Arunn's Notebook

Why do Toucans have large bill

Arunn Narasimhan

What can one do with the nose? If one were Cleopatra of Egypt, she could rule Rome. If one were the unfortunate Sphinx of Egypt, his form minus the nose could become the wonderment of the World. If one were Tycho Brahe, he could remove the nose, for polishing amidst a heated debate or duel, to distract his opponent. For, he lost his original nose in a duel and had a metal one fixed. If one were the Tamil Detective Sambu created by Devan, he could run his thumb and index finger over the nose to make a deduction that is often wrong. If one is a proboscis monkey inhabiting the island of pulau pulau bompa, it endures the ignominy of its nose looking similar to Rastapopulous, the villain of Tintin comics.

What can one do if one is a toco toucan with its nose and mouth wrapped into one big bill? Of course, it can use the bill for quick thermo-regulation by exchanging heat with the environment. In fact, in a matter of minutes toucans can release upto 400 percent of their metabolic heat through their bill.

In a study I liked very much, Prof. Tattersall and team have measured the temperature of toucan bills in several surrounding and body conditions to arrive at such conclusions. These results were published in July 2009 issue of the Science magazine [1]. Several webs news services [3 - 6] briefed on these findings before the paper was actually published.

This note is to discuss the published results in some detail.

Picture Credit: Ref. [2]

Sometime back we discussed why elephants have large ear flaps [7]. Elephant ears serve as means for their thermo-regulation. Elephant ears are believed to be highly vascular (with dense blood vessels), which enable them to transport heat from the interior of their body to the ears, for exchange with the environment. Elephants control their body temperature by losing the excess metabolic heat to the environment through their ears.

Toucans do a similar thing with their bills. Only quicker.

The bill of toco toucan, the largest of toucans (about 20 cm for the heaviest), is enlarged, uninsulated, and well vascularized with a network of superficial blood vessels. This enables blood perfusion as a dominant method of heat transport to them from their body. Like the ear flap of an elephant, the toucan bill has all the features of a thermal radiator.

Thermo-regulation in warm blooded animals is an active process that maintains constant body temperature. It can be considered an equivalent to a steady state heat transfer situation achieved through a balance between heat production (through metabolism), gain, and loss. Important thermo-regulation processes include shivering, sweating, vascular adjustment and breathing acceleration. Shivering induces muscle work and heats the body. Sweating and vascular adjustment are primary heat loss mechanisms. Birds don't sweat. So toucans can control their body temperature primarily by adjusting blood flow to the body surface and adjust the heat loss to the environment.

The blood flow is varied in a tissue region through local vasodilation and vasoconstriction where blood vessels stretch and shrink respectively, to control the flow rate. Prof. Tattersall and team have measured the surface temperature of the toucan bill to estimate the heat loss. The result of a typical experiment is shown in the figure below.

Figure 2: Toucan bill temperature measurement. Picture Credit: Ref. [1]

For all surrounding air temperature Ta, the eye region was nearly constant [30 °C to 36 °C] when compared to the body temperature of the toucan of ~ 38 °C to 39 °C. This indicated continuous blood flow to the naked skin around the eye regardless of the surrounding temperature. In the bill, the portion closer to the body (proximal) and tip portion (distal) showed more temperature variation with change in Ta. From C and D part of the above figure one can observe the proximal region of the bill was used mainly to lose heat to the environment when 16 °C < Ta < 25 °C. As the environment temperature rose, the proximal region alone is unable to sustain the necessary heat loss. The blood vessels in the distal region receives increased blood flow through vasodilation and becomes warmer. The increased surface area helps the bird cope with the rise in the environment temperature to lose the necessary excess metabolic heat energy. That the distal region begins to participate in the thermo-regulation is more evident in the temperature data for the juvenile toucans shown in E and F parts of the figure above.

Observe how the proximal region in E tries to manage losing the heat by steadily increasing its local temperature with respect to increase in the environment temperature. However, until after Ta > 25 °C, the proximal temperature in E begins to drop, only to be compensated by an increase in the distal (tip) region shown in F. I wonder how the toucan would feel as more blood flows suddenly through its bill towards the tip. I know how I feel with a running nose.

Toucans can do this thermo-regulation real fast. That is temporal changes in the adult bill’s surface temperatures are rapid and reversible, occurring within minutes. This is excellently captured in the figure below.

Figure 3: Blood perfusion in toucan bill. Picture Credit: (Ref. 1)

As the legend explains, in about two minutes the blood perfusion can be so high that the bill is all aglow with heat to be released to the environment when we compare the left and right images in the above figure. Such quick control of thermo-regulation was most evident when the toucans were observed while they were sleeping as the video below shows (obtained from supplemental material available on web [2]). It is a series of shots of thermal images taken over a few minutes while the toucan goes to sleep.

Watch how initially the bill is golden when the bird is awake and while preparing to sleep, it tucks the bill inside its feather, which is a possible insulator. As the toucan proceeds to sleep, the bill cools to blue suggesting a drop in the basal metabolic rate during sleep.

YouTube Link - Video Credit: Prof. Tattersall

In the next video (available in the webpage for [2]) another interesting aspect of toucans behavior can be observed. Sleeping toucans show transient changes in bill surface temperature without the bird actually waking up. This indicates the possible variations in the metabolic heat production and the sleep-state transitions associated with changes in thermo-regulation.

A few more interesting results are reported in the paper. In what is called as activity-induced vasodilation of the bill, the surface temperatures from a toco toucan in an outdoor aviary during flight was measured. Surface temperatures increased rapidly from ~ 30 °C to ~ 38 °C during a 10 minute flight at an average speed of 17 km/hr.

Next the heat loss from the bill was also estimated from the above temperature measurements using simplified models. These models with equations are explained in the supplementary material [2] provided with the paper.

The toucan bill is assumed as a cylinder at constant temperature exchanging heat with the environment through radiation and convection heat transfer. The details of this modeling is in similar lines as already done for the elephant ears. For the equations and math, read the separate note I wrote on this [8]. Results from these calculations indicate as a proportion of total heat loss, the bill accounted for 30 to 60 percent of heat loss in adults and juvenile toucans. As the figure below shows, adults on the other hand, could adjust bill heat loss to account for as little as 5 percent (Tbill – Ta ~ 0), and, for short periods, up to even 100 percent of total body heat loss. Juveniles are not this gifted and seem to be unable to control their vasodilation in the bill. Even 2-month-old toucans keep losing more heat from the bill than is necessary. Hence they shiver to produce additional heat even at as environment temperature as high as 26 °C.

Figure 4: Heat loss estimates from toucan bill. Picture Credit: (Ref. 1)

In short, toucans can lose as little as 5 percent or as much as 100 percent of their body heat through their bill, by opening or closing the embedded blood vessels.

However, heat loss estimates from the bill are highly variable. It depends on air speed and environment temperature Ta, due to strong forced convection possible at higher speeds. From the figure above one can observe that the heat loss estimates could account for as little as 25 percent (minimum) to as much as 400 percent (maximum) of resting heat production (RHP) in adults. This value, the authors mention, is the largest reported for an animal. But it is not clear whether toucan encounter velocities of 20 km/hr or 6 m/s in their habitat. Only for such wind speed values (the maximum in the above figure) heat loss is 400 percent of RHP.

In fact, the present calculation can predict even higher heat loss for higher wind velocities, as forced convection continuously increase with speed. It is not clear from the paper [1] if the 6 m/s is experimentally measured or used only in the model for estimates.

A similar concern exists for the heat loss calculations for elephant ears reported earlier [9]. For instance, when the ear is modeled as a flat plate convecting heat into the surrounding air blowing with a certain speed, the heat loss is proportional to the wind speed. Estimates from such a model reported in [9] (and discussed in detail in my earlier note [8]) require a wind speed of 12 m/s for the elephant to lose about 100 percent of the resting heat production. Whether such a flap rate is physically possible for the ear remains, as far as I know, unverified.

References

  1. Tattersall, G., Andrade, D., & Abe, A. (2009). Heat Exchange from the Toucan Bill Reveals a Controllable Vascular Thermal Radiator Science, 325 (5939), 468-470 DOI: 10.1126/science.1175553
  2. Tattersall, G. J. et al. - Supporting Online Material
  3. Toucan Beak Is New Kind of ‘Heating Bill’ - Wired post link
  4. Toucans use their enormous bills to keep their cool - Guardian News link
  5. Giant Toucan Bills Help Birds Keep Their Cool National Geographic news link
  6. Hot secret behind toucan's bill - BBC news link
  7. Why do Elephants have Big Ear Flaps
  8. More Heat Transfer from Elephant Ears
  9. Phillips, P. K., and Heath, J. E., Heat exchange by the pinna of the african elephant (Loxodonta africana), Comparative Biochemistry and Physiology Part A: Physiology, v. 101, 693-699, 1992. Abstract

[Thanks to Lakshmi for correcting the draft version]

© Arunn Narasimhan | Original version written ~ 2009 | Last revision on Apr 01, 2012

Home