Troodontids and owls: oh, the irony (part II)


It turns out that Cretaceous troodontid dinosaurs had asymmetrical ears. This makes them like owls, which also have asymmetrical ears. But not all owls have asymmetrical ears and, what's more, the story of ear asymmetry in owls is itself a pretty remarkable one....

Before getting distracted by godwits, I was talking about troodontids and their asymmetrical ears (and this itself came as a distraction, as beforehand I was talking about the evolution of blood-feeding in birds). The irresistible comparison that comes to mind is of course with owls, as owls also have asymmetrical ears (though not all of them do). The fact that troodontids evolved asymmetrical ears immediately falsifies the long-cherished notion that owls are unique among vertebrates in this respect. The asymmetry seen in owls differs, however, from that of troodontids in being restricted to the external ear; it is never present in the middle or internal ear.

Oh yeah, before I start, I want to say a big thank you to everyone who is reading, visiting and/or making comments. Sorry if I have yet to leave a response to a comment you're left. Sorry also if I owe you an email: I was already struggling to keep up with them, and since I've joined scienceblogs things have gone nuts. Anyway, if you like what I'm doing, please do tell your friends. Tetrapod Zoology seems to be growing in importance, and it would be nice to see it grow that little bit more: when the site first went live it was rated 60th in terms of number of hits, whereas today it is rated 28th. Right, back to those owls.


As mentioned, we begin with the remarkable fact that the ear asymmetry present in owls is not the same among all owls that have it: instead it turns out that owls are markedly different in the sort of asymmtry they possess. In some owls (including Eurasian eagle owls Bubo bubo* and most Strix species), the asymmetry is exhibited in the position of the external ear opening in the skin, in others (including some Asio species and Pseudoscops) it is the shape of the external ear opening that is asymmetrical, and in others (including the Tawny owl S. aluco and Ural owl S. uralensis) it is the skin folds around the ear openings that are asymmetrical. Actual asymmetry in the bony part of the ear has only been reported for the Ural owl, Great grey owl S. nebulosa, Tengmalm's owl Aegolius funereus and Northern saw-whet owl A. acadicus (Norberg 1977, 2002). Several species combine various of these different types of asymmetry. Asymmetry is not present in Athene, Ninox or Otus [adjacent pic of a Tengmalm's owl borrowed from here].

* Among Bubo owls, Norberg (2002) could only find asymmetry in B. bubo: it hasn't been reported in other Bubo species. However, further study is needed to determine whether B. bubo is really unique in this respect.

These markedly different types of asymmetry indicate that it didn't evolve once early on in owls, but at least five times, and perhaps as many as seven times. This makes things even more remarkable.

Why have asymmetrical ears? In simplified terms, it allows owls to better pinpoint objects that they are locating using aural cues. While all tetrapods that hear use interaural time delays to help pinpoint sound sources, owls with asymmetrical ears are also able to work out the elevation of a sound source, a pretty neat trick given that it allows them 'to capture prey in total darkness using acoustic cues alone' (Nishikawa 2002, p. 246). Norberg (2002) suggested that this specialized pin-pointing presumably evolved as owls first became expert at locating prey on the ground, and then on ground-living prey that inhabit dense forest, or hide in dense ground vegetation or under snow cover.

Here we come to the speculation I talked about at the start. If troodontids also have asymmetrical ears, does this mean that they were also specialized for the pinpointing of concealed prey based on acoustic cues? If so, then (combined with the fact that their hearing was probably very sensitive anyway) this is neat evidence supporting the idea that they were locating small mammals and other animals in undergrowth or leaf litter. The caveat here is that, as usual with dinosaurs, we don't have a sample size good enough to verify that this asymmetry was present across populations and species. I've only read about it in Byronosaurus and don't know if it's present in other taxa, which is why I'm especially intrigued by Castanhinha & Mateus' (2006) statement that it is present in some troodontid taxa (i.e., more than one). They also concluded that ear asymmetry in troodontids 'can be an analogy resulting from convergent evolution between troodontids and [owls], used for 3D directional acoustics'. So, we await the full paper with great interest. How incredibly cool.

i-18c7e1a75f47b9ecc2a3967cadcc4878-Luis alvarezsaur.jpg

What is 'the irony' I mentioned in the title? This is a reference to the fact that a group of peculiar European fossils from the Upper Cretaceous of Romania have been identified at times as belonging either to troodontids, or to owls. I just find it vaguely amusing that troodontids and owls turn out to have an unusual genuine similarity after all, albeit it in just one feature. The fossils concerned - dubbed Heptasteornis andrewsi and Bradycneme draculae - turn out to be neither troodontids nor owls, thus explaining the hilarious title of a paper that Gareth Dyke and I published on this material in 2004 (Naish & Dyke 2004). Heptasteornis is in fact an alvarezsaurid: a species of which is depicted in the adjacent Luis Rey image. I was going to talk about these Romanian fossils here as well but I have to stop there. In a bizarre twist of fate, the specific name for Bradycneme brings us back to that vampire theme again.

Oh well. So much for the shorter posts.

Coming very soon: those posts on vampires, Australian mega-cats, real live giant sea serpents, confessions of a quadrupedal hominid, those lost tree frogs, rhinogradentians, terrifying sex organs of male turtles, tortoises tortoises tortoises, and lots more (I did list loads of other stuff in the original version of this post but accidentally deleted them when I had to do an emergency edit on Jan 31st. Let me know if you have a copy of the original post).

Refs - -

Castanhinha, R. & Mateus, O. 2006. On the left-right asymmetry in dinosaurs. Journal of Vertebrate Paleontology 26 (Supp. 3), 48A.

Makovicky, P. J., Norell, M. A., Clark, J. M. & Rowe, T. 2003. Osteology and relationships of Byronosaurus jaffei (Theropoda: Troodontidae). American Museum Novitates 3402, 1-32.

Naish, D. & Dyke, G. J. 2004. Heptasteornis was no ornithomimid, troodontid, dromaeosaurid or owl: the first alvarezsaurid (Dinosauria: Theropoda) from Europe. Neues Jahrbuch fur Geologie und Palaontologie, Monatshefte 2004, 385-401.

Nishikawa, K. C. 2002. Evolutionary convergence in nervous systems: insights from comparativand phylogenetic studies. Brain, Behavior and Evolution 59, 240-249.

Norberg, R. A. 1977. Occurrence and independent evolution of bilateral ear asymmetry in owls and implications on owl taxonomy. Philosophical Transactions of the Royal Society of London B 280, 375-408.

- . 2002. Independent evolution of outer ear asymmetry among give owl lineages; morphology, function and selection. In Newton, I., Kavanagh, R., Olsen, J. & Taylor, I (eds.) Ecology and Conservation of Owls: Proceedings of the Owls 2000 Conference. CSIRO Publishing (Collingwood, Victoria, Aus.), pp. 329-342.


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Hi Darren!

I've been reading your blog (and some posts from Blogger) for the past few days, and so far I'm pretty hooked. I originally stumbled upon your blog via Olduvai George, via Pharyngula. I love the unique (as far as I know) feature of previewing upcoming posts at the end of a piece of writing.

Your fascination with cryptids, esoteric animal behavior, and dinosaurs means you captivate the 12-year-old in me. It's also a bit odd to read paragraph after paragraph of your work, then remember I'm not a professional biologist but I understood most of what you wrote. In short, you make a humble amateur feel smart, and that's something to be proud of.

Hopefully we'll meet often in the comments section!

Hi Poseidon, many thanks for your comment: thanks also for your control of the sea and earthquakes, love your work.

Seriously, that's a very complimentary bunch of things you've said. Glad you like the 'teasers': some of my friends just find them frustrating (perhaps understandable given that I've been promising to post on certain topics for about a year now). All the best and keep watching.

OK, it was this that utterly did me in. I adore Dougal Dixon's work. The sketches of sea monsters also had me giggling in nerdy glee.

I had no choice; you're lavishly praised and linked. I'll stop cluttering up the comments now, but thank you. Thank you for being on the internet.

I had a short- eared owl skull (Asio flammeus) with obvious asymmetry> I wish I had kept it-- the difference would show in a photo.

By stephenbodio@y… (not verified) on 29 Jan 2007 #permalink

Actually, I have grown quite fond of your repeated promises to discuss rhinogradentians and your perpetual failure to deliver on that score. I hope we can look forward to many more years of it. It could become a recurrent leitmotif. In years to come, people will blog about the great issue of when Naish will finally discuss snouters.

Hey, Poseidon -- great name. That would work pretty well as a suffix for a dinosaur name.

By Mike Taylor (not verified) on 30 Jan 2007 #permalink

Hallo Darren!

After all the anatomical features, it would be interesting to know what supposed style of life this animals had. Today I saw a very interesting documentation about the evolution of vertebrates, which had also a part about the ears of mammals. They showed some small canid (I think it was some kind of fennec fox or so) which used its higly sensitive ears to locate athropods in the underground. Perhaps troodontids had not only such sensitive ears to find prey at night, but also to detect small vertebrates and athropods under the earth, in rotten wood or in in humus-layers. Just an idea, but I could well imagine small troodontids hopping around to detect hidden prey, and using the feet (or even fingers?) to scratch it out.
BTW, are there any asymmetries in the skulls of toothed whales which would indicate an advance in their sense for acoustic? I just forgot for what resons their skulls show more or less strong asymmetries.


I'm a bit worried you were initially ranked at number 60. According to the SB homepage they only have 57 blogs! How bad is it to come worse than last?

Keep up the good work though, at this rate you can get to a rank beyond 1!

Thanks to all for your comments.

Steve: very intrigued by what you say regarding the Asio skull, as these owls are not supposed to exhibit bony asymmmetry. Are you sure it was an Asio? By the way, I still owe you a few emails - sorry about that.

Mike: good idea, and I hate to disappoint, but - one day - those promised rhinogradentian posts really will appear.

Sordes: the canid best known for using its huge ears to locate subterranean arthropods is the Bat-eated fox Otocyon megalotis, a termite specialist. But fennecs and others do it too. As for troodontids detecting 'concealed' prey that are in leaf litter, or in dense undergrowth: that's exactly what I'm proposing. Note however that their hand anatomy is totally discordant with the idea of digging, and their feet don't look too suited for it either. As for odontocetes, the function of the asymmetry is poorly understood but it is thought to be something to do with sending and interpreting noises. So far as I know it is restricted to the facial bones (and soft tissues) and doesn't extend to the ears. Interestingly - as in owls - there is some suggestion that asymmetry in odontocetes evolved more than once, though this is controversial.

David: good point. I learnt today that sb currently hosts 57 blogs. The page I was looking at lists all kinds of other files alongside blog names, hence my confusion.

Hey, Poseidon -- great name. That would work pretty well as a suffix for a dinosaur name.

That's what I love about science: the inside jokes. :-D

By David Marjanović (not verified) on 30 Jan 2007 #permalink