78 Scientific American, May 2019

PRECEDING PAGES: HÅK AN LILJENBERG

Alamy

T

he sun’s final rays filter through the leaves as night falls in the

dense, muggy rain forest. The descending gloam over Panama’s Barro

Colorado Island obscures the towering, spiky kapok trees, the palms

and the shrubs until human eyes can’t see much more than the small

patches of starlit sky through the canopy above. Crickets commence

their chorus as the howler monkeys hush for the night.

In the twilight, a nocturnal sweat bee, with bulging eyes, a

metallic green head and a pale brown abdomen, emerges from

her nest in a foot-long, hollowed-out stick. She’s hungry for nec-

tar and pollen. But before she flits off, she turns to look back at

the stick, which has a black-and-white-striped card above it,

placed there by scientists. Nearby stick nests also have cards,

but these are simply a flat gray.

After the bee flies off, zoologist Eric Warrant and his col-

leagues at Lund University in Sweden switch things around,

moving the striped card to another nest. When the bee returns,

she zooms right into the nest with the stripes, assuming it is

hers and demonstrating that sweat bees spot and use such visu-

al signals. “Even in the very dimmest intensities, they have no

problem seeing this,” Warrant says. (He notes that if the humans

tracking the insects did not wear night-vision goggles, they

would “literally crash into trees” because it is so dark.)

The remarkable night vision of these bees ( Megalopta gena-

lis ) stems only in part from eye adaptations such as larger lens-

es. Those do improve sweat bees’ light sensitivity. Still, the noc-

turnal insects find their nests at light levels where even those

peepers should not be sufficient. Warrant has concluded that in

addition to the bees’ eyes, the way in which their brain process-

es the little light available allows them to navigate after sunset.

For decades scientists assumed that most creatures must see

the same dim, colorless nightscape that people do. They thought

that nocturnal animals relied on other senses, such as smell and

hearing. Today a new wave of research is overturning that

assumption. “We always thought we knew how well animals

saw in the dark, but very few people had actually looked,” War-

rant explains. Once researchers started peering into this dark

world, they discovered that a wide variety of species see a star-

tlingly clear nightscape.

Moths, frogs and geckos, for example, can distinguish colors

at night when researchers themselves see nothing but shades of

gray. Being more sensitive to color variations gives them an

advantage because hue is a much more reliable way to distin-

guish objects, in bright or dim light, than noncolor indicators

such as intensity. It can help them find food, nests or mates in

the dark. “It’s just amazing that so many animals can be active

in dim light and still perform behaviors when we can’t,” says

Almut Kelber, a sensory biologist at Lund.

The secrets to night navigation reside between eye and brain.

Nerve cells in the optical systems of these animals add up scarce

bits of light to create a brighter picture and carefully prune

away other, noisy signals that would muddle the image. The

cells perform these summations by grabbing input from neigh-

boring spots in their visual field. They also sum up input from

single spots over a long time period, essentially slowing down

visual perception to make things much brighter.

IN LIVING COLOR

the eyes of people, along with those of most other vertebrates

and invertebrates, have cells that work as photoreceptors,

detecting light coming from outside. The cells are called cones

and rods. During the day, we use mainly cones, which send sig-

nals back toward the brain when hit by incoming photons of

red, green or blue light. They give humans excellent color vision,

but they do not respond much in the dark. In dim light, we rely

on rods, which are more sensitive because they work together

in groups, pooling the information from scant incoming light.

They tend to distinguish only shades of gray, however.

Warrant, Kelber and another Lund colleague, Anna Balkenius,

were the first to show, in a 2002 study, that an animal had color

vision at night. The researchers put insects called hawkmoths in

a cage in the laboratory and trained them to associate either a

blue or yellow artificial flower with a sugar-water reward. The

IN BRIEF

Several animals possess unexpectedly excellent

vision in dim light. Scientists once thought they

needed other senses to find food or mates.

These abilities include seeing colors. Creatures

such as moths, frogs and geckos detect color in

the dark to navigate the world.

Animals accomplish this feat by using neurons in

their optical systems to maximize sparse incoming

signals from their eyes.

Science writer Amber Dance

lives in the Los Angeles area.

© 2019 Scientific American