These low amplitude, tonal, and very low frequency “contact calls” are one of the most common silky sifaka vocalizations. They are often emitted during and just prior to group movement, while also being emitted in a variety of affiliative and resting contexts. Oddly, as in the case above, a fundamental frequency often seems to be absent; this mar be an anti-resonance in which properties of the vocal tract filter out the frequencies containing the fundamental.