If you want to sneak up on an animal, you might want to wheel up on a bicycle rather than try the tip-toe technique.

That's the tact psychologist Niko Troje's young daughter took when approaching wild rabbits in the neighborhood. But neither she nor Troje could say why the critters found bicycles less startling.

"I didn't have an answer for her then," said Troje, of Queen's University in Kingston, Ontario. "Now, I think I have one."

In a new study, Troje and his colleagues looked at how humans detect and interpret movement to hint at how animals do it.

They videotaped walking cats, pigeons and humans, focusing on certain points of motion.

When shown the digitalized video, people could easily identify the animal's size and the direction it was walking. This ability was conserved even when the digital points were scrambled.

Things got tricky when the image was flipped upside down, and people couldn't tell left from right based on the movements.

It seems humans, and likely other animals, are trained to interpret foot movement in a certain way.

"We believe this visual filter is used to signal the presence of animals that are propelled by the motion of their feet and the force of gravity," Troje said.

Scientists think this locomotion detector is part of an evolutionarily old system that helps animals quickly detect whether a potential predator or prey is nearby, even if it's just in their peripheral vision.

Other research suggests that even newly hatched chicks use this "life detector" system.

"It seems like their brains are 'hard wired' for this type of recognition," Troje said.

Some hunting animals, such as cats, creep along to disguise the foot component of their movement, allowing them to sneak up on prey more easily.

The study might also provide an explanation for phobias towards snakes, insects, spiders, and birds — animals that don't fit the "normal" movement patterns associated with the proposed life detector, Troje said.

The research is reported in the April 18 online edition of the journal Current Biology.

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