Toucans are able to alter the flow of blood through their large bills, which make-up 30-50% of their body area. The effect is very significant: the toucan bill is now understood to make a major contribution to temperature regulation.
The Toco Toucan(Source and video here)
The newly published research needs to be considered in the context of history: for many people have wondered about the unusually large beak (the toucan has the largest bill relative to the body size of birds). The authors draw attention to previous speculation:
"In the hall of animal oddities, the toucan's enlarged bill is the avian example of exaggeration, being a source of debate since Buffon labeled it a "grossly monstrous" appendage. Even Darwin was intrigued, stating that "toucans may owe the enormous size of their beaks to sexual selection, for the sake of displaying the diversified and vivid stripes of colour with which these organs are ornamented". More recent explanations for the oversized bill include fruit peeling, nest predation, social selection in the context of territorial defense, and, finally, serving as a visual warning."
Since this is Darwin's year, it is worth highlighting his comments which were made as part of an argument for sexual selection. The justification of his interpretation is (a) that the idea is not incredible; and (b) that there is no greater improbability in thinking toucans have large beaks because of sexual selection than in thinking male pheasants should be "encumbered with plumes" for the same reason.
"This leads me to remark that it is not at all incredible that toucans may owe the enormous size of their beaks to sexual selection, for the sake of displaying the diversified and vivid stripes of colour, with which these organs are ornamented. The naked skin at the base of the beak and round the eyes is likewise often brilliantly coloured; and Mr. Gould, in speaking of one species, says that the colours of the beak "are doubtless in the finest "and most brilliant state during the time of pairing." There is no greater improbability in toucans being encumbered with immense beaks, though rendered as light as possible by their cancellated structure, for an object falsely appearing to us unimportant, namely, the display of fine colours, than that the male Argus pheasant and some other birds should be encumbered with plumes so long as to impede their flight."
(Darwin, C. R. 1871. The descent of man, and selection in relation to sex. London: John Murray. Volume 2. 1st edition. Chapter 16. The quoted words are on page 227).
The reason why many have not found this hypothesis convincing is that both male and female toucans excel in bill size, and both sexes have bills that are brightly coloured. The other hypotheses that have been explored have not fared any better. Take the fruit peeling idea: toucans eat fruit, but is there an advantage in having a large bill to do this? There are many other frugivorous birds, but none display notable large bills. Before moving on to consider the new research, it is worth pausing to reflect on the adaptationist paradigm. What Darwinists have done is persuade people to think that plausible speculations deserve to be considered as science. The tried-and-tested empirical approach is replaced by just-so stories, which become the basis of adaptationist 'science'. People have the view that Darwin's strength was his detailed knowledge of empirical observations, but this case may help to illustrate why this image is false. Darwin's theory did not emerge from the data but was brought in as an interpretative screen through which the data was viewed. There were numerous places where the fit was not good, but people did not perceive them because of Darwin's skill in telling persuasive stories.
The researchers have documented the function of thermoregulation after noting the network of blood vessels between the horny and bony parts of the toucan bill. They chose to work with the toco toucan because it has the largest bill of all the toucans.
"The bill has a network of superficial blood vessels supporting the horny ramphotheca. Therefore, the toucan's bill combines all the important features of a candidate thermal radiator: It is enlarged, uninsulated, and well vascularized. It is, however, crucial that blood flow be adjustable in order to control heat exchange from the bill. We examined whether the toucan's bill can operate as a thermal window for heat loss, capable of being "opened" within and above the thermal neutral zone and "closed" to conserve metabolic heat at lower temperatures. We used infrared thermography to examine the effects of changing ambient temperature on the heat exchange profile of different regions of the bird's body."
The findings were spectacular:
"The bill radiated a great deal of heat at high temperatures and when the toucan flew, indicating that, like elephants and rabbits do with their ears, the toucans flush their bills with blood to cool down. At lower temperatures, the difference between air temperature and bill temperature dropped, meaning that the toucans were restricting blood flow to their bills. Based on its size, a toucan's bill can theoretically account for anywhere from 5% to 100% of the bird's body heat loss [. . .]. When the toucan is in flight, its bill is the most efficient heat-shedder ever reported, losing four times more heat than the bird produces while at rest. That's about four times more efficient than either elephants' ears or ducks' bills."
The earlier explanations assumed adaptation, whether the trait is the consequence of sexual selection, fruit peeling, nest predation, social selection - territorial defense or visual warning. These analyses are now revealed as over-simplistic. They all assume that the only thing to be explained is the large bill. Once a driver for adaptive change is found, a new just-so story is invented. The new explanation is full of complexity: the bill has an internal structure involving vascularity, and the bird has the ability to control the flow of blood so as to achieve thermoregulation. This is an integrated system with feedback mechanisms - and this is not amenable to the 'one driver - one trait' mentality that has dominated the thinking of adaptationists. These systems, however, fit readily within the design paradigm because here we recognise complex specified information.
Heat Exchange from the Toucan Bill Reveals a Controllable Vascular Thermal Radiator
Glenn J. Tattersall, Denis V. Andrade, and Augusto S. Abe
Science, 325, 24 July 2009: 468-470.
Abstract: The toco toucan (Ramphastos toco), the largest member of the toucan family, possesses the largest beak relative to body size of all birds. This exaggerated feature has received various interpretations, from serving as a sexual ornament to being a refined adaptation for feeding. However, it is also a significant surface area for heat exchange. Here we show the remarkable capacity of the toco toucan to regulate heat distribution by modifying blood flow, using the bill as a transient thermal radiator. Our results indicate that the toucan's bill is, relative to its size, one of the largest thermal windows in the animal kingdom, rivaling elephants' ears in its ability to radiate body heat.
Price, M. A Bird With a Big Air-Conditioning Bill, ScienceNOW Daily News (23 July 2009)
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