Hawkmoth Brains Slow Down in Order to See at Night
It’s difficult enough to see things in the dark, but what if you also had to hover in mid-air while tracking a flower moving in the wind? That’s the challenge that hummingbird-sized hawkmoths in the family Sphingidae must overcome while feeding on the nectar of its favorite flowers.
Using high-speed infrared cameras and 3-D-printed robotic flowers, scientists have now learned how this insect juggles these complex sensing and control challenges — all while adjusting to changing light conditions. The work shows that the creatures can slow their brains to improve vision under low-light conditions while continuing to perform demanding tasks. The research was published in the journal Science.
“There has been a lot of interest in understanding how animals deal with challenging sensing environments, especially when they are also doing difficult tasks like hovering in mid-air,” said Simon Sponberg, an assistant professor in the School of Physics and School of Applied Physiology at the Georgia Institute of Technology.
Scientists already knew that the moths, which feed on flower nectar during the evening and at dusk and dawn, use specialized eye structures to maximize the amount of light they can capture. But they also surmised that the insects might be slowing their nervous systems to make the best use of this limited light. But if they were slowing their brains to see better, wouldn’t that hurt their ability to hover and track the motion of flowers?
Sponberg and colleagues at the University of Washington studied this question by using high-speed infrared cameras and nectar-dispensing robotic flowers that could be moved from side-to-side at different rates. While varying both the light conditions and the frequency at which the flowers moved, the researchers studied how well free-flying moths kept their mouths — known as proboscises — in the flowers.
They also measured real flowers blowing in the wind to determine the range of motion the insects had to contend with in the wild.
“We expected to see a trade-off with the moths doing significantly worse at tracking flowers in low light conditions,” said Sponberg. “What we saw was that while the moths did slow down, that only made a difference if the flower was moving rapidly — faster than they actually move in nature.”
In the experiments, the moths tracked robotic flowers that were oscillating at rates of up to 20 hertz — twenty oscillations per second. That was considerably faster than the two-hertz maximum rate observed in real flowers. Because the moth’s wings beat at a rate of about 25 strokes per second, they had to adjust their direction of movement with nearly every wingstroke — a major sensing, computational and control accomplishment.
“This is really an extreme behavior, though the moth makes it look simple and elegant,” said Sponberg. “To maneuver like this is really quite challenging. It’s an extreme behavior from both a sensory and motor control perspective. This was an interesting example of how an organism can tune its brain to maintain its ability to gather food. The moths do suffer a trade-off by slowing their brains, but that trade-off doesn’t end up mattering because it only affects their ability to track movements that don’t exist in the natural way that flowers blow in the wind.”
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