Doesn’t that question sound silly? I thought it was a silly question, until I started thinking about it.
Yesterday, I was bicycling down the road and I thought I saw a dead snake on the road—I turned around and realized it was a rope. The fur should have been an easy hint, because there are no limbless mammals!
But why not? After all, other vertebrates have evolved to lose their limbs. Amphibians have evolved into caecillians. Snakes lost their limbs, and many lizards have as well. Eels lost most of their fins. So why have no mammals evolved into a limbless form? The amount of evolution in mammal limb forms is pretty wide. They can evolve into wings, and into flippers, and it seems that some mammals, like dolphins and otters, might be getting close to limblesslness.
Is there any specific reason that mammals have all kept their limbs? An easy guess is that it might have something to do with pregnancy and childcare. But, mammalian bodies have evolved to deal with that as well. Bats give birth hanging upside down. Cetaceans give birth in the water.
Maybe it is just a coincidence of evolution that we haven’t seen it yet, and 30 million years from now, there will be weasels and dolphin descendants that have evolved snake/eel like shapes.
Interesting question. The usual answer evolutionary biologists give to this sort of question is that most possible morphological forms never evolve, and wondering why any one variation didn’t happen is like wondering why any one house didn’t get hit by a meteor. In other words, there does not need to be a selective explanation for why something didn’t happen. That, I know is not a very satisfying answer. Another possible answer is that mammals (and birds) are more recent branches on the vertebrate tree than fish, amphibians, and reptiles, and simply have had less time to evolve major morphological changes. Another, more along your line of thinking, is that mammals (and birds) being homeothermic (warm blooded) need to consume a lot, frequently, and limbless things maybe tend to live in ways that are more wait-for-food-to-come by. Since it never evolved, it is hard to speculate plausibly about what it would be, how it would live, and what the advantaged and disadvantages would be.
Obvious point I didn’t think about:
Being long and skinny means more surface area. More surface area, for an endothermic animal, means needing a higher metabolic rate. Mustelids can burn twice as many calories as a more spherical mammal, for that reason. Mustelids are already on the edge of what you can squeeze out of a mammalian form—literally.
So maybe it is impossible for a mammal to become limbless. At least, a terrestrial mammal.
What about Tribbles? They’re legless mammals that are round, which seems great for surface area to volume ratio heat retention. OK, Tribbles aren’t real, but who says limbless has to mean slithery?
Good point. I was assuming based on the fur. Not a good move when dealing with alien species. They came from a planet with reptile predators, so mammals just came to mind. They have two scientific names: Polygeminus grex and Tribleustes ventricosus (I don’t know which is accepted).
Fun fact: “In the book ‘Evolving the Alien: The Science of Extraterrestrial Life’, a.k.a. ‘What Does a Martian Look Like?*’ Dr. Jack Cohen and Ian Stewart surprisingly elect the tribble one of the most realistic alien species in Star Trek and describe a thoroughly detailed alien ecosystem scenario to justify how such a creature could have evolved.”
i would think that the structure of the vertebrae and supporting muscles is probably what hinders terrestrial mammals from going limbless. it’s probably a lot easier to go without legs if you can move your body side to side rather than up and down. (it’s less of an issue in the water.)
My unsatisfying hypothesis is that perhaps mammals just don’t have the genetic capability to lose limbs - in the sense that the basic template can’t be modified that much. I believe in whales there is at least a vestigial pelvis and hind legs, which to me suggests that some genetic features can reduce limbs, but cannot change the basic structure.
Having said that, I’m not a neo-Darwinian in the sense that all evolution is the result of breeding ‘fitness’. As an example, the human semi hairless state might be just the result of some mutation. It may not have conferred an advantage, but it was no disadvantage. Perhaps it was linked to some other trait (upright walking, say) but perhaps the overall ‘thing’ just worked.
I think one of the biggest factors is probably how much the animal has to love, which results from thermoregulation. The few existing Limbless terrestrial verts I can think of are caecilians, snakes, and legless lizards. The ancestors of all of these probably lived a fairly sedentary lifestyle, close to the ground. They could only be sedentary because, as ectotherms, they didn’t need to move to find food as often as homeotherms. For homeotherms, the smaller they get, the more they need to eat, and the faster they need to move. Until more mammals than naked mole rats become ectotherms, I don’t think we’re going to get limbless mammals.
As for birds, they have not only the homeotherm problem, but also the wings problem. Thanks to wings, very few birds are solely terrestrial, which seems to be the first step to limblessness
But even birds like albatrosses have feet. I don’t see that the feet pose a disadvantage to them, and since they need to nest on land I suspect they will retain legs.
I think another key factor is that in mammals (and birds) the spine moves differently from reptiles, amphibians and fishes. It moves “up and down” while in reptiles it curls left and right. So slithering on the ground is not possible for a mammal as it would be for a reptile.
Think about how seals move on ice, they have to “bounce” on their belly and don’t move like snakes, it’s not very efficient so they don’t move on the ground very much. In water it works very well though. In fact completely aquatic mammals have evolved too (and they lost their posterior limbs, while the front ones have become fins), but as you may know, their tail is placed horizontally, differently from fishes where it’s vertical. Fishes swim by moving their body left and right while cetaceans move it up and down, It’s for the same reason.
Maybe I am missing something here, but tails. They are limbs and some species have evolved to lose them or have shorter versions. So, yes, you might say the loss of the 5th “limb” (tail) counts :)
Agree with first comment, it has to be connected with homeothermy, in water environment everything is different from the air and even soil.
Take mustelids as example, to be such slim and long they have to hunt a lot and pretty big prey sometimes, and they still have limbs and far from snake body plan.
I think birds like swifts are a better example @mamestraconfigurata, they hardly use legs other than when making nests and sleeping (still can sleep in flight though and if not breeding can not sit at all).
One thing that hasn’t been brought up yet is that mammals have a strong developmental constraint on the position of the neck-torso transition. Nearly all mammals have seven cervical vertebrae, with the only exceptions (sloths and sirenians) being highly specialized taxa with reduced metabolisms that help them avoid the detrimental side effects of having the wrong length of neck. Other tetrapods, by contrast, can have a wide range of variation in neck length—swans can have up to 23 cervical vertebrae, whereas macaws have only ten, and chameleons have only five whereas monitor lizards have nine. One prehistoric marine reptile, Albertonectes, had a whopping 76 cervical vertebrae.
Loss of the forelimb and pectoral girdle would entail a blurring of the distinction between cervical and thoracic vertebrae, so I wonder if the same developmental constraint that restricts mammals to seven-vertebra necks constrains mammals to keeping their pectoral girdle. Notably, the hind limb has been more or less completely lost twice in mammal evolution (sirenians and whales), but the forelimb has never been lost. It’s true that whales have vestigial traces of the hindlimbs, but they’ve only been around for a bit over 40 million years—snakes have been around for over 100 million years and some still have vestigial hindlimbs!
Another factor is that mammals probably just haven’t wound up with quite the right ecomorphology to lose limbs. Limblessness in tetrapods is often correlated with burrowing, but mammalian burrowers often use their limbs heavily in burrowing (such as moles), and so would not be under any selective pressure to lose their limbs. Why weasels haven’t lost their limbs is the real question here.
Also: the tail is not a limb and tribbles would not be mammals because any mammalian traits they may have are the product of convergent evolution, not shared ancestry.
legless lizard