A hunting falcon flying over a field might not see a camouflaged rabbit hidden in the grass. But if that rabbit loses its nerve and runs, then the falcon will see its meal. Movement gives away even the best disguises. How then could a camouflaged animal move without giving away its location? A recent study in Proceedings of the Royal Society B suggests short and fast bursts are the best strategy.
Wild predators and prey generally see each other in their peripheral vision first. Whether the falcon, in a split second, registers a peripheral blur and turns its gaze to the rabbit can be the difference between life and death. Natural selection should strongly favor any rabbit behavior that leads the falcon, via its peripheral vision, to either miscalculate the rabbit’s trajectory or overlook the prey once it stops moving, explains visual ecologist Jolyon Troscianko at the University of Exeter in the United Kingdom. Yet, he says, “We’ve done surprisingly little research on this.”
A growing literature is asking how motion affects the ability to blend in. Tests of camouflaged targets on a computer screen, for example, show that moving targets are easier to locate than stationary ones. But these studies put the target in front of viewers, which is their highest-resolution field of view, notes Ioan Smart, coauthor of the recent study. Few experiments have tested the more common scenario in which a viewer sees motion in their weaker peripheral vision.
So Smart, now a PhD student in sensory ecology at Abertay University in Dundee, Scotland, tested it in experiments with 18 volunteers when he was a master’s student at the UK’s University of Bristol. Each volunteer looked at a panel of computer screens and stared straight ahead until a target appeared onscreen and began to move in their peripheral vision, at which point the volunteer could look directly at it. Over 162 trials, the target moved and then winked off the screen. The viewer then moved a mouse cursor to the spot where they thought the target had stopped moving. Between trials, the target’s speed and duration of movement on screen varied. Its appearance also ranged from a black square standing out from a patterned background, to a grey square matching the average brightness of the patterned background, to a patterned square moving on a similarly patterned background. At the start of some trials, the target briefly flashed to mimic the startle displays of moths and other insects that show bright underwings in flight, which are thought to overwhelm and confuse predators.
The most difficult targets to localize turned out to be those moving fastest for the shortest periods of time, and with the same brightness as the background. Short, fast bursts were hard for humans to localize. That likely means they’re challenging for other species as well, since many animals also have a narrow band of high-resolution vision surrounded by weaker, lower resolution peripheral perception. The findings could explain why viviparous Eurasian lizards naturally move in short, hurried bursts followed by pauses, and why some cuttlefish turn a plain, uniform color before moving.
But whether these camouflage tactics are generalizable across species is still debatable, Troscianko says. He compliments the paper’s elegant design, but cautions that most animals can’t accelerate and decelerate in the space of milliseconds. A dart-and-stop strategy isn’t mechanically possible for large species, and he says, “More animals would do this if it really worked.” Future studies, he adds, might “test whether small animals which are hunted by visually guided predators tend to move in this fashion more often.”
Smart sees the study as a contribution to the “growing literature that investigates the interaction of motion and camouflage.” He’d like to see follow-up studies that add rustling foliage or other kinds of environmental movement to peripheral vision tests to see if they’d make a target harder to identify.