Bat Week and the Bats That Were Never Blind

"Blind as a bat" is an old saying, and it is wrong. Bats can see. The idea held on because the bats people noticed most hunt by sound in the dark, and for a long time the inside of a bat's eye seemed to back it up.

The Eyes Nobody Checked

Small insect-eating and nectar-feeding bats were long assumed to have retinas made entirely of rods, the cells that work in dim light and see no color. Early dissections turned up no cones, and that finding sat in the literature for decades.

In 2009 a team led from the Max Planck Institute for Brain Research in Frankfurt tested it on two small Central and South American species, Pallas's long-tongued bat and Seba's short-tailed bat. Both had cones. They were rare, about 3 percent of the light-sensing cells, but they were real and came in two kinds. One kind was tuned to ultraviolet. When the researchers recorded the eye's electrical response to light, it peaked in the ultraviolet, and the bats' corneas and lenses let that light through to the retina.

The authors suggested what the cones might be for: getting around at twilight, spotting predators, and, for the nectar feeders, finding flowers that reflect ultraviolet. A bat with ultraviolet vision is not unusual by animal standards. A mantis shrimp sees ultraviolet and polarized light, both invisible to us. It only looks surprising set against a proverb.

Sonar in Broad Daylight

The Egyptian fruit bat is a bigger animal with prominent eyes and excellent vision, and it echolocates too. That raised a fair question: if it can see, why bother?

Yossi Yovel's lab at Tel Aviv University followed Egyptian fruit bats through daylight in the city with cameras and microphones, in a study published in Current Biology in 2022. The bats kept calling in good light. They called more as they came in to land on trees, and faster still just before dipping down to drink from a pool. When a wall was put up in front of the pool, they went straight back to echolocating.

Yovel's reading is that sound gives a sharper measure of distance than sight does, and that the bat's brain combines the two. The bat is not choosing between its eyes and its ears. It is using both, the way a narwhal clicks to find its way and its food in water too dark for eyes to do the job alone.

A Fungus That Strikes in Winter

Bat Week runs October 24th to 31st every year. Government agencies and conservation groups across North America mark it together, among them the U.S. Fish and Wildlife Service, the U.S. Forest Service, the National Park Service, the Bureau of Land Management and Bat Conservation International. In 2021 the National Park Service reported partners across Latin America joining, from Mexico and the Caribbean to Brazil.

The threat that hangs over the week in North America is white-nose syndrome. It is caused by a fungus, Pseudogymnoascus destructans, that grows on the muzzle, ears and wings of bats while they hibernate. Since it was discovered in 2006, it has killed millions of bats. As of December 2025, the U.S. Geological Survey reported it confirmed in 41 states and five Canadian provinces. Twelve species have been hit, and populations of at least three have fallen by more than 90 percent. Tricolored bat numbers have fallen by about three quarters.

A hibernating bat lives on a fixed budget of fat, and the fungus drains it. In a 2014 study of little brown bats infected in the lab, a team including U.S. Geological Survey scientists found that sick bats burned fat at about twice the rate of healthy ones. Carbon dioxide built up in their blood, and their potassium climbed. The team's proposed explanation starts with the wings. A bat's wing membrane is thin, living skin, and the fungus erodes it. Their model holds that damaged wings lose water and release carbon dioxide poorly, and that as the disease advances the bat wakes more often to cope. Every waking costs fat that has to last until spring. A bat that runs out before the insects come back starves.

Winter survival is a gamble on stored energy for plenty of animals. A wood frog lets two thirds of its body water freeze and stops its heart altogether. A hibernating bat goes the other way, keeping its body ticking over as slowly as it can. White-nose syndrome makes that slow tick unaffordable. The U.S. Geological Survey calls a vaccine an urgent priority. Since 2019 its field trials have given an oral vaccine to more than 7,000 wild bats in seven states, and a sprayed-on version that bats take in as they groom is still being tested. Nothing is in routine use yet.

So the bat that people call blind can see ultraviolet, uses sonar to check what its eyes already tell it, and is dying in large numbers from damage to the very wings that make all of that possible.

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