A new study published Tuesday in Frontiers in Physiology sheds light on just how the dolphin accomplishes this remarkable feat.
Training Time — In the study, researchers trained three male bottlenose dolphins (Tursiops truncatus) to respond to three different hand signals. Each signal indicated a different interval of time that the dolphin should hold their breath.
"By conditioning, or training, the dolphins to associate three different signals to three types of signals, with time, the dolphins then associate these with the different tasks of long breath-hold, short breath-hold, and hold your breath — but as long as you want," Andreas Fahlman, lead author on the study and director of the Oceanogràfic Foundation, tells Inverse.
The researchers signaled to the dolphins roughly 5-10 seconds before they went underwater, and they regularly changed the signals between dives.
The dolphins seem to be able to adjust their heart rates accordingly, depending on the kind of dive the researchers signaled.
"We show that there is a connection between what we ask them to do and they then prepare differently if we ask them long or short dive," Fahlman says.
"What we show is that they both have a lower heart rate and a faster drop in the heart rate, and this [study] shows that they can vary their drop in heart rate depending on condition."
By reducing their heart rates, the dolphins avoid the typical decompression sickness, or the "bends," human divers encounter when they descend into the deep sea and face rapidly increasing water pressure.
How It Works — In a previous study, the same team researchers established the Selective Gas Exchange hypothesis, which explains how marine mammals — like dolphins — conserve oxygen and avoid decompression sickness.
"[The theory] proposed that by manipulating how much blood is directed to the lungs and to which region of the lung" the animals are able to "select which gas to exchange," Fahlman says.
"They can therefore still take up oxygen, remove carbon dioxide and avoid the exchange of nitrogen" — the cause of decompression sickness in humans.
By observing the different heart rates of the dolphins in each type of breath-hold, the scientists demonstrate how the dolphin's unique "dive response" is not merely a reflex, but an active response.
"The so-called “dive response” is supposed to be a reflex. As such, it should occur every time an animal dives, and it should be more or less the same," Fahlman says. "What we show is that the dive response is not a reflex, and, with this large variation in responses that they can decide to what extent they change the heart rate during a dive."
But it's not entirely clear whether dolphins are conscious of responding in this way on a cognitive level.
"We are trying to avoid the world voluntary, cognitive, or conscious, as, currently, such suggestion is without evidence," Fahlman says. "
There is no anatomical evidence or structure similar to other responses that are voluntary — for example, breathing. "
A lesson for human divers — The ability to regulate heart rate and withstand deep-sea compression is essential to the dolphins' survival. But humans also threaten dolphins with our manmade noises, such as the blasts produced during underwater oil excavations.
The blasts stress out the dolphins. And according to the Selective Gas Exchange hypothesis, stress affects these animals' ability to regulate their heart rate on long dives.
"When stressed, this ability to decide the heart rate probably no longer works to help reduce the risk of decompression sickness or the bends," Fahlman says.
By understanding the effect stress has on dolphins' heartbeat regulation, then we can modify our actions accordingly to minimize harm, Fahlman says. For example, instead of sudden explosions, we could instead produce gradually increasing noises during underwater explorations.
The data also hold implications for other species of marine mammals.
"These data are important as it shows that this ability is likely to exist in more species of cetaceans," Fahlman says.
Though dolphins live in water and humans on land, there is much we can learn from our marine friends.
"If the Selective Gas Exchange hypothesis is correct, then it would be difficult to see how this could help humans directly," Fahlman says. "However, a better understanding [of] how [dolphins] have found ways to reduce the risk may result in alternative mechanisms that could lead to similar results," he says.
Abstract: Previous reports suggested the existence of direct somatic motor control over the heart rate (fH) responses during diving in some marine mammals, as the result of a cognitive and/or learning process rather than being a reflexive response. This would be beneficial for O2 storage management and also allow ventilation-perfusion matching for selective gas exchange, where O2 and CO2 can be exchanged with minimal exchange of N2. Such a mechanism explains how air breathing marine vertebrates avoid diving related gas bubble formation during repeated dives, and how stress interrupts this mechanism and causes excessive N2 exchange. To investigate the conditioned response, we measured the fH during a breath-hold in three bottlenose dolphins (Tursiops truncatus) when shown a visual signal to perform either a long (LONG) or short (SHORT) breath-hold, or during a spontaneous breath-hold without a signal (NS). The average fH (ifHstart), and the rate of change in fH (difH/dt) during the first 20 s of the breath-hold differed between breath-hold types. In addition, the minimum instantaneous fH (ifHmin), and the average instantaneous fH during the last 10 s (ifHend) also differed between breath-hold types. The difH/dt was greater, and the ifHstart, ifHmin, and ifHend were lower during a LONG as compared with either a SHORT, or an NS breath-hold (P < 0.05). Even though the NS breath-hold dives were longer in duration as compared with SHORT breath-hold dives, the difH/dt was greater and the ifHstart, ifHmin, and ifHend were lower during the latter (P < 0.05). In addition, when the dolphin determined the breath-hold duration (NS), the fH was more variable within and between individuals and trials, suggesting a conditioned capacity to adjust the fH response. These results suggest that dolphins have the capacity to selectively alter the fH-response during diving and provide evidence for significant cardiovascular plasticity in dolphins.