The common expression “blind as a bat” suggests these flying mammals possess little to no vision, relying entirely on other senses to navigate the dark. This perception is a widespread misconception. Bats are the only mammals capable of sustained flight, evolving diverse sensory adaptations to thrive in their nocturnal environment. While their primary navigation tool in total darkness is indeed a sophisticated form of sonar, their eyes are fully functional and play an important role in their survival.
Bat Eyesight: Debunking the Myth
Bats are not blind; their visual systems are well-suited to their lifestyle in low-light conditions. Many species possess eyes designed to capture the maximum amount of light available at twilight or during the night. Their retinas are often dominated by rod cells, which are highly sensitive to low light levels, enabling them to see clearly when human vision fails.
Many bats can also detect ultraviolet (UV) light, a capability not common among mammals. This UV sensitivity aids in navigating at dawn and dusk or locating specific food sources. For instance, some flowers and insects reflect UV light, creating a visual signal that bats can use to find pollen, nectar, or prey.
Echolocation: Navigating by Sound
The system that truly allows bats to dominate the night sky is echolocation, a biological sonar. The bat emits high-frequency sound pulses, often through its mouth or nose, which are typically too high-pitched for humans to hear. The frequency of these calls can range from 9 to 210 kilohertz, depending on the species and the environment.
These sound waves travel outward until they strike an object, bouncing back as echoes. The bat’s highly specialized ears and brain interpret the properties of this returning echo to construct a detailed, three-dimensional map of its surroundings. The returning echo also carries information about the object’s size, shape, and surface texture.
Bats can detect the Doppler effect, which is the shift in frequency caused by relative motion. By analyzing this frequency shift, a bat determines if an insect is moving toward or away from it and at what speed. Some species can even adjust their outgoing call frequency to compensate for the Doppler shift, ensuring the returning echo falls within the narrow, most sensitive range of their hearing.
Two Categories of Bats: Different Sensory Priorities
Bats are traditionally divided into two main suborders: Microchiroptera (microbats) and Megachiroptera (megabats). Microbats, which comprise the majority of all bat species, are the primary users of laryngeal echolocation for navigation and catching prey. Most microbats are insectivores, and their smaller eyes reflect their greater dependence on sound for navigating clutter and targeting small, moving insects.
In contrast, megabats, often called fruit bats or flying foxes, generally do not use laryngeal echolocation. Instead, they rely heavily on their large, well-developed eyes, which are specialized for nocturnal vision, and an acute sense of smell. Megabats primarily feed on fruit, nectar, and pollen, using sight and scent to locate food sources. The variation in eye size and sensory focus across bat species illustrates that whether a bat prioritizes sight or sound is entirely dependent on its ecological niche and what type of food it hunts.