Vultures don’t exactly have the best reputation in the avian kingdom. Not only are they scavengers, doing nature’s dirty work clearing up the carcasses of dead and decaying animals, they can also be pretty ugly.
Indeed, when Charles Darwin after crossing the Atlantic in 1832, came across the turkey vulture, he was not impressed, noting “These disgusting birds, with their bald, scarlet heads, formed to revel in putridity.”
The sinister appearance of vultures is in part due to the lack of feathers on their heads and necks. Coupled with their hunched stance and prominent brows they’re not the sort of birds you’d want to pick a fight with.
For years scientists assumed that vultures had bald heads to prevent their feathers from becoming dirty when feeding on carrion. When eating vultures stick their heads into the body of a dead animal to get at the meat inside. Now imagine what happens when they pull their heads back out? They are covered with bits of innards, flesh, and blood. How much worse would that be if they had feathers on their heads?
However, other scavenging birds such as giant petrels aren’t bald, and neither are scavenging mammals such as brown bears or hyenas.
In 2008 a team from the University of Glasgow decided to investigate. As part of her PhD thesis Jennifer Ward studied the habits of the griffon vulture (Gyps fulvus) a large Old World vulture that lives in north Africa, southern Europe, and Asia.
Each day griffon vultures have to withstand a wide range of temperatures. In the desert of north Africa summer ground temperatures regularly reach 40°C. But within minutes of take- off griffon vultures can reach altitudes of over 2,000 m where the temperature drops to below freezing. To cope with this extreme change griffon vultures must have physiological and anatomical mechanisms to survive.
Ward and her team examined photos of wild birds and observed captive birds to find out which positions they adopted in different conditions. Changes in posture could increase their bare skin exposure from 7% to 32% and they modelled how heat would be lost from different parts of the birds’ bodies when in different postures.
When it was cold vultures hunched their bodies, tucking their necks in, and reducing the surface area through which they can lose heat, typical behaviour for birds when trying to keep warm.
When it was hot the vultures stretched their necks and spread out their wings to maximise their surface area. By exposing more or less of their bald skin on their heads and necks the team found that vultures could cut their heat loss in cold conditions by half, and in hot conditions could increase their heat loss by almost a quarter. When a vulture is unable to find any food that is a significant energy saving.
The scientists concluded that other species of vultures, such as the American king vulture, also have bald heads to help with thermoregulation. Other species of birds including emus, ostriches, and cassowaries may also have bald heads or large areas of bald skin such as wattles to help them control heat loss.
As well as bald heads vultures also tend to have bald feet and legs which again can help them get rid of excess body heat. They also have another way of keeping cool which is even more disgusting than the sight of some bare, wrinkled skin. On very hot days vultures will urinate on their legs. As the liquid evaporates it cools them down, in the same that we cool down when we sweat. Urinating on their legs also serves another purpose. Their urine is highly acidic and so it acts as a disinfectant cleaning off any potentially harmful bacteria they may have picked up when dining on the flesh of a decaying carcass.