Science: Hovering Hawkmoths Tune In to Flowers' Sway

The moths' visual processes slow down in low light so that the insects can keep track of wind-tossed flowers that provide nectar.

Researchers filmed the hawkmoth under different light conditions as it fed from a 3D, robotic flower. | Science /AAAS

The vision of the hawkmoth Manduca sexta — an agile insect flyer that feeds on nectar — is finely tuned to the swaying of flowers in the breeze, a new study shows. These findings help to explain how the moths' relatively small brains handle complex behaviors under the ever-changing light conditions on Earth.

The moths, which are sometimes mistaken for hummingbirds due to their nimble flight maneuvers and ability to hover, are known for foraging among wind-tossed flowers during the twilight hours of dawn and dusk. Researchers have suggested that hawkmoths compensate for the dim lighting by slowing down their visual processing. This tradeoff would also reduce the moths' reaction time, but there has been little evidence of such a tradeoff until now.

Using an artificial flower attached to a robotic arm, Simon Sponberg from the University of Washington in Seattle and colleagues tested the limits of hawkmoth vision in the laboratory. Their results, published in the 12 June issue of Science, suggest that the moth's sight and flight have evolved to perfectly match the movements of flowers — the insect's sole source of food.

"These findings give us a better idea of how animals' brains couple with their bodies to move through the environment…how they manage to maneuver with incredible grace and agility almost everywhere on the planet," said Sponberg. "We've been referring to the field as neuromechanics."

The researchers programmed their robotic arm to swing side-to-side at various frequencies so that it closely mimicked the motion of a flower blowing in the wind. They then took high-resolution videos of hawkmoths tracking the moving target and found that the moths' reflexes slowed by about 17% under dark, moonlit conditions compared to brighter, early-dusk light.

"From morning to midnight, the amount of light changes about 10 billion-fold. That's a huge range that animals have to deal with if they're going to be active."

Simon Sponberg

This reduction of visual processing is akin to opening a camera's aperture and slowing down its shutter speed to photograph something under dark conditions, explained Sponberg. "When you let in more light and expose an image for a longer period of time, the picture gets brighter. But it also gets blurrier and fast motions don't show up as clearly."

Sponberg and his team also discovered that hawkmoths had trouble tracking anything that moved back-and-forth faster than 1.7 times per second (a frequency of 1.7 Hertz or higher), regardless of light conditions. They went to the greenhouse at the University of Washington to explore this threshold of motion further, recording flowers — including five that hawkmoths pollinate — as they swayed in the breeze.

Surprisingly, the researchers observed that 94% of the flowers' movements were limited to 1.7 Hertz or lower. Taken together, the findings imply that hawkmoths may have avoided the usual pitfalls of slower visual processing by adapting their sight to the exact light and movement conditions of their natural environment.

"Light varies more than any other quantity on Earth," said Sponberg. "From morning to midnight, the amount of light changes about 10 billion-fold. That's a huge range that animals have to deal with if they're going to be active."

"Hawkmoths appear to have slowed the visual-processing parts of their brains down, but only to the point of 1.7 Hertz," he continued. "We don't yet know how this system evolved, and the drivers of these systems can be really tricky to identify. But what we are seeing is a really nice example of a brain and body, finely tuned to the environment in which they're working."