Most of the vertebrate groups have evolved a limbless form:
Fish (eels)
Amphibians (caecilians)
Reptiles
Birds have not, though that’s pretty hard to imagine.
But it’s not too hard to imagine (for me) a mammal form like the rat, ferret, weasel, or maybe even seal/sea lion, losing their limbs and elongating their bodies. But mammals have been around for hundreds of millions of years, and this hasn’t happened.
Could it have something to do with cold blooded vs warm bloodedness? That’s all I can think of, though I’m not sure how that would affect it.
It’s hard to see what the advantage would be in going legless. Most of those long, lithe mammals (ferrets, weasels, even cats, sorta) use their feet and especially claws a lot.
you could say the same about many reptiles, I suppose (a lot of lizards are awfully fast), but IIRC, many reptiles have their legs attached in a way very different from the way mammals do, and I could conceive of going legless (or almost legless – there are two-legged reptiles) and having an advantage. In fact, it’s happened more than once – there is are several types of legless lizards unrelated to snakes:
Just because it it “not hard to imagine” imposes no obligations on nature. There are no legless mammals (although naked mole rats seem to be getting close) because no mammals have happened to have the right genes in the right environment for the right length of time. Yet.
(Idea- mammalian burrowers tend to use their fore-legs to dig, not like reptiles who tend to force their snouts through the soil. A softer snout might be the reason no mammals have gone legless.)
I don’t say that there aren’t any because I can’t imagine them (I can --have a look at the cattails from A World Out of TimeA World Out of Time - Wikipedia ), but because I don’t think there would be an advantage to any existing mammals over their current mode of getting around – which is the same thing you’re saying (only you didn’t read me carefully enough)
Well, as Smeghead mentioned, there are plenty of legless animals in the sea, though even those have all retained some sort of forelimb as a flipper.
And I’m going to ignore fish – most don’t have big, sturdy, bony limbs, and there are multitudes of examples of fins in all conceivable geometries. Eels are just one point on a continuum.
So there are two groups of tetrapods that have repeatedly evolved limbless forms, the reptiles and amphibians. One characteristic of these herps (which is a crude lumping of these two groups) is that they move by wriggling their bodies from side to side. Even big land lizards move in a sort of S shape, bringing each leg forward by twisting their entire bodies. In contrast, most mammals move by swinging their legs back and forth beneath their bodies, and sometimes bending their entire bodies up and down*. That side-to-side wriggling is probably much more conducive to limbless locomotion, and thus you find lots of examples of lizards and salamanders that found it advantageous lose their limbs to better wriggle through the ground.
*This is true even with sea mammals – dolphins and whales propel themselves by pumping their tails and bodies up and down. As far as I know, all comparable sea creatures – big vertebrate top predators like sharks or tuna – move by wiggling side-to-side.
Good point. Also, don’t reptiles and amphibians generally have legs that stick out to the sides, making it require more effort to hold their bodies up than birds and mammals, with their legs usually mounted directly underneath, have to make to support their bodies? Maybe it’s easier to lose less efficient legs and leverage the side-to-side motion, but the really efficient mammalian and bird/dinosaur leg arrangement is effective enough to outcompete legless mutations?
To really answer this, you need to think about it this way, not what the animal looks like but what it takes the place of
So let’s think about snakes? What do they do? Where do they live? What do they eat? What is their overall purpose in their ecological niche?
Then think about it this way? What mammal or bird serves the same function. I think Ireland, New Zealand and Hawaii currently are the three places without snakes? So what birds or mammals in those places take the ecological function of snakes?
THAT mammal or bird would be the equivilent of the snake with or without legs.
Just the way the Moas took the place of larger herbivorous mammals in New Zealand. Moas certainly didn’t look like other herbivorous mammals but they served their function
Mammals also, in general, take much more care of their young then others. being able to carry, nurse, defend helpless children make arms harder to ‘evolve away’. I could see a snake being able to get to do those things (carry, nurse and defend) her young, but that’s because the snake is in that form to begin with, and just adding child raising. But for a ‘type’ of animal who’s major characteristic is based on childcare, it’s hard to see how limbs can be reduced to the point of them being eliminated.
Yes, but the children of mammals (and birds) also seem to require more parental interaction to be properly raised, and neither mammals and birds have a limbless form. And many will use their limbs as part of raising their children, if not carrying them. Loss of parental interaction would put the children at a big disadvantage, learning from parents and play with siblings is how many gain skills they need. Mammals also seem to spend a much greater percent of their life with children then others and need ways of handling them.
Again I can see a snake being capable to perform child raising in a mammalian fashion, but it seems like it’s because the snake is in that form already and did not evolve while also having to care for children. Mammals on the other hand would have to care for children while loosing functionality of hands and legs, or drop childcare, which is part of their success, teaching children leaned skills gives them a head start and a great advantage.
I wondered that too. All those legless creatures use their long, slender bodies for locomotion. A long slender body means a very high surface-to-volume ratio, which isn’t a big problem for cold-blooded creatures. It’s more of a problem for mammals and birds, who need to maintain a constant core body temperature.