May 18, 2016
When temperatures are scorching,
southern yellow-billed hornbills in the Kalahari Desert of southern Africa
dilate blood vessels in their beaks to thermoregulate and cool off, according
to a study published May 18, 2016 in the open-access journal PLOS
ONE by Tanja van de Ven from the Percy FitzPatrick Institute of African
Ornithology, University of Cape Town, South Africa, and colleagues.
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 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. To determine
whether hornbills might likewise use their beaks for this purpose, the authors
of this study raised the air temperature around
18 wild-caught southern yellow-billed hornbills and tracked their heat loss
with thermal imaging.
The researchers found that the
beaks accounted for up to 20% of the birds' non-evaporative heat loss, and
suggest that the benefits of heat loss from beaks on hot days likely varies
according to where the birds live. 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.
Tanja van de Ven notes: "We
have found that, like toucans, hornbills can use their beak as a controllable
thermal radiator. 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".
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