BEIJING, March 18 (Xinhuanet) -- It's for sure: humpback whale calves make sounds.

The nonprofit Cetos Research Organization studied humpbacks off Maui and Kauai, Hawaii, and said the sounds are directed at their mothers and may be expressions of curiosity or warnings of potential danger.

The sounds are not as complex as the continuous, repetitive and highly structured phrases and themes of older males, the researchers found. The calves instead produced a limited number of sounds that were short and simple in structure, according to the study.

The noises included repetitive grunts that increased in strength and were sometimes accompanied by bubble streams and seemed to function as an alarm call to the mother, the researchers found.

They say the sounds were produced more frequently during calmer periods when the mother was resting or during slow travel. "This tells us that calves do in fact communicate, and it tells us they are communicating to their mothers," Ann Zoidis, project director said.

Prior to Cetos' research, scientists had recorded sounds from whale pods that included calves. But Zoidis said they had difficulty pinpointing the particular animals responsible for the noises.

She said the common theory among scientists was that humpback calves did not produce sound.

She said Cetos researchers traced the sound back to its source, demonstrating that both male and female humpback calves make noises.

The group is continuing to study the sounds, including when they are emitted. The researchers are trying to determine whether they are a potential indicator of stress.