Some male humpback whales lengthened their songs while others ceased to sing altogether when exposed to low-frequency sonar tests off the coast of Hawaii in 1998, suggesting that sonar transmissions by the U.S. Navy could disrupt whale breeding and cause other behavioral changes, according to a new study published in the journal Nature. On average, the whales’ songs were 30 percent longer than normal, a strong shift given that the sonar was tested at less than full strength, said Patrick Miller, lead study author and a scientist at the Woods Hole Oceanographic Institution. Still, he said the researchers didn’t notice any “extreme reactions” in the whales such as breaching. Many environmentalists are calling on the Navy to end some of its uses of sonar, saying that it can disorient and kill whales.