When dolphins swim in the ocean, it seems effortless. Whipping their tails up and down, the elegant sea mammals propel themselves forward in a flawless glide that could make any human swimmer jealous. But that up and down tail motion puts a lot of stress on a dolphin’s body, compressing its organs and sending blood pressure pulses to its brain.
Today, researchers in Canada have a theory about how cetaceans (dolphins, whales and porpoises)manage to protect their brain of these swimming-induced blood pressure pulses. As described in a new article published in Science, all thanks to specialized networks of blood vessels known as “retia mirabilia”.
Scientists have long known that many animals carry retia mirabilia. The Greek physician Galen described the structures in the 2nd century CE and gave them their name, which translates to “wonderful nets”. Indeed, the retia mirabilia look like complex stringy nets made up of thin veins and thick arteries. They are found in a variety of mammals, birds, and fish, but rarely in humans.
In most animals that have them, the retia mirabilia serve as a temperature-regulating mechanism and have a unique structure. “You can almost imagine drawing a flower with a very large center – like a sunflower, for example – and thinking of it as a large central tube surrounded by several smaller tubes around that circle,” explains Sarah Kienle, a biologist at Baylor University, who was not involved in the recent study. “That’s basically what we’re talking about.”
This large central artery carries warm blood from the body’s heart to its extremities, while surrounding veins carry cold blood in the opposite direction, Kienle explains. And because they’re located right next to each other, heat is transferred between the artery and veins to make sure neither is too cold or too hot.
Flamingos are a classic example of animals that benefit from retia mirabilia, says Kienle. “Because they stay in the water at night, [retia mirabilia] in their legs help stop all the cool water from making their body temperature too cold,” she adds. Similar retia mirabilia have been found in marine mammals, helping to regulate the temperature of their fins, tongue and testicles.
Dolphins and other cetaceans have additional retia mirabilia that wind around their lungs, in their spine, and in their brains. These particular networks are quite different from those found in other animals. For one, the blood vessels involved are much larger, resembling a twisted mass of worms. On the other hand, they don’t seem to work as a temperature controller.
“This area – this region of the chest cavity leading to the brain – is much less studied and identified in mammals and especially in marine mammals,” says Kienle. She adds that there have been a number of hypotheses about the function of structures in this area, but no explanation has been well tested or widely accepted. The authors of the new Science paper think they have found the answer.
Researchers examined the internal biological structure of 11 different cetacean species, including fin whales and bottlenose dolphins. Some of the animals were dissected by these scientists, while others had been analyzed by other biologists in previous research. “All of them were animals that had already died,” most of them stranding themselves, says Robert Shadwickbiomechanics researcher at the University of British Columbia, co-author of the article.
The analysis of the entrails of all these cetaceans took time. “It took about 10 years for this study to materialize, more than 10 years, in fact,” says Wayne Voglbiologist at the University of British Columbia, who also participated in the study.
Based on their analysis, the researchers now believe that one of these previously puzzling retia mirabilia present around the brains of cetaceans likely developed as an adaptation to protect against the physical demands of swimming.
Whales, dolphins and porpoises evolved from mammals that once lived on land. Tens of millions of years ago, the ancestors of cetaceans rejected life on land in favor of the open ocean. The transition to an aquatic existence was no small feat for these mammals; it required a number of specialist adaptations.
One challenge these creatures had to overcome was the stress created by swimming on the body. As stated earlier, dolphins propel themselves forward by thrusting their large tail up and down, which causes such stress. This is also the case for other cetaceans today. “The body cavity is entirely under the spine, so when descending, everything below the spine is compressed,” Shadwick explains. “And on the upstroke he’s decompressed.”
This constriction and relaxation, Shadwick explains, is the source of enormous pressure, not only on the organs of cetaceans, but also on the surrounding blood vessels. Eric Ekdale, a biologist and paleontologist at San Diego State University, who was not involved in the study, compares this process to sit-ups. “When we do sit-ups or sit-ups, we’re compressing our abdominal cavity,” he says. “We breathe in, and then when we do the sit-up, we exhale, and that relieves some of the pressure.”
But marine mammals don’t have the luxury of exhaling. Except for times when they surface to breathe, cetaceans must hold their breath while swimming. How, then, do cetaceans deal with the internal pressures caused by their tail whips? In particular, how do they ensure that the blood pressure pulses generated by each downward blow will not cause brain damage when they reach the skull?
This is where the retia mirabilia come in. Shadwick and his colleagues hypothesize that one of these spongy networks that sits next to the cetacean brain attenuates pressure impulses as blood passes through it. Specifically, the researchers propose that this rete mirabile (the singular form of “retia mirabilia”) transfers impulses from veins to adjacent arteries in a way that protects the brain from damage.
To test this claim, the researchers developed a computer model based on the internal biological structures of the 11 species they observed. And indeed, they found that their hypothetical pressure transfer system worked: it was able to shield the animals’ brains from 97% of pressure impulses. They are now convinced that they have found the long-sought secret purpose of the “wonderful nets” of cetaceans.
Vogl also points out that seals, which belong to another group of marine mammals, do not have a rete mirabile around their brains. This further confirms the team’s hypothesis about the function of the network. While cetaceans swing their tails up and down, compressing their organs against their spine, seals swing their tails left and right, which doesn’t cause the same internal pressure. Seals don’t need to regulate swimming-related blood impulses – and if that’s what a cranial rete mirabile is for, that explains why seals don’t have one.
Vogl speculates that cetacean ancestors probably had retia mirabilia leading to the brain before they took to the oceans, but that this network served a different purpose on land. “I suspect he was probably thermoregulatory at some point and the function changed,” Vogl says.
But Ekdale, who studies the evolutionary transition from mammals to the ocean, isn’t sure. He suspects that the terrestrial ancestors of cetaceans did not have a retia mirabilia leading from the spine to the brain and that this network only developed after these mammals traveled to the oceans and had to adapt to swimming out of breath. “It’s probably a new structure, a new adaptation to life in water,” he says. But he admits it’s impossible to know exactly when this structure developed because soft tissues such as blood vessels are not preserved in the fossil record.
Despite taking a different stance on its origins, Ekdale says he finds the new paper a plausible explanation for the function of the once mysterious and undeniably wonderful network of blood vessels around the brains of whales and dolphins. “I think it’s kind of an interesting solution to the specific problem of a fully aquatic mammal,” Ekdale says.