Sulzer Technical Review Issue 2 / 2017

When Tortoises Fall From the Sky

July 19, 2017 | Herbert Cerutti

Some birds crack open the rigid shells of their prey by dropping it from a great height onto hard ground. Also, some apes ward off threats by hurling various objects at their enemies from tree branches.

Bird cracking a shell

Two thousand years ago, the Roman natural philosopher Pliny the Elder wrote about an eagle that would capture tortoises and smash open the shells of the animals by letting them fall from a great height. Modern scientists long considered this account little more than an amusing anecdote. However, it was confirmed by an astonishing discovery in 1980, when several zoo­logists observed golden eagles soaring high up in the sky with tortoises in their claws before dropping the prey onto rocky surfaces from a height of approximately 60 m. If the shell withstood the impact of the fall, the tortoise would be taken on another voyage into the sky. Sometimes up to three flights were necessary; one particularly robust shell needed to be dropped eight times before it finally cracked.

Animals that use the power of gravity

Bearded vultures, also known as lammergeiers, feed exclusively on carrion and bones. They can swallow the entire spine of a cow as well as bones that are up to 30 cm in length, which they then digest with the concentrated hydrochloric acid in their stomach. However, these vultures rely on the force of gravity to break up larger chunks of their food, such as the thigh of a deer. They ascend to a height of up to 100 m and drop the piece of carcass so that it smashes against the rocks below. The English zoologist Julian Huxley discovered an extraordinary site that had been a feasting ground for such vultures in Tanzania in 1961, when he came across a lava field strewn over with countless remains of shattered bones.

Techniques for cracking hard shells

There are many other species of birds that, like the golden eagle and lammergeier, know how to use the force of gravity to feed on their prey. Seagulls can get to the tasty flesh of shellfish by dropping them from up high. For example, the southern black-backed gull, also known as the kelp gull, drops large shellfish onto hard ground from a height of between 4 and 8 m, depending on the type of surface. Meanwhile, herring gulls plunge into shallow sea waters at low tide to capture common whelks (sea snails) before flying up many meters into the air and dropping them onto the stony beach. The herring gulls don’t use this technique to get at the sea snails themselves, though, but rather to gain access to the hermit crabs that often live inside empty sea snail shells.

Hurling stones

Some birds even behave like bomber pilots, using the dropping technique to open their food’s “packaging” on the ground. There are reports of black-breasted buzzards in Australia that swoop in at a low level to steal emu’s eggs from nests while the eggs are still being incubated. The buzzards then drop fist-sized stones onto the eggs from a high altitude. The use of this technique takes on a military dimension when birds carry out such aerial attacks not for the sake of obtaining food, but rather to fight off their enemies. A scientist was once studying a common raven’s nest containing six chicks on a rock face in Oregon. However, he was forced to abruptly end his study when the adult birds began to bombard him with stones upon their return.

Arsonists of the sky

There is a species of bird in Australia that is known to deploy a rather more uncanny method of hunting. Indigenous Australians tell tales of black kites that pick up smoldering sticks after there has been a bush fire. The kites then fly several hundred meters away to an area untouched by the fire to drop the burning stick on a patch of dry grass. Rodents and reptiles flee the fields that are now ablaze, allowing the waiting predators to conveniently pick off their prey. This seemingly impossible firebombing tactic was recently confirmed by zoological observations. 

When apes attack

Animals don’t always need wings to wage a battle from up high. For example, apes can use their elevated position in trees as a tactical advantage. Spider monkeys in South and Central America throw branches and feces at creatures on the ground in order to scare them away. And orangutans (Fig. 1) are also quick to ward off unwanted visitors by hurling heavy branches with a dangerous backhanded throw. Richard Davenport, an American professor of zoology, has reported on his experience of being bombarded with dangerous objects by a group of orangutans hiding in tree branches. The attack was continued for 15 minutes at a rate of ten projectiles per minute.

Orangutan sitting on a tree
Fig. 1 Sometimes orangutans attack from elevated positions.
Apes even make an appearance in historical accounts of war. During the Boer War in South Africa from 1899 to 1902, soldiers were often forced to flee a group of chacma baboons that were hurling stones at them. Seventy years later, zoologists experienced a similar attack spearheaded by a herd of 25 chacma baboons in a rocky valley in Namibia. The apes carried out 23 different waves of attack from cliffs, throwing a total of 124 stones at the researchers, who only managed to escape without injuries by rapidly dodging the stones and ducking out of the way. Each of the projectiles weighed at least one pound, and the baboons purposefully chose the most suitable stones to throw from their immediate surroundings or dug them out of the ground when they were short on ammunition.

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