In Desolation Island, the Doctor gets a chance to see a blue whale (Balaenoptera musculus, aver. length 90 ft., weight over 100 tons). As the animal swims past the ship, the Doctor only sees part of it as it rolls up to get a breath. Most whales spend only 5% of their time on the surface of the ocean. When they need to breathe, and whales are mammals - warm blooded, air-breathing - they just use a upward roll to put their nostril (blow-hole) above the water to breathe out and in again. This particular species of whale is the largest, most massive, animal that has ever lived, more massive than the largest Dinosaurs.
Blue whales are one of the baleen whales (whales without teeth). They have sheets of baleen hanging from their upper jaw in the place of teeth. And they use these to filter their food; primarily very small crustaceans called krill, out of the water. They live in the northern and southern oceans, feeding on these krill that can reach great numbers in the summer. In the late fall, winter, and early spring, the whales do not eat but live off the stored calories in their fat. Of course it is that fat that has made whales a prey for humans.
This is where I am going to leave you, standing by the railing with Doctor Maturin looking at the natural wonders of the world; Captain Aubrey is behind us performing astounding feats of seamanship. We are told that Patrick O'Brian has died, but Captain Aubrey and Doctor Maturin cannot die. They are two of the immortals like Don Quixote or Sherlock Holmes; they will live forever.
Do you want to see something that the Doctor missed?