Jessica Kendall-Bar, Drs. Dan Costa and Terrie Williams at the Long Marine Lab of UC Santa Cruz. NMFS Permit #19108
We can now monitor what happens inside diving animals in unprecedented detail thanks to a non-invasive way of recording blood flow and oxygen levels in their brain. The first study with this device shows seals consciously reduce blood flow to their blubber before diving.
Almost all mammals, including us, have the so-called dive reflex. It triggers changes in the body, such as reduced blood flow to the skin and a slower heart rate, that reduce oxygen consumption.
This reflex was thought to be an automatic response. In humans, for instance, it is triggered by breath holding and cold water on the face.
But recent studies suggest that at least some diving mammals have a degree of control. For instance, harbour porpoises slow their hearts more if they are planning to stay under longer.
We know little about other changes in the bodies of diving animals because they are very hard to study. So Chris McKnight at the University of St Andrews in the UK and colleagues developed a wearable, non-invasive device that uses near-infrared spectroscopy to monitor blood volume and oxygenation.
“It’s effectively like a Fitbit,” says McKnight. “It does not penetrate the skin.”
When the researchers attached this device to harbour seals, they found that the peripheral blood vessels of seals started to contract well before they dived – typically around 15 seconds before and sometimes as much as 45 seconds.
That means it must be under conscious control, says McKnight. “There’s no other stimulus.”
The seals also restored normal blood flow to the blubber several seconds before surfacing, again showing conscious control.
The study also revealed that when seals are feeding, they don’t bother to stay at the surface long enough to restore normal blood oxygen levels.
The team now plans to study other animals – including humans. The world’s top freedivers have set some astounding records in recent years. The record for breath holding without moving is over 11 minutes, and for swimming down and returning to the surface it is a depth of 130 metres.
McKnight suspects elite freedivers may have a degree of conscious control over the dive reflex, too.
Journal reference: PLoS Biology, DOI: 10.1371/journal.pbio.3000306