Sunday, July 24, 2011

Paleontology Myths, Part 1: The myth of ultralight bones

Over the next few months we intend to post a series of short commentaries exposing common myths in paleontological research that have biomechanical implications or relationships.  In some cases, these myths extend to work on living animals, as well.  Today's little "myth" is one of those that does, in fact, crop up in both studies on fossil animals and living ones.

Here is a cross section at the midshaft of the humerus of the pterosaur Bennettazhia; the trabecular braces  have been digitally removed to just show the outer shell of bone (the braces are little struts that run from one side to the other inside the bone):

As you can see, this bone is exceptionally hollow.  A similar degree of hollowness is seen in a few living birds, such as pelicans.  Many other birds also have hollow limb bones (though usually not quite as hollow as seen above in Bennettazhia) and nearly all birds have some hollow bones in the core of their body (vertebral column, especially).

The typical conclusion from observing these hollow bones is that birds and pterosaurs have (or had) light skeletons.  It's simple, clean, and intuitive.  Unfortunately, it's also wrong.  In reality, we have known since the 1970's that birds do not have lighter skeletons than that of other terrestrial vertebrates (e.g. mammals), and there is no evidence that pterosaurs had light skeletons, either (see work by Prange and others in 1979).  This may seem rather odd, but there are two key elements here:

1) A hollow bone is not necessarily lighter than a solid one if they do not have the same diameter.  Birds and pterosaurs tend to have wide bones.

2) A hollow bone is not necessarily lighter than a solid one, even given the same diameter, if the density of the material making up the bone is different.  Betsy Dumont showed very nicely in 2010 that birds have denser bone material than mammals.

So, as it turns out, while the bones of many birds and pterosaurs are/were extremely hollow, they are also wide, dense, and stiff - and this means that the total mass of bone is about the same as for a "typical" skeleton, like that of a mammal (i.e. thick walls and filled with semi-liquid marrow).  This might mean that the mechanical efficiency of pterosaur skeletons was very high - in other words, they may have had very strong skeletons for the same mass of material.  My work shows that this is true for part of the pterosaur skeleton (base of the wings, particularly) but no one has done an exhaustive look across the entire pterosaur skeleton to see if this holds true overall.

The issue of hollow bones and the potential for lightening the skeleton (or now) has been well addressed by Matt Wedel with regards to sauropods.  Check out his excellent blog post on it here.

But there you have it - birds don't have "light" skeletons, and it is likely that pterosaurs didn't, either.  As a parting bit of fun, here are some CT scans through the middle of the upper arm (humerus) and thigh (femur) bones of various birds, showing the extreme variation in the degree of hollowness:

Note: the humerus (upper arm bone) and femur (bone of the thigh) are shown in correct relative scale for each species, but the species are not shown to scale relative to each other - otherwise the Rhea and penguin would both be huge.



  1. With regards to the last image, one can take exceptional note that in flightless animals, the humerus and femur have the same relative diameter (penguin) or the femur is greater in diameter than the humerus (presuming all at midshaft length); the reverse is true in flighted animals, and even more exceptional in high-performance fliers (e.g., tropicbird). Very cool.

  2. Even though these recent studies suggest that hollowness doesn't affect mass, did this still have implications for avian evolution in the Mesozoic?

  3. Hollowness probably did have implications for avian evolution in the Mesozoic - we just aren't sure exactly what they were, because there are lots of possibilities beyond mass. In birds, the hollow bones are a form of so-called pneumaticity - so the bones are filled with air, and those air sacs connect to the respiratory system. Thus, right off the bat, expanding the air sacs into the bones might have been somehow important for respiratory function. It has also been suggested that there are thermoregulatory effects. On top of all that, there is the potential mechanical efficiency angle I alluded to, which is that the skeletons of hollow-boned birds may have greater strength:mass ratios, even if the total mass is unchanged.

  4. I should note that a great place to find more information about the evolution and importance of pneumaticity and hollowness in birds and pterosaurs is in the work of Patrick O'Connor from Ohio University. I highly recommend looking at his papers if you're interested in the subject. Pat is a very congenial individual, so if you're looking for something of his that's hard to get, just write to him directly.

  5. Speaking of possible myths related to hollow bones, is it true that animals with hollow, bird-like bones are unable to repair their bones if they become broken due to the lack of marrow? Disregarding the fact that a flying animal with a broken wing has a drastically shortened lifespan, anyhow.

  6. I am relatively certain that the marrow repair story is a myth. I can think of several cases of birds healing pneumatic bones (for example rehabilitated raptors - the humeri of birds of prey are air-filled). That said, healing might be slower or more complicated as result of pneumaticity. The primary factor for healing of bone is usually the presence (and density) of nutrient arteries to the bone, rather than marrow structure. Nutrient arteries exist in all living bones, including hollow ones. Great question, though - I had not actually heard that before.

  7. This is a smart blog.You have so much knowledge about this issue, and so much passion. You also know how to make people rally behind it, obviously from the responses.

  8. A hollow bone is not necessarily lighter than a solid one if they do not have the same diameter. Birds and pterosaurs tend to have wide bones.

  9. Its hard to find a good post on this topic.
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