Dolphins can slow their heart rate while diving and adjust their heart rate based on how long they plan to be underwater.
The authors of the work observed three male bottlenose dolphins (Tursiops truncatus), which were specially trained to hold their breath for different times in accordance with the commands.
We trained dolphins to hold their breaths long and short. When asked to hold their breath, their heart rate dropped before or immediately after the dive began. We also noticed that dolphins dropped their heart rate more and more rapidly in preparation for a long swim.
Andreas Falman, Doctor of the Valencia Oceanographic Park
The results indicate that dolphins, and possibly other marine mammals, can deliberately change their heart rate in accordance with the duration of the planned dive.
Dolphins have the ability to change our heart rate in the same way you and I can slow down our breathing, Fahlman suggests.
This, he adds, allows dolphins to retain oxygen while diving. It may also be key to preventing diving problems such as decompression sickness, which occurs mainly due to the rapid release of inhaled gas pressure.