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Birds can mistake some caterpillars for snakes; can robots help?

Researchers witnessed a hummingbird defending its nest from what it interpreted to be a snake, but was actually a caterpillar of the moth Oxytenis modestia.

This is exactly what happened in Costa Rica earlier this year, when researchers witnessed a hummingbird defending its nest from what it interpreted to be a snake, but was actually a larva of the moth Oxytenis modestia. The encounter is described in a new paper published in the Ecological Society of America's journal Ecology.

These moths, sometimes called the dead-leaf moth or the Costa Rica leaf moth, resemble flat dried leaves as adults. The caterpillars can inflate the top of their heads to expose a pair of eyespots. When disturbed, they raise their head up and move from side to side, increasing the snake-like appearance. In particular they resemble a green parrot snake, known to prey on nesting birds.

The attacking hummingbird's nest with eggs was about 10cm away from the caterpillar in a small tree. When the researchers went to look for an assumed snake, they instead found the caterpillar feeding on a leaf immediately above the nest.

Caterpillars and adults of a variety of butterflies and moths have eye-like spots that deter potential predators. Observations of how these eyespots affect animal interactions in natural settings are extremely rare.

The interaction took place on a strip of secondary growth between the Pacific and primary rainforest on the Osa Peninsula, Costa Rica. The authors believe that the comings and goings of the female rufous-tailed hummingbird (Amazilia tzacatl) around its nest may have disturbed the caterpillar, causing it to expose its eyespots, which in turn prompted the hummingbird to defend its nest using what is referred to as 'mobbing behavior' by birds, darting flights and pecking at a threat, commonly snakes.

One of the researchers believes future studies of this behavior can be conducted using a tiny, caterpillar robot to experiment with eyespots. With such a robot, researchers could vary the eye-like nature and contrast of spots on the head of the robot to test various responses of nest-defending birds. A study like this could definitively test the effects of eye-like versus other mimicry patterning for provoking or repelling defensive attacks.


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