Do Bats Drink Blood?: Fascinating Answers to Questions about Bats

Four: Bat Behavior

FOUR Bat Behavior Question 1: How does echolocation work? Answer: Echolocation is a sonar-like method used by many bats and a few other animals to navigate or search for food. Many bats use echolocation to find and catch flying insects, to avoid obstacles in their path, and to locate their roost in a cave or other dark place. Some bats also use echolocation to communicate with one another. When an animal echolocates, it makes a sound that moves out into the environment, strikes an object, and is reflected back as an echo. The reception and interpretation of different aspects of the echo provide the animal with information about the size, shape, texture, and movement of the object. The echolocation calls of most bats are not audible to us because they are ultrasonic, which means they are above the frequency humans can hear (above twenty kilohertz). The intensity or loudness of bats’ calls varies, ranging from 50 to 120 decibels, the equivalent of a loud whisper to the sound at a rock concert. Bats that emit low-intensity calls, like the pallid bat (Antrozous pallidus), are referred to as “whispering bats.” To avoid being deafened when they emit loud echolocation sounds, bats momentarily disengage the sound-detecting structures in the middle ear. Although components of the sounds used by some bats for social communication are audible to humans, other parts of these calls are ultrasonic. Investigating bats in their native Japan, Jie Ma and colleagues found that mother greater horseshoe bats (Rhinolophus ferrumequinum) and their pups use echo- 54 DO BATS DRINK BLOOD? location calls to locate each other as the mother moves into the cave where her pup is roosting. As the mother comes closer to her pup, their calls become synchronized. The term echolocation was coined by Donald Griffin at Harvard University around 1938 when he studied bats as they flew through pairs of vertical wires. He noted their accuracy in avoiding the wires, and using new electronic technology, he recorded the ultrasonic sounds made by the bats as they maneuvered between the wires. Then he put earplugs in the bats’ ears and let them fly again, and he found that the deafened bats were no longer able to avoid the wires, demonstrating that sound reception was crucial for navigation in these bats. An echolocating bat produces a series of short, highfrequency pulses of sound by contracting muscles in its voice box (larynx), and these sounds are sent out through its mouth or nostrils. Bats that emit echolocation calls through the nose have strange-looking folds and fleshy projections on the nose and face that may help to amplify or direct the sound. Each pulse of sound bounces off objects in the environment, returning to the bat’s ears as an echo that contains information for the bat. The sound waves of the echo are transmitted from the ear drum to the middle ear and then to the inner ear, where the movement of fluids in the cochlea stimulates sound-sensitive cells that send signals to the brain (see chapter 2, question 7: Why do bats have big ears?). George Pollak demonstrated that neural cells in a structure in the midbrain called the inferior colliculus are particularly sensitive to time differences, and it is the time delay between each call and its returning echo that tells a bat the distance to an object. Each cell is tuned to a very specific frequency, and the sound information is integrated and sent from the inferior colliculus to specialized areas in the auditory cortex where different characteristics of sound are processed . An echo will not reach both of a bat’s ears at exactly the same time, and the difference between the time an echo reaches each ear helps the bat to determine the exact location of an object. The tiny folds and projections in a bat’s ears also affect reception of the echo, providing additional information BAT BEHAVIOR 55 Figure 17. The echolocating pallid bat, Antrozous pallidus, sometimes captures prey on the ground. (Photograph courtesy of Bat World Sanctuary, www.batworld.org.) about the location of the target. Neural processing occurs in a fraction of a second after each echo is received, and the result is a steady stream of information for the bat about the size, location , and movement of objects in its environment. When hunting, the duration of each echolocation call and the time between...