Here, I give you a long-planned post on the patella (“kneecap bone”) of birds, which was my Royal Society Senior Research Fellowship sabbatical project for 2012-13. This is only a brief introduction to the anatomical issues at hand, err, I mean at knee…
Stomach-Churning Rating: 6/10; mostly skeletons/fossils, but there are a few images of the dissection of a guineafowl, which is fresh and meaty.
The question I am exploring, first of all, is simply how the patella evolved, because it seems to be present in almost all living birds. However, it is absent in all non-avian dinosaurs, and indeed most Mesozoic birds, too. There is barely a hint of any precursor structure (a “patelloid”) in other reptiles, but lizards evolved their own patella that is quite different (a flattened lozenge, not a rectangular structure lying tightly confined in a “patellar groove” on the femur as it is in birds). Mammals evolved the knobbly, hemispherical kind of kneecap that we’re familiar with, possibly on several occasions (a different story!). So the patella evolved at least three times in the lizard, mammal and bird lineages– and possibly more than once in each of these groups. And that’s about it for almost 400 million years of tetrapod evolution, except for a few very rare instances in fossils and sort-of-patella-like things in some frogs or other weirdos.
Fossil birds exhibit no clear presence of a patella until we come very close to modern birds on the avian stem of the tree of life (see below). And then, suddenly in modern birds, there is a lot of variation and not much good documentation of what kind of patella exists. This makes it challenging to figure out if the patella is ancient for modern birds or if it evolved multiple times, or how it changed after it first evolved– let alone bigger questions of what the patella was “for” (performance benefits, functional consequences, etc.; and developmental constraints) in the birds that first evolved it.
Considering that the patella is such an obvious bone in some birds, and certainly affects the mechanics of the knee joint (forming a lever for the muscles that cross it; homologous to our quadriceps muscles) and hence locomotion, it is a compelling research topic for me.
What follows is a pictorial guide to the patella of some birds, in sort of an evolutionary/temporal sequence (see my earlier post for a recap of some major groups), with a focus on animals I’ve studied more intensively so far (with >10,000 species, there is a lot that could be done):
- The early Cretaceous bird Gansus (from the IVPP in Beijing), represented by many beautifully preserved specimens, all of which lack a patella. This absence is characteristic of other stunningly preserved fossil Chinese birds, indicating that this is almost certainly an ancestral absence of a patella, until…
- The famed Cretaceous diving (flightless) bird Hesperornis, from Wikipedia/Smithsonian. Note the massive, conical/crested patella in front of the knee (jutting up and overlapping the ribs/vertebrae close to the pelvis; see also below). That elongate patella is characteristic of many diving birds that use foot-propelled swimming; it has evolved many times in this fashion. Other hesperornithiform birds show some transformational states in their anatomy toward this extreme one.
Check this out! More Hesperornis (cast), with the femur on the left and the patella on the right. The bloody patella is almost as long as the femur! That’s nuts. With kind permission from the Natural History Museum, London.
- Exhibited ostrich (Struthio camelus) skeleton in left side view showing the patella (white arrow), on exhibit atThe Natural History Museum at Tring, Hertfordshire, UK. Ostriches are remarkable in that they have this elongate patella (actually a double patella; there is a smaller, often-overlooked second piece of bone) and yet are rather basal (closer to the root of the modern avian family tree)– however, they obviously are specialized in ways other than this double patella, most notably their very large size, flightlessness, and elongate legs. So the unusual patella is more likely linked to their odd lifestyle than a truly primitive trait, at least to some degree (but stay tuned: what happened with the patella in other members of their lineage, the ratites/palaeognaths, is much less well understood!).
- Note that ostriches and Hesperornis together hint that the presence of a patella might have been an ancestral trait for living birds, but their patellae are so different that the ancestral state from which they evolved must have been different, too; perhaps simpler and smaller. Hence we need to look at other birds…
- Skinned right leg of a Helmeted Guineafowl, Numida meleagris, above. That whitish band of tissue in the middle of the screen, on the front of the knee, is part of what is concealing the patella. That is an aponeurosis (connective tissue sheet, like a thin tendon) of the muscles corresponding to our “quads” or our tensor fascia latae, detailed more below. Guineafowl are fairly basal and well-studied in terms of their bipedal locomotion, so they are an important reference point for avian form and function.
- Right guineafowl leg, with patella exposed. Here I’ve peeled away that white band of tissue and associated muscles, which have been reflected toward the bottom of the screen (AIL and PIL labels corresponding to the anterior and posterior parts of the Iliotibialis lateralis muscle). The tip of the scalpel is contacting the patella. It’s not much to see, but lies atop the bright yellow fat pad that cushions it against the femur. You should be able to see a groove in the end of the femur just above that fat pad, which is where the patella sits and slides up and down as the knee moves/muscles contract. This is called the patellar groove, or sulcus patellaris.
- Left leg of another guineafowl (with right tibiotarsus behind it, on the left) showing the patellae in articulation; in medial (inside) and cranial (front) views, respectively. The patella is the little rectangular bit of bone in the top middle of the screen, interposed between femur (thigh) and tibiotarsus (shank). With kind permission from the Natural History Museum, London.
- Right leg of a Cape Penguin (Spheniscus demersus) from the University Museum of Zoology in Cambridge, showing the big lumpy patella in this wing-propelled diver. They still walk long distances on land, so presumably a patella plays some role in their gait, helping to explain its large size, which like the ostrich and Hesperornis seems to be a novel trait. Notice the groove across the patella, made by the tendon of the ambiens (like our sartorius/”tailor’s muscle”), which crosses from the inside to the outside of the leg via this route. This groove is often considered a useful phylogenetic character in modern birds, as its contact with the patella (sometimes via a hole, or foramen) varies a lot among species.
Closeup of the knee/patella of the hornbill, Buceros sp., from above. Not much to squawk about, patella-wise, but it’s there.
And so we complete our quick tour of the avian patella, in its grand variation and humble beginnings.
Why does an ostrich have a patella and a Tyrannosaurus, Edmontosaurus or Triceratops did not? Why were birds the only bipedal lineage to evolve a patella (mammals and lizards gained a patella as small quadrupeds), and why did some bipeds like kangaroos “lose” (reduce to fibrous tissue, apparently) their patella?
These are the kinds of mysteries my group will now be tackling, thanks to a generous Leverhulme Trust grant on sesamoid bone ontogeny, mechanics and evolution. My group is now Dr. Vivian Allen as the postdoc, Sophie Regnault as the PhD student, and Kyle Chadwick as the technician and MRes student, along with numerous collaborators and spin-off projects. We’re looking forward to sharing more! But for now, I hope that I’ve engendered some appreciation for the avian patella, as the silly title indicates (“fella” used in the general sense of anyone!). This work is all unpublished, but some of this should be out in not too long, in much more lavish detail! Much as the patella is the “forgotten lever “of the avian hindlimb, it is the fulcrum about which a substantial part of my research group’s activity now pivots.
This was very informative, John. Thank you. I have to admit that my avian anatomy knowledge is so poor I was unsure whether they had patellae. After all, non-avian dinosaurs don’t…
Here’s a science question for you. I was told by a physical anthropologist once that people born with paralysis of the lower limbs never developed patellae. He was suggesting that, as sesamoids, patellae only develop in the presence of tension and movement in the patellar tendon. Is this true?
Thanks Ed! For human patellae, it varies depending on the disorder and the timing of paralysis. In general, according to data I am aware of, in mammals the patella will still form/ossify if the embryo is paralyzed. In contrast, this happens less often in birds. Intriguing difference!
But to make things weirder, there is a disorder in humans in which the patella and finger/toenails can fail to form. Why these 2 structures? I do not know. http://en.wikipedia.org/wiki/Nail%E2%80%93patella_syndrome
You might enjoy this dance of the patellas, filmed by myself (centre) and my daughters. http://www.youtube.com/watch?v=vr6JjZbIRew
oops I meant patellae.
Brilliant dancing patellae! I love it!
Fascinating stuff as always, John. Thanks for sharing. I must admit that, until I started reading your blog, I had thought that patellae were rather uninteresting or rather, might have thought that, if I had actually thought about them much.
I was completely unaware how freakish the patellae of arsefoots (arsefeet?) are. I have seen a pieced-together skeleton of a grebe but don’t recall noticing any knee-related weirdness.
Regarding the difference between avian and bipedal non-avian dinos in the presence/absence of patellae, could it be related to the reduction of the tail and the change in attachment of the caudofemoralis? If so, and Mesozoic birds had neither patellae nor proper tails, the next question is why did modern birds feel the (k)need to develop bony patellae when they did and why wait so long?
I felt that there was an opportunity here for a line about some Big Bird and Seamoid Street but was unable find it.
*Sesamoid Street, dammit. No cake for me.
I wish I’d thought of Sesamoid Street. I may steal that joke to use later– in fact I am sure I will!
Yes, one of the hypotheses we’re chasing is that there might be a link between limb sesamoids like the patella in birds and the change in posture from non-birds to birds. I’ve thrown that hypothesis out there before in my papers from early in the 00’s but have waited to really test it more mechanistically– first we need to know what factors explain the development of a patella in living birds (and absence of a patella in other living tetrapods).
Thanks John, for wonderful example of nature’s relentless variability. You ask the ultimate question: “why”. Implicit is the assumption that there is necessarily an answer – the “evolutionary advantage” awaiting unraveling. I’m not sure.
DARWIN was a pigeon breeder at heart, and thus selected for just one trait, ignoring all others. Nature is no pigeon breeder. It selects phenotypes, not traits (and badly at that – just think of genetic drift). So the various patellas may be telling tales – “just so stories” to be enjoyed.
And damn DAWKINS’ “selfish gene”.
Aldo
Sure Aldo, and in a short blog post I can’t get into all the nuances but yes, there may be no simple why (in terms of patellar function); that was what the brief mention of “developmental constraint” in my post alluded to. Although at some level there must at least be proximate causation and we’re looking at that, too.
John, does the size of the cnemial crest have anything to do with the size of the patella? Is it an ossification in the same tendon in birds?
Generally yes, to a degree there seems to be a correlation although I’ve never seen it addressed quantitatively. It varies; e.g. in some birds the two are big, some birds have big crest and little/no patella etc. It is partly same tendon but other things attach to the 2 cnemial crests in birds, too; e.g. distal muscles going to ankle/toes.
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Wow! I never knew that scientists studied patellae in so much depth. I loved biochemistry in college ( took it last because I was afraid of it!) and was attracted to your site as I set up my WordPress.
Thanks! Yes scientists study everything in crazy depth. The more esoteric, the better sometimes! 🙂
(I also was afraid of biochemistry in college)
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