People who easily get lost in the extraordinary darkness of a tropical forest can learn a lot from a bee that finds its way home in conditions ten times darker than starlight. Researchers at the Smithsonian Tropical Research Institute (STRI) research station on Barro Colorado Island in Panama and Lund University in Sweden show that sweat bees (Megalopta genalis) find their way home based on patterns in the canopy over their backs. This first report on back navigation in a flying insect, published in Current Biology, may be of particular interest to manufacturers of drones and other night-flying vehicles.
“One of the pioneers of the study of homing behavior in bees was Charles H. Turner, an African American scientist from the University of Chicago,” said William Wcislo, a research fellow at STRI, “he wrote in the Biological Bulletin in 1908,” Grave bees use this Memory to find the way home and carefully examine the surroundings of the nest to form memory images of the topographical surroundings of the burrow. ‘ But what we haven’t understood until now is that the signals the bees are memorizing are both over their heads and in front of their faces. “
Nobody knows why the sweat bees on Barro Colorado Island roam from flower to flower like crepuscular cats, collecting pollen only about 70 minutes before sunrise and after sunset. Mainly because it is 100 times darker on the tropical forest floor, where the bees live in dead twigs and tendrils, than it is above the treetops.
Eric Warrant and his team from the Vision Group at Lund University in Sweden have been working with Wcislo since the late 1990s to learn more about Megalopta’s optical superpowers. Their work showed that Megaloptas special compound eyes are 30 times more sensitive to light and concentrate light photons in a way that day-flying honeybee eyes cannot. By identifying pollen from bees’ bodies and their nests, they discovered that bees collect pollen from more than 60 species of plants: tall tree canopies such as Hura crepitans and Pseudobombax septenatum to coffee family undergrowths such as Faramea occidentalis.
“To a human observer, the most obvious visual cues in the forest at night are gaps in the canopy when we look straight up because the sky is much brighter than the forest below,” said Warrant. “We see a fairly complex pattern of criss branches traversing, but the bees’ view is much less complex. You see broad patches of light that vary in shape and position. We knew that ants could use treetop patterns as they walked through the woods, and we wondered if bees would do the same. “
Honey bees can’t, but to find out if megaloptic bees have another superpower, Sandra Chaib, a student at Lund University, moved to Barro Colorado Island to do the experiments.
To test this idea, Sandra first set up a special bee motel. Imagine the entrances to five rooms: Each entrance is an identical, circular gray disc. The door is a tiny round opening in the middle of the pane – the end of a stick nest.
In the first experiment, she asked if the bees could find their way home using a single landmark. She placed a black bar over the entrance to one of the nests. After the bees went looking for pollen, she rearranged the order of the nests, but the real nest was still indicated by the black bar above the entrance. When the bees returned, they chose the right nest.
Next, she made it a little more complicated by building some sort of canopy over each nest entrance – like the roof of a carport. She placed an awning with a pattern of black and white bars over the four empty nests, and another pattern over the occupied nest. When the bees went out, she changed the position of the nest again, and again when they came back they found the right nest. But she still hadn’t ruled out that the bees were using some other clue, like smell, to find their own nest. So she left the nest in the same place but placed the bee’s learned pattern over an empty nest … and the bees chose the empty nest to show that they were using the pattern as a guide, not as a smell or some other signal of the nest.
Her last experiment was the most elegant: this time two nests shared a common awning, a pattern of dark circles on a light background that represented the light and dark patterns in real tree tops. The entrance to an empty nest was under one end of the awning and the entrance to the occupied nest was under the opposite end. This time Sandra flipped the awning when the bees went away, reversing the pattern. The bees chose the wrong nest more than 80 percent of the time, demonstrating once and for all that they were navigating the above pattern.
Understanding how Megalopta navigates can be of interest for military applications. Most drones these days use GPS to navigate, but GPS and satellites can be disrupted or destroyed.
“Animals use a variety of different cues to navigate. Vision is obviously a very important one, but many animals have a magnetic sense and can measure the properties of the Earth’s magnetic field to find out where they are on the Earth’s surface and which way they are going to find oneself.” they want to travel. We have already learned about insects and other animals that can use the stars to navigate, pigeons can use smells to navigate … there are many different clues. The canopy is an indication of a drone can use to navigate, but a really robust navigation system will be one that uses a number of different indications like animals have to use all of these systems to navigate at the same time, “said Eric Warrant .
The STRI facilities are still closed due to COVID-19 restrictions, but researchers are eager to get out. “So far we only know how the bees recognize which room in the bee motel belongs to them when they get home,” said Wcislo. “We still have no idea how they move through the forest after they leave their nests.”
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A version of this study was put online in March 2021 before final publication in the print edition on August 23, 2021.
The Smithsonian Tropical Research Institute, based in Panama City, Panama, is a unit of the Smithsonian Institution. The institute promotes understanding of tropical biodiversity and its importance for human well-being, trains students for research in the tropics, and promotes conservation by raising public awareness of the beauty and importance of tropical ecosystems.
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