Blood vessels in the beaks of desert-living birds called hornbills dilate to help them cool off, according to a new PLOS ONE study published last week.
Mammals rely on sweat glands to keep our bodies from overheating, while birds typically use one of two strategies for dissipating heat: panting (evaporative heat loss) and dilating blood vessels in their beaks (non-evaporative heat loss). Previous studies revealed that when the temperature rises above 28°C (82°F), the supersized beaks of toucans account for about 60 percent of their total non-evaporative heat loss.
Hornbills have smaller beaks than toucans, but these are still disproportionately large relative to their bodies. These birds live throughout Afrotropical and Indomalayan areas, from arid savannahs to humid tropical forests. Southern yellow-billed hornbills (Tockus leucomelas) living in the Kalahari Desert of southern Africa are under intense pressure to conserve water – especially during the summer breeding season. That’s when females are confined to nest cavities, and the males are entirely responsible for provisioning the female and his chicks.
To see if hornbills use their beaks to cool off, a team led by Tanja van de Ven from the University of Cape Town used thermal imaging to monitor heat exchange around 18 wild-caught southern yellow-billed hornbills over a range of air temperatures. The birds were placed into a plastic chamber at an initial temperature of 15°C (59°F) , and this was increased all the way up to 45°C (114°F) over a span of about 120 minutes.
The team found that the hornbills dissipate heat through their beak when air temperatures are between 30.7°C (87.3°F) and 41.4°C (106.5°F).
Thermal images of a female southern yellow-billed hornbill at different air temperatures. Top left: When the air temperature is 15°C, the beak surface temperature matches the background. Top right: At 30.7°C, beak surface temperature changes, lower mandible first. Bottom left: At an air temperature of 32.2°C, the beak surface temperature is much higher than the rest of the body and the environment. Bottom right: When the air temperature is 43°C, the beak is cooler than the surrounding environment. van de Ven et al.
When the air is cool, the temperature of the beak surface matches the background. Beak temperatures start to change at 30.7°C (87.3°F), starting with the lower mandible. At an air temperature of 32.2°C (89.96°F), the beak surface temperature is much higher than the rest of the body and the environment – indicating that heat is radiating from the beak. At even higher temperatures, the beak becomes cooler than the surrounding environment.
The bill is responsible for as much as 19.9 percent of a hornbill’s total non-evaporative heat loss across its entire body surface. "Like toucans, hornbills can use their beak as a controllable thermal radiator," van de Ven said in a statement. "We think this might provide an advantage in the arid Kalahari by reducing the amount of water the birds need to spend on evaporative cooling." Non-evaporative heat dissipation may be more important for toucans living in tropical forests; the high humidity renders panting less effective. Since hornbills live in more arid settles, heat dissipation through their bills is slightly less critical. Toucans pant at 33.1°C (91.6°F), while hornbills pant at 37.4°C (99.3°F).
