of their evolution as the 'top monster on a planet full of monsters', where strong natural selection pressure and a strong background radiation (causing mutations) prevented the biological immortality from stifling the evolution of the species

The definition of perfection!

[Psyber]

Thanks for the messages above.

 

My own thoughts are...

 

1. Three-legged locomotion although possible (as is tri-radial symmetry etc) is probably not as efficient as having two, four, six, eight, etc legs. i.e., it is likely that there must be some selection factor that favours an even number of legs. However, as mentioned above, it could have been pure chance, and if starfish had colonized land first maybe we'd have something different.

 

Despite this, I can't imagine how three legs could be an advantage over two...

 

2. I was indeed referring to three sexes being involved in reproduction. I would guess that the advantages of mixing up genes (improved chances of resistance to pathogens etc) is fine with two sexes as each individual still passes on 50% of its genes to the next generation. If one moves to three sexes, any individual only passes on 33.3% of genes. The advantage of resistance is probably outweighed by the disadvantage of passing less genes on, so two sexes is the best.

 

Thus, I guess that three sexes (or more) involved in reproduction is probably unecessary anywhere in the universe. Nature doesn't like waste afterall !

 

Here's another one for you (still keep commenting on the first two though!)

 

An individual of an alien race, once 50 years old, loses its ability to reproduce and becomes a pure warrior to defend the colony (it develops a hardened carapace, huge size, immense strength etc).

I don't think this can evolve if it occurs in the post reproductive phase. Am I correct?

You've read the one about the "PAK" too, hey!

[bombus]

No. Even when not reproducing, he can affect his relatives, either positively or negatively. If the colony had more need of the warriors than it had of the food to feed them, then they would be positively affecting their relatives in the colony; hence those who were better fighters past their reproductive years would be likelier to pass on their genes than if not.

 

Yes, I can see what you are saying, and it sounds believable. Basically, those individuals that can protect their offspring better than others are more likely to pass on their genes to the next generation because their offspring don't die before they breed. This could presumably work at the group level (not just at the family level).

 

However, according to accepted science, evolution only works at the level of individuals passing on more of their genes.

 

On that note, surely it would be an advantage for an individual in a colony to be a late developer so delaying the onset of a post-reproductive phase and carry on mating, while others (who have become warriors) defend his genes.

 

Thus in the next generation there would be more late developers than there would otherwise be, ad infinitum???

[Mr Skeptic]

That seems about right, so long as the late developers don't end up taking over and so leaving the whole colony defenseless. I'd agree that it would work somewhat at the group level, but I'd suggest that the warriors would go to extra trouble to protect their own family. If there were social expectations, this could complicate things too.

[bombus]

That seems about right, so long as the late developers don't end up taking over and so leaving the whole colony defenseless. I'd agree that it would work somewhat at the group level, but I'd suggest that the warriors would go to extra trouble to protect their own family. If there were social expectations, this could complicate things too.

 

The thing is, at what point does it become a disadvantage to the individual to be a late developer. Probably never! Thusly the species would 'evolve' into exinction (if it lived on such a violent world). An evolutionary bottleneck, such as cheetahs are probably in: cheetahs sacrifice power for speed. At some point in the future they will not be able to catch AND kill their prey (which can keep getting lighter and faster - unlike the cheetah).

 

The extra effort to defend their families would complicate matters though.

[Edtharan]

However, according to accepted science, evolution only works at the level of individuals passing on more of their genes.

This doesn't fully explain things though. Social insects produce sterile offspring that don't pass on their genes, and these provide for the colony. How could such a thing evolve if evolution only worked at the level of the individual.

[bombus]

This doesn't fully explain things though. Social insects produce sterile offspring that don't pass on their genes, and these provide for the colony. How could such a thing evolve if evolution only worked at the level of the individual.

 

Well, the queen bee (or wasp, ant, termite) and drones pass on the genes. All the workers are offspring, but never pass on their genes. But their effectiveness depends on the queen's 'fitness' and the 'fitness' of the drone that she mated with. 'Successful' (fit) queens produce effective workers, who ensure that her fertile offspring (young queens and young drones) leave the nest at the right time and spread her genes (well, 50% of them).

 

The sterile offspring are really no more than 'arms and legs' of the queen (and drone). The better your arms and legs, the better chance of passing your genes on.