Diving mammals can spend extended periods of time underwater. How do their bodies keep oxygen flowing when they are submerged? Angelo Incitti, MS, a research associate at the National Marine Mammal Foundation in San Diego, presented his research at the American Physiological Society’s 2026 Comparative Physiology conference last month and discusses his team’s efforts to answer this question.

Q: What are you studying?

A: We are examining blood flow through some of the body’s smallest blood vessels, called capillaries, in diving marine mammals. We are working with bottlenose dolphins and California sea lions at the U.S. Navy Marine Mammal Program that are trained to perform 3- to 5-minute simulated dives. We collected voluntary blood samples and intravital microscope images before, during and after each dive to look at capillary structure and blood flow in states of rest, hypoxia (low oxygen) and oxygen recovery.

Q: Why is it important to study tiny blood vessels of diving animals?

A: As some of the best divers on the planet, we know these animals possess unique adaptations that allow them to load, store, deliver and conserve oxygen with remarkable efficiency. As we complete data collection, we hope to use additional metrics from the videos to estimate rates of oxygen delivery and make comparisons between these species. 

Q: How do you look at blood flow in an animal while it’s underwater?

A: We had to get very creative! We built a custom station that the dolphins swam onto while presenting their fluke (tail) so we could take blood samples. We positioned the intravital microscope camera under their tongue while they rested on the station. We then lowered the station beneath the surface of the water (to simulate a dive) while the animals’ fluke remained on deck for blood samples to be continuously collected. After three minutes, we submerged the camera and collected intravital microscope data during the dive, before raising the station and collecting the final set of data after the dive.

The sea lions were trained to lie on their side and submerge their head in a large tub, where we completed a similar process. The entire process was done voluntarily by the animals, which took months of training.

Q: What can watching individual red blood cells tell you about blood flow during a dive?

A: It allowed us to measure changes in blood flow before, during and after dives in each species and potentially estimate how much oxygen is being delivered to the tissues. We are currently working on new analysis approaches needed to estimate rates of oxygen delivery.

Q: Your study is the first to examine blood flow using a microscope camera in marine mammals. Why is that important?

A: These data have never been collected in marine mammals. They serve as baseline measures of blood flow in the capillaries, including how fast red blood cells move, how many pass through the vessels, how curvy the vessels are and how densely packed the capillaries are in these species.

Q: What’s the most important thing you’d like people to know about your study?

A: Learning more about microcirculation in marine mammals may improve our understanding of how oxygen delivery works in mammals.