Tetrapod Zoology

i-6f8b81bdc580c9a526d2a3969a1fedb0-Anhima_cornuta_Lydekker_wikipedia_June-2010.jpg

Another waterfowl fact. This one is gonna be brief. In the previous article we looked at the wing spurs of Plectropterus. They’re pretty cool, but they’re far from unique, and even more incredible are the much larger, dagger-like spikes seen in screamers…

Screamers (Anhimidae) are a small group (three species) of South American* waterfowl… well, they’re not referred to as ‘waterfowl’ as often as are other members of Anseriformes, but I think they should be. One species (Horned screamer Anhima cornuta: shown in adjacent illustration) has – as you might guess – a horn! (though, technically speaking it’s not a horn at all). Anyway…

* Fossil species are known from North America and Europe.

i-03d73f77d885562b32a1504b8e0157ba-anhimid-carpomet-resized_June-2010.jpg

The wing spikes are pretty impressive; they’re long enough to be really obvious even when the birds have their wings completed folded. In the Black-necked or Northern screamer Chauna chavaria [carpometacarpus shown here], a big, curved spike projects from the base of the alula, and a separate, stouter, straighter claw grows from the distal end of the major metacarpal. During combat, the birds will stab rivals with these spikes, and on occasion the spikes snap off and can then remain lodged in the tissues of the opponent. Does this render the attacker spike-less for the rest of its days? NO – the spikes can grow back. Many sources state this latter fact, but I’ve never seen any ‘good’ data on it: how long does it take the spikes to grow back? Do they break off frequently? Do all species suffer from this breakage? I’d love to know more [photo of Southern screamer C. torquata below by Mo Hassan; stolen without permission from here on The Disillusioned Taxonomist. The wing spikes in this species are smaller than those on C. chavaria].

I’ve mentioned the clubs and spikes present on some bird wings a few times now, but have never properly elaborated. Do not fear, as elaborating is what I am going to do in a soon-to-appear article. Stay tuned.

i-f3361c50fd966301f1d9c5f95ab8ea28-Southern_screamer-Mo_Hassan_June-2010.jpg

Another waterfowl fact next! For previous Tet Zoo entries on waterfowl see…

Comments

#1 mo
June 20, 2010

It doesn’t look like a duck/goose/swan at all.
#2 John Harshman
June 20, 2010

Indeed it doesn’t. But obscure little bits on its bones do look that way, and so do all the little bits of its genome. Evolution is funny that way.

Of course screamers, to our eyes, look more like chickens than ducks. Since chickens (well, galliforms) are the closest relatives of anseriforms, the question arises whether a chicken-y appearance is primitive in anseriforms. We really don’t know. Storrs Olson was of the opinion that duckiness was primitive, and claimed to find a host of vestigial duck characters in screamers, including bill lamellae. But how, by direct inspection (which was Olson’s whole argument*) do you distinguish vestigial characters from incipient ones? The oldest anseriform fossils — Maastrichtian presbyornithids — are more ducklike. But phylogenetic analysis, which can come in handy from time to time, puts screamers well outside the presbyornithid node, so that doesn’t help a bit. (There’s a still unpublished fossil “screamer”, but it’s Eocene and so is even less helpful.) The fossil record, as usual, is too incomplete to draw such conclusions from.

*Why can’t I find the citation for this on the web? Do I have to resort to actual paper? Too tedious for now. I think it’s in the Auk, a long time ago.
#3 Jerzy
June 20, 2010

In the field, horn of Horned Screamer looks very odd, thin and straight, most like car antenna.

BTW, I always wondered why European and American zoos have heaps of very bland southern screamer, but never striking horned screamers?

Screamer claws grow roughly in place where birdlike dinosaurs had claws. Maybe some fossils have specializations that their claws were used primarily for fighting?
#4 Alan
June 20, 2010

I am not sure why there are no captive Horned Screamers in European or US zoos, but a check on ISIS shows that there are very few reported hatching for Southern Screamer in the last 12 months. It looks like they are hard to breed in captivity, at least outside range countries. I am not sure how they behave when nesting, but given how aggressive other galloanserae can be when they have eggs or young I suspect lack of space for breeding pairs may discourage people keeping a species when there are more manageable ones available, especially where the species is listed as Least Concern by the IUCN.
#5 Darren Naish
June 20, 2010

Thanks for comments. Comment 1: the Olson reference in question is actually Olson & Feduccia…

Olson, S. L. & Feduccia, A. 1980. Presbyornis and the origin of the Anseriformes (Aves: Charadriomorphae). Smithsonian Contributions to Paleobiology 323, 1-24.

They show the allegedly ‘vestigial’ lamellae of screamers in Fig. 5, p. 8. Available online (free to all) here.
#6 John Harshman
June 20, 2010

I swear there’s also an Olson paper in the Auk where he puts down the idea of Galloanserae. But I may be mixing that up with the one where he announces that piciforms are polyphyletic:

Olson, Storrs L. 1983. Evidence for a Polyphyletic Origin of the Piciformes. The Auk, 100(1): 126-133.
#7 Zach Miller
June 20, 2010

Are these genuine claws? Do they have a bony core and a keratin sheath? How is it that they’re growing from the sides of the hand bones? This is freaky stuff, and really makes me wonder about developmental pathways for these claws.
#8 Jose
June 20, 2010

A fact that I found interesting about this subject: the dagger-like structures on this species haven’t evolved from the claws of the fingers (and thus, they’re not vestigial), instead, they are brand new structures located in another parts of the “hand”. At least, it appears to be so…
#9 Mike Keesey
June 21, 2010

…the question arises whether a chicken-y appearance is primitive in anseriforms…

Since tinamous (which are palaeognathes) have a chicken-y appearance (they were originally classified as grouse), I’d assume a chicken-y appearance is more or less ancestral for all of Aves (crown clade).
#10 Hanneke Meijer
June 21, 2010

Male solitaires (Pezophaps solitaria) from Rodriguez also show a bony knob on the radial side of the metacarpal. Presumably, this bony knob was used in male-male combat and many skeletons do show injuries possibly related to a blow by such a knob.
#11 Mike Dickison
June 21, 2010

I hope there’ll be a mention of the flightless Jamaican ibis Xenicibis, at one point thought to be quadrupedal, but probably a clubber.
#12 Darren Naish
June 21, 2010

I’ll say again… dammit, you people gotta stop ruining all the surprises for everyone else!!
#13 Nathan Myers
June 21, 2010

I’m going to go out on a limb and guess that these things are unpleasantly noisy, rather than (e.g.) especially fast.
#14 Anonymous
June 22, 2010

“During combat, the birds will stab rivals with these spikes, and on occasion the spikes snap off and can then remain lodged in the tissues of the opponent. Does this render the attacker spike-less for the rest of its days? NO – the spikes can grow back. Many sources state this latter fact, but I’ve never seen any ‘good’ data on it: how long does it take the spikes to grow back? Do they break off frequently? Do all species suffer from this breakage? I’d love to know more.”

Interesting related note, something similar seems to be going on with Stegosaurus. Ken Carpenter did a paper where he noted that about a quarter of steggie thagomizers had their tips broken off, and went off to describe an Allosaurus vertebra where the thagomizer pierced it, got stuck, and warped the development of the bone. Strangely enough, the Allosaurus didn’t show any signs of infection on the bones. The breaking off of the various spikes and points in screamers and steggies probably isn’t evolutionarily intentional, but its a nice bonus (if you can grow it back, that is).
#15 Zach Miller
June 22, 2010

I think that paper is in “Horns & Beaks,” or maybe “The Carnivorous Dinosaurs?” One of those compilation books, anway. It’s a cool paper indeed, though I think the allosaur caudal transverse process DID show evidence of infection, or at least very strange healing growth. It’s like the transverse process tried to grow new bone AROUND the hole, not into it. I was surprised how many stegosaur spikes are found with the tips broken off. It probably hurt!
#16 Mo Hassan
June 28, 2010

You’re welcome to use my photos any time, Darren.