For animals that rely on sight to navigate their surroundings, little or no light can present particular challenges.
Animals can experience a large range of light conditions during a single day, with intensities capable of varying by more than 10-billion fold.
Special adaptations to low-light can allow an animal to increase what it sees, but this is often at the detriment of their other capabilities such as flight.
Scientists from the Georgia Institute of Technology and University of Washington in the US wanted to find out what sort of “trade-off” makes this behaviour possible in the hawkmoth (Manduca sexta), an agile flyer that feeds on nectar at dawn, dusk and through the evening.
They discovered that it was able to slow its brain to see better in dim light, while still being capable of hovering in mid-air and tracking the movement of its favourite flowers.
This is really an extreme behaviour, though the moth makes it look simple and elegant
Their findings – reported in the journal Science – suggest the moth and the plants it pollinates co-evolved to provide a perfect match of movement, flight and sight that enables them to perform so well in the dark.
“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 lead author Dr Simon Sponberg, from the Georgia Institute of Technology.
In a nocturnal hawkmoth’s eyes, light is collected from different lenses and focused on a single photoreceptor, making them more sensitive to light.
The authors also suggested the moths may be allowing their brain more time to process visual information as light decreases, but this should be at the expense of the insects’ ability to respond to moving flowers.
The team made robotic 3D printed flowers, filled with nectar to see how well the moths could track and feed from the flowers under different light and motion conditions.
To manoeuvre like this is really quite challenging
They found that the moths tracked the flowers about 17% slower in dim moonlight, compared with brighter early dusk conditions.
But they also found that the moths’ tracking ability depended on the flowers’ movements. The moths were capable of following a flower moving at frequencies less than 1.7 Hertz in both light conditions. When the flower moved at a frequency higher than this the moths struggled to keep up with it.
When they measured the wind-blown movements of the hawkmoth's favoured flowers in the wild they found that 94% of their motion was below 1.7 Hertz. This means that the flowers’ natural movement presents the perfect low-light tracking conditions for the moths, as the insects don’t suffer any drop in performance as a result of slowing their visual processing to see better.
“This was an interesting example of how an organism can tune its brain to maintain its ability to gather food,” Dr Sponberg said.
“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.”
Dr Sponberg added that a surprising result of the work was seeing how well an animal with a tiny brain can combine dynamic movements with adjusting its brain processes to different light levels. A discovery, as he explains in the video above, that could help the next generation of “micro air vehicles” operate efficiently under a broad range of lighting conditions.
“This is really an extreme behaviour, though the moth makes it look simple and elegant. To manoeuvre like this is really quite challenging. It’s an extreme behaviour from both a sensory and motor control perspective,” Dr Sponberg said.