Why are bats blind?

Bats have both excellent hearing and good eyesight. Public domain image, source: CDC.

Bats are not blind and can in fact see quite well using their eyes. While most bats do have advanced ears that give them a form of vision in the dark known as echolocation, these good ears does not require them to have bad eyes. Bats use their good hearing to find food in the dark of night, and their good eyes to find food during the light of day. The vision of bats is tuned to low-light conditions such as is present during dawn and dusk. While some bats may not have as good color vision as humans, their overall vision may be better than humans during dawn and dusk.

In their book on bats, authors Barbara Schmidt-French and Carol Butler state, "Someone with poor vision is commonly called ‘blind as a bat,' but the expression is inappropriate since bats can actually see quite well, with visual acuity varying from one species to another. Both megabat and microbats rely on vision during social interactions with one another, to watch for predators, and for navigating across landscapes. Megabats have large eyes and depend on vision to orient themselves during flight and to find food. Most microbats use echolocation to navigate and find food, and they tend to have smaller eyes, although they, too, use vision during their daily activities and to detect objects outside the effective range of echolocation, which is about thirty-three to sixty-six feet (ten to twenty meters). Some bats are also capable of visual pattern discrimination, which may assist fruit or nectar bats in finding food." Taken literally, the comment, "you are as blind as a bat," should mean that you have excellent vision in low light conditions, although it is usually meant to imply you have bad vision overall. This phrase perhaps originated from the fact that bats have rapid, erratic flight patterns that look like a blind person bumbling about.