Two biological puzzles

January 14, 2020 • 9:15 am

Here are two questions to ponder while I am doing other things today. The first comes from Matthew, whose words are indented:

Here’s a question which might be good to pose to readers.

Why are there no live-bearing birds? Live-birth has evolved many times in squamates, so is clearly within mutational reach of the reptilian genome (and interestingly, it generally leads to social behaviour). It has been argued that birds lay eggs because they would be too heavy to fly if they were carrying around young inside them. Apart from the obvious problem that bats manage fine, if that argument is right, you might expect some flightless birds to have been live-bearing. But they aren’t. Maybe they were in the past? Any hand-wavy explanations?


And I have my own question:

Why are there no herbivorous snakes?  There are lots of snakes in the world and they slither in the grass, but none of them eat it—or any other vegetation. They are all carnivores.

This is particuarly puzzling in light of the fact that the relatives of snakes—lizards—often eat a great deal of vegetation, and at least one species—the marine iguana of the Galápagos—eats only vegetation (algae; though rarely they’ll eat other stuff). So it is possible for reptiles to evolve into herbivores. (Many of the dinosaurs were plant-eaters.) Why haven’t snakes done it?

Neither Matthew and I know the answers here (after all, these questions bear on mutational possibility, evolutionary history, physiology, and so on), but the questions are interesting to ponder. They do show that not all conceivable “niches” get filled by evolution.

Here’s a nice video of a marine iguana (Amblyrhynchus cristatus) foraging; I saw many of these when I visited the Galápagos some years ago. It is also the only marine lizard. There are other marine reptiles like saltwater crocodiles, sea snakes, and of course marine turtles, but to my knowledge this is the only lizard that forages in the sea (they live mostly ashore).

113 thoughts on “Two biological puzzles

  1. These are very interesting and well thought out questions!

    The thought of mass came to mind, and then I see the argument about bats. So. Hmmm.

    For #2, I’m wondering if it’s simply easier to *eat* animals, compared to possibly high quantities of effort to rip up grass, leaves, etc – they don’t really have teeth for grinding, and their digestive system isn’t like annelids that have a gizzard… do lizards have a microbiome that digests the lignin and cellulose? Maybe snakes don’t have that. I wonder though about fruit.

    1. For #2, I’m wondering if it’s simply easier to *eat* animals,

      Are you meaning that plant foods have a lower energy density than animal foods? But the exact same issue of comparing them to other squamates (from which snakes have descended – though I’m not at all sure that either squamates or snakes are monophyletic groups, rendering phylogenetic bracket like arguments a bit wobbly) occurs. The energy requirements of many squamates are relatively high compared to snakes – which are largely ambush predators – yet various squamates have gone down the vegetarian route.
      What are the relative energy requirements of grazing algae off rocks immersed in the relatively cold waters of the Humboldt Current versus sitting beside a mouse trail, waiting for dinner to come within reach.

      1. No

        I mean if we watch a snake eat a mouse, and then imagine if it was eating grass, my intuition says it would be more difficult for the snake to grasp and grip a collection of grass blades, to rip it, then work them all down its throat without choking or the blades getting all over the place. It would take the snake a long long time to get the nutrition it needs, even if it had gut bacteria that help it meet the need. the mouse, in comparison, fits the mouth well, the … teeth? What are they?… can get a grip, the muscles can shove the mouse down – and all in a fraction of the time.

        But I don’t even own a snake as a pet, Ive only seen the nature shows.

  2. I am not a scientist so really have no idea. However for question 2, because snakes have 10,000+ muscles (compared with humans, who have around 700), a diet high in protein and fat would be necessary to stay healthy?

    1. Neither am I, but you do not need to be a scientist by employment to approach a problem or a question in a scientific way!

      You make a valid point.

      Plenty of snakes have live young. I wondered if some dinosaurs might have had live young.

      1. It is possible, but we know for certain that a wide range of dinosaurs (including the extant avian ones) took the “egg” route.
        To forestall a likely trip-up, there are numerous fossils known of ichthyosaurs which were clearly pregnant, and an appreciable number of such fossils know which appear to have been fossilised in the process of giving birth (more likely, decomposition gases were forcing the foetus out of the body cavity ; whether the foetus was dead at the time isn’t clear). Ditto for various plesiosaurs.
        Neither ichthyosaurs nor plesiosaurs are dinosaurs. They are somewhere in the paraphyletic mess that is “reptiles”, but they’re not in the much tighter group that is “dinosaurs”.
        I can’t think of hearing a report of a dinosaur fossil being discovered which incorporates fossils of a smaller specimen of the same species, in an appropriate position in the body cavity. Which doesn’t prove anything much. However for calibration I have heard several recent-ish reports of multiple specimens being discovered fossilised together in a burrow structure. But they have been of multiple species in one burrow. The burrow-ologists mentioned in discussions that they were intrigued by the idea of finding adult+juvenile of the same species in one burrow, as a pointer towards parental care, but I haven’t heard of such a discovery. Yet.

    2. An interesting idea about muscles. A quick counter example is an elephant, which has 40K muscles in its trunk alone. It still eats only vegetation. So it wouldn’t be that large of an impediment (with a huge body to process the vegetation).

      1. Huge bodies are not needed to process vegetation in general – “said the mouse to the elephant” (I haven’t quoted any Flanders & Swann for ages).
        I was about to muse on whether you needed a large gut to process low grade, cellulose dominated plant material. But then I thought of termites.
        Elephants are big because they don’t have the right gut flora. Koala are not particularly big, live on very low-nutrient foods (eucalyptus leaves) and are critically dependent on having the right gut flora, which discovery has made a big difference to treatment of burn-victim koalas because the antibiotics to treat infection of the burns wipes out the gut flora, necessitating faecal transplants as part of the treatment.

          1. Are the relevant bacteria (or fungi, or both) likely to survive in a soup with huge numbers of lactobacilli?
            I don’t know, but I assume that they put the faecal samples inside a water-soluble capsule, inside an acid-soluble capsule … the aim being to get the bugs alive, to the duodenum. Since the capsules are a pretty well-established technology, why get involved in a complex piece of bioengineering? KISS!

        1. Elephants most certainly do have the ‘right gut flora’. Larger size = lower relative metabolic requirements = the ability to survive on entire trees rather than grass seeds.

          1. As an aside, I watched a small herd pass through a stand of trees in South Africa. They pulled the trees down and ate the roots, leaving the top lightly grazed. No wonder they require a large range.

    3. I am not sure why the absolute number of muscles should have a particular bearing on diet and protein requirements. Mammals may have fewer muscles but relatively bigger ones for example?

  3. For the second puzzle, herbivorous snakes, I would have to ask why legs were not useful to the ancestral snake.

    I would say they allow a different angle of attack to eat plants, but surely this is about the snake’s jaw? Does it have the mouth & jaw to browse like the marine iguana? The ancestral snake was already a committed carnivore I would guess.

    1. Also, I guess all the herbivore niches might have been occupied – there is no ‘snake herbivore’ niche to fill?

    2. That’s what I was thinking. Maybe I’m wrong, but don’t all snakes swallow prey whole? It seems a much farther leap to go from that to browsing on vegetation, rather than starting from a jaw that already tears bits of flesh off some bones.

      1. Ummm … I remember reading a paper on that a while ago …
        Here we go. Do you have access to Nature online, or ink-on-dead-tree (hint : is your friend, but not the publisher’s)? If not, I can mail you the PDF.
        NATURE, VOL 400, 12 AUGUST 1999 p655 “The origin of snake feeding”, Lee, Bell & Caldwell.
        It’s a more complicated story than my memory was telling me.
        Abstract :Snakes are renowned for their ability to engulf extremely large prey, and their highly flexible skulls and extremely wide gape are among the most striking adaptations found in vertebrates.
        However, the evolutionary transition from the relatively inflexible lizard skull to the highly mobile snake skull remains poorly understood, as they appear to be fundamentally different and no obvious intermediate stages have been identified. Here we present evidence that mosasaurs —large, extinct marine lizards related to snakes—represent a crucial intermediate stage. Mosasaurs, uniquely among lizards, possessed long, snake-like palatal teeth for holding prey. Also, although they retained the rigid upper jaws typical of lizards, they possessed highly flexible lower jaws that were not only morphologically similar to those of snakes, but also functionally similar. The highly flexible lower jaw is thus inferred to have evolved before the highly flexible upper jaw in the macrophagous common ancestor of mosasaurs and snakes — for accommodating large prey. The mobile upper jaw evolved later — in snakes — for dragging prey in to the oesophagus. Snakes also have more rigid braincases than lizards, and the partially fused meso- and metakinetic joints of mosasaurs are transitional between the loose joints of lizards and the rigid joints of snakes. Thus, intermediate morphologies in snake skull evolution should perhaps be sought not in small burrowing lizards, as commonly assumed, but in large marine forms.

        That’s 20 years old, but it should be a good start for the citation indices if you want to follow up on the subject.

      2. I’m thinking the same thing. Perhaps snakes do it that way because they can. They can/must swallow their prey whole and live off of that for a good while. Eating meat probably gives this reptile the kind of power-packed protein and haemoglobin it needs. This also affords them the advantage of hiding out/lying low during the digestion period, saving energy while potentially avoiding life-threatening situations.

        Just guessing here – Perhaps birds started to lay eggs as a way to share the incubation of eggs with their mates, though it does seem that that duty falls to the female in many species of birds.

        These are merely layperson guesses!

    3. the ancestral snake.

      I’m not a snakeologist, but I’m pretty sure I’ve heard systematists saying that leglessness has evolved multiple times in the squamate line, and that “snakes” are thus polyphyletic. There is no “ancestral snake” from whom all present snakes are descended and which had no non-snake descendents.

  4. Archosaurs and relatives in general are/were not viviparous (not seen in crocodiles or turtles, as well as birds, no evidence for any dinosaur). All of these critters have a heavily calcified eggshell, as opposed to the more leathery eggshell of squamates (and monotremes), so probably a derived amniote condition. One hypothesis (I don’t know how well this holds up to scrutiny these days) is that the calcium in the shell is derived from the yolk of the embryo, rendering the reduction/loss of the eggshell (as would be the condition in an intermediate ovoviviparous form) difficult or impossible.

    Now I write that, it doesn’t make any sense! Still, if you’re committed to having a highly calcareous eggshell to start with, then you sort of can’t start losing it utero and gain any maternal nutrition, as a small amount of loss (as an intermediate condition) isn’t going to help. Not so with a more leathery eggshell

    With regards to the snakes, it might be difficult to build a big gut, necessary for fermentation of herbage(or at least holding t for a long time)into a limbless animal. Yes, snakes have an expanded gut after eating a meal, but then they are sessile for a long while, whereas a herbivore has to forage more continually.

    1. Many turtles have leathery eggshells that absorb water during development, whereas others (tortoises, mud turtles) have brittle calcareous eggshells. But true, none are live-bearing.

      1. I wonder if this isn’t more a a life-history thing than a physiological constraint. Chelonians all seem to have a Type III life mortality profile – something you tend to see with long-lived, heavily-defended adults that can count of repeated reproductive episodes, especially when the defensive method only works for animals above a certain size. Given that most offspring won’t live long enough to reproduce, it makes sense to adopt an r-strategy.

    2. Viperids (and similar heavy-bodied venomous forms) retain faecal matter in the hindgut for long periods – it acts as ballast when they strike. So retention of matter in the gut isn’t unknown for snakes and hindgut fermentation isn’t forbidden by this consideration, at least. I think that you’re right, though, that vegetation is just not nutrient-dense enough to make it worth snakes’ while, given other constraints on their anatomy.
      Interestingly, all of the limbless lizards that I know about are carnivores as well.
      I wonder if metriorhynchid crocodilians were viviparous – they certainly don’t have the sort of anatomy that would have permitted them to easily come ashore and dig nests on the beach.
      The New Zealand gecko species are all, as far as I know, viviparous, despite a chalky egg shell being plesiomorphous for gekkonids. Nonetheless your physiological constraint for viviparity in archosaurs sounds interesting and plausible. But natural selection always seems to find ways around this sort of thing, even if only in a few cases.

    3. it might be difficult to build a big gut, necessary for fermentation of herbage(or at least holding t for a long time)into a limbless animal.

      One word, but six legs : termites.
      Though whether the chemistry that their bacterial symbionts use is “fermentation”, or something different is unknown to me. I think they use different classes of bacteria to those in ruminant mammals.

  5. Have argued, with myself at times, these two questions on numerous occasions. If snakes had a fossorial origin then the size and complexity of a gut to digest plant matter would preclude a narrow body. Snakes have everything slimmed down including loss of bilateral symmetry (think lung) so a simple alimentary canal would mean a carnivorous diet. I do though seem to recall accounts of some species of snake eating fruit on occasion – digestion of the sugars but not cellulose would not require a complex gut. But now I am feeling old, recalling the Garth Underwood papers on the origin of snakes.

  6. “…It has been argued that birds lay eggs because they would be too heavy to fly if they were carrying around young inside them. Apart from the obvious problem that bats manage fine, if that argument is right, you might expect some flightless birds to have been live-bearing…”

    Here’s my (non-scientist) guess:

    Bats are grouped with placental mammals who probably later developed wings. Birds are on an evolutionary line that are egg laying and have coped well enough with that, a good enough adaption for whatever environment that they find themselves in.

    I’m prepared for a D- and will stand at the back of the class wearing a dunces cap facing the wall. 🙂

    1. Yes, but I bet that penguins would be viviparous if there was any way that they could be! Explains why there are/were no fully marine birds or dinosaurs (as opposed to mammals and lepidsaurs).

    2. I agree with you on the bats: they were, with near certainty, already placental before they evolved powered flight.
      Moreover, note that bats, contrary to most birds, have weak hind legs. I have this notion (correct me if I’m wrong) that bats, contrary to birds cannot ‘take off’ from the ground, but need a high point to launch themselves. If ‘grounded’ they need to climb up to a higher place for the launch.
      Hence the ‘Egg Weight’ argument is not definitively dismissed by comparing to bats.

      Pterosaurs were different still, as was discussed in 2 earlier posts:

        1. My post got lost after trying to post it!

          I made the comment that vampire bats can take off from the ground, and that New Zealand bats(mystacinids) are more terrestrial than other bats with more robust hind legs, and even hunt on the ground. Maybe trying to link to a video was the problem. I’ll post this, and if it works will try and post the video

          1. Nope, the video will not post.

            Meanwhile, I’d also noted that bats came from a quadrupedal ancestry rather than a bipedal one, as birds did.

  7. Snakes are optimized for sneaking up on things and eating them whole. Eating plants requires chewing which is not something snakes do.

    Came up blank on why there are no live-bearing birds. I’m very interested to know the theory behind that one.

    Bonus question: Why are there no flying snakes? LOL

    1. There is a least on species of snake which can glide, a behavior thought to be the predecessor to flight in birds. (I’m not saying snakes are evolving flight)

      1. The video isn’t posting, but from Wiki, “Chrysopelea, more commonly known as the flying snake or gliding snake,”

  8. Good questions. One can suppose that penguins, for example, might benefit from live births because then they wouldn’t have to spend a long time ashore in rookeries and they don’t have the weight constraints that flying animals have. So you’d think there’d be some pressure for live births in them, at least.

    I don’t have an answer to either question, but dollars to donuts in both cases it is likely due to energy budget constraints; maybe (for reasons I do not know) it is too energetically costly for birds to have live young or for snakes to be herbivorous. What could those constraints be? Maybe gas exchange in eggs permits the development of bird embryos that cannot happen in an avian womb. Maybe the size of the snake gut can’t extract enough energy from plants. It seems to that any solutions I come up with could be dealt with by mutation and evolution; both birds and snakes have been around long enough.

    It’ll be interesting to see what folks here think.

    1. 1. There may be a minimum body weight for a pup/chick to survive in cold waters (though the emperor penguin may be getting near the weight of the smallest sea mammals – sea otter and Baltic seal).
      2. Maybe there’s no available evolutionary route from a Penguin’s complex breeding behaviour.

    2. Maybe gas exchange in eggs permits the development of bird embryos that cannot happen in an avian womb.

      I think that point needs some more development. In the event that birds were to move towards viviparity, wouldn’t development of some structure functionally equivalent to a placenta be a necessity? As it is, there is a range of degrees of development at which a chick comes out of the egg, from ones that can scamper around within minutes of hatching, to ones that need weeks to even open their eyes. So the potential to gain by actually developing the chick internally is definitely there.

      1. Birds, like all amniotes, have a chorio-allantoic membrane. That’s what forms the placenta in mammals, and in some squamates

        1. Which is why I’d expect that something similar would happen in birds if they were to develop viviparity. The mineralisation of the shell would decrease (or the mechanism that mineralises the egg after production of it’s other structures would delay it’s initiation) while the membrane develops a nutrient transfer role. If I recall correctly, the various structures of the egg are laid down from embryo and yolk outwards (indeed, I can’t think of a way it could be otherwise without there being some major circulatory system develop in the egg, which I don’t see unless I accidentally bite into a fertilized egg at breakfast time). So, the membrane would be the penultimate structure added and the shell (mineralised or not, “leathery”) the last structure. Then it’s “tatties over the side”.

  9. At one point I asked the question about birds in my Evolution class. It was actually a formal assignment where the students had to formulate hypotheses. Following the assignment, they read and discussed the paper by Blackburn & Evans (see below). Most of the students opted for the “eggs too heavy for flight”. However, while it seems too general and vague, phylogenetic constraints are certainly part of the story. For example, endothermy is clearly a constraint on mammalian life history patterns.

    Daniel G. Blackburn and Howard E. Evans, “Why are there no Viviparous Birds?,” The American Naturalist 128, no. 2 (Aug., 1986): 165-190.

    1. I wonder if eggs are useful during dry periods?

      Is there an advantage to the adults to brood? Does it help them build up a store of energy for the exhausting period of feeding the hatchlings?

    2. Besides, there are plenty of ground dwelling birds. Did the ancestral ratites ever fly?

      Oh – “The finding that tinamous nest within this group, originally based on twenty nuclear genes[20] and corroborated by a study using forty novel nuclear loci[22] makes ‘ratites’ paraphyletic rather than monophyletic.[23] Since tinamous are weak fliers, this raises interesting questions about the evolution of flightlessness in this group.”

  10. I put it all down to bad luck. Bad in the sense of no lucky mutations or, if they happened, they got drifted away.

      1. I found another bonus Mencken quote that is great :

        ” The final test of truth is ridicule. Very few dogmas have ever faced it and survived. …”
        “On Truth” in Damn! A Book of Calumny (1918), p. 53″

        same source.

    1. In other words, just because it hasn’t happened yet, or happened but didn’t survive for long, doesn’t mean it can’t happen in the future. Evolution is going on all the time.

  11. I’m with Dominic in thinking the specialization of the snake jaw is too extreme to allow a path to a vegetarian diet along which each step is at least as “fit” (as evolutionary biologists use that term) as the previous step, and preferably an improvement. And I’m with Diki in thinking that the argument from bats is weak. They come from a lineage that long since gave up egg laying. Moreover, we don’t need to believe that carrying the developing embryo to term for a live birth would make flight impossible, just too costly (e.g., in terms of energy expended).

    1. I’m with Dominic in thinking the specialization of the snake jaw is too extreme

      There are at least two designs of snake jaw. I posted one Nature link up-thread (NATURE, VOL 400, 12 AUGUST 1999, p655) but while I was digging that one out, I also found NATURE, VOL 402, 25 NOVEMBER 1999, p369. This describes a different jaw anatomy, particularly in terms of how the dentary (upper jawbone ; “reptiles” have distinctly more bones in their jaws than mammals, in whom a number of the bones have moved into the ear) is attached to the bones of the cranium.
      First Para : (it’s an article, not a paper)

      Most snakes transport prey through the mouth by using asynchronous ratcheting movements of their upper jaws. In contrast, we have found that threadsnakes (members of the basal snake clade Scolecophidia) have a unique feeding mechanism in which the tooth-bearing elements of the lower jaw rotate synchronously in and out of the mouth, dragging prey into the oesophagus.

      I’d forgotten about threadsnakes before doing that search.

      Threadsnakes (family Leptotyphlopidae) are tiny, burrowing serpents that feed predominantly on the larvae, pupae and adults of social insects. […] To obtain sufficient quantities of ant brood, the snakes must invade nests that are tenaciously defended by worker ants. Large, aggressive ants can seriously injure or even kill these small snakes, which as adults are usually less than 2.5mm in diameter and 1.5g in weight.

      Interesting beasts.

  12. I think there is less lability for these features in the taxa concerned then is indicated in the posing of the questions.

    1. Lack of viviparity in birds. Although, as rightly noted, viviparity has evolved multiple times in lizards and snakes (squamates), the latter are not especially closely related to birds. Crocodilians are the nearest living relatives of birds, and all are oviparous. It is, of course, hard to know the reproductive habits of extinct organisms, so viviparity could have been more widespread, but many dinosaurs (also close to birds) are known to have laid eggs. No archosaurs (birds + dinosaurs + crocs + pterosaurs + various others) are known to have been viviparous, so lack of viviparity is a feature of a large and old clade. Turtles as well are all egg-layers. The only other reptiles besides squamates that are viviparous that I can think of at the moment are ichthyosaurs. Thus within reptiles and their descendants (amniotes), viviparity has evolved once or twice in mammals, once in ichthyosaurs, and many times in squamates. So while viviaprity is clearly in the ‘mutational space’ of reptiles, it is not phylogenetically widespread within the group.

    2. Lack of herbivory in snakes. Among lizards, folivory is really a well-developed lifestyle only among iguanas, a single family (or subfamily) with a few dozen species, and involves gut specializations for digestion. Frugivory occurs in a number of other lizard families (geckos, anoles), and is more phylogenetically widespread than leaf-eating, but seems to be supplemental to a mostly carnivorous/insectivorous diet. So while herbivory is big for iguanas, it’s a bit of a one off for lizards in general.

    The two features have thus not evolved terribly often. The difficulty of doing so could be due to mutational constraints, ecological/functional constraints, or a combination. And I could go on about such possible constraints: for example, I could note that fleshy tongues (lacking in snakes) are used by geckos and tortoises in feeding on plant matter; and that monitor lizards, with teeth and tongues like snakes, can use inertial feeding (throwing the head back) to swallow; but that snakes depend on a gripping of the prey that works on dead bodies and eggs, but probably not on leaves. (Snakes could probably swallow fruit, so obviously swallowing mechanisms can’t be the whole answer.)

    So, I don’t think it’s that ‘easy’ for amniotes to evolve viviparity (unless you’re a squamate) or that it’s ‘easy’ for squamates to evolve folivory.

    The preceding should be considered supportive of, and supplementary to, the remarks of Christine Janis above.

    1. Interesting! Do YOU think that eggs in birds with their hard shells enabled them to exploit drier environments?

      1. Any advantage that mineralised shells provide over non-mineralised shells is likely to be pretty marginal. Plenty of lizards, snakes etc get by in desert environments where birds are very rare.
        Also, the various clades of Permian period “reptiles” (from various of which our current mammals, dinosaurs (feathered, twittering things engaged in strafing shopping streets), squamates, crocodilians and a few others evolved) have no record that I know of mineralisation. If they hadn’t been fossilised, we’d probably call them “leathery”. Many of the non-marine deposits of this period show evidence of having been laid down under arid, oxidising conditions.

        Mineralised shells have a complex pattern of pores in their surfaces. The patterning of these pores is one of the characteristics used for classifying fossil shell, and when it is found with (sufficiently) intact embryo fossils, you can link pore patterns to dinosaur classes. Then, isolated shell fragments can be assigned with some degree of confidence to a particular group, so you can claim to have found, for example, a sauropod nesting site, or a theropod one.

        In a mineralised shell, water loss is managed, actively, by the cells of the “chorio-allantoic membrane” (that Christine Marie Janis reminded us of upthread), not passively by the mineralised shell.

        Hmmm, it has been a very long time since I looked at a shell fragment under the microscope. Where’s my hand lens?

        1. Now I remember what I meant to say in my initial post (and why it didn’t make any sense!) OK, the situation for archosaurs is that the heavily mineralized shell is meant to be the source for the calcium in the embryo (rather than the yolk), and hence any reduction of the shell would be disadvantageous.

          This was an old idea from (I think) Gary Packard, and I have no idea if it is still viable, although it makes for a nice story. But if there’s some truth to it, then it might explain why you never see eggshell reduction in archosaurs (+ turtles), which would be the necessary first step to viviparity. In fact, I’m not sure you even see egg retention in archosaurs (the most basic form of ovoviviparity).

          The chorioallantoic membrane is involved with gas exchange. Like water gain or loss, this is passive, by diffusion (or osmosis): what the CA membrane does, because it’s vascular, is to speed up the diffusion process by transporting the oxygenated blood away from the (inner) surface of the eggshell back to the embryo (speeds up diffusion because this process increases the concentration gradient across the shell).

  13. My two cents is that vivaparity evolves from oviparity when the eggs become overly vulnerable to neglect or predation. Because birds are mostly co-equal parents (ninety percent are socially monogamous), oviparity is safer than the anatomically dangerous notion of live birth. Other adaptations like a decreased incubation time or chick precociousness, have likely arisen in polygynous birds with vulnerable eggs that are as safe or safer than the costs of vivaparity.

    1. But that doesn’t explain why no archelosaurian (a word I just discovered to include turtles and archosaurs) is viviparous. Sea turtles might have a great advantage in being viviparous (as are sea snakes). And it remains true that none of the extinct marine reptiles, may of which were known to be viviparous, were archelosaurians.

      Live birth doesn’t appear to be ‘dangerous’ for lizards, and birds of course have huge eggs for their size (so oviparity there might not be too convenient, either).

      I think perhaps a more interesting question is why therian mammals became viviparous!

      1. Good point.
        It is thought that the eutheria, the placentals, became so due to a retroviral infection. The genes initiating the syncytium of the trophoblast, and hence the placenta, are of retroviral origin.
        Most of the viviparous squamates and sharks do not have a placenta, but eggs that hatch within the body (if I’m not mistaken, that is), which is quite different.

        1. Yes — But —-

          The retroviral infection in at least the ancestor of extant placentals is expressed in the trophoblast, but all mammals (even monotremes) have a trophoblast of some sort. What those syncitium genes do is aid in immunosuppression, and may enable the fetus to suppress the maternal immune reaction and to be retained for longer in the uterus (i.e., past one estrus cycle). Note that some marsupials have a brief chorioallentoic placenta (in addition to the earlier yolk sac or choriovitelline placenta seen in all therians)at the end of gestation. Some simpler version of these genes are seen in marsupials, so may have been involved in the initial viviparity in a therian common ancestor. What is more interesting/puzzling is how this appears to be an all-or-nothing thing for therian mammals, as opposed to having been evolved many times in squamates.

          Viviparous squamates and sharks may indeed have membranes that function as a placenta, whether from the chorioallentois (in squamates) or the yolk sac (in sharks). That is, if they’re truly viviparous, not ovoviviparous. Dan Blackburn has written a lot about this stuff.

          1. Function as a placenta, exchanging oxygen and ‘food’ from the mother’s blood stream? I didn’t know that, very interesting though.

            1. No, it’s that the genes are expressed in the chorioallantoic placenta, which appears to relate to the maternal tendency to reject foreign material (i.e., the fetus in this case) being suppressed.

      2. Evolving to live birth may be tricky due to the risk of ectopic implantation or even non implantation, if the intricate processes of fertilization, zygote migration, implantation, uterine mucosal preparation, etc. are not all worked out.

  14. Some excellent ideas have been offered. I don’t have much to add, except to note that – it seems to me there should,/i> be vivaporous birds, given that the only thing required is the loss of a complete shell building process. A smallish mutation could trigger a shortened egg building cycle, followed by a few more mutations that do no more that eliminate, not invent processes. The initial driver could be that it simply is less energy consuming to reduce eggs and eliminate them. That said, I’m unsure if holding the young within the body for extra development time takes more energy than carrying food to the nest. That would be, taking the baby to the food instead of the food to the baby.

    1. I’m unsure if holding the young within the body for extra development time takes more energy than carrying food to the nest.

      When the adult is away foraging, the hatchling(s) in the nest are decidedly vulnerable.

      q.v. cuckoos.

  15. I think it is amusing that consideration of both questions each touch upon a factor that makes substantial use of a single element of the periodic table:

    question #1 brings up eggs. Calcium carbonate is used to make eggs, and the calcium has to come from the diet.

    question #2 brings up teeth. Teeth in humans are formed from a number of things, e.g. hydroxyapatite..[Google searches…], of which a substatial fraction is calcium, e.g. in the form of calcium …[Google search] phosphate. I don’t know if every species uses the same synthetic process to form teeth, but genuine teeth (in my non-expert view) all contain compounds made from calcium. Cartilage [searches Google…] seems not to contain calcium. lots of other proteo-glycan/protein though.

    … so perhaps that’s a coincidence, but it’d be interesting to follow the calcium, see if there are large discrepancies in living species that hinge upon the biological management of calcium.

    1. Calcium carbonate is used to make eggs, and the calcium has to come from the diet.

      Bird eggs, yes. But other vertebrate eggs, not necessarily. Many groups have unmineralised eggs, and I wouldn’t rule out the possibility of other minerals being used in other phyla. Since insects use chitin for everything short of writing materials, I wouldn’t be at all surprised to see them using it for egg casings too, possibly bolstered with calcium phosphate minerals.

      Calcium is a common element (Z=20 ; 13th in abundance in the Sun ; 64 atoms for every 1000 of silicon and 28,000,000 of hydrogen). It would be surprising if organisms hadn’t developed some way of managing it, even if only to exclude it from cells because of any toxic effects. Magnesium is a lot more common (#7 in abundance ; 1000 atoms on the same scale), and outside the vertebrates is often used as part of structure of precipitated calcium carbonate minerals.

      It’s not just calcite out there, there is also aragonite (orthorhombic rather than trigonal) and vaterite (much rarer, hexagonal) ; calcite and aragonite both occur in high- and low- magnesium forms (~20 to 4 atom % and 4 to trace) ; all 4 forms have different stability fields in temperature, pressure and salinity fields and in the presence of water can interconvert, which can be detected texturally under the microscope. Or with one of several staining solutions (I only ever used Alizarin red – it does an adequate job and you only need three wells in your spot dish – good old Feigl!). Or if you happen to have it, a cathodoluminescence attachment for your ion microprobe (which didn’t arrive until the year after my graduation). But you are expected to be able to identify grains of shell debris to the phylum (sometimes order) level under the petrological microscope, and to work out the approximate deposition and recrystallisation history for Mr Joe Random Limestone. At least, you are if you’ve got a carbonate petrologist on the faculty. Fortunately, it was an open-textbook exam, so if you’d spent the previous 4 years assiduously writing marginalia in your copy of “Introduction to the Rock-Forming Minerals”, you should be able to work it out.
      Sometimes, knowing the order of the common fossils can even give you an approximate age for the specimen – what with the way living organisms speciate and go extinct and varying their calcium chemistry is a useful trick for handling changes in water temperature, pressure or salinity.
      Did someone mention the sheer amount of things we do know about the natural world, which our friends, the Creationists, dismiss out of hand as being made up. That rather riles me sometimes. I can’t imagine why.

      1. Hmmm, I need to find a composition listing for “Bulk Silicate Earth”. Solar abundances are a useful estimator for what our galaxy consists of, but for planets which have lost a significant amount of “volatiles”, maybe not so useful. One of the discussions on early Earth atmospheres I was going through at the weekend was proposing the Earth has lost on the order of 500 bar of atmosphere, mostly CO2, as a consequence of the (putative) Moon-forming impact. That’s a non-trivial composition change.

      2. “Bird eggs, yes. But other vertebrate eggs, not necessarily. Many groups have unmineralised eggs, ”

        In general:
        Birds : no teeth.
        Other vertebrates: teeth

        I recall discussion of this here but I’m in haste…

        1. I doubt that is significant. Which has more calcium – your set of teeth, or one of your hands? Actually, your set of wrist bones, or your pair of heel bones would probably be a fair match for the calcium content of your teeth.

          1. “Which has more calcium”

            What do you mean?

            I mean there is apparently a biosynthetic pathway for these structures. How these pathways are used – if they are homologous at all – is what I am interested in knowing. It’s also a matter of resource management economy. If teeth and eggs are too much for the mother to manage together, one of them might be manageable.

            But these are hasty thoughts – I haven’t done a literature search.

            1. Compared to the rest of the skeleton, the (Ca) investment in the teeth is trivial.
              Is the C-A membrane that (sorry, name not to-hand) mentioned sufficiently active that it could initiate transfer of calcium to the process of mineralising the skeleton in the last few days of development? Mineralisation in the human skeleton is continuing well into the human’s 20s if not 30s. I’m not sure how long it lasts in avian dinosaurs.

              1. “Compared to the rest of the skeleton, the (Ca) investment in the teeth is trivial.”

                yes but my point is not just how large or how much calcium is used in a selected individual’s body part, but how the biosynthetic pathway is managed by the organism to produce the part in the first place. there’s economy at work – the teeth are only as big as they are because the female needs to make eggs at some point.

                I need to read more about which biosynthetic pathways produce teeth, bones, and eggs, and other calcium-containing structures.

              2. The avian dinosaurs grew a special type of bone in various skeletal positions (sufficiently conservatively between modern avian dinosaurs and Tyrannosaurs to allow the bone type to be recognised in the dinosaurs (my memory is telling me that it was either the specimen called “Sue”, or “B-rex” under it’s 2 kilotonnes of spoil). In modern avian dinosaurs, that m…, m…, m… (I’ve forgotten ; it’ll come back) bone functions as a calcium buffer between the diet and egg laying. Which is what you’re talking about.
                But the volume of that bone is a few percent of the skeletal volume in adults. A lot less than individual size variation.

    1. In amniotes, at least, I think oviparity. Those three additional extraembryonic membranes (i.e., chorion, allantois, and amnion) seem to be as much for mechanical support within a large terrestrial egg as they are for things like getting transferring oxygen to the embryo/storing wastes.

      1. By a long time. Well over a hundred million years, even if you’re restricting yourself to land-dwelling animals.

  16. Some snakes only eat eggs…but I guess that still means they’re carnivorous…but obligate carnivores? I guess so since eggs are animal matter. Snakes also eat insects, but I don’t think there are any that exclusively eat insects. I guess this is just a stupid comment that in a round about way is saying: I have no idea why ALL snakes are carnivorous. It is a good question though, as is the bird/egg laying question.

    Thanks to the readers above who have shed some light on both these biological puzzles.

    1. There are insectivorous snakes–the smaller ones, no surprise there. Two off the top of my head are the thread snakes (probably all of the blind snakes) and the shovel-nosed snake of the Mojave Desert, which specializes on cockroaches.

  17. Most birds seem to engage the males in the egg brooding process. Perhaps live-bearing reduces the input of the males at this development stage and is thus not as efficient?

    PCC said hand-waving theories are okay.

  18. The thing is, not only is no archelosaurian (now my favorite new word) viviparous (so male engagement may not be the key issue), but it really does seem that there is something preventing viviparity in birds. It would surely be advantageous for penguins, if they could manage it.

  19. Golly.

    I can contribute nothing to the biological discussion as such but I find this downright fascinating. I was, oddly enough, just wondering why there are no viviparous birds a while back. I’ve spent a lot of time thinking about the porosity of egg shells, and what sorts of materials might diffuse through them, and what some bird on its way to live birth might look like, but I am reminded, again, that you biologists actually know so doggoned much about the issues, and that if you don’t know the answer, it’s unlikely that anything I have to say is going to untie any Gordian knots that just happen to be lying around.

    Now, I am glad to be a friend of Christine’s, and I share with her the odd hobby of arguing with creationists, despite the futility of it. Discussions like this remind me that as ignorant as a typical creationist may be of biology as compared to me, I am likewise similarly ignorant compared to those who actually know something worth knowing. I am so glad to be able to take in and more or less understand the comments in this thread, which are illuminating.

    To digress just a bit, it raises two points for me about that creationist-arguing world. The first is that I think that creationists by and large have no idea just how much is actually understood about living things. If they did understand, they’d understand how ill-suited they are to just stomp in and extinguish the dominant paradigm for the sources of diversity in living things. I wish I could get a few of them to read something like this thread, though I know that mostly they’d say, “see? The scientists really don’t know ANYTHING. They’re just guessing!”

    But the weirder point is that this all reminds me of a recent book which may actually be the worst book I have ever seen the Discovery Institute publish. That’s quite a bar to clear, but Foresight, by Marcos Eberlin, is a masterwork of inanity. It comes to mind right now because Eberlin quite literally, and without a great deal of elaboration, raises the chicken-and-egg problem. There must have been a first egg. But you need a chicken to make it! Yet, to have the first chicken, you need to have had an egg already! You see? A completely air-tight, unsolvable puzzle! Along the way Eberlin commits a wide variety of crazy howlers, including thinking that “homology” and “homoplasy” are the same thing. And did you realize that the human cervix poses the same problem as the chicken and egg? The first human baby had to be born through a fully-functioning cervix! But how could there be a fully functioning cervix already when the first human baby was born? I wish I could say that this was an oversimplification of the book’s arguments. It is not.

    I reviewed Eberlin’s book over at Amazon. It’s such a stinker that nobody more qualified than I am has even bothered to rebut its silliness. And of all the doggoned things, who should respond to it but Jonathan Wells, a man who has the distinction of being even a bit less honest than the average Discoveroid?

    Anyway: thanks to everyone for your comments here. I am enjoying the heck out of reading it all.

    1. Now, I am glad to be a friend of Christine’s, and I share with her the odd hobby of arguing with creationists, despite the futility of it

      It is quite possibly the only blood sport which is ethically defensible. Pretty popular around here.

      the worst book I have ever seen the Discovery Institute publish. That’s quite a bar to clear

      Since the TV is playing a Sherlock Holmes story, I feel safe in deducing that you have a devout attachment to many less painful forms of self-induced flagellation as well. Actually reading that dreck is a serious task to undertake – up there with PCC(E)’s choice to actually read the Bible cover to cover.

      Jonathan Wells, a man who has the distinction of being even a bit less honest than the average Discoveroid?

      Isn’t he the one who claims to be a geologist of some sort? I seem to remember wasting effort on him some years ago. Definitely a waste of oxygen and food.

      untie any Gordian knots

      I am having a mental image of Gordian snakes.

      1. I think you might be thinking of Stephen Meyer, whose undergrad background was geology. Wells is the Moonie who went to get his Ph.D at Berkeley so as to arm himself for the fight against “Darwinism.” Both highly dishonest people, but I think Meyer is better at it — I think you could find people of reasonable intelligence, but no biological training, who could be bamboozled by Darwin’s Doubt, but it would be hard to be fooled by one of the books by Wells unless you not only lacked a good understanding of biology but also were a complete and utter imbecile.

      2. “Since the TV is playing a Sherlock Holmes story, I feel safe in deducing that you have a devout attachment to many less painful forms of self-induced flagellation as well.”

        I have been known to stay in a tavern when a Steve Miller tribute band started playing. But in my defense, there was a beer that needed finishing. And Ghee-Bone-A-Raga-Me-Bay-Beh is at least catchy.

        But I don’t think of this so much as self-flagellation as flagellation of idiots. I feel reasonably good about just calling this stuff out — when one of these books goes up on Amazon and every reviewer says it’s brilliant and that in a hundred years it will be regarded as the watershed work that undid the Darwinian deception, that makes me sad. And, at my low scientific pay grade (retired lawyer who does not want to see our nation run by fundamentalist halfwits — yes, I know, a bit late to the party) I can’t contribute to the work of science but I bloody well can DEFEND its integrity against these ridiculous assaults. Somebody, I think, has got to do it, and if I do occasionally have a few brain cells commit suicide because they no longer wished to live in a world where they are compelled to read Eberlin, Wells, Meyer, et al., I figure that they at least died in a good cause.

        1. Ah, an altar to the braincells sacrificed to Creationist-stomping. There’s an idea.
          Obviously, there would have to be libations.

          1. Indeed there would. Preferably a pint of Wadworth’s 6X, quaffed on a cold evening in Devizes.

            Unfortunately, I live in Seattle, so can do that less frequently than the situation calls for.

  20. Very interesting questions. I don’t know the answers nor am I an expert on the subject, but I have a potentially silly question myself – why are there no autotrophic animals or some other multicellular macroscopic mobile organisms with nervous systems? I don’t include kleptoplasty by some snails that ingest algae and keep the chloroplasts for some time, but that’s a good example of what I mean – why endosymbiosis of cyanobacteria or algae with an animal didn’t happen since obviously it happened more than once in algae or why the alge/plant lineage didn’t evolve into mobile life since the ancesters of plants and animals are mobile… I can think of many reasons why (from “it just didn’t happen by chance” to large autotrophs make enough food as they are so they don’t need to move etc.) but I still wonder if there’s a deeper reason… especially for animals who don’t use light to make organic mollecules…

    1. Ummm, if you have a large autotroph (or a small autotroph “hoping” to grow up and be a large autotroph), once it has chosen it’s place to settle down and grow up, what does it need a nervous system for? It has chosen it’s place to live with an adequate flux of whatever nutrients it needs, something to take the waste away … and something to have sex with (if it needs something other than itself). And once those needs are met … what need does it have for the physiological expense of a nervous system.
      It’s not an autotroph, but I’m thinking “tunicate” here. And many of the coelentorates (corals). But not the ones that engage in the eating of some fish while not eating Nemo-oids (Harlequin Clownfish, if I recall correctly) – they need some degree of nervous system for that, and indeed, corals do have one. It’s not big, it’s not complex, and it’s probably blissfully ignorant of Riemannian curved geometry, but they can do basic integration and/or differentiation, which is more than a lot of teenage hoominz.

  21. Eggs and weight for flying birds — the weight should matter, but we should think of weight of the clutch. Birds are able to rapidly lay up reserves in one egg, lay it in nest and immediately start on another, often producing an egg a day until the clutch is complete.

    So, if a small songbird produces six eggs, each 5% of the mother’s weight, it never has to carry more than 10% extra weight in flight [though actually will have increased fat reserves going into nesting]. A ‘placental’ bird might have trouble maturing a similar ‘litter’ of same-sized offspring..

    Other reasons probably connect with archosaur commitment to thick eggshell long before they took flight.

  22. I merely wish to thank prof Coyne for keeping this huge website afloat during all these years; and for keeping scientific modes of thought alive in laypeople’s minds like mine.

  23. Very interesting questions!

    I now see that spiders are often omnivores (and I knew about the more herbivore exception ), so that clade isn’t analogous. Here was an interesting attempt at explanation: .

    “At least part of the secret is that nearly all these herbivores (at least, those that eat leaves and parts of plants that have high cellulose content) have something in common: they don’t actually digest their own food. … To be fair, about 15-20% of snakes give live birth, and some have limited parental care and are social at times. But generally speaking, opportunities for microbe transfer between snakes are few and far between. What’s more, restrictions of functional morphology probably restrict snakes’ ability to evolve herbivory.”

    “Herbivory by a snake has actually kinda-sorta been reported in the peer-reviewed literature once or twice. … The more compelling case comes from a study done on island Cottonmouths (Agkistrodon piscivorus) by Harvey Lillywhite and colleagues, published in the journal BioScience in 2008. During a study of pitviper scavenging in the intertidal zone conducted on Florida’s Seahorse Key, Lillywhite observed Cottonmouth turds containing relatively large amounts of seaweed (up to 54 grams, or pretty much the entire turd). This seemed like a lot of material to be secondarily or accidentally ingested, and Lillywhite speculated that the Cottonmouths might be eating seaweed because it smelled like fish. Then he and his team went further: to test this idea, they presented Cottonmouths snakes in the laboratory with marine plant materials with and without fish present. The snakes thoroughly investigated the algae lacking fish scent for several minutes, with frequent tongue flicking, pushing, and probing, but they did not attempt to ingest it. When the presentation was repeated using plant materials rubbed with a dead fish or loosely enveloping a piece of fish, the snakes voluntarily swallowed the marine plants that had contacted fish, whether or not the fish was still present.”

  24. On birds, there has been good enough explanations offered, some very good! I wanted to add that I seem to remember the idea that harder, thinner shells evolved due to birds evolved better feathers. They could lie on and protect clutches to a greater extent than earlier reptiles.

    Maybe the trait is pretty much locked in since the egg has to handle being trampled, moved on et cetera.

  25. Regarding snakes, maybe a gene for digesting plants got damaged in one of their ancestors, and that mutation has been passed along to all offspring. This might be similar to the problems humans have with not being able to generate their own Vitamin C, as was mentioned in a video posted here a couple of days ago.

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