Close-up ofa female saffron's schnoz. The small notch near the end of the bill is anindividual quirk.

A keel-bill toucan shows offhis overgrown beak.

A plate-billed mountain toucan profiles herpeculiar beak. The yellowpatch on her back isthe mark ofthe female.

A young saffron toucanetflaps offwith her fav­orite food. The lack of tail and over-grown feetare marks ofthe juvenile.

enE.~

~Q>0>oa:>,.0oo..cQ..

Apetemerald toucanet debates whether ornot to eat his bit ofapple.

A crimso'n-rumpedtoucanet peeps out ofherpalm log nest hole.

36

TheLong Billof theTOucan

byRoger Paul Williams

andElaine Radford

New Orleans, Louisiana

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Ever since the first human saw atoucan, people have wondered why abird would have such an awkward­looking bill. Adding to the confusion isthe fact that, once having grown thebeak, the ingenious toucan puts it to usein a variety of ways, many of which areprobably unrelated to its original func­tion. We'd like to leap into the resultingfray with a suggestion of our own: wethink the toucan evolved its bill as acooling system.

The toucan's bill is a light, porousstructure rich in blood vessels, inter­nally similar to the smaller beaks ofother softbills. Like all birds, the toucanhas no sweat glands and hence mustpant when overheated, using theevaporation of moisture within its openmouth as a cooling mechanism. But thetoucan has a real problem regulating itsbody temperature; not only does itdwell in the hot, humid rain forests ofCentral and South America, whereevaporation is a slow and difficult pro­cess, but its plumage consists of fine,almost fur-like feathers which can actlike an insulating blanket. Perhaps thetoucan's answer to the dilemma was thedevelopment of a long beak with broadinternal surface area, allowing theblood circulating therein to cool moreefficiently as the bird gaped.

There's little doubt that the bill is, atleast sometimes, put to this purpose.During the record-breaking heat waveof ew Orleans' summer of 1984, ourpet emerald toucanet indulged inshowers and baths up to five times a day.So closely does this bird link a wet billwith coolness that he has learned to askfor the shower mister by gaping aswidely as he can, as if yawning. Onceunder the spray, he concentrates on

37

catching the droplets within his man­dibles by yawning and stretchingVigorously.

But is there any evidence that this isthe original reason for the bill's evolu­tion? We believe so. In addition to theenvironmental factors already de­scribed, there is a rough correlation be­tween habitat and bill size. The largertoucans of the genus Ramphastos livein the hotter jungle regions, while thesmaller, shorter-billed toucanetsgenerally inhabit the cooler mountainslopes. This trend reflects the need forgreater heat loss (and hence larger bills)in the hotter, more humid sea-level rain­forests. one of the alternate hypo­theses addresses this observation.

Of course, competing theoriesabound. The most obvious was also thefirst discredited; like most birds, thetoucan has a poor sense of smell.

Another dimly regarded pOSSibility isthat old standby, "protective colora­tion," which has been applied at onetime or another to every unexplainedbit ofplumage in the classAves. The firstornithologist to perform an in-depthfield study on toucans dismissed thistheory out of hand. In The Life Historyof the Toucan, Josselyn Van Tynewrites, " ... the actions of a toucanusually defeat any [such] purpose....At the first sign of danger, the toucanalmost invariably begins to bob abouton its perch and croak loudly, adver­tising its presence to all within half amile or more." Protective coloration isuseless to an animal that can't sit still.

A more seriously considered idea isthat the bill evolved for purposes ofidentification. In the field, some speciescan scarcely be distinguished from oneanother unless the bill is visible; thekeel-billed and the chestnut-mandibled(Swainson's) are such a pair. Yet thespecies identification thesis seemssomehow inadequate. While it canhardly be argued that the toucans don'tuse this handy tool for recognition, wedon't feel this theory tells us why thebill came about in the first place. Afterall, the fact that humans might use theshape of a nose to help recognize oneanother doesn't mean that Roman nosesevolved for that purpose.

At any rate, Van Tyne's studies sug­gest that the bill developed before themarkings. In the genus Ramphastos, forinstance, we find that the most primi­tive species, the red-breasted toucan,has a dull green bill, while colorful pat­terns run riot on the beaks of more ad­vanced species such as the keel-billed.Species recognition, in that case, isalmost certainly a secondary function

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of the overgrown bill.Similar objections arise to the hypo­

thesis that the bill is a convenientfeeding device. Why not a slender,more easily negotiated beak like that ofan ibis if ease in eating is the goal? Intruth, when watching the acrobaticsnecessary for toucans to feed with theirunwieldy bills, we're amazed that any­one could consider this appendage adinner-time advantage. It's quite com­mon to observe a toucan or toucanetknocking itself clear off its perch whilefighting a morsel into position forswallowing.

Although the theory that the long billevolved for use in battle is a colorfulone, we must be careful not to confuseobservations of avicultural specimenswith what really takes place in the wild.While breeding pairs of captive toucanshave been known to injure and even killtheir aviary companions, such eventsare rarely reported in nature wherethere is more room for a potential vic­tim to flee the clumsy toucan. And aswild adult toucans are almost neverattacked by predators, ornithologistshaven't had much opportunity to seethese fearsome-looking weaponsemployed in actual fighting. Toucansseem to rely on bluff, perhaps feintingbut not actually striking. Given thefragility of this structure, it's probablyjust as well that it isn't usually pressedinto service as a bludgeon.

Therefore, although toucans do usetheir bills at some time or another forone or more of these purposes, none ofthese older ideas fully explains how thebig beak got there in the first place.Unlike most of these competing forces,the cooling function theory follows theclassic form of an evolutionary drivingforce. It was there, in some measure, atthe beginning since all birds gape to cooloff; any small increase in bill size wouldbe rewarded with a definite smallincrease in heat tolerance; and themagnitude of the advantage to theotherwise well-insulated toucan makesit believable that it would be worth theinconvenience to evolve the bill.

Finally, the geometry of the bill(broad rather than slender) is exactlywhat one would expect to maximizethe interior surface area and henceevaporation potential. Given the con­gruence of these facts, it's easy to see

. how the bill might have graduallyevolved to the comical dimensions weobserve today, allowing the toucan tolater modify its acquisition to allow forthe activities suggested by the alternatetheories. You see, the toucan simplywent to extremes to keep its cool. •

39