Researchers working in Hawai'i waters have reported the first confirmed evidence that humpback whale calves produce sounds.

The purpose of the grunts and squeals recorded by the Cetos Research Organization has not been deciphered, but they could be alarm calls to summon the calf's mother or an expression of curiosity, said Ann Zoidis, director of the research project.

"We found that calves make sounds most often when separated from their mothers, but they are not always alarm calls. Sometimes they occurred when curious calves approached the divers and even made eye contact with them," she said. "In those cases, the sounds appeared to be in reaction to a novel object."

Marine scientists for years have been using single hydrophones to collect sounds from whale pods that included calves, Zoidis said, but it was difficult to pinpoint the individual animals responsible for the noises.

Cetos built a two-element hydrophone array that allowed researchers to do angular measurements to the sound source.

Zoidis, who also heads the Cetos Research Organization, said the study verified for the first time that humpback whale calves, both male and female, make sounds.

"This might seem obvious but actually had not been documented previously, and, in fact, the common theory in the scientific literature was that they do not make sounds," she said.

The federally and state-permitted research was conducted off Maui and Kaua'i during winter months from 2004 to 2008 by Cetos snorkelers equipped with the two-element hydrophone and video cameras to record calf sounds and behavior.

BURBLING TO MOM

Researchers found that humpback calves do not "sing" as older males do when they produce continuous, repetitive and highly structured phrases and themes. The recorded calf vocalizations were simple in structure, of short duration and limited in repertoire, according to the study.

The "social sounds" made by the calves included repetitive grunts that increased in amplitude and/or bubble streams and seemed to function as an alarm call to alert the mother, the study said.

In many cases when a calf approached close to a diver, it did so silently and there was no reaction from its mother resting at deeper depths, according to Zoidis. As the animals hovered near the diver, the calves produced grunts and squeals, often while twisting and turning in an apparently relaxed interactive mode, she said.

However, on three occasions when calves emitted repeated grunts with increasing incidence and amplitude, twice with bubbles and once with an accompanying jaw clap, the mother quickly surfaced, approached and seemed to herd the calf away from the diver.

Researchers also found vocalizations more common in lone mother-calf pairs than in situations where such a pair was accompanied by one or two escorts or in a competitive group typically consisting of five or more adult animals.

Zoidis said this could be because in competitive groups "there is so much action and pursuit of the female that calves are more focused on keeping up and staying out of the fray." Calves more frequently produced sounds under calmer circumstances when the mother is resting or during slow travel, she said.

"This tells us that calves do, in fact, communicate, and it tells us they are communicating to their mothers," she said. "It also shows they are curious and very definitely notice the divers and interact with them."

STILL TO BE TRANSLATED

Cetos is conducting further analyses of the acoustic characteristics and behavioral context of the calf vocalizations, including whether they are a potential indicator of stress, which could prove useful in shaping management policies to protect the endangered marine mammals.

An article on the group's research findings appears in the March issue of the Journal of the Acoustic Society of America.

Zoidis said a byproduct of the research is the data collected on the amount of time calves spend at or near the surface, where they are more vulnerable to vessel strikes. Cetos is trying to get funding to analyze the additional information, she said.

The nonprofit Cetos Research Organization, based in Bar Harbor, Maine, and Oakland, Calif., has been doing research in Hawai'i since 2004. Although the research group was formed in 2003, its volunteer senior scientists include longtime marine mammal researchers such as Zoidis who have been working in Hawai'i waters since at least 1993.

Federal regulations prohibit approaching within 100 yards of humpback whales by any means (power boat, sailboat, surfboard, kayak, swimming, etc.). Researchers are required to obtain permits from the National Marine Fisheries Service and the state of Hawai'i to break the 100-yard distance requirement.

Reach Christie Wilson at cwilson@honoluluadvertiser.com.