You have to hand it to bats. They are a highly successful species packed full of special abilities. They have fortified immune systems, and they are the only flying mammal on Earth. Of course their echolocation abilities are phenomenal. Scientist have discovered that bats have the largest vocal range of any mammal. One species can emit sounds from 1 kilohertz to 120 kilohertz--a seven octave range--while humans mostly have three. A study from Denmark and published in PLOS Biology has determined how bats produce such a wide range of sound using high-speed videography.
Myotis daubentonii, named after French naturalist Louis Jean-Marie Daubenton, has two sets of vocal structures. One produces the high pitch 'shrill' used to locate prey and navigation. The other produces low pitched 'growls' used to express discomfort and antagonistic social calls. The research team used excised larynxes to study how air moves through them, but also live bats which they gently prodded with paintbrushes to induce the mammal to emit low pitched sounds.
The two registers are distinct physically and also adapted to the distinct purposes, High-frequency sound waves travel much faster for short distances, which is suited for locating pursued prey or avoiding collision with obstacles. Low-frequency waves travel farther, which allows communication with distant bats. Daubenton bats have vocal folds and membranes. Vocal membranes are extremely thin and low-mass allowing them to vibrate rapidly. Vocal folds are much heavier. These so-called "false vocal cords" were found to produce the growls used for social communication. Not all bat species have this arrangement, using instead clicks or hisses.