What Is the Difference Between CF-FM Calls and CF Calls

Multiple Choice

What is the difference between CF-FM calls and CF calls in bats?

A) CF-FM calls involve long calls at a single frequency; FM calls involve a modulation of frequencies.
B) CF-FM calls involve a modulation of frequencies; FM calls use a single signal at a high frequency.
C) CF-FM calls require a loud initial call followed by a decrease in amplitude; FM calls use only a single amplitude.
D) CF-FM calls involve a series of calls at different amplitudes spaced out in a rhythmic pattern; FM calls use a single signal at a low amplitude.