One of the less obvious difficulties of ocean life is getting a good night's sleep. Since whales must make a conscious decision to breathe, it isn't feasible for them ever to be completely unconscious. They might not wake up in time to take a breath! Marine biologists have determined that whales get around this problem by letting only one-half of the brain sleep at a time. In this way, the whale is never completely unconscious, but it still gets all the rest it needs.
We can't know for sure what this rest state feels like, but most likely, it is something like the semi-conscious state we experience as we begin to fall asleep. We're pretty close to unconsciousness, but have enough awareness of our surroundings to wake up completely if we need to. It's not uncommon to see a whale logging, swimming slowly along the surface with very little movement. Presumably, whales behave this way when they are at rest.
Another problem with ocean life is water pressure. Because of the pull of gravity, water exerts greater pressure at lower depths than it does at higher depths; essentially, you feel the extreme weight of all the water above you. Humans and other land mammals can only swim down to a relatively shallow depth before the increased pressure crushes their bodies. This is because the air that mammals carry in their lungs exerts only a certain amount of outward pressure (pressure from the inside of your body). As the difference between the outward pressure in the lungs and the inward pressure of the water increases, there is a greater force pushing in on the sides of the body. At a certain point, this force exceeds the structural integrity of the rib cage, and the rib cage collapses. Obviously, this would kill a human being.
Whales can withstand this pressure because their bodies are more flexible. Their ribs are bound by loose, bendable cartilage, which allows the rib cage to collapse to some degree under high pressure that would easily snap our bones. A whale's lungs can also collapse safely under pressure, which keeps them from rupturing. When the lungs collapse, the air inside them is compressed, maintaining a balance between inward and outward pressure. These adaptations are particularly important to sperm whales, which dive to depths of 7,000 feet (2,133.6 m) or more, hunting for the giant squid that live at these great depths.
In addition to adapting to the difficult conditions of ocean life, whales have also adapted to take full advantage of its rich resources. In the next sections, we'll look at some of the remarkable features that help whales catch their prey.