A team of scientists led by Tanja van de Ven from the University of Cape Town has found that southern yellow-billed hornbills (Tockus leucomelas) in the Kalahari Desert use their bills as an adjustable thermal radiator.
Southern yellow-billed hornbills (Tockus leucomelas) keep cool by losing heat through their beaks. Image credit: Chris Eason / CC BY 2.0.
While mammals have sweat glands to keep their bodies from overheating, birds may rely on panting (evaporative heat loss) and dilating their blood vessels (non-evaporative heat loss), particularly in their beaks.
Toco toucans (Ramphastos toco) are an example of the latter: their outsized beaks account for up to 60% of their non-evaporative heat loss at air temperatures above 28 degrees Celsius.
Although not as extreme as toucan beaks, hornbill beaks are still quite large relative to their bodies.
“Active regulation of heat transfer from the beak has recently been demonstrated in a toco toucan, with the large beak acting as an important contributor to heat dissipation,” Dr. van de Ven and co-authors wrote in a paper in the journal PLoS ONE.
“We hypothesised that hornbills likewise use their large beaks for non-evaporative heat dissipation, and used thermal imaging to quantify heat exchange over a range of air temperatures in eighteen desert-living Southern yellow-billed hornbills.”
“We found that hornbills dissipate heat via the beak at air temperatures between 30.7 and 41.4 degrees Celsius.”
“The difference between beak surface and environmental temperatures abruptly increased when air temperature was within 10 degrees Celsius below body temperature, indicating active regulation of heat loss.”
Thermal images of a female Southern yellow-billed hornbill (Tockus leucomelas) at different air temperatures. Image credit: Tanja van de Ven et al.
In tropical forests where toucans typically live, for example, beak heat loss could be key, as humidity may make panting less effective.
For those Southern yellow-billed hornbills living in the desert, beak heat loss could be important as, in contrast to panting, it may help to conserve scarce water.
“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,” Dr. van de Ven said.
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van de Ven T.M.F.N. et al. 2016. Regulation of Heat Exchange across the Hornbill Beak: Functional Similarities with Toucans? PLoS ONE 11 (5): e0154768; doi: 10.1371/journal.pone.0154768
