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).
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).
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.