Moreover, all diving animals, from birds to reptiles to mammals, experience a drastic slowng of the heart rate when diving. In seals, whose normal surface heart rate may be seventy to eighty times a minute, the heart slows to six and ten beats a minute upon diving.
여기에서 When porpoises venture into lower waters, their heart rate slows down. 이게 ture 든데 도대체 왜 그런지 모르겟어요..