If these mysterious sounds can be conclusively linked to humpback whales, this new research would add a whole new dimension to the already complex repertoire found in this species’ communication. PHOTO BY JASON STURGIS, NOAA PERMIT #13846.

“Imagine a heartbeat,” said Dr. Jim Darling, research biologist of Whale Trust Maui, describing the series of low frequency sounds known as “pulse trains” he recorded near humpback whales off the coast of Maui.

These mysterious sounds are the subject of a recently published paper in the Journal of Acoustic Society of America (JASA), the result of studies supported by Whale Trust and the National Geographic Society.

At around 40 Hz, the sounds are much lower frequency than typical humpback whale sounds (80-4,000Hz). While other large whales and some land mammals such as elephants are known to use very low frequency sounds – even infrasounds below human hearing – this is the first report of their potential existence in humpbacks.

Just above the lower threshold of human hearing (20 Hz), these sounds can be easily masked by ocean waves, vessel noise or the more typical sounds of nearby humpbacks during their busy winter assembly off Maui.

Darling describes them as “ethereal” and difficult to hear except on exceptionally quiet, calm days at sea.

If these mysterious sounds can be conclusively linked to the humpbacks, this would add a whole new dimension to the already complex repertoire found in this species’ communication.

“The first time I heard them, or realized I heard them, was in 2005 when recording social sounds from an active group of eight whales,” explained Darling. “Although I have recorded samples since, it took a long and particularly good recording of a male-female pair in 2013 to convince me they were real.”

Humpback whales are well-known for singing long, complex songs, for shorter grunts, groans, whistles and throbs called “social sounds,” and even for sounds resulting from physical slaps made by flukes and flippers or bubble trains.

Separate underwater video taken at the same time as the 2013 sound recording showed no bubble production or other behaviors that might provide a ready explanation.

Recorded only a few times each year off Maui over the last decade, Darling cautions that while researchers cannot be 100 percent certain the humpbacks produced these sounds, on this last instance, they were recorded within 100 meters of the whales. Any fluctuation in volume appeared to match the whales’ approach to the boat, and no other species of whales were known to be in the area at the time.

Sound plays a huge role in the lives of humpback whales, not only in the winter breeding areas like Hawaii, but also on summer grounds when they may use group-specific feeding calls, or at times enigmatic click trains when foraging.

“If verified,” explained Darling, “this new sound offers another piece of the puzzle that is humpback whale society.”

Darling added, “We have a long way to go on understanding this, but it does remind us of how very, very little we know about these animals.”

Dr. Darling will be one of the renowned featured presenters at Whale Trust’s tenth annual Whale Tales educational event on Feb. 12-15, 2016 at The Ritz-Carlton, Kapalua.

He will share his latest findings into the similarities and differences of humpback whale songs across the Pacific Ocean. Presentations are free and open to the public.

Whale Trust Maui is a Maui-based nonprofit organization whose mission is to promote, support and conduct scientific research on whales and the marine environment, and broadly communicate the findings to the public.

Research led by Whale Trust Maui scientists has been featured in documentaries on the BBC, Discovery Channel, National Geographic International, National Public Radio, The Today Show, PBS, NHK (Japanese National Broadcasting) and in National Geographic Magazine (1999, 2007). The team is also featured in the IMAX film “Humpback Whales 3D” (2015).

For more information, visit: www.whaletrust.org.