New right whale mothers living in the North Atlantic Ocean tone down their underwater vocalizations, and "whisper" to their young calves to avoid attracting predators, according to a study.
The researchers, including those from Duke University in the US, said that the critically endangered whales had few natural predators, but the calves were vulnerable to orcas and sharks.
The study, published in the journal Current Biology, noted that new whale moms, when communicating with their calves, drastically reduced their "up call" which they typically used to engage with other right whales over long distances.
The researchers said that the up call typically consisted of a short, loud nearly 2-minute long "whoop" sound that rose sharply and traveled long distances underwater.
However, they added that the same call was modified while communicating with their young ones into a very quiet, short, grunt-like sound that is audible only over smaller distances.
The sounds were previously unknown to scientists, and "can be thought of almost like a human whisper," said Susan Parks, lead author of the study from Syracuse University in the US.
Parks said that the variant of the call allowed the mother to stay in touch with the calf without advertising their presence to potential predators in the area.
To make this observation, the researchers attached small, noninvasive recording tags via suction cups onto North Atlantic right whales in calving grounds off the coasts of Florida and Georgia in the US.
The researchers also attached tags to juvenile and pregnant whales found in the area, and to mother-calf pairs.
"The mothers significantly reduced the number of higher-amplitude, long-distance communication signals they produced compared to the juvenile and pregnant whales," said Douglas Nowacek, co-author of the study from Duke University.
Nowacek added that the right whale mother-calf pairs relied on acoustic crypsis -- a type of behavior meant to avoid detection -- to reduce the risk of eavesdropping by predators lurking in the nearby waters.
"Right whales face a number of challenges, including a very low number of calves born in recent years, combined with a number of deaths of reproductive females by collisions with large ships or entanglement in fishing gear," Parks said.
According to the researchers, only about 420 North Atlantic right whales exist in the wild today, with 30 confirmed deaths in the past three years, making any additional death a huge blow to the species' chances of survival.
Parks said that many behavioral traits of right whales are still unknown, and added that more studies on the front would improve conservation efforts.
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