Why are bird bones hollow? Why is palaeontology hollow?

“CP” recently posted on why bird skeletons are light, and if so why (they’re not, and she gave references). She then moved on to the question of why birds have air-filled, i.e. pneumatic, bones. As usual, she follows the habit amongst members of the Society of Vertebrate Paleontology, of pretending tracts significiant to the subject but inconvenient to their views or self-esteem do not exist, so she won’t acknowledge my book, where I gave some interesting answers to these questions (though it may of course be that no-one at for example Bristol and/or Southampton University, has told her of it).

CP took the view that birds have hollow bones because this makes them stiffer:

For now, the important thing is this: birds have hollow bones which make them more stiff.

It’s well-known that moving material from the centre of a beam to the outside can strengthen it. This is why space-frames are so common in engineering. However, this does not distinguish birds from mammals since mammals also often make the edges of a bone solid, filling the core with spongey format bone filled out with gunk, or just filling it entirely with gunk such as marrow (which generates blood cells).

So it turns out many mammal bones aren’t solid bone either. Of course they’re not. Mammal DNA isn’t stupid: it can evolve mechanically efficient shapes as well as archosaurs can.

Two questions remain though:

The first is: why are some bird bones empty even of gunk, holding just air?

It’s because air vessels inside birds tend to roam as they mature. Mammals can’t do this because their lungs have to expand, contract and bend, so salients or hernias from the edge of a lung would get tend to get pinched, stretched and torn, if ever they developed the habit of insinuating themselves into cracks and crannies in bones for example. That whole developmental style for the edge of lungs or air channels in the thorax is a no-no for mammals.

However, birds’ ancestors were two-legged, so their trunks didn’t bend when they moved. (Two-legged was convenient if you had big feathers on your fore-limbs, and it was possible because it was always warm at the start of the Triassic… which allowed warm-bloodedness to evolve easily… which allowed the facility of permanent dynamic equilibrium required by obligate bipeds.) Since the ancestors of birds (and pterosaurs, and dinosaurs) didn’t need to bend their lungs, those groups were able to invade their postcranial bones with air.

Actually, to be honest, this had started even before bipedality evolved, because an important insect hunting mechanism in the ancestry of archosaurs seems to have been flicking the head and neck forward, much as herons do, though with shorter necks. This gave great incentive for lightness, first of neck bones, and eventually even of those head bones that couldn’t communicate with the mouth, nose or ear. I think bone pneumaticity appears earliest in the neck in both their phylogeny and ontogeny.

It was this that made it possible for birds, but not mammals, to enjoy the full benefit of air-filled bones. As I explain in “The Secret Dinobird Story:…” chapter 7 section 2:

In most bird species, some bones contain air. These air chambers are connected to the air sac system, but air never normally flows through them (even though it has been possible for a bird to survive by breathing entirely via a break). Pneumatic bones connected to the breathing system would limit the effectiveness of the lung protection system outlined above for deep diving birds, and for some, the increased buoyancy would also be disadvantageous. Most diving birds, and some others (for example gulls) lack pneumatic bones. Parts of the body which move backwards and forwards rapidly, for example limbs, are best kept as light as possible, which is an explanation of the value of air in such cases where the most efficient use of bone is as a hollow structure. Mammals often replace matter from the centre of limb bones with yellow marrow, which is comparatively light, and perhaps air would be used instead if they had a mechanism for putting it there. (Tellingly, it is the more fat-rich yellow marrow that tends to be found in limb bones; the more water-rich and therefore denser red marrow is found elsewhere.) Some pneumatic bones in birds do not oscillate rapidly, for example certain vertebrae in many species. Perhaps there is simply no tissue that could be usefully installed in such bones (the bird was after all born without anything useful there, and providing access channels for any functional tissue may be awkward), while the weight of some useless non-gas may still be worth shedding. Some rapidly oscillating bird bones aren’t pneumatised, even amongst non-diving birds.

A second question is: why are some bird bones extraodinarily thin-walled, apparently designed to fulfil just a single engineering purpose, unlike mammal bones which look more robust all round?

It’s because birds don’t have a robustly tactile lifestyle (except for example penguins, and their skeletons show it). Birds don’t usually have to risk hitting the ground hard since they have parachutes. They may seem to fly in fast and land hard, but they carefully ensure impacts are made at a safe speed, and with the feet. Some birds of prey hit their targets hard but only in the way they want to – it’s not like a lion having to risk being rolled on by a zebra, or a large climbing mammal falling out of a tree. One supposes that birds as prey concentrate on evading contact instead of struggling effectively once they’ve been caught.

So pneumatic bones do actually make birds lighter. But their air-sacs don’t. From “The Secret Dinobird Story:…” chapter 7 section 1:

Schmidt-Nielsen (1971) debunked the fallacy that avian airsacs reduce the body’s density and thus aid flight (although a caption in the paper restated it), but it is still being repeated decades later in for example Gee (2000), and even in more authoritative works. Lowering the density does not assist climbing flight if the mass stays the same: adding an air-sac just means some other organ has to be moved elsewhere but unless it is done away with, it must still be lifted in flight; and if it is removed there is no need to replace it with an air sac. Reducing weight may be of little value to incipient fliers at the gliding stage since being heavier makes gliders faster without sacrificing much range. But low weight is a great advantage to powered fliers since it reduces the energy requirements for gaining height through air, perhaps most critically when taking off.

CP points out that one subject of her posting: why or indeed whether birds do have lighter skeletons than mammals, is answered: actually, they don’t. But in the penultimate paragraph she seems to consider re-inventing my 2012 wheel:

Now the title of this post is the evolution of the lightweight skeleton of birds, and I haven’t talked at all about evolution yet. So where does evolution come in, you might ask? Well I think, and I’m not alone in thinking this, that the hollow pneumatic skeleton of birds (and in fact pterosaurs, the extinct flying reptiles I study) evolved not purely as a weight-decreasing method, but likely in a more complicated intertwined way of increasing strength, decreasing weight, and improving the respiratory system while flying. This is certainly not a novel idea, but it’s about time this idea of the hollow bird skeleton evolving purely as a means to decrease mass be put to rest. I’ve seen it several times on “science” shows, and it’s brought up constantly in the media. It’s not all about mass reduction, but likely a complicated number of things that affect each other.

It did deserve sorting out – that’s why I did it. If she wants to reconsider the issues afresh, fine. But in science, misleading readers about the theoretical landscape by trying to airbrush other workers, or more importantly theories, is fraud. And so is claiming funding to do it.

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