The common misconception that bats are blind is inaccurate. All bats possess functional eyes and can indeed see, even during daylight hours. While their visual capabilities differ significantly from humans and vary among species, bats integrate sight with other senses to navigate their environment effectively.
Dispelling the Myth: Bat Eyesight
Their visual acuity, however, varies depending on the bat’s classification. Megabats, or fruit bats, generally have larger, well-developed eyes and rely heavily on their vision for navigation and finding food, particularly in dim light and even during the day. Some fruit bats possess color vision, which aids in differentiating ripe fruit from leaves. Conversely, microbats, primarily insect-eaters, typically have smaller eyes, but their vision is still effective, especially in low-light conditions.
Their retinas are dominated by rod photoreceptors, which are highly sensitive to light and enable excellent vision in dim environments. While rods are crucial for night vision, bats also possess cone photoreceptors, which allow for some color vision and enable them to see in brighter light. Studies have shown that bats have a high density of rod cells, ranging from 300,000 to 800,000 rods per square millimeter in their retinas, far more than humans. Even microbats, which primarily use echolocation, use their vision for longer-distance navigation and to detect cues like sunrise and sunset.
The Power of Sound: Echolocation
Echolocation is a sophisticated biological sonar system that allows bats to perceive their surroundings by emitting high-frequency sounds and interpreting the returning echoes. This ability is particularly pronounced in microbats, serving as their primary sensory tool for navigating and hunting in complete darkness. Bats produce these ultrasonic sounds using their larynx, or sometimes through their nose, and these calls range in frequency from 14,000 to over 100,000 Hz, well beyond human hearing.
Once emitted, these sound waves travel through the air and reflect off objects in the environment. The bat’s highly sensitive ears then capture these echoes, processing information such as the time it takes for the echo to return, its intensity, and changes in its frequency. By analyzing these details, bats construct a detailed “sound map” of their surroundings, determining the size, shape, texture, distance, and even the movement of objects, including tiny insects. This remarkable precision allows them to detect objects as fine as a human hair. While echolocation is exceptionally effective for close-range detection and tracking in cluttered environments, vision complements it by providing information for larger distances and overall spatial awareness.
When Bats Take Flight During the Day
Despite their ability to see during the day, most bat species are primarily nocturnal. This nocturnal behavior offers several advantages, including reduced competition for food, as many of their insect prey are active at night. Additionally, being active at night helps bats avoid diurnal predators like birds of prey, which are typically asleep or less effective hunters in the dark. The large surface area of their wings also means that daytime activity could lead to overheating, as they would absorb more heat than they could radiate.
However, bats are occasionally observed flying during daylight hours for various reasons. Fruit bats, for instance, are sometimes active during the day, foraging for food, and some species like the Samoan fruit bat are regularly active both day and night.
Disturbances to their roosts, such as human activity or extreme temperatures, can force bats to emerge during the day. Young bats may also fly during the day as they learn to navigate and forage. Furthermore, hunger due to food scarcity, particularly in hot weather, or illness and injury can compel bats to fly outside their typical nocturnal schedule.