Roy Adams commented on Playing Possum by Susana Monsó
How animals conceive of death and dying—and what it can teach us about our own …
Page 148 "In contrast to our own case, for dolphins breathing is not a reflex, but each inhalation is instead the result of a voluntary decision, which is why they have at their disposal a fairly easy means to end their own lives."
Some internet research confirms that dolphins and whales are both voluntary breathers.