When dolphins dive deep to search for prey, their respiration stops. Their heartbeat slows. Yet they are still able to power all of the functions they need to hunt. No one is entirely sure how they do it, because no one has ever been able to measure their blood oxygenation levels during these deep dives. It’s a logistically complex challenge; how do you attach a sensor to a massive sea-dwelling creature without doing harm? The rudimentary data that researchers have gathered so far has come from heart rate monitors bound to dolphins’ bodies with thick, wide neoprene straps—dolphin girdles, essentially.