If you’re like me, you’ll have asked yourself many times, “Jerry, why do toucans have such ridiculously big bills?” (See Figs. 1 and 2.) The first answer that might strike you is that the bill — like bodies, plumage ornaments, and other traits in birds — was driven to extreme size by sexual selection. But that won’t wash because male and female toucans have identical-sized bills, and if the male’s bill is brightly colored, so is the female’s. (There are several dozen species of toucans in five genera, all Central or South American.)
The next most obvious hypothesis is diet: maybe toucans eat a type of food that requires large bills to handle. But that doesn’t seem likely, either. Toucans are frugivores (fruit eaters; Fig. 3), and there’s no obvious reason why they need such a big bill to handle fruit. Indeed, there are many frugivorous birds, like parrots, and none of the others have such hypertrophied beaks.
A paper in today’s Science gives a clue: the bill is a radiator. You might suspect this because the bill is full of small blood vessels and is uninsulated. But can the birds control the flow of blood to the bill as needed?
The authors used thermal-imaging video cameras to record the temperature of the birds’ bills and bodies in rooms adjusted to different temperatures ranging from 10 degrees C to 34 degrees C (the species was the toco toucan, Ramphastos toco, which has the largest bill of all toucans). What the authors found, and what you can see in the movies below, is that the toucan can adjust blood flow to the bill depending on ambient temperature. When the room heats up, the surface of the bill heats up rapidly, allowing body heat to be dumped. The reverse happens at cooler temperatures.
When birds are flying — a time when they produce 10 to 12 times more metabolic heat than when they are resting — the bills can heat up by as much as 6 degrees Centigrade. And when the birds get ready to sleep, a time when their body temperature is reduced (this saves metabolic energy), the surface of the bill transiently heats up, allowing them to dump heat (see movie #1 below). There are also, as you can see in movie #2, transient changes in bill temperature during sleep, presumably to regulate body temperature (like many birds, the toucan tucks its bill under its feathers while asleep, presumably also to buffer heat loss).
Movie 1. Body heat moves to the bill right before the bird goes to sleep (note bill glowing bright orange, while body stays darker; temperature scale to right). “Heat dumping to the bill during entry into sleep. Thermal imaging video demonstrating transient movement of body heat to the bill during initiation of sleep in a toco toucan. Time-lapsed data obtained at 10-s intervals. Total frames = 724, total length = 2 hours.”
Movie 2. “Sleep-state transitions witnessed as changes in bill temperature. Thermal imaging video of transient changes in bill temperature that occur during sleep while the bill is tucked between the wings. Time-lapsed data obtained at 10-s intervals. Total frames = 724, total length = 2.7 hours..”
Now none of this answers the question of why the beaks are often brightly colored. That probably has the same answer to the question of why some other non-dimorphic birds, like parrots, are also brightly colored. There are lots of theories (ease of recognizing members of your own species is one), but, in short, we don’t know why. And we also don’t know why toucans, but no other species, have beaks this large. Why do toucans need to thermoregulate more than other species? A final question — one that probably can’t be answered — is this: did natural selection increase bill size because that increase directly helped with thermoregulation, or is the thermoregulatory function an exaptation, a beneficial byproduct of a feature selected for some other reason?
Fig. 1. The ridiculously large bill of the toucan. This is a keel-billed toucan, Ramphastos sulfuratus.
Fig. 2. The toco toucan, subject of this study. Is that a banana in your mouth or are you glad to see me?
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Fig. 3. Toucan Sam
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G. J. Tattersall, D. V. Andrade, A. S. Abe. 2009. Heat exchange from the toucan bill reveals a controllable vascular thermal radiator. Science 325:468-470.
Damn! This paper kills my favorite hypothesis of crypsis, convergence with bananas.
I think I’ve seen TV footage of toucans using their bills to pluck fruit from small branches that wouldn’t take the weight of such a large bird. So the primary benefit of the large bill could just be feeding efficiency. Probably easier to grow a long bill than a long neck!
“if the male’s bill is brightly colored, so is the female’s”
Apparently toucan play at that game.
:p
When the room heats up, the surface of the bill heats up rapidly, allowing body heat to be dumped.
Cool.
(I know…lame.)
Kudos to the authors for this study. I wish I could read the Science article directly.
You can read it directly, I think. Click on the link!
No, sadly:
In his defense, I’m guessing it’s been a while since Prof. Coyne didn’t have a subscription to Science.
If you have ever eaten a freshly picked perfectly ripe fruit you will know why toucans have huge bills, the sudden gush of the sweet syrupy liquid and the moist pulp will bring an instant smile to your face, the more fruit that gets stuffed in the greater the sensation. Happy toucans are better lovers equals more happy giant billed babies.
“What the authors found, and what you can see in the movies below”
What movies?
Not the pix – the Movie 1 & 2 subheads are active linx.
Hornbills, widely distributed in the Old World tropics, also have hypertrophied bills. They frequently have an additional dorsal keel or process on the bill, making it even larger. They eat fruit (among other things), and the nearest reference I have to hand says “Sexes alike or similar”. I’ve seen a pair at the Milwaukee Zoo, but can’t recall how dimorphic they are. It would be an interesting group on which to test the radiator hypothesis.
And all this time I thought they had big bills because they had expensive tastes.
A duck walks into a 7-11 and asks for a tube of Chapstick.
The clerk asks, “Will that be cash or charge?”
The duck replies, “Just put it on my bill!”
I thought for sure the first joke would be about Fruit Loops. 😛
Yup, Sean, I was going to talk about the extreme Rockwell hardness rating of late Pliocene Fruit Loops, but I’m not going to mention it now…
“And we also don’t know why toucans, but no other species, have beaks this large. Why do toucans need to thermoregulate more than other species? A final question — one that probably can’t be answered —…”
Goddidit. No wonder why IDiots are winning: scientists keep telling people about stuff they do not know! Maybe Mooney et al. are right…
🙂
I could have sworn it said “balls.” But, that misunderstanding aside, toucans sure do look cool.
Heh.
“But [the sexual selection argument] won’t wash because male and female toucans have identical-sized bills, and if the male’s bill is brightly colored, so is the female’s.”
I’m sorry, but there is no reason why a trait which is sexually selected by females should not also appear in the females themselves. There are plenty of species of brightly coloured birds (e.g. the European robin) where both species are equally brightly coloured. Many female sheep and deer have horns and antlers. Daughters get half of their genes from their fathers, so you would expect them to display certain of their fathers’ traits (even those sexually selected by their mothers).
(Actually, the horns/antlers were a bad example, because they are not selected by the females – but you catch my general drift.)
Agreed re sexual selection operating on both sexes. Jerry, it beggars the imagination to believe that ANYTHING other than sexual selection drives the bright and often highly varied coloration of toucan bills. Of course, that leaves the possibility that something else drove the evolution of large bill size first, with sexual selection “painting” the bills afterward. There is nothing about the biology of toucans that suggests a physiological requirement for a massive, highly expensive heat exchanger. But the suggestion by Anton Mates elsewhere on this posting that long bills assist in nest robbing of oropendulas is a very interesting idea. Which came first in driving the evolution of massive bills … sexual selection or nest robbing … probably can never be known. Though comparisons of the feeding ecology of toucans with their molecular phylogeny might help.
“Why do toucans need to thermoregulate more than other species?”
A special ability doesn’t necessarily require a special need. Maybe all birds need to thermoregulate equally, but toucans are just better at it.
Not sure I agree. On your (admittedly hypothetical) account, you’re faced with explaining why all birds don’t have as large bills as toucans have. It seems more plausible that there’s something special about the toucan’s evolutionary position that has generated such a large bill.
It all goes to show that when the
going gets hot you too can run up a
big bill in the jungle.
As Richard Carter says, there’s no reason why sexually selected traits must also be dimorphic.
Toucans are also frequent nest robbers, and occasional predators on small animals like lizards and bats. A long bill comes in very handy there, for snagging distant prey and probing inside nests. One of the Attenborough documentaries–the “Demands of the Egg” episode in “Life of Birds,” IIRC–has a great sequence of a toucan inserting its bill into the long “neck” of an oropendula nest and extracting eggs.
Tattersall et al. mention this, of course. There’s no reason why diet, social/sexual selection and thermoregulation couldn’t all be factors here.
I actually have a serious question. (I know, shock horror.)
I read recently that cats are unable to detect what we call “sweet.” The gene’s either not there or blocked, I don’t recall off hand.
However, my cat loves to eat mice, lizards, insects and birds. Obviously such fare tastes good to him, in spite of the gourmet trout and shrimp I buy for him.
So, my question is this. Do pandas have a “taste” for bamboo, koalas for eucalyptus, toucans for fruit and so on?
My cat goes ape for catnip, too, although I’ve tried using it as a spice in soup to no effect.
I believe that the genes for sweet reception in cats are there, but inactivated (I may have posted on this before). Your question is a good one, but deals with subjective perceptions and so is almost impossible to answer. We will never know what a panda really “tastes” when it chomps on bamboo. I think all we can say is that it must not be unpleasant, and, indeed, must be something that the panda seeks out because that’s all it can digest. I think one can make a strong inference that selection would mold subjective perceptions of flavors to make them agree with what the animal is evolved to eat.
Taste is a question that came up over and over as I raised my kids. We fed them a variety of things from enchiladas to curry to fish and chips and they pretty much eat anything these days.
Their peers, however, raised on hot dogs, macaroni and cheese and canned foods are far less adventurous.
So, I agree, taste alone is not an overriding factor. Might one then raise a koala on Big Macs?
“Might one then raise a koala on Big Macs?”
Only if you sat one on a burger, and then lifted them up.
Koalas have very Catholic tastes.
(Don’t tell that to M&K, Ok?)
Why the assumption that big bills were selected for… maybe toucans evolved from huge birds and their bodies shrunk over time.
That sounds a bit like my much maligned hypothesis that elephants evolved from large gray worms.
🙂
There is actually a great documentary showing various mammal embryology that this reminds me of– http://channel.nationalgeographic.com/episode/in-the-womb-animals-2864
I wonder if the toucans embryology is very different from other birds… is it born from huge eggs to accommodate it’s huge beak or is there just wild growth after hatching.? Are the beaks colored at birth? (‘time to use my google fu)
Just so…
Cool baby pix: http://www.strayreality.com/toucanbaby.htm
It has big eggs and is born with a big beak that his duller colored than in adult hit and proportionately shorter.
And I found these cool embryo pix of other animals… including a penguin. http://www.dailymail.co.uk/sciencetech/article-1077492/It-aint-half-snuggly-Mum-The-ground-breaking-pictures-animals-capturing-life-womb.html
“adulthood” not “adult hit”
(Damn!)
New born toucans!: http://www.avesint.com/99Tocochix.html
they look pterodactylic–
“newborn”… not “newborn”. (I think it’s time for me to go to sleep.)
Oh wow, they’re both ugly and cute at the same time – I wasn’t sure that was even possible.
ever seen a pug?
yes. have you seen a horse
I’ll ditto Richard Carter’s point that a lack of sexual dimorphism is not evidence that it is not sexually selected. And sexual selection can go both ways, and can even reinforce the same characteristic.
Parsimony makes me go with exaptation as the most likely explanation for the radiator function.
I see you’ve added a picture of Fruit Loops. Nice touch!
To pick a non-trivial nit, Jerry wrote
How can we rail at creationists for the pejorative phrase “It’s just a theory” when we misuse the word like that? There may be lots of conjectures and hypotheses about toucan beak coloration, but no theories.
It’s losse usage like this that allows people like Behe to argue in court that
Loose usage of the word allows this kind of obfuscation.
There is nothing wrong with the way Coyne used the word theory. One can have a failed theory or an unproven theory. No theories are actual indisputable facts. All can be superseded by better ones.
The next most obvious hypothesis is diet: maybe toucans eat a type of food that requires large bills to handle. But that doesn’t seem likely, either. Toucans are frugivores (fruit eaters; Fig. 3), and there’s no obvious reason why they need such a big bill to handle fruit. Indeed, there are many frugivorous birds, like parrots, and none of the others have such hypertrophied beaks. ta almost correct your post! not only that the toucan eats only fruit! it is the staple food fruit eggs of other birds and insects. image 3 will have nothing to do with the bird is only a grain marketing.
I read an article once stateing that Toucans were co-evolved with Oropendolas and maybe other hanging nest maker birds. Toucans have long bills of a length matching the depth of the bird they co-evoloved with. Toucans prey on eggs out of hanging nests, and the hanging nest is an attempt to keep snakes ond other birds out of the nests. Toucans cna dip into nests of the right species, that male nests of the right depth, and steal eggs. They are egg theives. I can’t find the citation, sorry.