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Caterpillars disguise themselves as bird POO to avoid being eaten - and it triples their chance of survival

Caterpillars of Japanese moths contort their black and white bodies at rest
These postures make them look like bird droppings on a branch or leaf
Researchers found that birds actively avoid caterpillars that do this
Scientists found the disguise can increase triple their chances of survival

Published: 11:21 EST, 26 May 2015 | Updated: 11:44 EST, 26 May 2015

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Large numbers of insects employ camouflage in an attempt to avoid becoming bird food, but not many try to look like something that has been digested already.

Scientists have discovered a group of caterpillars that have a rather unique, and unappetising, way of defending themselves against predators - they make themselves look like bird droppings.

The caterpillars of certain moths in Japan have distinctive black and white colourings, which on first glance appears to make them stand out against the green foliage they live on.

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Although this might look like a bird dropping dangling delicately from a leaf, it is actually the caterpillar of a moth called Apochima juglansiaria. Its black and white colouring makes it stand out but researchers found that by bending its body while resting on the leaf, birds tend to be tricked into thinking it is a dropping

However, the moth larvae also contort their bodies while resting on branches or leaves, making them look just like a splatter of bird poo.

Now researchers have found the deception is remarkably effective as birds actively avoiding these caterpillars when they strike their bendy poses.

THE BIRD THAT PRETENDS TO BE A POISONOUS CATERPILLAR

Biologists have found that the young of dull grey bird called the Cinereous Mourner have evolved an unusual tactic to avoid being eaten by predators.

Rather than relying upon their parents, which measure around seven inches (20cm) in length, to protect them, the chicks mimic poisonous caterpillars.

On hatching, the chicks are covered with a distinctive mass of bright orange and strangely shaped, spiky feathers.

This gives the chicks the appearance of a large and hairy caterpillar that use bright colours to warn predators in the Amazon jungle that they are toxic.

To add to the effect, however, researchers found the chicks begin to move in strange ways after a few days, writhing their heads much like a caterpillar.

Cinereous Mourner chicks, above, move their heads around when disturbed like a caterpillar

And with good reason - not many self-respecting birds would want to eat another animal's droppings.

Dr Toshitaka Suzuki, an evolutionary biologist at The Graduate University for Advanced Studies in Kanagawa, Japan, said: 'Our results showed that, for the caterpillars with bird dropping colouration, birds attacked the bent ones almost three times less often than straight ones.

'Birds have four types of cone cells (in their eyes), whereas humans have three.

'Therefore, the colouration that is conspicuous to human eyes would also be easily detectable by birds.

'Birds misrecognise these caterpillars as real bird droppings, particularly when they bend their bodies.

'This is the first demonstration that the adoption of a specific posture can help caterpillars masquerade as inedible objects.'

Many animals use mimicry in an attempt to confuse and deter predators.

Some caterpillars are known to mimic twigs by holding their bodies rigid on the branches they rest on.

Others use distinctive colours to look like a more dangerous animal such as a snake, while moths and butterflies uses eyespots on their wings to fool predators.

Round-tailed horned lizards are also known to flatten their bodies when threatened to make themselves look more like stones.

But there are not many species that attempt to look like a pile of dung.

Yet the caterpillars of some species of Japanese moths, including Macrauzata maxima, Apochima juglansiaria and Acronicta alni, appear to do this – at least to human eyes.

To test whether the poo-like appearance of the caterpillars was actually putting off birds, the researchers, whose work is published in the journal Animal Behaviour, set up an experiment using model caterpillars made from pastry.

They pinned the artificial caterpillars - either coloured in the same way as those that mimic bird droppings or like green caterpillars - in a variety of positions on tree branches.

This rather unappetising mess is actually the caterpillar of the Japanese moth Macrauzata maxima

The researchers created models of caterpillars out of pastry to avoid harming real creatures during their research. The images above show the different poses they used. They found that birds attacked the black and white coloured models least when they were bent while body position made little difference to green models

They found that bent dropping coloured model caterpillars were snatched up by hungry birds just 10 per cent of the time while straight ones were attacked 27 per cent of the time.

They found that the posture of the green caterpillars did not affect their survival rate.

Dr Suzuki said: 'The adoption of bend posture specifically improves the survival rate of bird dropping caterpillars, but not other types of caterpillars.'

The researchers also measured the colour difference between their models and the branches to see if birds could spot them.

Don't touch me! Caterpillar shows off defence mechanism

The results showed that the birds could clearly see the bird dropping colouration against the background but when combined with a body posture that mimicked bird poo, it fooled them.

Dr Suzuki added: 'These caterpillars use bent posture when they rest on leaves or branches.

'Other caterpillars seem to use different postures when at rest. For example, twig-mimicking caterpillars hold their bodies rigid on the branches, which may enhance their resemblance to real twigs.

'Other cryptic, green caterpillars do not bend when at rest. Thus, resting posture seems to be evolved with body coloration.'

See the chick that pretends to be a poisonous caterpillar