why didn’t cetaceans didn’t evovle into having gills??

Aug 30, 2013


Discussion by: iskander.cha

Hi
Well first of all i must point that i don't have much knowledge when it comes to biology ( also my English isn't that good so would be happy if my mistakes were corrected).
After i finished the 11th chapter of the book " the greatest show on earth" i couldn't help but wonder about the reason for which cetaceans didn't develop gills, and the first think that came to my mind is: the functions they acquire from the transformation of gills ( into what they are now, and here i will be discussing larynx obviously since it is what have been introduced to me through this book) are more important than developing gills.
So here is how i looked at it: the functions they have acquired is indispensable for them, unlike what i found when i looked in the internet , where everybody was comparing gills and lungs to see which is more efficient for survival
From what i know ( mostly about dolphins and whales) communication is really important for cetaceans and also they use echolocation so an individual who has lost such an organ as the larynx would be disadvantaged in both reproduction and survival. this of course assuming that it will revert to having gills and not redevelop new ones ( since from what i understand the probability is higher this way).

13 comments on “why didn’t cetaceans didn’t evovle into having gills??

  • 1
    Kim Probable says:

    Evolution works with what it has, and in this case, gill arches in fish evolved to become parts of the inner ear, lower jaw, and larynx (possibly limbs, too, I think) in other vertebrates. Their nerves and muscles have also been repurposed to support these structures. Cetaceans already had a way to breathe, so there wasn’t much selection pressure for that, and losing the lower jaw and ear would have been disastrous, since both are so heavily used for echolocation. (Toothed whales receive vibrations through their lower jaw, and those vibrations are then transmitted to the ear.)

    Basically what they have is better than having gills, and it’s more advantageous for them to keep what they have rather than have it repurposed for something else. They can breathe very rapidly and have an incredible volume of air exchanged with each breath. It’s so efficient that I don’t know if gills would really do much for them.



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  • 2
    SaganTheCat says:

    As I understand it, there is not enough available oxygen in water to maintain metabolism in warm-blooded mammals, so a whale with gills could not survive. Secondly there’s the big question of how do you evolve gills? I could imagine a scenario where an aquatic mammal uses both air and water, which could go on to evolve into a fully water-breathing animal in the distant future, if there was some way of extracting enough oxygen from water through existing tissue that a small difference in an individual’s ability would have a measurable effect on its reproductive success rate.

    As far as I know, such a mechanism doesn’t exist, presumably sticking my paw into water some oxygen is able to become absorbed into a few cells but probably fewer cells than i lose during a grooming session. I just don’t think there’s an opportunity for nature to select against.

    Most importantly though; why? Millions of years of cetacean evolution has demonstrated that lungs are “good enough” which is pretty much the limit to natural selection. from our perspective we could see the benefits of not having to come up for air, especially if you intend to spend your life away from land but evolution doesn’t have foresight. it acts on what’s there to work with. lungs are good, you can hold your breath for a bit and go out hunting. a gene that makes lungs a bit bigger, or with a few more blood vessels will be selected for in the environment of semi-aquatic mammals, its effect is immediate and far reaching. Deeper dives, longer swims, more strength, more mating, more offspring, more big-lung genes.

    The calculation of which is more efficient is the wrong place to start. nature has no drawing board. the purpose of the porpoise is to make pups, not extract oxygen. oxygen extraction is a means to an end and any mutation that improves its ability to extract oxygen from water may well be deleterous in any number of ways.

    That said, evolution works on whats there already. one part can evolve into something else that we can only recognise with hindsight. no one looking at the gills of a cambrian animal would guess some of them were on their way to becoming wings of a future insect. For all we know, some sort of gill is in the early stages of evolution. chances are, the species will go extinct before anyone ever finds out



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  • 3
    steve_hopker says:

    In reply to #2 by SaganTheCat:

    As I understand it, there is not enough available oxygen in water to maintain metabolism in warm-blooded mammals, so a whale with gills could not survive. Secondly there’s the big question of how do you evolve gills? I could imagine a scenario where an aquatic mammal uses both air and water, which c…

    Perhaps there has not been long enough – whales maybe branched from a common ancestor with hippos about 40-50 million years ago (
    (http://evolution.berkeley.edu/evolibrary/article/evograms_03: http://en.wikipedia.org/wiki/Evolution_of_cetaceans), whereas fish took a lot longet to emerge?



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  • 4
    SaganTheCat says:

    In reply to #3 by steve_hopker:

    In reply to #2 by SaganTheCat:
    Perhaps there has not been long enough – whales maybe branched from a common ancestor with hippos about 40-50 million years ago ( (http://evolution.berkeley.edu/evolibrary/article/evograms03: http://en.wikipedia.org/wiki/Evolutionof_cetaceans), whereas fish took a lot longet to emerge?

    yes as i said above, maybe something is starting to evolve that way but the sentence “fish took a lot longer” is a non-sequitur. fish didn’t evolve from a non-gilled ancestor. they didn’t evolve gills, gills evolved fish



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  • 5
    Alan4discussion says:

    The transformation from fish to land mammals via amphibious fish, amphibians, and reptiles took millions of years.

    Evolution modified what it has to work with a tiny step at a time. There are plenty of modern cetations (as well as fossils) – seals etc which show the deveopment from amphibious mammals to fully marine creatures.

    The obvious obstruction to re-evolving gills, is that any contact between blood and seawater will have a massive chilling effect which would be disabling to a warm-bloded mammal. – especially in the polar waters where many whales live for at least part of the year.

    If we look at the fossil record, it is clear that whales evolved from smaller Hippo-type, and seal-type amphibious ancestors, before scaling up, to better retain body heat and establish physical dominance.

    http://ngm.nationalgeographic.com/2010/08/egyptian-whale/whale-animation

    http://ngm.nationalgeographic.com/2010/08/whale-evolution/mueller-text

    http://ngm.nationalgeographic.com/2010/08/whale-evolution/barnes-photography



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  • Why didn’t cetaceans evolve to have gills?

    To put a fun and different twist on this question, we really do not know that such a development didn’t once begin but then turned out to be an evolutionary dead end. Would I bet my house on such a speculation? Probably not, but I appreciate your question for prompting me to think a little differently.

    Mike



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  • 7
    Neodarwinian says:

    Natural selection is the blind watchmaker and does well enough with the materials at hand, lungs in this case. The fish shape was rather easy with the tetrapod material at hand, but you have been told what fills were and became, so the ” going back, ” so to speak would have been, among other things, inefficient energy wise and morphologically unnecessary.



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  • 8
    downshifter says:

    SaganTheCat said pretty much what I was thinking, so take all his stuff and insert it and then I’ll just add some musings to it. 🙂

    Another interesting question is, why haven’t/didn’t marine reptiles develop “gills” either? (Ichthyosaurs, plesiosaurs, mesosaurs, and other equally unspellable critters) Or modern marine reptiles such as sea snakes and sea turtles? (I put “gills” in quotes for the obvious reason that they would develop something new, rather than re-manifest their original gills.)

    Without disagreeing with anything SaganTheCat said, there still seems to be a case of convergent evolution going on here that would be interesting to investigate.

    It’s worth noting that some sea turtles appear to be evolving in precisely this direction. It’s a bit of a snigger that some turtles can take in oxygen near their bums. While looking for a quote about that, I found out that it’s actually more extensive than that. I’ll just quote my source:

    “In addition to using their lungs for breathing, turtles have developed supplemental forma of respiration. Some aquatic turtle species bring water in through their nasal passages and into their mouths and throat where oxygen is extracted by the pharynx. This is done through the pharyngeal lining which acts as a gill. Some turtles take water into their anal opening where two sacs are filled and emptied with water, causing a slow current which enables oxygen to be collected. One fresh water turtle in Australia has cloacal gills for respiration.”

    From here: The Oceanic Resource Foundation



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  • 9
    Archaic Torso says:

    Like Kim said. Also any genetic changes during the (enormous and complex) transition from breathing with lungs to using gills for gaseous exchange would have to confer an advantage in terms of fitness (or at least no disadvantage) at each stage in order to be retained. This would be unlikely since they already have very efficient lungs to do the job.



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  • Evolution has to work with what is already there. The vestigial gills were already pressed into re-use. So it would have to start from scratch.

    Also consider that the sharks have some very efficient gills, but they don’t work unless the shark keeps moving. This may be as big a drawback as having to surface. The lungs might work so well that primitive aux gills would not be helpful. Consider that warm-blooded seals, cetaceans, walruses, narwhales etc. all consume oxygen faster than any cold-blooded fish. I doubt you could pull enough oxygen from the water with gills for a cetacean. Consider that many fish augment their gills by breathing at the surface.

    Cetaceans can live in very muddy water since they don’t ingest it. Fish have to keep their gills clean.

    In computer design you can start from scratch from time to time. In evolution you cannot, despite how desperately you need a redesign.



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  • I dunno – Human: “Cetacean, why don’t you have gills ?”
    Cetacean responds: ” gills, shmills, give me krills!”
    Why don’t birds have propellers? Why don’t primates have rotating arms?, Why don’t chelonians have Kevlar shells? Why didn’t ants invent television? etc, etc. Why are apparently rational human beings capable of being so illogical? Why, why, why? Sounds like the language development of a three year old. How, in contrast, can be an interesting question.



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  • 12
    downshifter says:

    In reply to #11 by rzzz:

    Why, why, why? Sounds like the language development of a three year old.

    rzzz, I don’t know if you intended it, but your response sounds rather harsh. I think it’s perfectly reasonable to wonder why these creatures developed fins, eliminated (reduced) rear legs, developed flukes, developed higher myoglobin concentrations in the skeleton, developed tear glands that secrete a substance to protect against salt water (or got rid of them altogether for whales), got rid of skin glands, changed the eye lens, extensively modified its hearing and sound production, developed baleen in some cases, developed the ability to drink salt water, and probably a gazillion other differences that a cetologist could point out that I can’t…

    …but DIDN’T develop the ability to breathe underwater.

    I feel pretty confident that wondering “why” this is the case is what led to studies that suggest there isn’t enough oxygen in the water to sustain them, etc.



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  • 13
    odalrich says:

    As I didn’t have a scientific education, the only reply that occurs to me is: give cetaceans a chance; perhaps with a little bit of more time they will develope gills, horns or nothing at all ( more likely they’ll be extinct in some years time, taking into account the present state of our seas). In any case we won’t be here to see it and tell the tale.



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