Imagine hearing a heartbeat like sound in the ocean, but not knowing the source.
Whale Trust researchers spent a decade listening for these sounds and wondering what they were. Finally, on a calm glassy day these sounds were recorded within a few meters of a pair of humpback whales. Even more convincing, the sounds increased in volume as the whales got closer and became softer as the whales swam away. This led to the publication of a new paper describing these so-called mystery sounds for the first time.
Are These Whale Sounds?
To be clear, we still cannot be 100% confidant that these sounds were made by humpback whales … but the circumstantial evidence is intriguing and prompting us to want to know more. A couple of our best examples are below.
Why Are These Sounds Not Heard More Often?
One of the questions that has arisen is why these sounds are not heard more often. This is a very good question. Indeed, we wonder why we don’t hear them more often. At least part of the answer lies in just how difficult they are to hear, even under perfect listening conditions. The sounds are only slightly above the threshold of human hearing, and easily masked by background ocean sounds from waves and wind, from other whale sounds, and by human noise from boats. Another reason is that there are really not that many people listening.
Why Is All This Important?
If verified, and these sounds are indeed another communication channel for humpback whales beyond the familiar song and social sounds, it could completely change how we view and interpret whale behavior on the breeding grounds.
The Sounds…Can You Hear Them?
Below are spectrographs (pictures of sounds) that illustrate these bouts of low frequency pulses (sounds). The spectrograph displays frequency (how high or low the sound is) on the vertical axis and time on the horizontal axis.
The degree of boldness of the sound pattern indicates the relative intensity or closeness of the sound to the hydrophone. The pulse patterns typically are heard around 40 Hz. Humans can hear from about 20 Hz to 20,000 Hz, so these sounds approach the lower threshold of our hearing.
How Best to Listen
Use earphones, or connect your computer to the biggest woofer you can find. These are base sounds, so they can be difficult to hear amongst all of the other humpback whale sounds (both song and social sounds) that you hear clearly.
Watch the cursor on the spectrograph as it passes over the bright yellow sounds to know when you should be hearing these lower frequency sounds. Listen for a series of pulses: babum, babum, babum. Think heartbeat.
Example 1: February 12, 2005
Recorded off Maui near a competitive or surface-active group of 8 whales (usually comprised of one female and multiple males). During this encounter lasting 70 minutes, the low frequency pulses were present on 8 of 9 recording tracks. One of the tracks illustrated below.
Example 2: March 8, 2013
Recorded over a 22-minute period while a pair of adult whales, a male and a female, circled around our research boat. The low frequency pulses were heard during the first 13 minutes of this recording; none were heard after that.
The bouts of pulses ranged from a few seconds to over 90 seconds in duration with variable gaps in between them. They increase and decrease in intensity is dependent on the position of the whale relative to the hydrophone.
One of the most exciting parts of this study was the ability to place the sounds within a behavioral context. Play the video below to see the context that we recorded these sounds in on March 8, 2013.
This video shows a male and female pair circling our research boat. The hydrophone cable is just visible hanging from the corner of the boat.
Note: the sounds and video are not synchronized, that is the pulse recording and video were taken with the same whales over the same 20-minute period but independently.
Video Credit: Jason Sturgis/Whale Trust Maui. NOAA permit #13846. Courtesy “Humpback Whales”, Pacific Life and MacGillivray Freeman Films.