April 18, 2011

How Fiddler Crabs Spot Killer Birds

Fiddler crabs would all end up as dinner for seagulls were it not for their sophisticated threat detection system, vision scientists have found.

Researchers in The Vision Centre have announced an important advance in understanding how simple animals distinguish between threats and non-threats by looking for a combination of visual cues.

"These crabs can't distinguish shapes and must constantly compete with birds that are bigger and have sharper sight," says Dr Jan Hemmi from the ARC Centre of Excellence for Vision Science at the Australian National University. "They have to run to their burrows to escape passing birds as often as every two or three minutes.

"However, many birds that fly over crab colonies are just on their way to feed over the open ocean, so how do these crabs stay vigilant without succumbing to paranoia?"

The world first research into how crabs see birds is throwing new light on the visual cues of that small prey animals use to recognize the danger signals given off by potential predators.

By using dummy prey and synchronized cameras, the scientists recorded the events from the view-point of the crabs, while monitoring their responses.

"We used dummy predators in the form of styrofoam balls and the crabs were scared witless of them "“ that's how bad their eyesight is," Dr Hemmi says. "So instead of focusing on the shape of the bird, which they can't discern, they look at its height, the speed of its movements when it swoops, and how it flies.

"As soon as the predator flaps its wings, the flickering light is detected by one ommatidium (photoreceptor or vision cell) of the crab's eyes, which is akin to one pixel on a computer screen."

The size and relative speed of the bird detected by the crabs' photoreceptors tells it whether the bird is getting closer "“ or simply passing over. A flapping bird might indicate an approaching threat "“ while a soaring bird might be just passing.

"Crabs' eyes are two centimeters above the ground," he says. "So one of the rules they follow is that every moving object above their eyes' horizon could be a threat and their eyes are tuned to detect very small changes above the horizon.

"But they have also become very selective in discriminating between dangerous and non-dangerous situations, and they can do so because they attend to very subtle differences in the behavior of different birds and insects that fly across the mudflat."

Dr Hemmi says that the new insights into how simple animals see and react will help in the design of machine vision for robots.

"If the robot is tasked with seeing very small approaching objects, imitating the visual system of crabs can save a lot of computation work that the robot would otherwise have to do. Processing less of the "Ëœgeneral picture' and more of what is relevant means less computation for the machine, and a faster reaction time."

The paper "Natural visual cues eliciting predator avoidance in fiddler crabs" by Jochen Smolka, Jochen Zeil and Jan M. Hemmi was published in the journal Proceedings of The Royal Society B.

The Vision Centre is funded by the Australian Research Council as the ARC Centre of Excellence in Vision Science.


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