Whale mothers choose nursery sites in shallow waters where predators cannot “eavesdrop” on communication between a mother and her young, researchers say.
Each winter, the whales migrate thousands of miles to these bay habitats to give birth and care for their young. But why choose such shallow nursery grounds that may be within dangerous proximity to human activity and where food supply is scarce?
While scientists have speculated that the up to 50-foot-long whales choose these locations for lack of predators and warmer and calmer waters, the researchers uncovered a new potential motive. They hypothesize that shallow, sandy, near-shore waters have reduced acoustic propagation, meaning vocal signals don’t travel as far at these sites.
That allows whale mothers to communicate with their nearby young, while not being heard by predators off in the distance.
Since questions remain about why baleen whales migrate such long distances every year, the researchers say their results shed new light on their migratory behavior.
Understanding habitat use and selection also allows researchers to better focus conservation and management efforts, which is critical for endangered whale species like the right whale in the North Atlantic.
The researchers gathered data at three nursery sites across three continents in the southern hemisphere (South America, Africa, and Australia) where southern right whale nurseries are commonly spotted. They found that the depth at which right whale mothers and their young are often observed has the most limited acoustic detection range for their calls.
“Animals that communicate using sound must balance the need to be heard by their intended audience and the risk of being overheard by eavesdroppers such as predators,” says Julia Zeh, a PhD candidate in biology at Syracuse University.
Changes to sound production behavior to reduce detectability by eavesdroppers is known as acoustic crypsis. Southern right whales have commonly used three forms of acoustic crypsis to avoid predators: reduction in call amplitude; using signal frequencies which are difficult for eavesdroppers to detect and/or localize; and reduction or complete ceasing of acoustic signal production, effectively going silent to avoid detection.
In their paper, the team propose a fourth method of acoustic crypsis centering around southern right whales’ habitat choice.
“We found that southern right whale mothers and calves spend time in specific locations where they can hear each other, but other animals can’t hear them,” Zeh explains. “These results follow on some interesting recent papers that recorded quiet calls, or essentially whispers, from right whale mothers and calves.”
Future research will be aimed at determining how common a habitat selection approach to acoustic crypsis may be.
Source: Syracuse University