Top Image: Yellow-billed hornbill, Dean Portelli.
While you and I rely on sweating to cool off, birds have two primary thermoregulatory strategies: panting (evaporative heat loss) and dilating their blood vessels (non-evaporative heat loss), particularly in their beaks. Bird beaks are dynamic structures and most birds show varying degrees of blood vessel dilation and constriction in their beaks. It’s thought that birds with unusually large beaks might use these structures to actively dissipate heat.
One such bird is the hornbill. Hornbills are widespread across Africa, living in habitats ranging from arid deserts to humid tropical forests. In a new study, researchers looked at southern yellow-billed hornbills (Tockus leucomelas) inhabiting the Kalahari Desert to see if they use their disproportionately large beaks to lose heat.
Tanja van de Ven of the University of Cape Town, South Africa, and colleagues used thermal imaging to track heat changes in 18 wild-caught hornbills over a range of temperatures. They found that hornbills dissipate heat via the beak at air temperatures from about 30-40° C. The difference between beak surface and environmental temperatures abruptly increased when air temperature was within about 10° C below body temperature, indicating active regulation of heat loss.
“I observed this rapid surface temperature increase of the beak of the hornbill as temperatures approached about 30°C,” says van de Ven. “This indicated the capacity of the hornbill to use the beak as a controllable thermal radiator.”
This makes them similar to another giant-beaked bird, the toco toucan (Ramphastos toco). A recent study showed that toucans also dilate blood vessels in their beaks to facilitate heat loss. Although both bird species share the ability to regulate heat exchange through their large beaks, toucan beaks account for more of the total non-evaporative heat loss (60% for toucans, compared to 8% for hornbills).
van de Ven and colleagues say this difference can be explained by three different possibilities. The first one is that the toucan beak is much larger in proportion to the body than the hornbill beak: The toco toucan’s beak represents 30-50% of its total body surface area, whereas the yellow-billed hornbill’s beak accounts for less than 5% of its total body surface area. Second, the hornbill beak is much harder than the toucan beak due to a thicker and stronger layer of keratin. This could affect the efficiency of thermal conduction; thinner beak structure means blood vessels are closer to the surface.
Third, toucans dilate the blood vessels in their beaks at lower air temperatures than hornbills, which creates a more favorable gradient for moving heat from the body to the outside. This difference in threshold temperature could be due to differences in environmental conditions, namely, humidity. van de Ven and colleagues speculate that non-evaporative heat loss through the beak might be key in the tropical forests where toucans live, as the humidity might make panting less effective. Thermal radiators such as beaks may be more important for species in humid environments.
“This leads us to hypothesize that the beak heat exchange capacity might even differ between populations of the same species residing in habitats with different temperature and humidity conditions,” says van de Ven.
Glenn Tattersall, one of the authors of the toucan study, says this hornbill research is a natural extension of testing the thermoregulatory hypothesis in the wild in another bird species.
“Birds don’t just inhabit thermal environments; they inhabit more complicated environments,” says Tattersall. “This is a nice first step in guiding us along that way.”
The outcome of this study has implications for more than just explaining the physiological capacities of different species. Many biologists are concerned with managing or predicting animals’ responses to climate change.
“If we can use beak size and watch birds’ use of their beaks as thermal radiators, then we can build an energetic picture of how they are coping with climate change,” says Tattersall. “Looking ahead to how birds are going to respond to temperature changes, we need to have a handle on what is a manageable temperature and what is a stressful temperature. This kind of thermal imaging study is a good way to do that.”