Filter feeder gap between the two rows of baleen plates lets the water in together with the prey, while the baleens filter out the water. Rorquals such as the blue whale, in contrast, have smaller heads, are fast swimmers with short and broad baleen plates. To catch prey, they widely open their lower jaw — almost 90° — swim through a swarm gulping, while lowering their tongue so that the head's ventral grooves expand and vastly increase the amount of water taken in. Baleen whales typically eat krill in polar or subpolar waters during summers, but can also take schooling fish, especially
Janjucetus snout is wider, which may have been a precursor to the large mouths of modern baleen whales. Like other baleen whales, "Janjucetus" did not possess the ability to echolocate, however it may have had a large line of fat along its lower jaw, similar to modern toothed whales (Odontoceti), which would mean it could detect ultrasonic signals. It had unusually large eyes for baleen whales compared to its body size, which were positioned high up on the skull; likewise, it probably relied on good eyesight instead of echolocation to navigate. "Janjucetus" did not have baleen, and instead had large teeth.