Tetrapod Zoology

Why do some owls have ear tufts?

i-6e83652c0f747afaa5152f1e4e965965-ear tufts to mimic a mammal hypothesis.jpg

It’s going to be a busy week, and already I’ve been totally unable to finish any of the planned articles. But I’ll do what I can. In an effort to produce something short and sweet, here’s a look at a question I’ve been pondering for a while: why do some owls have ear tufts? There are currently round about 225 living owl species, and of those about 50 have what we commonly call ear tufts. As with so many structures present in animals, we have little idea as to what these are for, and little hypothesis testing has been done. However, several very interesting hypotheses exist…

My favourite hypothesis (though that does not mean that I consider it the most likely) is that the presence of ear tufts enhances an owl’s ability to mimic a mammal, and hence to appear more frightening to mammalian predators (Mysterud & Dunker 1979). This hypothesis [hinted at by the pictures shown above] predicts the co-occurrence of mammalian predators with tufted owls, but because many tufted owls are found in places where there are no mammalian predators, it was deemed unlikely in Perrone’s review of ear tuft significance (Perrone 1981). The mimicry hypothesis also predicts that having ear-like structures is useful if you want to frighten an attacking mammal, and that seems doubtful to me (if only because animals clearly take more notice of glaring eyes and a gaping mouth than the presence or absence of ears).

i-220dc7033ce7d9c35fb1fafd8d7ea1c2-Bubo virginianus wikipedia July m2008.jpg

Rather than playing a role in threat displays, maybe ear tufts aid species recognition: Sparks & Soper (1970) and Burton (1973) mentioned the idea that ear tufts might help provide a given owl with a distinctive silhouette, thereby allowing other owls to help recognise members of their own species by sight. While it seems plausible that species recognition might have played a role in the evolution of ear tufts, it looks unlikely that this is the primary explanation for their presence given that owls in general locate one another by vocalisation, and apparently find each other long before visual contact is made.

Finally, there’s the camouflage hypothesis. Because some owl species erect their ear tufts when alarmed by approaching danger (while at the same time adopting a vertically elongate body shape different from their more rotund resting pose), it’s been suggested that ear tufts help break up the owl’s outline and make it appear more like a broken branch. Perrone (1981) noted that the camouflage hypothesis would only apply if ear tufts were present in those owls that roost during the day: diurnal owls roost at night and, in theory, shouldn’t require visual tricks to conceal themselves from predators. Tufted owls should also roost in places where the tufts might aid in concealment; presumably, in locations where the ear tufts might help break up the owl’s shape (like in trees, and not in cavities or on the ground). In other words, the hypothesis predicts that ear tufts should be restricted to tree-roosting nocturnal owls. And this does (mostly) appear to be the case: tufted owls are all forest-dwelling and nocturnal (though note that not all populations of all tufted species are forest-dwelling).

i-e0708acd2c020ec66511466281351928-Eastern screetch owl wikipedia.jpg

At the moment, then, the camouflage hypothesis best fits the data: ear tufts are selectively advantageous because they improve an owl’s ability to resemble a broken branch while roosting. Given that nocturnal owls are already cryptically coloured and do their best to avoid the diurnal attentions of other animals, it follows that additional cryptic features would be to selective advantage. However, we still don’t have a handle on the whole story: if ear tufts are such a good idea for tree-roosting nocturnal owls, then why aren’t all tree-roosting nocturnal owls tufted? They’re not (Perrone 1981). Why not? As always, more work is needed. While preparing this article I learnt that Santillan et al. (2008) have just published a short article on the role of ear tufts in the Ferruginous pygmy-owl Glaucidium brasilianum, and this might include some relevant new data. Alas, I have yet to see this article [Eastern screech owl Megascops asio shown here, from wikipedia].

Owls have been discussed on Tet Zoo a few times before. For the recent spread in Britain of the Eurasian eagle owl Bubo bubo go here (and here for an update), for a discussion of cranial asymmetry go here, and for a look at fish owls go here.

Refs – -

Burton, J. A. 1973. Owls of the World: Their Evolution, Structure and Ecology. Dutton, New York.

Mysterud, I. & Dunker, H. 1979. Mammal ear mimicry: a hypothesis on the behavioural function of owl “horns”. Animal Behaviour 27, 315.

Perrone, M. 1981. Adaptive significance of ear tufts in owls. Condor 83, 383-384.

Santillan, M. A., Sarasola, J. H. & Dolsan, M. 2008. Ear tufts in Ferruginous pygmy-owl (Glaucidium brasilianum) as alarm response. Journal of Raptor Research 42, 153-154.

Sparks, J. & Soper, T. 1970. Owls: Their Natural and Unnatural History. Taplinger, New York.

Comments

  1. #1 Jerzy
    July 14, 2008

    Question:
    How do we know that dinosaurs didn’t have visible ears?

  2. #2 Jerzy
    July 14, 2008

    Just a random thought:
    No species of owl has pointed crest. Crests occur in almost all bird orders – except owls. I guess shape of sound waves flowing around owl head makes it better to have two ear-tufts instead of one crest. Would be testable using experimental modeling.

    If camouflage or threatening predators would be dominant function, you would expect that owl chicks (which leave nest when flightless and actively threaten intruders) would have especially long ear tufts. But they have not – head feathers develops last on an owl.

  3. #3 Darren Naish
    July 14, 2008

    Hi Jerzy, thanks for your comments.

    How do we know that dinosaurs didn’t have visible ears?

    Dinosaurs would have had visible ears just like those of most extant reptiles (including birds). However, are you referring to pinnae (ear flaps, like those of mammals)? We can be confident that extinct dinosaurs didn’t have these because..

    1. None of their living relatives do.
    2. They are unique to mammals, and not widespread among tetrapods.
    3. They are clearly absent from the mummified dinosaurs we have.

    Crests occur in almost all bird orders – except owls. I guess shape of sound waves flowing around owl head makes it better to have two ear-tufts instead of one crest. Would be testable using experimental modeling.

    But sound doesn’t flow ‘around the head’ given that the ears are located low down and near the edges of the facial disk. Furthermore, the owls with the most acute hearing (barn owls, Great grey etc.) lack ear tufts altogether. A lot of work has been done on owl hearing, and no correlations between aural acuity and ear tufts have been noted.

    If camouflage or threatening predators would be dominant function, you would expect that owl chicks (which leave nest when flightless and actively threaten intruders) would have especially long ear tufts. But they have not – head feathers develops last on an owl.

    For the mimicry/threat hypothesis, I agree that ear tufts on juveniles might be predicted (and in fact favoured, given that juveniles need to defend themselves from mammalian predators more than do adults). For the camouflage hypothesis, ear tufts are predicted for those owls that sit out on branches during the day. Do juveniles do this? No, they mostly remain concealed in nests. By the time the owls fledge and sit out on branches, they have ear tufts (example: in eagle owls, ear tufts are developing at c. 30 days, yet young don’t leave the nest until c. 40-45 days and don’t adopt the branch-sitting behaviour of adults until after 60 days).

  4. #4 Hai~Ren
    July 14, 2008

    Interesting. I’d always assumed that ear tufts helped in hearing, but didn’t quite think about all the other owl species that did not possess ear tufts.

    Have you watched that often-circulated clip from some Japanese television show of a white-faced owl (my guess is Ptilopsis granti) that shrinks when startled by a jack-in-the-box? The ear tufts don’t seem to change their appearance as the owl tries its best to pretend to be a piece of wood, but I have no idea about how the ear tufts behave in other species:

    http://youtube.com/watch?v=LQce5mgX4rU

    A number of those species that possess ear tufts are the largest owls (such as the eagle owls Bubo). I don’t see them being much at risk of predation. Maybe the camouflage is more to minimise the chances of getting discovered and mobbed by diurnal birds?

  5. #5 djlactin
    July 14, 2008

    Very interesting post that presents and examines several interesting and plausible hypotheses. But

    /enter fantic mode/

    gaaack! I HATE IT when knowledgeable people emit teleological junk like this:

    “…owls have evolved ear tufts in order to mimic mammals, and hence to appear more…”

    “…maybe ear tufts have evolved to aid species recognition…”

    “…owls evolved ear tufts to help them to resemble broken branches while roosting…”

    This construction implies that the owls got together and debated: “Hmmm, how should we change to (mimic mammals; aid species recognition; resemble broken branches)?”, and that after reaching consensus, they went out and evolved according to plan. It implies a goal to evolution.

    This sloppy message is everywhere: “Skunks evolved bad smell to deter predators”; “Apes evolved big brains to outsmart their troop-mates.”

    When evolutionists say things like this, it’s no surprise that less knowledgeable people get the horribly wrong impression that creatures evolve to attain a goal. (And/or that individuals evolve to accomplish this goal!)

    Evolution is a consequence of selection on variation. Period. What happens happens because all intermediate steps had an advantage over competing traits.

    Therefore, THIS is how to do it right:
    “…that ear tufts help break up the owl’s outline and make it appear more like a broken branch. …” [and that selection on existing variation therefore fostered fixation of the trait in the population].

    /exit fanatic mode/.

  6. #6 Darren Naish
    July 14, 2008

    Ok ok…

  7. #7 Darren Naish
    July 14, 2008

    Right, having gone back and edited the text (hopefully satisfactorily: let me know) I’m not entirely sure why the wording I used was such a problem. Maybe that’s because I’m not very clever. But saying ‘owls evolved ear tufts to mimic mammals’ is – so far as I’m able to tell – short-hand for ‘the presence of ear-like structures proved selectively advantageous for owls, thereby encouraging the retention and accentuation of this trait’, there being the assumption that the development of increasingly specialised ear-like tufts was a gradual process with even the earliest proto-ears being advantageous. In other words, I think this is down to semantics more than teleology, and I’m not convinced that ‘owls evolved ear tufts to mimic mammals’ is necessarily a misleading description of one hypothesis.

    Having said that, I will keep your words in mind and will try and avoid this sort of thing in the future.

  8. #8 Jerzy
    July 14, 2008

    So pinnae leave no markings on skull altogether., e.g. attachment sites of muscles which move ears? And what we assume about ears of extinct mammals comes from conjecture of living ones and rare soft tissue impressions?

    For your info: Many eagle owls Bubo and eared owls Asio nest in open nests. Their chicks leave nests long before flying and without ear tufts and perch nearby. Ear tufts would be very useful then. You can often see them on animal photos, because they are impossibly cute.

  9. #9 Richard Hing
    July 14, 2008

    Owls are cool as fuck.

  10. #10 Tom Long
    July 14, 2008

    Always the intellectual.

  11. #11 Craig York
    July 14, 2008

    Probably a daft notion, but I wonder if the tufts might not function as a kind of flight sensor? ” Mmm. Bit of a
    crosswind, better crank the left wing up a hair…”

  12. #12 Allen Hazen
    July 14, 2008

    Fascinating post! I’d never thought about the question. … The (umm) function of ear-tufts in lynxes and kaibab squirrels would have to be something else: neither, as far as I know, spend much of the day perched stationary on branches pretending to be dead wood. If I’d been asked, I would have guessed their ear-tufts were like peacock tails, advertising to potential mates “Look how healthy I am.”

    On the philosophy of science front… Teleological language is everywhere in biology. Didn’t you recently attend a conference on FUNCTIONAL anatomy? Among people who can be assumed all to know that biological teleology is supposed to be reducible to (“reducible to” = philosopher-speak for what you meant by “short hand for”) a story about selective advantage, even talking about “decisions” can be useful: Toby White’s Palaeos essays contain some very amusing passages about the “career choices” of different taxa while being as rigorously evolutionary as anything on the web! … If djlactin is really allergic to teleological language, he/she should have stopped reading at the end of your first paragraph, where you raise the question of what owls’ ear-tufts are “for”!

  13. #13 DDeden
    July 14, 2008

    Long tufts help with the auditory zooming (enlarging the ear disk) function while enlarging the facial profile at little cost (similar to how in chimp dominance battles the hair sticks up), I’d think.

    Well that’s what I use mine for anyway.

  14. #14 Argosty
    July 14, 2008

    DDeden: “Long tufts help with the auditory zooming (enlarging the ear disk) function while enlarging the facial profile at little cost (similar to how in chimp dominance battles the hair sticks up), I’d think.”

    No, because the ear tufts do not form a part of the facial disk, do not play any role in hearing at all, are not present in all owls, and are not present in the owls with the best hearing.

  15. #15 Zach Miller
    July 14, 2008

    I love owls, and I have oft wondered what purpose those tufts serve.

  16. #16 kad
    July 14, 2008

    Excellent article. Like some others here, I always assumed the “ear” tufts aided in hearing and/or triangulation. I now see that is wrong but I think the competing hypotheses are flawed as well.

    On a side note, when I was an interpretive naturalist with the National Park Service I once attended a program at a wildlife rescue center where an ornithologist described the acuteness of an owl’s visual and auditory senses in the following manner (paraphrasing):

    “Say there is an owl perched high above home plate in the Houston Astrodome and a mouse is scurrying along the outfield fence some 400 feet away. Imagine it would be pitch dark inside the Astrodome except for one lit birthday candle on the pitcher’s mound. This single candle affords enough light for the owl to see the mouse scurrying along the outfield fence. Blow out the candle. Now even an owl can’t see in the absence of all light but the mouse is not safe. The owl can still hear the mouse moving along the fence.”

  17. #17 jcdoss
    July 14, 2008

    The obvious thought that they affect hearing because they look like ears is a good one, but what about vision? Perhaps they could be used to block stray light by cocking the head at particular angles. As the bird looks straight ahead like in the photos you supplied, they look pretty useless. But if the bird is looking down towards the ground (and prey), perhaps they would be in the right position to block oblique rays of a setting sun, for example.

  18. #18 Mark Lees
    July 14, 2008

    I think Hai-Ren makes an important point with regard to the possibility that the “ear-tufts” act as camouflage against predators. He states “A number of those species that possess ear tufts are the largest owls (such as the eagle owls Bubo). I don’t see them being much at risk of predation.”

    The fact is that many of the largest owls have ear-tufts, while many of the smaller which would seem to be more at risk from predators and hence morein need of camouflage lack them. Also from experience owls tend to be difficult to locate while roosting whether they have ear-tufts or not – indeed over years of birding I have never noticed any tendancy for those with ear-tufts to be harder to find.

    Also I’m not convinced by the comment that “the hypothesis predicts that ear tufts should be restricted to tree-roosting nocturnal owls. And this does (mostly) appear to be the case: tufted owls are all forest-dwelling and nocturnal”

    Locally we have 2 ‘eared’ owls – Long-eared Owl (Asio otus) and Short-eared Owl (Asio flammeus) – as the names indicate one has ‘long’ obvious ear-tufts, while the other has ‘short’ ear-tufts only noticeable in some circumstances, but nonetheless does have them. Long-eared is a nocturnal bird roosting in forests, but short-eared owls are often found in open country, heathland and scrub, and is partly diurnal. Interestingly short-eared owls seem to erect their ear-tufts when alarmed or alert – in circumstances where there seems to be no camouflaging effect.

    Sorry not to be able to add anything constructive – but I really think the camouflage theory is weak.

  19. #19 DDeden
    July 14, 2008

    Owls perch on branches, listening and watching, that’s their specialization. The fact that those with the best hearing lack tufts precisely proves my point, it’s not aural acuity, but zoom-ability (I don’t know the technical term). Natural selection at work. Review these comments:

    Crests occur in almost all bird orders – except owls. I guess shape of sound waves flowing around owl head makes it better to have two ear-tufts instead of one crest. Would be testable using experimental modeling.

    But sound doesn’t flow ‘around the head’ given that the ears are located low down and near the edges of the facial disk. Furthermore, the owls with the most acute hearing (barn owls, Great grey etc.) lack ear tufts altogether. A lot of work has been done on owl hearing, and no correlations between aural acuity and ear tufts have been noted.

    If camouflage or threatening predators would be dominant function, you would expect that owl chicks (which leave nest when flightless and actively threaten intruders) would have especially long ear tufts. But they have not – head feathers develops last on an owl.

    For the mimicry/threat hypothesis, I agree that ear tufts on juveniles might be predicted (and in fact favoured, given that juveniles need to defend themselves from mammalian predators more than do adults). For the camouflage hypothesis, ear tufts are predicted for those owls that sit out on branches during the day. Do juveniles do this? No, they mostly remain concealed in nests. By the time the owls fledge and sit out on branches, they have ear tufts…

  20. #20 DDeden
    July 14, 2008

    Unlike hawks, eagles, vultures, condors, falcons which are very aerodynamic, owls are notably unaerodynamic, but instead are aurodynamic / earodynamic.

  21. #21 Eric Snively
    July 14, 2008

    Cool question. I work with raptors including great horned owls, and observe how the owls use their tufts all the time. They’re not static accoutrements, but mobile and reflect the owls’ moods. If you spend enough time with the birds you learn to associate tuft position with circumstance and evident emotional state; the behavioural correlations are precise.
    For example, when the owl is unstressed and alert the tufts are in their normal erect posture. When the bird is stressed (hot and/or nervous and panting, especially after being wrangled or when riding in a car), the tufts are retracted flat against the head. When eating, the owl retracts its tufts, but not as flat as when stressed. When the owl is frightened (surprised by a handler, or when encountering a carnivoran on the ground), the tufts are slightly retracted. The owl will hiss, click its beak loudly, and puff up its feathers, but not raise its tufts to imitate a coyote’s pinna or anything. The tufts go way up when the owl is alert to imminent prey; then you have to be VERY careful not to let them eviscerate kindergarteners or thrash your glove and forearm. The tips of their claws are blunt (more so in a snowy I work with than in great horneds) but no one bothered to tell the owls. Their digital flexor tendons are ossified and their grip amusingly strong.
    We can infer testably from these observations that great horned owls use their tufts for intraspecific communication. We can infer definitively that great horned owls are right bastards.

  22. #22 Darren Naish
    July 14, 2008

    Many thanks to all for comments.

    After reading all of this and thinking about the subject some more I don’t think that one hypothesis explains the diversity of ear tufts seen across the group, but I never thought that anyway: as I said above – and as Perrone (1981) noted – the camouflage hypothesis doesn’t explain everything because ‘more than half of the nocturnal, forest-dwelling owls lack tufts, despite the advantages these structures presumably confer’. At the moment I reckon that crypsis plays a role for some lineages (and is perhaps supported by the fact that members of forest-dwelling lineages end up with reduced ear tufts when moving into open environments), but the case for intraspecific communication looks good in others.

    Perrone finished his paper by saying that more study was needed and that we still didn’t have all the answers. So bring on more research and more hypothesis testing, we’re still in the early phases.

  23. #23 Neil
    July 14, 2008

    I did post before but it didn’t come up for some reason: Could the ear tufts be simply a sign maturity? Or are they fully formed in Juveniles?

  24. #24 Joe B
    July 14, 2008

    Darren:

    “But saying ‘owls evolved ear tufts to mimic mammals’ is – so far as I’m able to tell – short-hand for ‘the presence of ear-like structures proved selectively advantageous for owls, thereby encouraging the retention and accentuation of this trait’”

    This type of wording is so commonly used (as you say) because it is shorter and to the point, however it adds to the misconception of what evolution / natural selection is all about. Think of the following to ideas:

    1) Giraffes evolved long necks in order to reach the leaves of tall trees.

    2) Giraffes of the long neck variety were much better at reaching the leaves of tall trees. As a result they were the ones that lived. The shorter giraffes died because they were not as capable of feeding themselves.

    Common sense tells us that giraffes did not decide to grow long necks. Evolution simply does not work in this manner.

    It’s your use of the word ‘to’ in your sentence which implies some sort of intent.

    Why did Owl’s evolve tufts? The answer is genetic mutation. The real debate is do tufts provide an advantage? And if so, how?

    Keep in mind that tufts do not necessarily provide any advantage at all. If tufted versus non-tufted varieties have an equal (or very close to equal) chance of survival then it is entirely possible to see both varieties co-exists for a time. Remember that all the species that have died off DID exist for a while. When they WERE in existence they were successful but only NOW do we think of their differences as disadvantageous. Not all differentiation requires an explanation.

  25. #25 Nathan Myers
    July 14, 2008

    Teleological language is convenient when used by experts; I presume they would be made fun of by their peers were they to ascribe impossible motivations. However, it’s very easy for people like me to slip into foolish reasoning, and making fun would be distracting and anyway not very sporting. So, one reason to avoid it is educational.

    That doesn’t mean we must twist ourselves in knots. In place of “what are tufts for?”, it suffices to say “what are tufts good for?” In context, “Owls with ear tufts can mimic mammals”.

    The faulty reasoning that teleological language (and even teleological reasoning expressed defensively) encourages in experts and in people like me, alike, is to imagine that having discovered a use for something, we have discovered the use for it. Thus, Eric S. very helpfully describes tuft mobulity, and identifies undisputably social uses for them, but is there any reason to think that these are the only uses, or that those uses came first? Having evolved mobile tufts, it might be surprising were owls not to use them socially, in addition, perhaps, to whatever else led them to be selected for in the first place.

    My cats demonstrate very elaborate social uses for their pinnae (curiously parallel to owls’) that do not detract from their use the better to hear with.

  26. #26 Nathan Myers
    July 14, 2008

    s/mobulity/mobility/

    @Joe B: I think this particular differentiation does require an explanation. I confess I would be disappointed were it to turn out to be the same as for so many other birds’ peculiarities: sexual selection.

  27. #27 Alec T
    July 14, 2008

    What about body language? Tufts can be held either high or flat to show that they’re frightened or to show aggression, along with adapting the “vertically elongate body shape” you describe abov. Maybe they just use their “ears” the same way cats do.

  28. #28 Alec T
    July 14, 2008

    …or it could be sexual selection.

  29. #29 Dr Vector
    July 15, 2008

    Er…possibly I’m just airing my ignorance here, but does anyone know what function ear tufts serve in mammals? If so, that seems like it might have some bearing on the owls. If not, then we’ve got a pair of related mysteries, and maybe we’ll get to the answer faster if we consider the lynxes etc. along with the owls.

    Also, seriously, could we crank down the Teleology Concern-O-Meter about 3000%? Cuz, sure, you can say, “Carnivores of sharp-toothed variety were better at piercing and slicing meat. As a result they were the ones that lived. The carnivores with flatter teeth died because they were not as proficient at killing and processing prey”, or the equivalent, every time instead of saying “sharp teeth are for eating meat” BUT
    (a) IMHO it’s not worth tying oneself up in circumlocutionary knots and destroying the flow of the prose just to avoid any possible misreading, and
    (b) also IMHO, it makes you sound like a git.

    You can unload a gun, but you can’t unload language.

    (Lest anyone think that this comment was functionless twaddle, as its creator I affirm that it was written with a purpose, and that purpose was to ignite a teleological language war. Let the flames begin!)

  30. #30 carel
    July 15, 2008

    I agree with Eric Snively’s remarks. Where birds from many other orders use crests in threat displays, I’ve seen many owl spp. threaten, but have never seen ear tufts used in this way. Of the “eared” owl spp. that I know well, all have down analogs as juveniles that are morphologically very different from the adult feathered tufts, but are used in the same ways: form disruption while roosting and (less importantly, I think) intraspecific signaling. The “ears” of the African Lophostrix are fused into what comes closest to a single owl “crest.”

  31. #31 carel
    July 15, 2008

    The “ears” of the African Lophostrix are fused into what comes closest to a single owl “crest.”

    Oops. I meant Jubula, not Lophostrix — sorry.

  32. #32 Dave Godfrey
    July 15, 2008

    Do owls with ear tufts form a clade, or are they more widely distributed around the tree?

    If they crop up all over then that’s a potential mark in favour of camoflage and against the sexual selection hypothesis (and possibly the communication one too)- in that case its the sort of thing that might only occur once, but is retained by the descendants, rather than evolving in parallel multiple times. Are there differences between the “ears” of male and female owls? Again one might expect to see this if they’re a sexual characteristic, but wouldn’t necessarily see it if their primary function is communication.

  33. #33 bugguy aka djlactin
    July 15, 2008

    ” Maybe that’s because I’m not very clever. ”
    I would certainly NOT adopt this hypothesis.

    “But saying ‘owls evolved ear tufts to mimic mammals’ is – so far as I’m able to tell – short-hand for ‘the presence of ear-like structures proved selectively advantageous for owls, thereby encouraging the retention and accentuation of this trait”

    That’s what I tried to say to my graduate supervisor as he had me stuffed into the iron maiden….

    The problem is that this ‘shorthand’ is not perceived as such by ‘the laiety’. ‘They’ hear; ‘they’ accept. Almost every intelligent non-biologist that i have ever met (this includes myself before — expect for the ‘intelligent’ part — before about 4th year university) takes this statement literally.

    In fact, many (most?) arguments that I have had with people who disagree with evolution have resolved to this point: “HOW could any creature decide to evolve into [some specified form]?” or “What do you mean ‘the moths turned black?’” “Jack Chick cartoon: T. rex thinks: “Think I’ll evolve another eye!” Or that every creature aims to evolve into humans (“So why are there still monkeys? Hah!”)

    Try it: engage a naive but intelligent friend on the idea of evolution. See what they think it is. I have watched more than a few eyes open when I reveal the true nature of the process.

    We (actually, I mean “biological science bloggers who discuss evolution occasionally”… which EXCLUDES me, so ‘we’ is inappropriate… but I digress) must be careful not to propagate this mis-simplification. Simple but effective alternatives like you have substituted: “Creatures who had this trait had an advantage in surviving or making babies over the ones who did not, so over generations, the trait became more common in the population.”

    Yeah, it takes a few more words, but precision trumps concision. (Preciseness beats conciseness?)

  34. #34 djlactin
    July 15, 2008

    Dr Vector:

    I’m thinking about cats here. Many years ago I encountered a very odd gentleman whose job was tending captive Cats in an animal farm in Sequim Washington. His observation was that the tips of the ear tufts were at the same level as the (higher) tip of the rump. He proposed that the turf warned the cat to duck his backside when running through an opening. My professor treated the idea with disdain, but can it be dismissed so easily?

    But I have another hypothesis: “Chicks dig it.”

  35. #35 Nathan Myers
    July 15, 2008

    Bugguy/djlactin re-raises an interesting point that I think deserves more exploration. (Craig York gets prior credit.)

    The tufts manifestly have a social role, and probably a role in mating, and might also have a role in camouflage, but even all together those do not suffice to explain them. To consider the question, we need more facts.

    Almost everyone who has mentioned a sensory function for the tufts only considers hearing, but the only reason to think so is that they are placed where cats’ ears are. Matt (Dr. V.) suggests the function is the same as in cats, echoing Darren’s hint in the pictures at the top. Cats use whiskers and tufts as feelers when sneaking about in the dark. Do owls do any terrestrial stalking, or do they only
    attack from the air? When they do attack from the air, and miss, do they try again for the same prey, or do they just fly away?

  36. #36 Darren Naish
    July 15, 2008

    Cat ear tufts: general thinking is that these primarily serve a signaling function. In fact caracals (which have the biggest and most obvious ear tufts of any mammal) employ them in elaborate ‘head-flagging’ displays. Kitchener suggested that (for whatever reason) some cats have switched from using the tail as a signal to using the ears, and it’s notable that ear-tufted cats have the shortest tails. But does this tell us anything about owls?

  37. #37 Barn Owl (tuftless)
    July 15, 2008

    Do owls with ear tufts form a clade, or are they more widely distributed around the tree?

    I don’t know about clades, but from a quick glance at species within a few owl genera, Tyto and Athene owls all appear to be tuftless, whereas all Otus and nearly all Bubo owls (Snowy Owl is an exception) have ear tufts.

    The hypothesis that ear tufts can be used to signal moods within a species makes the most sense to me. Owls can change the orientation of the feathers in their facial discs and facial ruffs to maximize sound localization, so moving ear tufts with facial muscles would just seem to be an extension of this. The same epithelial-mesenchymal interactions that pattern feather germs and feather growth on the owl’s facial disc during development could give rise to distinctive ear tufts in some species-since owls are altricial, this might be a straightforward study to do. Map the facial feather germs on tufted and tuftless owl species; look at expression and spatial regulation of genes known to be involved in feather morphogenesis in other birds (e.g. chicken, duck).

  38. #38 Boltzmann's elf
    July 15, 2008

    Having done some rehabilitation work with them, I have a hard time believing that predation has a significant influence over the average life-span of the Great Horned Owl (Bubo virginianus). At least before the shotgun and rifle. On the other hand, they definitely seem to use them to express their emotions, so I would endorse the signaling hypothesis.

  39. #39 Nathan Myers
    July 15, 2008

    I feel obliged to mention that our family cat, Zen, has most excellent ear tufts.

    Once the early innovator succeeded in evolving ear tufts, for whatever purpose, their signaling function would provide enough continuing value to keep them. (They can’t cost much to maintain.) Therefore, their presence on B. virginianus doesn’t demonstrate much. The question is, what drove them into existence before the owls discovered their use for signaling? Maybe signaling, but it makes it hard to rule out hypotheses for their origin when they remain useful even when the animal moves on to a different niche.

    I wonder if mice freeze when they see the outline of a cat’s (fox’s, weasel’s) head.

  40. #40 cfrost
    July 16, 2008

    Would it be possible for a competent anatomist to determine whether ear tufts might have a sensory purpose? – Enlarged nerve tracts, etc.

  41. #41 Lars Dietz
    July 17, 2008

    According to the molecular phylogeny chapter by Wink & Heidrich in “Owls of the World” by Koenig, Weick & Becking, Strigidae falls into two clades, although the support is not high. One of these include Athene, Glaucidium, Surnia, and Aegolius (no tufts), the other Bubo, Otus, Asio, Megascops, Strix etc. In this clade Otus s. str. is most basal, and in general the topology looks like tufts have been reduced independently at least in Strix and Pulsatrix. The question in why, as Strix owls are clearly nocturnal forest owls.

  42. #42 Graham King
    July 18, 2008

    Great blog, Darren! On an important question: Why do owls have ear tufts?
    Clearly, to make them look like cats (and vice-versa).
    And… why, then, this?

    (1) to provide amusement for parents and older siblings – when young children who have successfully learnt categories ‘dog’ ‘cat’ ‘bird’ etc can yet be fooled into misidentifying SOME feline and avian exemplars (cats and owls) as each other – at least, in pictures or photos, selected for their ambiguity. Thus teaching the young ‘uns that they don’t know everything yet, and that the world is a stranger place than they may blithely suppose.
    -Humility and prudence; important lessons, both.

    (2) to provide common features, for easy recognition by humans, of their (cats’ and owls’) useful common rodent pest control abilities. “Got a vermin problem mate? You need to get one of those round-faced big-eyed pointy-eared jobs. Soon sort it out for you.”
    -A handy all-purpose rule of thumb.

    (3) to provide a single instantly-recognizable predator Face of Death for rodents: for them to beware of, teach their young to beware of, and in extremis -when confronted by The Face close-up- to know to swiftly review their life and make their final peace with their Maker. “Behold, the big-eyed round-faced pointy-eared Nemesis approacheth! Thine end is nigh.”
    -A merciful provision by a loving Providence.

    (Teleological? Yes, but surely that means it’s kinda… logical?)

  43. #43 mithril
    July 22, 2008

    could the tufts serve a sensory function, not unlike the whiskers on some mammals?

    if the tufts could detect the minute changes in wind patterns to allow a forest dwelling owl to avoid trees/branches on dark nights and in the shadows of a forest, for example, presumably it would be a trait that would be advantagious.

    i’d imagine the best way to check that idea would be to disect a deceased member of a Tufted owl species and look for concentrations of nerve ending around the feather tufts..followed by behavioral studies comparing both tufted and non-tufted species in the same enviroments.

  44. #44 DDeden
    September 20, 2008

    The lesser prairie chicken has similar ear tufts. They don’t perch on branches, they avoid trees which may harbor raptors, they leck in openings, they lift ear tufts during dominance displays but not while hiding from airborne predators. Raptors, predatory mammals (coyotes) and fences are the 3 major killers per one study.

    lesser prairie chicken male

  45. #45 Martyn
    May 30, 2010

    The ear tuffs possibily make the owl ‘larger’ when disturbed or frightened. remember the largest of owls would have had predators at one time. They also serve to ‘break up’ the shape of the owl. Evolution has changed the way different animals look over time and thus some will have them and others wont. im sure there are many reasons for them rather than just one and thats why there are differences from species to species.