To Charles Darwin it was emblematic of sexual desirability. To French naturalists it was "grossly monstrous". But even to the uneducated eye, it was conspicuous as an alarmingly oversized appendage.
The toucan's bill is one of the most bizarre products of evolution, but in the past 200 years scientists have failed to agree why it has grown so huge. Some argue it helps the birds collect hard-to-reach fruit, while others say it is a warning to rivals, or helps them raid other birds' nests for food.
Research by Canadian and Brazilian scientists puts forward a completely new theory that helps explain how the toucan got its bill. In a report in the journal Science, they claim the appendage doubles as a giant radiator that keeps the birds cool in the heat of the tropics.
Glenn Tattersall, a comparative physiologist at Brock University in Ontario, used a heat-sensitive camera to film toco toucans, which have the largest bills of all the toucans. The adults' bills can grow to 20cm – a third of their body length.
The thermal camera revealed that the birds use their bills to control their body temperature by adjusting blood flow into the appendage. By opening or closing blood vessels in the beak, the birds can lose as little as 5% or as much as 100% of their body heat through their bill.
The study puts toucans' beaks on a footing with elephants' and rabbits' ears as nature's solution to life in a hot climate.
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20 sec video at link, plus more of article
http://www.guardian.co.uk/science/2009/jul/23/toucan-bill-beak-cooling-heat