Heart rates and gas exchange in the Amazonian Manatee (Trichechus inunguis) in relation to diving

Unrestrained Amazonian manatees (Trichechus inunguis) maintained a constant heart rate during diving and exhibited a slight tachycardia during breathing. ‘Forcing’ the manatees to dive caused a marked bradycardia. They exhibited a more pronounced tachycardia during breathing after ‘forced’ dives and hyperventilated during recovery dives. Manatees are capable of dives exceeding 10 min duration without having to resport to anaerobic metabolism, and even after 10 min dives recover within 3–4 short dives. The ability of manatees to make long dives, in spite of relatively poor O₂ stores, is due to their low metabolic rate, while the rapid recovery is aided by their high CO₂ stores which minimizes CO₂ storage in the body. In manatees the changes in alveolar O₂ and CO₂ pressure ( $PA_{O_2 } $ and $PA_{CO_2 } $ ) in relation to dive time are slower and more variable than in other marine mammals. The lower rate of change is probably due to the manatees' reduced metabolic rate, while the greater variability is due to their breathing pattern, in which both ventilation and body gas stores influence alveolar gases.