Some bats have better vision than you!

06.14.24

By Alyson Brokaw

Ghost Bat (Macroderma gigas) hanging from roof of cave, northern Australia
Australian ghost bats can see details around two cycles per degree. Jean-Paul Ferrero/ Auscape/ Minden Pictures.

First things first – bats are NOT blind! Despite the common phrase, all bats have functional eyes and can see. Their vision, like their hearing, varies from species to species. Visual cues like the movement of fluttering wings can help orient hunting bats, while light cues help bats know when to leave the roost for the night. Bats may even have some interesting sight-related superpowers that we humans can only dream of!

Seeing Details

Every year or so, I sit in my ophthalmologist’s office, reading off a nonsense sequence of letters from a board across the room. This chart of rows of increasingly smaller and smaller letters is called the Snellen chart and is used by my doctor to measure how well I can discern details, also referred to as visual acuity. Normal human visual acuity is benchmarked as 20/20 – so what’s a bat’s visual acuity? 

Since scientists can’t ask bats to read letters off a chart, they instead measure visual acuity in either minimal separable angle or cycles per degree. The more cycles per degree, the better a bat (or any animal) is at resolving detail. Human visual acuity is between 45 and 60 cycles per degree, depending on lighting conditions. The rufuous horseshoe bat (Rhinolophus rouxi), an insect-eating bat with small eyes, has an estimated acuity of 0.35 cycles per degree,. Meanwhile, larger-eyed bats like the carnivorous Australian ghost bat (Macroderma gigas) and the non-echolocating fruit-eating Indian flying fox (Pteropus giganteus) can see details around two cycles per degree. 

Rods and Cones

Inside our eyes, in the retina, are special light-sensitive nerve cells that respond to light coming into the eye and send that information to our brains. These special nerve cells are also called photoreceptors, and vertebrates (including mammals like bats and humans) have two types: rods and cones. Rods respond specifically to the amount of light in the environment, letting us see shapes and objects in dim and dark conditions. Because they are nocturnal animals, it is not surprising that bat’s eyes have lots of rod cells. Compared to humans, who have an estimated peak density of about 150,000 rod cells per mm2 , bats pack between 300,000 and 800,000 rods per mm2 in their tiny retinas. 

Cones need brighter light than rods to activate and are useful in daylight conditions. Visual acuity is dependent on cone cells, which is why it becomes harder to see details as the sun goes down (and could be why bats, active mostly at night, don’t have the acuity of daylight animals like humans). 

Living in Color

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Greater horseshoe bats have lost their ability to discriminate between blues and violets.
Yushi & Keiko Osawa.

Cone cells are also responsible for color vision. Inside cone cells are special proteins called opsins, which respond to different wavelengths of light (and therefore different colors). Mammals have two main types of opsins: long-wavelength-sensitive (LWS) and short-wave-sensitive-1 (SWS1). LWS cones detect long wavelength colors, like reds and yellows, while SWS1 cones detect blues and purples. Thanks to these proteins, most mammals can distinguish between colors like blue and red, but not middle colors like greens (certain primates like humans are exceptions thanks to gene mutations that resulted in new LWS proteins). 

While bat eyes are dominated by rods, up to two percent of bat photoreceptors are cones. By looking at the genetic sequences of the different opsins in bats, scientists can estimate what colors different species can see. So far, all bats studied have functioning LWS genes, with highest sensitivity in the orange/red wavelengths of the color spectrum. These longer wavelengths of light are better in low-light conditions, making them useful after sunset. In contrast, not all bats can see the color blue. Some bats, like the greater horseshoe bat (Rhinolophus ferrumequinum) and the white-winged vampire bat (Diaemus youngi) have lost function in their SWS1 genes and thus their ability to discriminate between blues and violets. 

Bat Visual Superpowers

Pallas's long-tongued bat
The nectar-feeding Pallas’s long-tongued bat (Glossophaga soricina) can discriminate between colors down to about 310 nm.
Jennifer Barros.

Bats may not be able to see green like humans, but they may be able to detect wavelengths of light invisible to us. Human S-cones (the ones that see blue) have peak sensitivity around 430 nanometers (nm). Light wavelengths below 400 nm, or ultraviolet light, is beyond human perception, but not bats! At least seven species of insect-eating bats have demonstrated behavioral responses to ultraviolet light (~365 nm), while the nectar-feeding Pallas’s long-tongued bat (Glossophaga soricina) can discriminate between colors down to about 310 nm

Some bats even have heat vision – though this superpower doesn’t actually have anything to do with the eyes but rather special thermal receptors on the nose. Common vampire bats (Desmodus rotundus) can sense heat. They do this using a variation of a special nerve receptor called TRPV1, which is responsible for the burning sensation we feel when we touch something hot or eat something spicy. Vampire bats have two versions of this protein, one of which is tuned to a lower heat threshold, letting them sense heat from blood vessels under the skin of their prey from up to 20 centimeters away. 

So next time you hear the phrase “blind as a bat,” just remember – some bats can probably see better than you!

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