You see that when the mammals dives deep, the heart rate slows, then when the mammal comes back up to the surface of the water to breath, the heart rate speeds back up. The animals remains at the surface for a short period of time with that high heart rate before being able to dive again. Finally, the animal preforms its deepest dives first, then dives less deep over time. All of this is a response to having enough oxygen in its blood, dumping carbon dioxide waste at the surface and regaining oxygen, and over time being less efficient at this process.
