To keep their newborn babies safe from falls and injuries, human moms might lay them to sleep in cribs with high sides. Whale moms use another safety strategy for their young calves: They migrate thousands of miles to raise them in shallow-water nurseries.
The shallow water is between 16 feet deep and 33 feet deep. But the zooplankton that adult right whales eat does not grow much in these places. Which means the moms are “just fasting the whole time they’re nursing their calves, and they use up all their energy stores,” says Julia Marie Zeh. She’s a marine biologist at Syracuse University, where she studies baleen whales such as right whales and humpbacks — as opposed to toothed whales such as sperm whales and dolphins.
Why would these animals make this puzzling choice? Zeh and her colleagues had a theory. They already knew that whale moms and babies didn’t “talk” much to each other. And when they did, they tended to whisper. That meant a whole lot of quiet was going on in the nursery bays, where Zeh has seen dozens of the huge mammal pairs hanging out together. (The average right whale mom is 52 feet long and 80,000 pounds; newborns are 14 feet long and 2,000 pounds and grow quickly.)
So the researchers ran an experiment. They used a computer to model the way sound travels in shallow water compared with how it travels in deeper water in the open ocean. They discovered that in shallow water, sound loses energy quickly. This is because it is absorbed by the ocean bottom and is also reflected off the water’s surface.
“Close to shore is where whale sounds travel the least,” says Zeh. “Another animal would have to be very close in order to hear them.”
Eavesdropping animals could include orcas, which eat whale calves. They also could include right whale males, which are interested in mating with the moms. “They can get very aggressive, and a calf can get caught in the middle and injured,” Zeh says.
Understanding the way sound travels (or doesn’t) in these whale nurseries gets scientists such as Zeh a step closer to understanding why these animals migrate as far as they do instead of staying put. It also helps them communicate how to better help whales survive in the wild.
Many whale species were seriously affected by whaling starting in the 19th century. Some southern right whale populations have rebounded. But their North Atlantic right whale cousins are still highly endangered, with fewer than 350 animals estimated to be left. They can get tangled in fishing lines and are vulnerable to being struck by ships.
“We can take some of what we’re learning with southern right whales and apply that to aid Northern Atlantic right whales,” says Zeh. “If we can understand which habitats are preferred by these whales, and why, we can understand which areas are important to protect.”