Humpback whales have for years left marine biologists dazzled and mystified with their eerily beautiful yet bafflingly complex array of songs. Though they have always been known for their wide vocal range, humpbacks in the waters around Hawaii have recently been observed producing bursts of sound at much lower frequencies than any previously recorded, adding to the sense of mystery surrounding their choral behavior.

The new sound was recorded on a number of occasions between 2005 and 2013, always at the height of the winter assembly, which sees around 10,000 humpbacks gather in the region each year to mate and give birth. For this reason, James Darling of Whale Trust Maui speculates that the peculiar call may be somehow “involved in breeding behavior patterns,” yet admits that many questions remain unanswered.

Describing the whales’ behavior in the Journal of the Acoustical Society of America, Darling explained that underwater audio recordings revealed low-frequency “pulses” of sound, strung together in groups called “packets”, which combine to form “bouts”. While previous recordings of humpback whale song have revealed a frequency range of between 80 hertz and 4 kilohertz, those witnessed by Darling registered just 40 hertz.

Speaking to National Geographic, Darling explained that it took him several years to identify the source of the sound, believing at first that it may have been generated by helicopters or submarines. While he still cannot confirm beyond doubt that it is indeed being produced by the whales, he says it’s “very unlikely” that anything other than the humpbacks is responsible for the rumblings, which he compares to “listening to a heartbeat with a stethoscope.”

The purpose of the newly-discovered vocalization is unknown, and adds to the mystery surrounding whale song in general. For instance, songs, which can last for up to half an hour and are performed exclusively by males, tend to be coordinated among whales living within the same area as one another, who typically perform almost identical vocalizations. However, the songs often change dramatically from one season to the next, making it difficult for researchers to decipher their function or meaning.

The bouts recorded by Darling were highly variable in length and structure, ranging from 3 to 93 seconds, with gaps between the packets of between 6 and 96 seconds. “The question arises as to whether the low frequency (ca. 40 Hz) pulses are a part of the humpback whale's song and social sound repertoire, or if they are a different “genre” entirely,” he ponders.

For an idea of exactly what these pulses sound like, check out the recording below.

_{Recording of whale sounds in Maui, including low-frequency pulses as described by James Darling. Credit: cdellamore}