Energetic costs of bill heat exchange demonstrate contributions to thermoregulation at high temperatures in toco toucans (Ramphastos toco)

Body temperature regulation under changes in ambient temperature involves adjustments in heat production and heat exchange rates between the animal and the environment. One mechanism involves the modulation of the surface temperature of specific areas of the body through vasomotor adjustment. In homeotherms, this thermoregulatory adjustment is essential for the maintenance of body temperature over a moderate temperature range, known as the thermal neutral zone (TNZ). The bill of the toco toucan (Ramphastos toco) has been described as a highly efficient thermal window and hypothesized to assist in the thermal homeostasis of this bird. Herein, we directly evaluated the contribution of heat exchange through the bill of the toco toucan and role of the bill in the delimitation of the TNZ. To do this, we measured metabolic rate (MR), via oxygen consumption, over a range of ambient temperatures from 0 to 35 degrees C. MR measurements were made in birds with the bill intact and with the bill insulated. The limits of the TNZ did not differ between treatments, ranging from 10.8 to 25.0 degrees C. The MR differed among treatments only at elevated temperatures (30 and 35 degrees C), reaching 0.92 +/- 0.11 ml O2 g-1 h-1 (mean +/- s.d.) for the intact group and 1.13 +/- 0.13 ml O2 g-1 h-1 for the insulated group. These results indicate that although heat dissipation through the bill does not contribute significantly to widening of the TNZ, it may well be critically important in assisting body temperature regulation at higher temperatures extending above the upper limit of the TNZ.

Automated summary

Body temperature regulation in the toco toucan involves adjustments in heat production and exchange rates between the bird and its environment. The bill of the toco toucan has been hypothesized to play a role in thermal homeostasis. This study directly evaluated the contribution of heat exchange through the bill and found that while it doesn't significantly widen the thermal neutral zone, it may be important in regulating body temperature at higher temperatures.